CHEM 351-2023.inorganic chemistry for universityey

CHEM 351 INORGANIC POLYMERS AND ELECTRON DEFICIENT COMPOUNDS

2 Course Outline Inorganic Polymers, Rings and Cages . 2. Electron-Deficient compounds e.g. Boron compounds. 3. Introduction to nanotechnology / chemistry

3 REFERENCES 1. Inorganic and Organometallic Polymers by Ronald D. Archer Concepts and Models of Inorganic Chemistry by Bodie E. Douglas, Darl H. McDaniel and John Alexander. Concise Inorganic Chemistry by J. D. Lee. Chemistry of the Elements by N. N. Greenwood, A. Earnshaw . Non-Metal Rings, Cages and Clusters by J. Derek Woollins . Electron Deficient Compounds by K. Wade. Advanced Inorganic Chemistry by F. A. Cotton, Geoffrey Wilkinson et al . Textbook of Nanoscience and Technology by B.S. Murty , P. Shankar, Balder Raj, B.B. Rath, James Murday . Nanoscience Materials in Chemistry, Wiley 2001

4 Course Objectives Provide the necessary skills for the synthesis, classification as well as the physical and chemical properties, and the uses of I norganic Polymers, Rings and Cages. Provide the skills to understand and appreciate the theoretical basis of structure and bonding as well as the physical, chemical properties and importance of electron deficient compounds. Provide some insight into nanotechnology/chemistry

Mode of Delivery 1. Direct Delivery in lecture rooms 2. On-line Lectures 3. Online lecture notes / power point materials 5

Mode of assessment 1. Class Assignments 2. ‘Impromptu’ Quizzes and Tests 3. Online Assignments/Tests 3. Class tests 4. Mid-Semester Examinations 5. End of Semester Examinations 6

7 Inorganic polymers Introduction By considering just the name one can say that they are non-organic or non-carbon containing polymers. The most obvious definition for an inorganic polymer is a polymer with a skeletal structure that does not include carbon atoms in the backbone. A Polymer that has inorganic repeating units in their main polymeric backbone. It is a giant 3D or 2D network structure made up by a number of covalent bonds but with an absence or near-absence of hydrocarbon units in the main molecular backbone.

Types of Inorganic Polymers 8 de Lill D. T. and Carraher , Jr C.E.

9 The Need for Inorganic Polymers Over Organic Polymers ! Organic Polymers: React with oxygen or ozone over a long period of time and lose their advantageous properties. Burn, often with the release of toxic smoke. Degrade when exposed to ultraviolet or gamma radiation. Sometimes soften at unacceptably low temperatures, or they swell or dissolve in organic solvents, oils, or hydraulic fluids.

10 Inorganic polymers: Inorganic elements can have different valences than carbon, and this means that the number of side groups attached to a backbone may be different from the situation in an organic polymer . This will affect the flexibility of the macromolecule its ability to react with chemical reagents its stability at high temperatures its interactions with solvents and with other polymer molecules.

The bonds formed between inorganic elements are often longer stronger resistant to free radical cleavage reactions than are bonds formed by carbon. 11

Some Important Inorganic Polymers Natural Inorganic Polymers Portland cement • agates • talc • zirconia • quartz • diamond • carboranes • carborundum • aluminosilicates Synthetic Inorganic Polymers polythiazyls • polyphosphazenes • poly(boron carbonitride)s • polysiloxanes • polysilanes • poly(sulfur nitride)s • polyphosphonitriles • polycarbosilanes 12

13 Classification of Inorganic polymers Classification based on: 1. connectivity 2. dimensionality 3. chemical constituents

14 Classification on the basis of connectivity Ray defines connectivity as the number of atoms attached to a defined atom that is a part of the polymer chain or matrix. This polymer connectivity can range from 1 for a side group atom or functional group to at least 8 or 10 in some metal-coordinated and metal-cyclopentadienyl polymers, respectively.

15 Connectivity of 1 Anchored metal-containing polymers used for catalysis can have connectivity values as low as 1 with respect to the polymer chain. Note that the metal can have other ligands as well, but in as much as they do not affect the polymer connectivity, the metal is defined as having a connectivity of 1.

Poly-(Sulphur nitride) (c) Poly( dichlorophosphazene ) (b) Linear polyphosphate (d ) A portion of a silicone chain where R is an alkyl organic group 16 . Connectivities of 2 C

Boric acid M A synthetic Silver polymer 17 Connectivity of 3 Arsenic (iii) sulphide

EDIT IN GOOGLE SLIDES Click on the button under the presentation preview that says "Use as Google Slides Theme" . You will get a copy of this document on your Google Drive and will be able to edit, add or delete slides. You havek A portion of asbestus chain Ultraphosphoric acid 18 Mixed Connectivity of 2 and 3

Vitreous silica has silicon atoms with a connectivity of 4. Boron and aluminium phosphates and many other three dimensional polymers have connectivities of 4 for at least one type of atom in the polymer . 19 Connectivity of 4

A number of polymeric inorganic species have mixed connectivity of 3 and 4, including some borate glasses, where the counter cations provide the counter charges for the four oxide ions connected to at least some of the boron atoms as shown in Figure . Presentation design slide ) 20 Mixed connectivity of 3 and 4

21 Connectivity of 6 Examples of connectivity of 6 include metal coordination polymers having metal atoms or ions joined with two tridentate ligands. A tridentate ligand is a ligand that has three atoms that are coordinated to the same metal atoms or ion.

22 Mixed connectivity of 4 and 6 Orthophosphates and arsenates of titanium, zirconium, tin, cerium, thorium, silicon, and germanium have mixed connectivity of 4 and 6.

23 Connectivity of 8 Metal coordination polymers of zirconium(IV), yttrium(III), and several lanthanide ions [cerium(IV), lanthanum(III), europium(III), gadolinium(III), and lutetium(III)] have been synthesized that possess connectivity of 8 because two tetradentate ligands are coordinated to each metal ion that is part of the polymer chain.

24 Classifications by dimensionality Pittman used this classification for polymeric species containing metal atoms in their backbones. Here the dimensionality will be used for all types of inorganic polymers. i . 1-D Polymeric structures ii. 2-D Polymeric structures iii. 3-D Polymeric structures

25 1-D Polymeric structures A linear chain polymer is categorized as a one dimensional (1-D) polymer even though it may have twists and turns in the “linear” chain. Simple polymer chains in which all of the atoms in the chain have a connectivity of 2 are classed as 1-D polymers. However, a linear chain polymer with one or more atoms of each repeating unit having a connectivity of more than 2 is also possible. For example, a polymer with benzene rings in the chain will have some carbon atoms with a connectivity of 3.

26 2-D Polymeric structures Simple inorganic species with a connectivity of 3 often lead to sheet or two dimensional (2-D) polymers as shown in Figure for boric acid & arsenic sulphide .

27 On the other hand, connectivity do not always determine the dimensionality. For example, the aqueous iron(II) oxalate polymer has the 1-D linear structure, but the analogous 2,5-oxyquinonate complex of iron (II) has a 2-D structure is as shown below. Iron (ii) 2,5-oxyquinonate

28 3-D Polymeric structures Inorganic polymeric networks in which bonding occurs in three dimensions are well known. Starting with quartz (SiO 2 ) as a prime example, the most common characteristic of such species is insolubility- unless decomposition occurs during a dissolution process. To have a true 3-D polymer, at least some of the atoms must have a connectivity of 4 or more. Some polymers, such as polysilynes are pseudo-3-D as a result of 3-D ring formation to relieve steric strain.

29 Prussian blue is a classic example of a mixed Fe(II) and Fe(III) 3-D polymeric structure, with each iron ion surrounded octahedrally by six cyano-ligands.

30 Classification on the basis of chemical constituents According to this classification method inorganic polymers are classified on the basis of the following parameters: 1. Wholly inorganic polymers 2. Inorganic-organic polymers 3. Organometallic polymers 4. Hybrid organic-inorganic polymers

31 Wholly inorganic polymers Inorganic polymers in this class constitute the major components of soil, mountains and sand, and they are also employed as abrasives and cutting materials (diamond, silicon carbide (carborundum), fibers (fibrous glass, asbestos, boron fibres), coatings, flame retardants, building and construction materials (window glass, stone, Portland cement, brick and tiles), and lubricants and catalysts (zinc oxide, nickel oxide, carbon black, silica gel, aluminium silicate, and clays).

32 Inorganic-organic polymers Inorganic polymers containing organic portions attached to inorganic elements in their backbone. The area of inorganic-organic polymers is very extensive. Some examples of this class are: polysilanes , polysiloxanes , polyphosphazenes.

33 Polysilanes Polysiloxanes Polyphosphazenes

34 Organometallic polymers Organometallic polymers are made of over 40 elements including main group of metals (Si or Ge), transition metals or rare earth elements in addition to the 10 elements (C, H, N, O, B, P, halides) which is found in organic polymers. The variations of organometallic polymers seem endless. Organometallic polymers are new materials which combine the low density and structural variations and functional group varieties of organic materials with electrical conductivity and the high temperature stability features of inorganic compounds.

35 Different structures found in organometallic polymers

36 Hybrid organic-inorganic polymers Hybrid organic-inorganic networks are multifunctional materials offering a wide range of interesting properties. Since there are countless different combinations of the organic and inorganic moieties, a large number of applications are possible by incorporation of inorganic building blocks such as silica networks, porous materials and metals.

37 Π- conjugated polymers prepared via organometallic condensation reactions

INORGANIC CARBON POLYMERS As carbon forms the basis for most synthetic polymers, it also forms the foundation for a number of important inorganic polymers. Diamond Elemental carbon exists in many different forms (allotropes), including the two best known: diamond and graphite. Natural diamonds form when concentrations of pure carbon are subjected to great pressures and heat by the earth’s mantle. They are particularly known as the hardest known natural material with the highest bulk thermal conductivity. 38

The diamond structure consists of a three-dimensional lattice of tetrahedrally arranged sp 3 -hybridzed carbon atoms arranged as a face-centered cubic crystal. As a result of its rigid structure, diamonds are generally highly pure with few contaminants. They are highly optically dispersive, so only a little impurity causes a relatively large color change. Introducing small amounts of boron causes them to become semiconducting, leading to their use in the production of transistors. 39

While the largest gem-quality diamonds are mainly found in nature, most diamonds are artificially made. These are small, often the size of a grain of sand. Commercially, because of their high strength, they are employed in cutting, shaping, grinding, boring, and polishing. Graphite Graphite occurs as two-dimensional sheets of hexagonally fused benzene rings composed of sp 2 -hybridized carbon. The bonds holding the fused hexagons together are “ordinary” covalent bonds. 40

The sheets, however, are held together by weaker π−π interactions, which can be explained to reinforce concepts such as dipoles and polarizability. While the bonds between carbon atoms in the sheet are extremely strong, the intra-sheet interactions are significantly weaker. This allows the sheets to easily slip past one another, and this “slipperiness” of the layers accounts for graphite’s use as a good lubricant for clocks, door locks, and hand-held tools and as the “lead” in lead pencils. 41

Carbon Nanotubes Carbon nanotubes are becoming one of the most important materials because of the exceptional properties and abundance of the feedstock, carbon. CNTs are generally classified into two groups: Multi-walled carbon nanotubes, MWCNTs, are comprised of 2 to 30 concentric graphitic layers with diameters ranging from 10 to 50 nm [and] lengths that can exceed 10 micrometers. Single-walled carbon nanotubes, SWCNTs have diameters ranging from 1.0 to 1.4 nm, with lengths that can reach several micrometers. 42

Depiction of a single layer of graphite (graphene, left) and how it can be folded to produce a carbon nanotube (here, zigzag conformation, middle). Top-down views (right) of a single-walled CNT versus a multiwalled CNT. 43

Coordination Polymers and Metal−Organic Frameworks The concept of network inorganic solids can be extended to include coordination polymers and metal−organic frameworks (collectively, CPs). CPs are the result of using multifunctional organic moieties to connect metal centers or clusters into one-, two-, or three-dimensional compounds CPs are commonly synthesized by hydrothermal or solvothermal means, where the reagents are combined into a Teflon lined autoclave and heated under autogenous pressure.

The microwave synthesis of CPs is becoming more common, but it is difficult to obtain single crystals using this method. More traditional crystallization methods, such as slow evaporation and solvent layering, have also been successful in producing CPs. If a CP has permanent porosity, it is typically called a metal−organic framework (MOF). MOFs possess some of the highest surface areas ever recorded, which coupled with their porosity leads to applications in molecular storage and separations, catalysis, and sensing, among others

Simplified way of depicting how an organic metal center or cluster can be combined with an organic “linker” moiety to produce one-, two-, or three-dimensional network solids.

Simple depiction of the self-assembly (top) to produce GWMOF-6 (bottom; yellow polyhedra = LnO 9 , black lines = C, blue dots = N, hydrogen atoms omitted) using HSAB considerations and the chelate effect.

Composition of the MOF-5 PBU (top), polyhedral representations of the PBU and SBU of MOF-5 (middle), and overall extended structure of MOF-5 (bottom) 48

Semiconducting Polymers Semiconducting polymers are not technically inorganic polymers, however they are used to merge and reinforce several concepts in inorganic chemistry Beginning with the structure of polyacetylene with a simple repeat unit C 2 H 2 , showing both cis and trans conformations, is an easy way to introduce the semi conductivity concept. This alternating structure leads to a fixed carbon backbone, but with a cloud of electrons spread, or delocalised , across the polymer chain. 49

Theoreticians proposed that, as the length of the chain tended to infinity, the conductivity of the material should increase dramatically. A quick review of the atomic structure of carbon and how the 2s and 2p orbitals can hybridize into sp orbitals can be beneficial. The sp 2 -hybridized orbital has a full valence complement once electrons from carbon and hydrogen are included, leaving one electron in a higher-energy p z orbital. If this simple molecular diagram is extended to an “infinite” number of C atoms, then band formation occurs. 50

With a half-filled p z orbital and thus a half-filled p z band, metallic conduction would be expected from these kinds of organic polymers based on sp 2 -hybridized carbon. The delocalization of electrons in these kinds of polymers is not complete, with some localization of electron density on individual C nuclei. The use Peierl’s theorem, which states that 1-D metallic conductors (like organic semiconductors) are unstable relative to their semiconducting states, and thus, there will be some way to split the half-filled p z band into a full valence band and empty conduction band with a band gap separating them. Even though metallic conductivity is theoretically expected in these systems, in reality they demonstrate semiconducting behavior. 51

Depiction of how organic carbon comes together to produce semiconducting organic polymers, which is useful in extending the basic concepts of band theory. 52

SILICATES These are metal derivatives of silicic acid H 4 SiO 4 . They are prepared by fusing metal oxides or metal carbonates with sand (SiO 2 ). All the silicates have the SiO 4 4- anion formed by SP 3 hybridization of Si-atoms with the various silicates differing from one another in the manner in which the SiO 4 4- anions are linked together. 53

Similarities and differences between silicon and carbon Unlike elements in the other groups which show a general trend of variation in properties down a group, elements of Group 14 show similarities as well as differences down the group, especially the first two and the last two members. The first two members, i.e.. C and Si are generally similar in chemical properties. However, their behaviour is not the same as those of the last two typical metallic members, i.e.. Sn and Pb . 54

Although both C and Si atoms tend to form covalent bonds, sp 3 -sp 3 Si - Si overlap is not as effective as sp 3 - sp 3 C − C overlap and as a result, bond enthalpy of C-C bond is 347 kJ mol -1 whereas Si-Si bond is just 226 kJ mol -1 . Hence, the fact that carbon is capable of forming long -C-C- chains does not mean silicon also forms stable -Si-Si- chains. On the other hand, sp 3 - sp 3 Si - O overlap is of the right order to form strong Si-O bonds, as reflected by a high Si-O bond enthalpy of 466 kJ mol -1 . Thus, similar to carbon which forms -C-C-C-C- chains (catenation) and hence polymers, silicon forms -Si-O-Si-O chains and hence polymeric silicates ( Heterocatenation ). 55

Silicate Minerals in the Earth’s Crust CRUST MOSTLY Oxygen O and Silicon Si 27% of all known minerals are silicates 40% of common minerals are silicates >90% minerals in the earth’s crust are silicates

CLASSIFICATION One classification based on the arrangement of the silicon tetrahedron within the structure divide the silicates into six classes. SiO 4 4- ( i ) Orthosilicates (Nesosilicates) (ii) Pyrosilicates (disilicate anion) ( Sorosilicates ) (iii) Cyclic or ring silicates (metasilicates, cyclosilicates) (iv) Chain silicates (Inosilicates) Pyroxene and Amphibole (v) Two-dimensional sheet (layer) structure (Phyllosilicates) (vi) Three dimensional or framework silicates (Tectosilicates)-Aluminosilicates 57

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Silicate Mineral Classification

Aluminosilicates When some of the Si 4+ ions in silicates are replaced by Al 3+ ions. For each Si 4+ ion replaced by Al 3+ , the charge must be balanced by having other positive ions such as Na + , K + and Ca 2+ ions. Radius ratio is 43 close to CN 4 to 6 e.g. Sanidine, [( K,Na )AlSi 3 O 8 ] 4 Orthoclase, [( K,Na )AlSi 3 O 8 ] 4 Albite , [NaAlSi 3 O 8 ] 4 Anorthite , Ca[Al 2 Si 2 O 8 ] The alkaline ions are held in place to balance the charges due to the presence of Al 3+ ions instead of Si 4+ ions 60

The ions replace ions in the chains of the corner shared tetrahedron of the SiO 4 group However bonding between Al and Si can be different Silicon atoms or ions tend to be bonded to 4 oxygen atoms in a tetrahedral fashion, but aluminium ions tend to be bonded to 6 oxygen atoms in an octahedral fashion Important Aluminosilicates 1) Micas 2) Fuller’s earth 3) Talc 4) Zeolites 61

Micas Micas are amphiboles made up of sheets of tetrahedra in which SiO 4 4- is replaced by (AlO 4 ) 5- tetrahedra They are 1. chemically inert 2. thermally stable 3. have high dielectric constants Hence are used in the furnace windows electrical appliances fillers for rubber plastics and insulation 62

The general formula X 2 Y 4-6 Z 8 O 20 (OH, F) 4 In which: X is K, Na, or Ca or less commonly Ba, Pb, or Cs Y is Al, Mg, or less commonly Mn, Cr, Ti , Li, etc. Z is mainly Si or Al, but may also include Fe 3+ or Ti 63

Fuller’s earth 64 Fuller’s earth consists chiefly of hydrated aluminum silicates that contain metal ions such as magnesium, sodium, and calcium within their structure Montmorillonite is the principal clay mineral in fuller’s earth, but other minerals such as kaolinite , attapulgite , and palygorskite also occur and account for its variable chemical composition . It has a strong absorptive power and cation exchange properties Used as adsorbent and cation exchanges

65 Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France

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Talc Talc is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg 3 Si 4 O 10 (OH) 2 . The mineral talc Mg 6 (OH) 4 (Si 4 O 10 ) 2 is a familiar phyllosilicate found in metamorphic rocks , and its structure is similar to the mineral lizardite . Each talc layer consists of two aluminosilicate tetrahedral sheets and one “trioctahedral” sheet, with a layer spacing of 0.94 nm . The mineral pyrophyllite Al 4 (OH) 4 (Si 4 O 10 ) 2 represents the “dioctahedral” end-member. Talc is used in a wide variety of products that we see every day. It is an important ingredient in rubber, a filler and whitener in paint, a filler and brightening agent in high-quality papers, and a primary ingredient in many types of cosmetics. 68

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Zeolites General formula for the composition of a zeolite is M x /n [(AlO 2 ) x (SiO 2 ) y ] . mH 2 O where cations M of valence n neutralize the negatively charged zeolite framework. SiO 2 tetrahedra are electrically neutral ( e.g., quartz) Substitution of Si(IV) by Al(III) creates an electrical imbalance and neutrality is provided by an exchangeable cation 70

Synthesis of Zeolites Template synthesis: The cavity and channel size is controlled and maintained by building the framework around a specific organo-ammonium cation. These cations can ultimately be converted to volatile products at about 500°C and the cage retains its framework structure 71

Zeolite Structure: Framework Alumino silicates When a tetrahedral silicon (IV) is replaced in a silicate by a tetrahedral aluminum (III), the framework attains an extra negative charge. This will be compensated by a cation such as Na + . Such compounds are termed framework aluminosilicates 72

3 Dimensional Perspective of Zeolites: 73 Also known as β cage 8 hexagons and 6 squares Lowenstein’s Rule: The AlO 4 - tetrahedra are always interspersed with SiO 4 tetrahedra Or Al-O-Al units does not occur in zeolites The inside cavity is known as α cage having 6 octagons 8 hexagons and 12 squares

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75 Zeolite Y or Faujasite Zeolite A or Linde-A Na 12 (AlO 2 ) 12 (SiO 2 ) 12 . 27 H 2 O Na 2 (AlO 2 ) 2 (SiO 2 ) 5 . 10 H 2 O Channel size 4.1 Å Al: Si ratio 1:1 Used as an ion exchanger for water softening (in detergents) due to high Na + content. Dehydrated form used for absorption of moisture and small volatile molecules. Reusable and environmentally safe Channel size 7.4 Å Al: Si ratio 2:5 Catalysis of larger molecules. Metal complexes can be even made inside the cavities

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77 Si 50% - Al 50% Cations Na, Ca Polar 3-D straight channels Si 93 % non-polar Hydrophobic 1-D channels Si x% - Al y% Cations Na, Ca Polar 3-D entangled channels ZSM 5 Faujasite Linde A Ring 8 Ring 10 Ring 12

78 Classical main group inorganic polymers New Polymers Based on Main Group Elements Polysilanes Polysiloxanes Polyphosphazenes Poly( carbosilanes ) Polygermanes & Polystannanes

Polysilanes Wurtz coupling polymerization Dehydrogenative coupling polymerization Ultrasonic polymerization 79 Methods of Synthesis

80 E lectrochemical polymerization Electrochemically induced electron transfer can be used in place of a redox reagent or light to achieve the same purpose; i.e.. to form free radicals capable of inducing polymerization.

81 Properties  -electron delocalisation is present in polysilanes that contrasts organic polymers (e.g. polyacetylene ) that have  -electron delocalisation. Physical properties arising from delocalisation including strong electronic absorption, conductivity, photoconductivity and photosensitivity. The exclusive Si-Si bond allow sigma electron delocalization.

82 Solubility can be tuned with varying attached groups Thermal resistant up to 573 K Low sigma-sigma excitation The Si-Si bond can be broken by UV light

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84 For conduction to occur a band must be partially filled. This can be achieved by: Adding electrons to the conduction band by reaction with a reducing agent. 2. Removing electrons from the valence band by reaction with an oxidising agent. 3. Promoting electrons from the filled valence band to the empty conduction band using heat or light.

85 In polysilanes the band gap is such that 300-400 nm light can excite electrons from valence band into conduction band. The band gap is sensitive to: 1. Chain length. Band gap decreases and absorption wavelength increases (lower energy). Reaches limit at ~ 30 Si atoms. 2. Conformation of polymer chain: trans conformation leads to absorption at lower wavelengths (better overlap).

86 Application of polysilanes 1. Application based on electrical properties i ) Can be employed as hole transporting materials or emitters in organic light emitting diodes (OLED) ii) Can be employed as part of the active layer in multi-layered OLEDs – as in photovoltaics. 2. Application based on Precursor to silicon-carbide reactivity i ) Precursor to silicon carbide – pyrolysis of a mixture of polysilanes and poly( carbosilanes ) at 1670 K .

87 3. Photo initiator in free Radical reactions i ) Homolytic cleavage on exposure to UV light form free silyl radical. ii) Can react with olefinic monomer to initiate free radical polymerization. iii) Polymerization of styrene and several acrylate polymers. 4. Fabrication of micro lens array 5. Coatings 6. Photoresists in Microelectronics

88 7. 7.Precursors to silicon containing materials.

89 Though the applications of polysilanes are numerous, commercialization has not happened. This due to the difficulties in controlling their synthesis with regard to molecular weight, PDI (polymer dispersity index) and impurities as well as the high cost of these elaborate methods and purification processes involved

Polysiloxane or Silicones Silicon (Si) Silica siloxanes 90 Silanes Silsesquioxanes

91 Synthesis method & Ring Opening Polymerization or Equilibration reaction

92 Properties Highly thermally and chemically stable. Water repellent. LMW silicon polymers are soluble in organic solvents. Good insulators. Resistant to oxidation. Have non-sticking and anti-foaming properties.

93 Applications Most important inorganic polymers with regard to commercial applications. Medical: prosthetics, catheters, contact lenses, drug delivery capsules. Non medical: e.g. elastomers, adhesives, lubricants, water repellents, moulds, cosmetic implants, for water proofing in electrical condensers. Also used for various purposes at low temperatures.

PHOSPHAZENES ( Phosphonitrilic compounds) These are cyclic compounds of phosphorus and nitrogen of general formula [PNCl 2 ] n . The reaction produces a mixture of ring compounds (NPCl 2 ) n , where n = 3,4,5,…. and fairly short linear chains. The most common rings (n=3 and 4) contain six or eight atoms. The former are flat and the latter exists in ‘ chair and boat conformations. 94

Analogous bromo -compounds may be prepared in the same manner except that bromine should be added to suppress the decomposition of PBr 5. The fluoride must be prepared indirectly using NaF in nitrobenzene i.e.. The iodides are not known 95

96 Other Synthetic methods Ring - opening polymerization

97 Neilson and Wisian -Neilson method Synthesis operates at room temperature and allows molecular weight control De Janger method N- Silylphosphoramine R= alkyl or aryl Matvjaszewski method N-silyl-P-( triethoxy ) phosphoramine

STRUCTURE Nitrogen atoms are SP 2 hybridized and two such hybrid orbitals are involved in  -bonding. Similarly each phosphorus is SP 3 hybridized and such hybrids are involved in  -bonding. There is then one “in plane”  –bonding involving the lone pair of N (SP 2 orbital) in xy plane and the vacant dxy or dx 2 -y 2 of the P-atom. 98

99 (a) Hybridization of N, showing a lone-pair of electrons in sp 2 and three electrons for bonding in P z and the other two sp 2 hybrids. (b) Possible interactions in the xy plane (ring plane) between the N lone-pair electrons and vacant d orbitals on adjacent P atoms (shown looking down the z-axis on to the xy plane.

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There are two “out of plane” interactions: Heteromorphic interactions: The singly occupied Pz 1 orbital on the nitrogen overlaps with the dxz or dyz orbital of phosphorus (p  –d  bonding) (b) Homomorphic interactions: There are interactions of Pz orbitals of two nitrogen through the dyz orbital of the phosphorus in between. 101

102 Perspective representation of possible  bonding (heteromorphic p  -d  interactions between N(p) and P( dxz ) orbitals and possible homomorphic p  -d  interactions of N(p) through the P( dyz ) orbitals)

Tetrameric Phosphonitriles or cyclotetraphosphaza-tetraenes These are [NPCl 2 ] 4 and [NP(NMe 2 ) 2 ] 4 The tetrameric rings of the above compounds are more flexible than those of the trimers . There is enough strain and they exhibit non-planar structures without serious deterioration of the P-N  -bonding. The chloro -derivatives are useful for introducing other substituents at the P-atoms in the ring. 103

E.g. When hexachlorobis ( ethylamino ) tetraphosphazatetraene is treated with excess dimethylamine in chloroform the product that results is not only the full amination of both the (PCl 2 ) groups and one of the PCl group but also one of the ethylamino ( NHEt ) group bridges the opposite PCl group with the elimination of HCl. This arises from an internal trans-annular attack by one of the NHEt groups. The product is the only known bicyclic phosphazene compound. 104

This bicyclo -structure is reminiscent of ADAMANTANE The amide however undergoes elimination of NH 3 with increasing temperature to form cross-linking between rings or possibly rapture of rings to form linear polymers. 105

Very little is known about these polymers. However relatively low molecular weight polymer believed to be a 3-ring species with the formulation Cl 9 P 6 N 7 has been isolated 106

INORGANIC RUBBERY POLYMERS (1) If excess phosphorus pentachloride is used in the preparation of phosphazene , then it is possible to isolate linear polymers as indicated below: (2) When the phosphonitrilic compound is heated it is possible to obtain linear polymers. NB: The reactive chlorine are still susceptible to nucleophilic attack and displacement 107

If R = CH 3 CF 3 ( trifluoroethane ) then the product obtained is a water repellent polymer which is used to fabricate artificial blood vessels and prosthetic devices. Although the hydrolytic stability of the phosphazene polymers make them attractive as structural materials, it is possible to create hydrolytically sensitive phosphazenes that may be useful medically in slow release drugs. Steroids, antibiotics, catecholamines ( eg dopamine & epinephrine) have been linked to phosphazene molecules with the intention that hydrolysis will provide these drugs in a therapeutic steady state. 108

Examples 109

OTHER HETEROCYCLIC RINGS & CAGES 1. S-N Ring compounds S-N ring compounds are prepared by ammonolysis 110

The corresponding tetramer is also known i.e.. (2) Tetrasulphurtetranitrides The ammonolysis of sulphur monochloride (S 2 Cl 2 ) either in solution or in an inert solvent or heated over solid ammonium chloride yield tetrasulphurtetranitrides ; S 4 N 4 . S 4 N 4 is a bright orange solid insoluble in H 2 O but soluble in some organic solvent. 111

The structure has been found to have two non bonding S-atoms at a distance of about 2.58 Å. This is considerably shorter than the sum of the van der waal radii which is 3.60 Å. Although this nonbonding distance is longer than the normal S─S bond which is about 2.06 Å, some interactions must occur between the trans S-atoms. All the S─N bond distances are equal and about 1.62 Å indicating extensive delocalization rather than alternating discrete single and double bonds. 112

Fluorination of S 4 N 4 It produces tetrathiozyltetrafluoride i.e.. Reduction of S 4 N 4 113

All possible S x (NH) 8-x isomers except N─N bonds are known. 114

Oxidative chlorination of S 4 N 4 NB: It is unexplained that chlorination produces the ClSN trimer while fluorination retains the tetramer unit 115

SULPHURNURYL CHLORIDE ( NSOCl ) 3 ( NSOCl ) 3 can be prepared by reacting the trithiazyl chloride with SO 3 ( NSOCl ) 3 may also be prepared from sulphamic acid. 116

Application of polyphosphazenes Fuel Cell: Polyphosphazene is currently the highest performing membrane material for Methanol based Proton-Exchange Membrane (PEM) Fuel Cells.

This fuel cell type is ideal for miniature power supply and is a leading candidate for automotive applications. Specially-fabricated polyphosphazene membranes possess high proton conductivity, slow methanol transport, and stability (thermal, mechanical, and chemical). They out-perform most other membrane materials in methanol environments and have demonstrated minimal methanol crossover, better thermal performance in high-temperature environments, and minimal chemical and mechanical degradation.

Medical: Medical applications include: Drug Delivery, Biological Membranes, Coatings, and polymeric medical devices and components such as prosthetics and implants Drug delivery systems utilize polyphosphazene's ability to accept grafts of active components to create a 'carrier-molecule' for pharmaceutical uses. In addition, a substituent capable of influencing the behaviour of the chain (i.e.. accelerating hydrolysis), can be added, enabling the rate of release to be a function of the polymer characteristics and substituent ratios.

Other Uses Polyphosphazenes are used as flame retardants, additives, performance polymers, and in specialty applications. (The exceptional performance of polyphosphazene derivatives under extreme temperature conditions, their inertness to chemical environments, and their non-flammability, make them suitable materials for applications in hostile landscapes.) Their excellent combustion behaviour such as low smoke emission, non corrosiveness, low toxicity of gases, and their ability to withstand a diversity of hostile chemical environments make them suitable materials for use in fluoroelastomeric seals, gaskets, O-rings, and in insulating foams.

Other products include specialty rubbers, flame resistant materials, polymer conductors, lubricants, liquid crystal polymers, catalyts , paints, adhesives, photocuring polymers, self-stabilized polymers, and additives. Polyphosphazenes are being used to make membranes more thermally, mechanically, and chemically stable, as well as to enhance selectivity and overall performance. (These are mainly used in electrodialysis, microfiltration, ultrafiltration, and reverse osmosis applications. )

Poly( carbosilanes ) The scheme below shows the reductive coupling reaction in the synthesis of poly carbosilanes - reactions of the ethoxy group of (a) provide at least four other polycarbosilanes . The monomer synthesis for polymer (b) is also shown 122

Uses i ) In the manufacture of ceramic foam ii) Used as catalytic support for TiO 2 catalysts iii) For the synthesis of silicon carbide fibers

Polygermanes and Polystannanes Like polysilanes , polygermanes and polystannanes which are made out of germanium and tin atoms respectively ( ie group 14 congeners of polysilanes ) are being studied for use as electrical conductors.

Polystannanes Polystannanes are the only known polymers with backbones made entirely from metal atoms. Synthesis Three common synthesis routes used to prepare polystannanes : Polymerization of tin dichlorides by Wurtz or Wurtz -like reactions, Electrochemical reactions Catalytic dehydropolymerization of tin dihydrides

INORGANIC RINGS The most important ring system of organic chemistry is the C 6 H 6 ring either as a separate entity or in polynuclear hydrocarbon such as napththalene , anthracene, phenanthrene, etc. 128

In Inorganic Chemistry, there are at least two analogues of benzene namely: borazine (B 3 N 3 H 6 ) and phosphazenes (P 3 N 3 X 3 ). 129

BORAZINE OR BORAZOLE Borazine is an unsaturated compound of B and N atoms with the formula B 3 N 3 H 6 . It is isoelectronic and isostructural with benzene, having delocalized electrons and aromatic character. The physical properties are also similar. However despite the resemblance in structure, there is little chemical resemblance between borazine and benzene. The difference in electronegativity of B and N atoms is influential. 130

In borazine the  -electrons are concentrated on the N-atoms and there is a partial positive charge on the B-atoms which leaves them open for electrophilic attack on the N-atom. Consequently borazine in contrast to benzene readily undergoes addition reactions. Also unlike benzene the  -electrons are not derived from all six atoms of the ring but from the 3 nitrogen atoms. 131

Synthesis of borazines (1) Stock’s Method In this method diborane (B 2 H 6 ) and NH 3 are heated in 1:2 molar ratio at low temperature (-120  C) to obtain diammoniate of diborane (B 2 H 6 .2NH 3 ) which is addition compound (adduct). When the adduct is heated at 200  C, the borazine is obtained 132

(2) By heating BCl 3 with NH 4 Cl (or RNH 3 Cl) 133

(3) By heating a mixture of LiBH 4 and NH 4 Cl 134

STRUCTURE B: 1S 2 2S 2 2P 1 Neutral atom *B: 1S 2 2S 1 2P x 1 P y 1 P z SP 2 hybrid B - : 1S 2 2S 2 2P x 1 P y 1 P z SP 2 hybrid N: 1S 2 2S 2 2P x 1 P y 1 P z 1 neutral atom and SP 3 hybrid N + : 1S 2 2S 2 2P x 1 P y 1 P z SP 2 hybrid 135

In this structural formula the formal – ve and + ve charges have been assigned to the B and N atoms respectively. These illustrations are isoelectronic with carbon (in SP 2 hybridization) so that the borazine has the same skeletal configuration as in benzene. All B-N bond distances are 1.44 Å which is between the calculated B-N single bond (1.54 Å ) and B-N double bond (1.36 Å ). The valence bond approach describes the structure in terms of two canonical forms whereas a molecular orbital description involves three (3)  -orbitals embracing all six (6) atoms in the hexagon. These delocalized orbitals differ somewhat from their benzene analogues because the constituent 2P z atomic orbitals of B and N are not identical in energy. 136

Physical properties Borazine is indeed a close analogue of benzene. -- Similarity of the physical properties of the alkyl substituted derivatives is more remarkable. -- For example the ratio of the absolute boiling point of the substituted borazine to similarly substituted benzene derivatives is 0.93  0.01. Such similarities lead to a description of borazine as an inorganic benzene. 137

Physical properties of borazine and benzene Chemical properties The chemical properties of borazine and benzene are quite different. 138 Borazine Benzene Molecular weight 80.5 78.1 b.p . (  C) 55.0 80.10 m.p . (  C) -56.2 5.51 Critical temperature (  C) 252 288.0 Liquid density at b.p . (g/cm 3 ) 0.81 0.81 Crystal density at m.p . (g/cm 3 ) 1.00 1.01 Trouton constant (J/K mole) 89.5 88.2 Surface tension (Nm -2 ) 0.0311 0.0310 Dipole moment Intermolecular distance (pm) 144 142 Bond distance to H (pm) B-H (120), N-H(102) C-H (100)

Both have  -clouds of electron density delocalization over all the ring atoms. However because of the difference in electronegativity between B and N, the  -cloud in borazine is described as being lumpy with more electron density localized on the nitrogen atom N. This partial delocalization weakens the  -bonding hence N retains some of its basicity whereas B some acidity. As a result of this, polar species such as HCl can therefore attack the double bond between N and B. NB: The different electronegativity of B and N turn to stabilize bonding to B by electronegative substituents and bonding to N by electropositive substituents. 139

The tendency for borazine to undergo addition reaction rather than substitution is well contrasted by the electrophilic substitution reaction of benzene (i.e. halogenation of benzene) as indicated below: 140

Borazine analogues of naphthalene, and related hydrocarbons have been made by pyrolysis of borazine or its passing through a silent electric discharge. Naphthazine Biborazonyl There is also the four membered as well as the 8-membered rings like 141

142

CYCLOBORAZINES Unlike hydrogenation of benzene yielding cyclohexanes , straight forward hydrogenation of borazines do not yield cycloborazines , but rather yield polymeric materials of indefinite compositions. Substituted derivatives of saturated cycloborazine form readily by addition reactions. 143

REACTIONS OF BORAZINES (1) Addition reactions: (a) One molecule of B 3 N 3 H 6 adds three molecules of H 2 O, ROH, RX or HX in the cold without a catalyst --The more negative group is generally attached to B, since B-atom is less electronegative than N-atom in B-N bond. --For example when HX derivative is heated at 50-100  C it loses 3H 2 molecules to yield B,B’,B”- trihaloborazine . 144

--This addition reaction shown by borazine is not shown by benzene. (b) One molecule of borazine adds 3 molecules of X 2 at 0  C and gives B- trihalo -N- trihaloborazine which on heating at about 60  C loses 3 molecules of HX and forms B- trihaloborazine i.e.. -- Addition reaction occurs in borazine quite more readily due to the considerably more reactive nature of borazine to benzene. -- This reaction can be compared with that shown by benzene where substitution takes place. i.e.. 145

(2) Hydrolysis Borazine is slowly hydrolyzed by H 2 O to produce H 2 , B(OH) 3 and NH 3 . -- The hydrolysis is favored by increase in temperature 146

(b) Under proper conditions borazine reacts with 3 molecules of H 2 O to produce B-trihydroxy borazine (OH) 3 B 3 N 3 H 3 in which the OH groups are attached to B-atoms. (3) Hydrogenation Benzene can be hydrogenated to produce cyclohexane, C 6 H 12 , Borazine on the other hand can be converted to cycloborazine only by special techniques such as shown: -- Direct addition yields polymeric materials with indefinite composition. 147

BOROXINE Iso -electronic with borazine is boroxine H 3 B 3 O 3 which is formulated as [XBO ] 3 Boroxine is planar but has less even  –delocalization than borazine and possess a six membered ring. Boroxine can be prepared by the explosive oxidation of B 2 H 6 or B 5 H 9 , or high temperature hydrolysis of boron. Although this compound is thermodynamically unstable at room temperature with respect to B 2 H 6 and boric oxide, nevertheless it can be characterized. 148

A boron–phosphorus analogue of borazine has been synthesized rather recently. The electronegativities of B and P are similar, unlike those of B and N. As a result polarization should be less extensive in this compound than borazine. The B 3 P 3 ring is planar with equal and shortened B-P bond suggesting significant aromaticity. 149

HOMOCYCLIC INORGANIC RING COMPOUNDS Several elements form homocyclic rings. The thermodynamically stable form at room temperature consists of S 8 rings. The oxidation of several non-metals in strongly acidic systems produces poly-atomic cationic species of the general type Y n +m 150

The best characterized of these are the S 4 +2 , Se 4 +2 , Te 4 +2 . The structures of these compounds have been shown to be planar and has been shown that all these species are square planar. The structures are stabilized by the Hückel sextet of  -electrons. Se 8 +2 is known to be bicyclic. The transannular bond is 2.84 Å which is longer than those of the ring 2.32  0.02 Å but are considerably less than the sum of the van der waals radii of about 3.80 Å. 151

Other ions of this type, presumably cyclic, though less thoroughly studied are the S 8 2+ , S 16 2+ , Sb 4 2+ , Sb 8 2+ , Te 6 2+ etc. But have been suggested as products of mild oxidation of some non-metals. The oxidation of red phosphorus with hypo-halides in alkaline solution produces the anion of an interesting phosphorus acid. This acid has been shown to be cyclic. 152

CYCLIC OXOCARBON ANION [(CO) n ] -2 (-4) The oxocarbonate ion C 5 O 5 2- was the first member to be synthesized. It was isolated in 1825 by Gmelin and thus shares with benzene the honour of being the first aromatic compound discovered. It was the first inorganic substance discovered that is aromatic. It is a bacterial metabolic product and was possibly the first organic compound synthesized. 153 Benzoquinonetetraolate , THBQ

All of these oxocarbon anions are aromatic according to simple MO calculations. The aromatic stabilization of the anion is apparently responsible for the fact that squaric H 2 C 4 O 4 is about strong as sulphuric acid. NB: Oxalic acid (H 2 C 2 O 4 ) containing C in a comparable oxidation state but not aromatic. K a1 ≈ K a2 for squaric and sulphuric acid. The K a2 of oxalic acid is 3 orders of magnitude smaller. 154

NON-METAL CAGED COMPOUNDS The simplest caged type molecule is found in white phosphorus which is a P 4 molecule. This molecule is more stable at room temperature and is a tetrahedron of phosphorus atoms. Such a structure requires bond angle of 60  In as much as the lowest inter orbital angle available using only s and P-orbital is 90, the smaller bond angle in P 4 must be accomplished either through the introduction of considerable d-character or through the use of bent bonds. 155

The former involving d-orbitals requires considerable promotion energy and is therefore unlikely. The latter involving bent-bonds result in the loss in bonding energy of some 96 KJmol -1 due to strain but is thought to be energetically favoured . In any event the molecule is destabilized and quite reactive. P 4 cages react readily with O 2 to form a mixture of oxides and can also be converted into a more stable allotropes. 156

Both are anhydrides. 157

Only one phosphorus sulphide P 4 S 10 is isoelectronic and isostructural with the phosphorus oxide. This is obtained by mixing P 4 and S 8 in appropriate stoichiometric quantities. Other sulphides are obtained by the reactions below: 158

ELECTRON DEFICIENT COMPOUNDS INTRODUCTION The formulas and structures of several kinds of compounds can be predicted with the aid of the valence bond theory, molecular orbital theory, the octet rule, and the 18-electron rule. However the electronic and molecular structures of one large class of compounds cannot be understood in these terms. At the very time G N. Lewis proposed the electron-pair bond, Alfred Stock was preparing a series of compounds whose formulas gave no hint as to their structures and whose structure- once determined could not be accommodated by a simple valence-bond model. Stock was able to prepare and characterize B 2 H 6 , B 4 H 10 , B 5 H 9 , B 5 H 11 , B 6 H 10 and B 10 H 14 . These compounds could be divided into two groups hydrogen “rich” of general formula B p H p+6 ; and a hydrogen “poor” of formula B p H p+4 . A third series of very stable anions B p H p 2- (which can be thought of as derived via deprotonation of B p H p+2 ) has been prepared (p = 6 to 12) 159

160

ELECTRON DEFICIENT COMPOUNDS These are compounds with too few electrons for a Lewis structure to be written with an octet around the central atom e.g. compounds of group 1, 2 and 13 elements of the periodic table especially compounds of boron. Electron-deficient compounds are compounds in which the number of valence orbitals exceed the number of valence electrons. (e.g. BH 3 , B 2 H 6 , AlH 3 ) 161

ELECTRON PRECISE COMPOUNDS Compounds with the correct number of electron pairs for bond formation with none left-over as non-bonding electron pairs on the central atom; i.e.. all valence electrons of the central atom are engaged in bond formation. E.g. carbon and carbon group of compounds (group 14) (CH 4 , C 2 H 6 , SiH 4 , SnH 4 ). ELECTRON RICH COMPOUNDS Compounds which have more electron pairs than are needed for bond formation with the extra electron pairs being present as non-bonding electron pairs on the central atom. E.g. compounds of group 15, 16, 17 elements (NH 3 , H 2 Se, H 2 O, H 2 Te, H 2 S, PH 3 , HCl, AsH 3 , HF, SbH 3 , HBr, HI). 162

HYDROBORANE CLUSTERS In a cluster atoms form a cage-like structure There are a great number of known neutral and anionic hydroborane clusters These structures are often described as being polyhedral or deltahedral A deltahedron is a polyhedron that possesses only triangular faces, e.g. an octahedron 163

THE SIMPLEST HYDROBORANE-B 2 H 6 This is an electron-deficient compound held together by two 3c-2e bonds. Higher boranes are prepared by pyrolysis of B 2 H 6 in the vapour phase. 164

CLASSIFICATION OF HIGHER BORANES (Electron counting) The building blocks from which the deltahedron is constructed is assumed to be a B -H unit. The electrons in the other B-H bond are ignored in the counting procedure but all the others are included whether or not it is obvious that they help to hold the skeleton together. By skeleton we are referring to the framework of the cluster with each B-H group counted as a unit. If B atom happens to carry two terminal hydrogen atoms (H T ) only one of the B-H bonds is treated as a unit. 165

E.g.1 B 4 H 10 ≡ (BH) 4 H 6 If the structure or shape is 4(B-H) units 4 x 2e- = 8e-s 4 BHB’s 6H 6 x 1e- = 6e-s 2 additional BHTs 14e-s 1 BB 7 bonds E.g.2. B 5 H 11 ≡ (BH) 5 H 6 5(B-H) units 5 x 2e- = 10e-s 6H Atoms 6 x 1e- = 6e-s 16e-s 8 bonds 166

WADE’S RULE Boranes of formula [ B n H n ] 2- will be found to have the CLOSO (Greek word for “caged”) structure with a B-H unit at each corner of a closed deltahedron and no BHB bonds in the closo structure. Such structure are known to have (n+1) skeletal electrons. These series of anions is known for n = 5 to 12 Trigonalbipyramidal [B 5 H 5 ] 2- Octahedral [B 6 H 6 ] 2- Icosahedral [B 12 H 12 ] 2- NB: The closo hydroborates and carboranes are often thermally stable and fairly unreactive. 167

168

2. Boranes with the formular B n H n+4 (BH) n H 4 the NIDO (Latin word for “nest”) structure (i.e. can be viewed as a closo structure which has lost one vertex or corner and may have a BHB or a BB bond. The compounds in this series contain (n+2) skeletal pairs of electrons e.g. B 2 H 6, B 5 H 9 , B 6 H 10, B 10 H 14 etc. In general, the thermal stability of NIDO borane is intermediate between that of closo and Arachno baranes. 169

170

3. The boranes with formula B n H n+6 ≡ (BH) n H 6 the Arachno (Greek word for “spider’s web”) structure and are like closo boranes less two vertices and may also have BHB’s bonds. They have (n+2) cornered polyhedron requiring (n+3) skeletal electrons. They are the most unstable. E.g. B 4 H 10, B 5 H 11, B 6 H 12, B 8 H 14, n -B 9 H 15, i -B 9 H 15 171

172

4 . The boranes of formula B n H n+8 ≡ (BH) n H 8 the HYPHO (Greek word for “net”) structure have the most open structure in which the B atom occupy the n corners of an (n+3) – cornered polyhedron requiring (n+4) skeletal electron pairs. No neutral boranes has yet been definitely established in this series but known compounds of B 8 H 16 and B 10 H 18 may prove to be hypho-boranes and several adducts are known to have hypho-structures. 173

5. The boranes with formula B n H n+10 ≡ (BH) n H 10 the Klado structure (Greek word for “branch”). They have (n+4) cornered polyhedron requiring (n+5) skeletal electrons. Linkage between two or more of these polyhedral borane clusters is indicated by the prefix CONJUNCTO- (Latin name for “ I join together”). They have the formula B n H m . At least five different types of interconnected borane clusters have been identified and have the following features; Fusion by sharing a single common B atom e.g. B 15 H 23 . 174

(b) Formation of a direct 2-centre B-B σ –bond between 2 clusters e.g. B 8 H 18 i.e.. (B 4 H 9 ) 2 , B 10 H 16 i.e.. (B 5 H 8 ) 2 , (3 isomers), 175

B 20 H 26 i.e.. (B 10 H 13 ) 2 (11 possible isomers of which most have been prepared and separated, 176

anions of these sub group are represented by the three isomers of B 20 H 18 4- i.e.. (B 10 H 9 2- ) 2 , 177

(c) Fusion of two clusters via 2B atoms at a common edge e.g. B 13 H 19 , B 14 H 18, B 14 H 20 (d) Fusion of two clusters via 3B atoms at a common face; no neutral borane or borane anion is yet known with this conformation but solvated complex (MeCN) 2 B 20 H 16 .MeCN has this structure. 178

(e) More extensive fusion of 4 B atoms in various configurations e.g. B 20 H 16 , B 20 H 18 2- 179

SUMMARY TYPE FORMULA SKELETAL ELECTRON PAIRS CORNERS OF POLYHEDRON EXAMPLES Closo [ B n H n ] 2- n+1 n [ B n H n ] 2- to [B 12 H 12 ] 2- Nido B n H n+4 n+2 n + 1 B 2 H 6, B 6 H 10, B 5 H 9 Arachno B n H n+6 n+3 n + 2 B 4 H 10, B 5 H 11 Hypho B n H n+8 n+4 n + 3 B 8 H 16 , B 10 H 18 Klado B n H n + 10 n+5 n + 4 B 6 H 16 , B 7 H 17 Conjuncto B n H m B 8 H 18, B 15 H 23 etc. 180

Using Wades Rule E.g. [B 5 H 5 ] 2- Closo structure 5(B-H) 5 x 2e = 10e -s overall charge 2e- = 2e - s (5+1)e- pairs  12e - s i.e.. From the formula [ B n H n ] 2- with (n+1) pair skeletal electrons (ii) B 5 H 9 ≡ (BH) 5 H 4 Nido structure 5(B-H) 5 x 2e = 10e -s 4H 4 x 1e = 4e-s (5+2)e - pairs  14e - s From B n H n+4 with n+2 skeletal electron pairs 181

182

STRUCTURAL CORRELATION Very useful structural correlation between the Nido and Arachno compounds is based on the observation that clusters having the same number of skeletal electrons are related by removal of such B-H groups and the addition of the appropriate number of electrons and H atoms. This type of process relates the octahedral closo [B 6 H 6 ] 2- anion to the square pyramidal nido-B 5 H 9 borane which is in turn related to the butterfly-like arachno-B 4 H 10 . 183

184

185

NOMENCLATURE Neutral boron hydrides are named borane; a Greek prefix indicates the number of B atoms and an Arabic numeral in parenthesis gives the number of hydrogen atoms (e.g. B 5 H 11 is named as pentaborane (11)). The Arabic number is omitted if only borane containing a particular count is known – e.g. B 2 H 6 is usually referred to as diborane . Anionic species are named as hydroborates . Greek prefixes separately indicate the number of H and B atoms; the charge on the anion is given a parentheses. E.g. B 5 H 8 - is octahydropentaborate (-1). The structural type is sometimes specified. i.e.. B 5 H 8 - is also octahydro-nido-pentaborate (-1). E.g. B 5 H 9 pentaborane (9) B 5 H 11 pentaborane (11) B 5 H 10 pentaborane (10) 186

THE BONDING PROBLEM IN BORANES Localized Bonding picture Retaining the valence bond concept of the relationship between bond distance and bond order a problem is immediately encountered on examining the known structures of boron hydrides. The coordination number of each boron (and some of the hydrogens) exceed the number of low-energy orbitals. Ideally a bonding picture for electron-deficient compounds should allow the same straight forward prediction of geometry, reactivity, stoichiometry, redox properties, acidity etc. that the valence bond approach permits “ for regular compounds”. 187

Early attempts to account for the electronic structure of diborane the simplest member of the class, included the observation that B 2 H 6 is isoelectronic with ethene C 2 H 4 . In this view we could regard the two bridging H’s in the structure as protonating the double bond of B 2 H 4 2- . Subsequent research has confirmed the acidic nature of bridge H’s in the boranes . 188

However this bonding picture is difficult to extent to the higher boranes . A straight forward application of valence bond theory to the electronic structure of diborane requires some 20 resonance structures. Clearly, as the task of describing more complex molecules begins to require unmanageable numbers of canonical structures, hence the simple valence bond approach loses its utility 189

PFIZER  –BONDED (SP 3 ) MODEL Two of the SP 3 -hybrid orbitals of each boron atom are used in bonding with the terminal hydrogen and all these are involved in 2c, 2e bond. The two points towards the bridging H and interact with the 1S-orbital of H to form 3c, 2e bonds (Hence the bonding in B 2 H 6 is diamagnetic due to the absence of unpaired electrons). 190

BANANA (SP 2 hybrid) Model This model is better suited to the observation that H T BH T angle ≡ 122 ◦ far apart that they are also 2c, 2e. Generally bonding in boranes consist of the following: BHB (3c, 2e) ≡ BBB can be in two forms. (a) closed – 3c – BBB (b) Opened – 3c – BBB bond 191 H H

MOLECULAR ORBITAL APPROACH In simple covalent bonding theory molecular orbitals considerations, MOs are formed by the linear combination of atomic orbitals (LCAO); e.g. two atomic orbitals combine to give one bonding and anti-bonding MOs and orbitals of lower energy will be occupied by the electron pairs. This is a special case of a more general situation in which a number of AOs are combined together by the LCAO methods to construct an equal number of MOs of differing energies, some of which will be bonding, some possibly non-bonding and some anti-bonding. In this way 2-centre, 3-centre and multicentre orbital can be envisaged 192

In borane chemistry two types of 3-centre bond finds considerable application: B-H-B bridge bonds and central 3-centre B-B-B bonds. The figure below shows the formation of a 3-centre B-H-B orbital  1 from an SP x hybrid orbital on each of B(1), B(2) and H 1S orbital,  H. The three AOs have similar energy and appreciable spatial overlap, but only the combination; (B 1 ) + (B 2 ) has the correct symmetry to combine linearly with (H). 193

The three normalized and orthogonal MOs have the approximate form: Bonding :  1  ½[(B 1 ) + (B 2 )] + 1/2 (H) Non-bonding (anti-bonding):  2  1/2 [(B 1 ) - (B 2 )] Anti-bonding :  3  ½[(B 1 ) + (B 2 )] - 1/2 (H) 194

Formation of a bonding central 3-centre BBB bond  1 and schematic representation of the relative energies of the 3 molecular orbitals  1 ,  2 and  3 . The approximate analytic forms of these MOs are: Bonding :  1  [(B 1 ) + (B 2 ) + (B 3 )]/3 Anti-bonding :  2  [(B 1 ) - (B 2 )]/2 Anti-bonding :  3  [(B 1 ) + (B 2 ) - 2(B 3 )]/6 For closo and for larger open cluster boranes it becomes increasingly difficult to write a simple satisfactory localized orbital structure, and full MO treatment is required. 195

MO Description of bonding in B 2 H 6 The MO scheme for one of the B–H–B bridging three center two electron bonds. 196

197 The non-bonding orbital is actually of slightly lower energy than shown and so has slight bonding character. This arises from the fact that the orbitals involved in the terminal B–H bonding have the correct symmetry to overlap with the bridging bond orbitals, resulting in a stabilization of the ‘nonbonding’ orbital.

198

199

200

201

202 In all 18 AOs on the 6B-H atoms combine to give 18 MOs of which 7 (n+1) are bonding framework MOs and the remaining 11 are anti-bonding.

These diagrams also indicate why neutral closo -boranes B n H n+2 are unknown since the 2 anionic charges are effectively located in the low lying inwardly directed a 1g orbital which has no overlap with protons outside the cluster i.e. above the edges or faces of the B 6 octahedron. Replacement of the H t by 6B further builds up the basic three dimensional network of hexaborides MB 6 just as replacement of the 4H t in CH 4 begins to build up the diamond lattice. The diagrams also serve, with minor modification to describe the bonding in isoelectronic species such as closo-CB 5 H 6 - , 1,2-closo-C 2 B 4 H 6 , 1,6-closo-C 2 B 4 H 6 etc. 203

Similar though more complex diagrams can be derived for all closo-B n H n 2- (n=6-12). These have the common feature of a low lying a 1g orbital and n other framework bonding MOs: in each case , therefore (n+1) pairs of electrons are required to fill these orbitals as indicated by Wade’s rules. It is a triumph for MO theory that the existence of B 6 H 6 2- and B 12 H 12 2- were predicted by Longuet -Higgins in 1954-5 a decade before B 6 H 6 2- was first synthesized and some five years before the (accidental) preparation of B 10 H 10 2- and B 12 H 12 2- were reported. It is a general feature of closo-B n H n 2- anions that there are no B-H-B or BH 2 groups and 4n boron atomic orbitals are always distributed as follows: 204

n in the n(B- H t ) bonding orbital (n+1) in framework bonding MOs (2n-1) in non-bonding and anti-bonding framework MOs. As each B atom contributes one electron to its B- H t bond and two electrons to the framework MOs, the (n+1) framework bonding MO are just filled by the 2n electrons from nB atoms and the two electrons from the anionic charges. 205

TOPOLOGICAL APPROACH TO BORON HYDRIDE STRUCTURE – styx numbers Lipscomb et al established a systematic procedure for obtaining the valence structure of more complex boron hydrides incorporating three- centre bonding. The procedure involve essentially determining the total number of orbitals and electrons available for bonding. The number of B-H bonds and B-H-B three centre bond is then counted and the requisite orbitals and electrons are assigned. The remaining orbitals and electrons, considered to be available for frame-work bonding, are distributed among two- centre B-B bonds and three- centre B-B-B bonds. 206

A systematic prescription for accomplishing this is outlined below: Consider a neutral borane whose formula can be written as B p H p+q . The molecule consists of p(BH) groups and q “extra” hydrogens distributed between bridging positions and BH groups (converting them to BH 2 groups s = number of B-H-B bonds t = number of B-B-B bonds y = number of B-B bonds x = number of BH T bonds 207

Several relations can be formulated between structural features and available orbitals and electrons called equation of balance: For hydrogen balance: q = s + x ………(1) i.e.. All the “extra” Hydrogens must be in B-H-B or BH T units For orbital balance: p = s + t ………(2) The structure contains p boron atoms, each must participate in one three- centre bond if it is to attain a complete octet. This can be either a B-H-B or B-B-B For electron balance: The total number of electron pairs available for framework bonding is p from the BH groups plus ½q from the extra ‘H’s. These must be just enough to occupy the s + t + y framework bonds and the x BH 2 bonds. 208

Hence p + 1/2q = s + t + y + x ………..(3) Substituting (1) and (2) into (3) we obtain p – 1/2q = t + y ……………………..(4) y = ½(s – x)…………………..(5) In general; s  x but s  q also s  q/2  q/2  s  q NB. These equations are diophantine equations 209

Applying the equation of balance to a compound of given composition allows us to determine a set of styx numbers that specify a valence structure. eg 1. For B 2 H 6  (BH) 2 H 4 p = 2; q = 4 And we have : 4 = s + x; 2 = s + t; 0 = t + y  y = -t The only possible solution is s =2, t = 0, y = 0, x = 2 (written 2002) and the structure corresponds to 210 s t y x 4 -2 2 3 -1 1 1 2 2

eg 2. For B 5 H 11  (BH) 5 H 6 : p = 5; q = 6 This formulation gives (4) different styx numbers 6 = s + x; 5 = s + t; 2 = t + y  y = 2 – t; y = ½(s–x) i.e.. (3203), (4112), (5021), (6-130), 211 s t y x 6 -1 3 5 2 1 4 1 1 2 3 2 3

For 3203  5B, 3BHB, 2BBB, 0 BB, 3BH T OR For 4112  5B, 4BHB, 1BBB, 1BB, 2BH T 212

For 5021  5B, 5BHB, 0BBB, 2BB, 1BH T 213

In choosing the best structures the following additional considerations must be kept in mind: Every pair of adjacent B’s must be bonded to each other through a B-B bond, B-H-B, or B-B-B bonds. 2. Pairs of B atoms bonded by a B-B bond may not be bonded to one another by B-B-B, or B-H-B bond. 3. Nonadjacent pairs of B atoms may not be bonded by framework bonds. 4. Other things being equal the preferred structure is the one with the highest symmetry. These considerations eliminate structures (4112) and (5021), leaving the structure (3203) . 214

SYNTHESES AND REACTIVITY OF NEUTRAL BORON HYDRIDES The best way to synthesize B 4 H 10 , B 5 H 9 , B 5 H 11 , and B 10 H 14 , is to pyrolyze diborane , B 2 H 6 under carefully controlled conditions. The fact that such an approach is feasible becomes clearer if we organize the borane family somewhat differently. The chemical relationships between boranes may be seen better when we combine the B p H p+q formulation with another general formula (BH) n (BH 3 ) x , where n can assume values between 0 and 10 inclusively and x assumes the values 1, 2 or 3. 215

216

In the table we have some known and unknown boranes and these are listed according to their n, q and x numbers. The usefulness of this tabular form become apparent when it is recognized that it shows that two boranes may be converted to one another or higher boranes can be made from simpler ones by the application of one or more of the following reactions in their proper sequence. 217

(A) Gain or loss of BH 3 This is used to convert a borane of a given n and x to a borane of the same ‘n’ but different ‘x’. (B) Gain or loss of H 2 This is used to convert a borane of a given ‘n’ and ‘x’ to one of the next higher or lower ‘n’ and ‘x’. (C) Gain or loss of a BH unit This is used to convert a borane of a given ‘n’ and ‘x’ to a borane of higher or lower ‘n’ but the same ‘x’. 218

GENERAL SCHEME 219

220

PROPERTIES OF BORON HYDRIDES PHYSICAL PROPERTIES Boranes are colourless , diamagnetic, molecular compounds of moderate to low thermal stability. The lower members are gases at room temperature but with increasing molecular weights they become volatile liquids or solids. Their boiling points are approximately the same as those of the hydrocarbons of similar molecular weights. 221

The boranes are all endothermic and their free energy of formation  G f ◦ is also positive: Their thermodynamic instability results from exceptionally strong interatomic bonds in both elemental B and H 2 rather than the inherent weakness of the B-H bond. The bond energies of typical boranes are B-H T -380, B-H-B – 440, B-B – 330 and B-B-B – 380 kJmol -1 compared to the bond energy of 436 kJmol -1 for H 2 and heat of atomization of crystalline boron of 555 kJmol -1 of B atoms ( ie . 1110 kJmol -1 of 2B atoms). 222

Physical properties of some boron compounds 223 Nido - boranes Arachno - boranes Compound mp bp  H f ◦ /kJmol 1 Compound mp bp  H f ◦ /kJmol -1 B 2 H 6 -164.9 ◦ -92.6 ◦ 36 B 4 H 10 120 ◦ 18 ◦ 58 B 5 H 9 -48.8 ◦ 60 ◦ 54 B 5 H 11 122 ◦ 65 ◦ 67 (93) B 6 H 10 -62.3 ◦ 108 ◦ 71 B 6 H 12 -82.3 ◦ 85 ◦ (extra) 111 B 8 H 12 Decompose above -35 ◦ - B 8 H 14 Decompose above 30 ◦ - B 10 H 14 99.5 ◦ 213 ◦ 32 n-B 9 H 15 120 ◦ 120 ◦ -

CHEMICAL PROPERTIES Boranes are extremely reactive and several are spontaneously flammable in air. Arachno - boranes tend to be more reactive (less stable to thermal decomposition) than nido-boranes and reactivity also diminishes with increasing molecular weight. Closo -borane anions are exceptionally stable and their general chemical behavior has suggested the term” three dimensional aromaticity. 224

Boron hydrides are extremely versatile chemical reagents but the very diversity of their reactions make a general classification unduly cumbersome. Instead, the range of behavior will be illustrated by typical examples taken from the chemistry of the three most studied boranes : B 2 H 6 , B 5 H 9 and B 10 H 14 . 225

Chemistry of diborane , B 2 H 6 B 2 H 6 occupies a special place because all other boranes are prepared from it. It is also the most studied and synthetically useful reagent in the whole of chemistry. 226

PREPARATION B 2 H 6 gas is most conveniently prepared in small quantities by the reaction of I 2 on NaBH 4 in diglyme [(MeOCH 2 CH 2 ) 2 O] or by the reaction of a solid tetrahydroborate with an anhydrous acid: (ii) When B 2 H 6 is used as reaction intermediate without the need for isolation or purification the best procedure is to add Et 2 OBF 3 to NaBH 4 in a polyether such as diglyme . 227

(iii) On the industrial scale by the direct reduction of BF 3 with NaH at 180  C and the product formed trapped out as it is formed to prevent subsequent pyrolysis: 228

229

REACTIONS OF B 2 H 6 Combustion Care should be taken in these reactions because B 2 H 6 is spontaneously flammable. Has a higher heat of combustion per unit weight of fuel than any other substance except H 2 , BeH 2 and Be(BH 4 ) 2 (ii) Pyrolysis B 2 H 6 undergoes complex pyrolysis in sealed tubes at temperatures above 100  C forming a variety products depending on the conditions. The initiating step is the unimolecular dissociation equilibrium; 230

Initiating step: Stable intermediate B 4 H 10 is then followed by B 5 H 11 A complex series of further steps gives B 5 H 9, B 6 H 10, B 6 H 12 and higher boranes culminating in B 10 H 14 as the most stable end product together with polymeric materials BH x and a trace of icosaborane B 20 H 26. 231

(iii) Cleavage reactions Bonds are readily cleaved even by weak ligands to give either symmetrical or unsymmetrical cleavage products. Symmetrical products ( Homolytic ) Unsymmetrical products ( Heterolytic ) 232

The factors governing the course of these reactions are not fully understood but steric effects play some role eg . NH 3 , MeNH 2 and Me 2 NH give unsymmetrical cleavage products whereas Me 3 N gives the symmetric cleavage product Me 3 NBH 3 . Symmetrical cleavage is the more common mode and thermochemical and spectroscopic data lead to the following sequence of adduct stability for LBH 3 . PF 3  CO  Et 2 O  Me 2 O  C 4 H 8 S  Et 2 S  Me 2 S  py  Me 3 N  H - The relative stability of sulphide adducts is more notable and many other complexes with N, P, O, S etc. donor atoms are known. 233

The H - is a special case since it gives the symmetrical tetrahedral ion BH 4 - isoelectronic with CH 4 . The BH 4 - ion itself provide a rare example of a ligand that can be unidentate , bidentate or tridentate ( eg . [cu 1 (  1 - BH 4 )(PMePh 2 ) 3 ], [Cu 1 (  2 – BH 4 )(PPh 3 ) 2 ]; [ Zr IV (  3 – BH 4 ) 4 ]). In addition to pyrolysis and cleavage reactions, B 2 H 6 undergoes a wide range of substitution, redistribution and solvolytic reactions: 234

(iv) Hydrobaration Addition of B 2 H 6 to alkenes and alkynes in ether solvent at room temperature. Hydroboration is regiospecific , the boron atom showing preferential attachment to the least substituted carbon atom (anti- Markovnikov ). Protonolysis of the resulting organoborane by refluxing it with an anhydrous carboxylic acid yields the alkane correponding to the initial alkene. Oxidative hydrolysis with alkaline hydrogen peroxide yields the corresponding primary alcohol: 235

(v) Thermal isomerization Diborane is an electrophilic reducing agent which preferentially attacks a molecule at a position of high electron density. Internal alkanes can be thermally isomerized to terminal organoboranes and hence to terminal alkenes (by displacement) or to primary alcohols; 236

In the case of heteropolar double and triple bonds the boryl group BH 2 normally adds to the more electron rich atom i.e.. O atom in carbonyl and N atom in C  N and C  N. Thus after protonolysis aldehydes yield primary alcohol and ketones yield secondary alcohols, although in the presence of BF 3 complete reduction of C  O to CH 2 may occur. Likewise nitriles are reduced to amines, oximes to N- alkylhydroxylamines , and Schiff’s bases to secondary amines. 237

(vi) Reductive cleavage: Reductive cleavage of strained rings such as those in cyclopropanes and epoxides occur readily and acetals (or ketals ) are also reductively cleaved to yield an ether and an alcohol: 238

(viii) Removal of atoms: Removal of O atoms occur either with or without addition of H atoms to the molecule. Thus phosphine oxides give phosphines and pyridine-N-Oxides gives pyridine without addition of H atoms, whereas aromatic nitroso compounds are reduced to amines and cyclic diones can be successively reduced by replacement of C  O by CH 2 eg . 239

CHEMISTRY OF NIDO-PENTABORANE, B 5 H 9 Preparation: B 5 H 9 can be prepared by passing a 1:5 mixture of B 2 H 6 and H 2 at sub-atmospheric pressure through a furnace at 250  C with a 3-s residence time (or at 225  C with a 15-s residence time) there is a 70 % yield and 30% conversion. Pyrolysis of B 2 H 6 for days in a hot/ cold reactor at 180  C. Apex-substituted derivatives 1-XB 5 H 8 can be readily prepared by electrophilic substitution ( eg . Halogenation or Friedel Craft’s alkylation with RX or alkenes) 240

(iv) 2-XB 5 H 8 results when nucleophilic reaction is induced by amines or ethers, or when 1-XB 5 H 8 is isomerized in the presence of a lewis base such as hexamethylene tetramine or ether: (v) Further derivatives can be obtained by metathesis e.g. 241

REACTIONS Lewis bases B 5 H 9 reacts with lewis bases (electron pair donors) to form adducts e.g. with PMe 3 to give [B 5 H 9 (PMe 3 ) 2 ]. (ii) Weak Bronsted acid: B 5 H 9 acts as a weak acid. The acidity increases with increasing size of the borane cluster and arachno-boranes are more acidic than nido-boranes . Nido: B 5 H 9  B 6 H 10  B 10 H 14  B 16 H 20  B 18 H 22 Arachno : B 4 H 10  B 5 H 11  B 6 H 12 The experimental ordering B 4 H 10  B 10 H 14 and B 5 H 9  B 6 H 12 provides a connection between the two series. 242

B 5 H 9 can be deprotonated at low temperature by loss of H  to give B 5 H 8 - provided a sufficiently strong base such as a lithium alkyl or alkali metal hydride is used. (iii) Cluster expansion : (iv) Cluster degrading : 243

(v) Subrogation of a {BH} Subrogation of a {BH} unit in B 5 H 9 by an ‘isoelectronic’ organometallic group such as {Fe( 5 -C 5 H 5 )} can occur and this illustrates the close interrelation between metalloborates , metal-metal cluster compounds and organometallic complexes in general. Eg. [1-{Fe(CO) 3 }B 4 H 8 ] ; [1-{Co( 5 -C 5 H 5 )}B 4 H 8 ] ; [2-{Co( 5 -C 5 H 5 )}B 4 H 8 ] . 244

Nido-decaborane , B 10 H 14 Decaborane is the most studied of all polyhedral boranes and one time (mid 1950s) was manufactured on a multi-ton scale in the US as a potential high- energy fuel (HEF). It is now obtained in research quantities by the pyrolysis of B 2 H 6 at 200  C in the presence of catalyst of Me 2 O. 245

It is a colourless , volatile, crystalline solid insoluble in water but readily soluble in a wide range of organic solvents. Its structure is regarded as derived from the 11B atom cluster B 11 H 11 2- by replacing the unique BH group with two electrons and appropriate addition of 4H  . 246

Molecular orbital calculations give a sequence of electron charge densities at various B atom as 2, 4,  1, 3,  5, 7, 8, 10  6, 9 though the total range of deviation from charge neutrality is less than  0.1 247 S = 4 t = 6 y = 2 x = 0

Chemical properties The chemistry of B 10 H 14 can be conveniently discussed under the headings (a) Proton abstraction, (b) Electron addition, (c) Adduct formation, (d) Electrophilic substitution: (e) Nucleophilic substitution (f) Cluster addition reaction 248

(a) Proton abstraction B 10 H 14 can be titrated in aqueous/alcoholic media as a monobasic acid: pka 2.70. Proton abstraction can also be effected by other strong bases such as H - , OMe - , NH 2 - etc. X-ray studies on [Et 3 NH] + [ B 10 H 13 ] - established that the ion is formed by loss of a bridge proton as expected and this results in considerable shortening of the B(5)-B(6) distance from 179 pm in B 10 H 14 to 165 pm in B 10 H 13 - . 249

Under more forcing conditions with NaH a second H  can be removed to give Na 2 B 10 H 12 ; B 10 H 12 2- ; the anion acts as a normal bidentate ( tetrahapto ) ligand with many metals. (b)Electron addition Electron addition to B 10 H 14 can be achieved by direct reaction with alkali metals in ether, benzene or liquid NH 3 . 250

A more convenient preparation of the B 10 H 14 2- anion uses the reaction of aqueous BH 4 - in alkaline solution. Calculations show that this conversion of nido-borane to arachno -cluster reverses the sequence of electron charge density at the 2,4 and 6,9 positions so that for B 10 H 14 2- the sequence is 6, 9,  1, 3,  5, 7, 8, 10  2, 4, this is paralleled by changes in the chemistry. 251

(c) Adduct formation B 10 H 14 2- can formally be regarded as B 10 H 12 L 2 for the special case of L = H - . Compounds of intermediate stoichiometry B 10 H 13 L - are formed when B 10 H 14 is deprotonated in the presence of the ligand L : 252

The adduct can be prepared by direct reaction of B 10 H 14 with L or by ligand replacement reactions: Ligands L and L’ can be drawn virtually from the full range of inorganic and organic neutral and anionic ligands and indeed, the reaction severely limits the range of donor solvents in which B 10 H 14 can be dissolved. 253

The approximate sequence of stability is: SR 2  RCN  ASR 3  RCONMe 2  P(OR) 3  py  NEt 3  PPh 3 . Bis -ligand adducts of moderate stability play an important role in activating decaborane for several types of reactions e.g : 1. Substitution 254

2. Cluster rearrangement; 3. Cluster addition; 4. Cluster degradation In the cluster degradation reaction it is the coordinated B atom at position 9 that is solvolytically cleaved from the cluster. 255

(d) Electrophilic substitution: Electrophilic substitution of B 10 H 14 follows the sequence of electron densities in the ground state molecule. Halogenation in the presence of AlCl 3 leads to 1- and 2- mono-substituted derivatives and 2, 4- and 1, 2- di-substitution. Friedel Crafts alkylation with RX/AlCl 3 (or FeCl 3 ) yields mixtures such as 2-Me B 10 H 13 , 2, 4- and 1, 2- Me 2 B 10 H 12 , 1, 2, 3- and 1, 2, 4-Me 3 B 10 H 11 and 1, 2, 3, 4-Me 4 B 10 H 10 . 256

(e) Nucleophilic substitution This occurs preferentially at the 6(9) position ; e.g. LiMe produces 6-Me B 10 H 13 as the main product with smaller amounts of 5-Me B 10 H 13 , 6, 5(8) –Me 2 B 10 H 12 and 6, 9-Me 2 B 10 H 12 . (f) Cluster addition reaction B 10 H 14 undergoes numerous cluster addition reactions in which B or other atoms become incorporated in an expanded cluster. 257

A more convenient high yield synthesis of B 10 H 12 2- is by the direct reaction of amine boranes with B 10 H 14 in the absence of solvents. Heteroatom cluster addition reactions are exemplified by the following: The structure of the highly reactive anion [Al B 10 H 14 ] - is thought to be similar to the nido- B 11 H 14 - with one facial B atom replaced by Al. 258

(The open face comprises of fluxional system involving the three additional H atoms) The metal alkyls act somewhat differently to give extremely stable metalloborane anions which can be thought of as complexes of bidentate ligand B 10 H 12 2- . 259 [AlB 10 H 14 ] -

Many other complexes [M(B 10 H 12 ) 2 ] 2- and [L 2 M(B 10 H 12 ) 2 ] are known with similar structures except that, when M = Ni, Pd , Pt, the coordination about the metal is essentially square-planar rather than pseudo-tetrahedral as for Zn, Cd and Hg. 260

HETEROBORANES In many known compounds one or more BH groups in a deltahedral framework are replaced by atoms of a different elements. The structure of these species are related simply to those of the parent boron hydrides except that atoms of differing size may cause some distortions of the polyhedral framework. 261

Nido boranes (BH) p H 4 Arachno boranes (BH) p H 6 Thus a nido borane is a fully protonated (BH) p 4- and the arachno borane is a fully protonated (BH) p 6- When we look at (BH) p c- formulation this suggest that a great range of compounds containing the (BH) units or some other group also capable of donating 2e -s to the polyhedral framework could be obtained. Thus a BH unit can be replaced by CH + , P + , S 2+, N + or O 2+ (having two electrons for framework bonding plus two more in a bond to an oxo -H or as a lone pair ). 262

CARBORANES Closely related to the polyhedral boron hydrides is a large family of carboranes (carbaboranes), which are clusters that contain both B and C atoms. TYPE 1: methyldiboranes (Me n B 2 H 6-n ) where n= 1, 2, 3, 4 not 5 or 6. Similar derivatives of diborane in which the borane group replaces the terminal hydrogen of the parent borane are also known. 263

Type II These are actual carboranes where both B and C feature in the electron deficient molecular skeleton. Closo-carboranes – C 2 B n-2 H n (n=5 to n=12) Dicarba-closo-boranes These are neutral species and isoelectronic with B n H n 2- Eg . C 2 B 3 H 5 264

The closo-B 4 C 2 H 6 may be generated from a closo B 6 H 6 2- by the removal of 2BH units and adding 2CH + groups or units. C 2 B 4 H 6 265

NIDO CARBORANES [C 2 B n-2 H n 2- ] These have n cage atoms and n+2 pairs of cage bonding electrons and n+1 corners. Structurally they adopt an incomplete cage structure, nido or nest structure i.e. one cage corner is left vacant though when obtained as metal salts, the metal cation may occupy the vacant site in the crystal. Isoelectronic species which might all be expected to have the nido structures are: B n H n 4- , CB n-1 H n 3- , C 2 B n-2 H n 2- , C 3 B n-3 H n - , C 4 B n-4 H n . 266

HEXABORANE (10) B 6 H 10 is the limiting member of a series of five compounds containing six cage atoms and 8 pair of bonding electrons. 267

ARACHNO CARBORANES Very few of these are known. The series are: B n H n 6- , CB n-1 H n 5- , C 2 B n-2 H n 4- , C 3 B n-3 H n 3- , C 4 B n-4 H n 2- , C 5 B n-5 H n - and C 6 B n-6 H n . The parent hypothetical anion B n H n 6- are effectively the skeletons of a (BH) p H q in which q=6, eg . B 4 H 10 and B 5 H 11 . These have n cage atoms and n+3 pairs of cage bonding electrons i.e. the right number for a cage with n+2 corners. They accordingly adopt the arachno structure in which two cage corners are left vacant e.g. C 2 B 7 H 13 , C 2 B 8 H 10 , C 2 B 3 H 7 2- 268

Preparation and Reactions of carboranes The most important preparative route is the reaction of boranes with acetylenes The isomers of the closo compound C 2 B 10 H 12 have icosahedral geometry and exhibit extremely high kinetic and thermodynamic stability. 269

The 1, 2-; 1, 7-; 1, 12- isomers have the common names o- m- and p- carboranes respectively. o- carborane has been given the following symbol in literature 270

Reactions at B center in carboranes parallel those of boranes bridge proton abstraction and electrophilic substitution, including halogenation. The terminal H attached to electrophilic C are relatively acidic . Hence these C centers can be metalated 271

The metalated products retain structural integrity and can react with nucleophiles to produce a large number of C-substituted derivatives. Thermal isomerization occur. For dodecarborane , the diamond-square-diamond mechanism has been proposed for the isomerization of the 1,2- to 1,7- by Lipscomb but the 1,12- isomer cannot be generated by this mechanism. Moreover, the activation energy required to pass through the cubo -octahedral transition state is likely to be rather too high. 272

The diamond-square-diamond mechanism consists of a pair of triangular faces at right angles which open into a square and rejoin with a different pair of vertices connected. 273

An alternative proposal which can lead to both the 1, 7- and 1, 12- isomers, is the successive concerted rotation of the 3 atoms on a triangular face. Yet a third possible mechanism that has been envisaged involves the concerted basal twisting of two parallel pentagonal pyramids, comprising the icosahedron. It is conceived that the various mechanism operate in different temperature ranges or that two (or all three) mechanisms are active simultaneously. 274

O- carborane is attacked by a strong base that partially fragments it, generating a nido anion that retains structural integrity. B 10 C 2 H 12 + NaOEt + 2EtOH Na[B 9 C 2 H 12 ] + B( OEt ) 3 + H 2 The 1,7-isomer can be obtained by thermal rearrangement of the anion or by starting with m- carborane . NaH in tetrahydrofuran deprotonates the anion, giving a dianion B 9 C 2 H 11 2- . Na[B 9 C 2 H 12 ] + NaH Na 2 [B 9 C 2 H 11 ] + H 2 Assuming for convenience sp 3 hybridization of the five atoms on the open face, a set of MO’s reminiscent of the Cp - anion may be constructed. The MO’s are occupied by six electrons. 275

Hawthorne has exploited the analogy between B 9 C 2 H 11 2- (the dicarbollide ion) and Cp - to prepare metal dicarbollide complexes related to the metallocenes . 276

A general technique for preparing such complexes involves excision of a BH group through base degradation and reaction of the resulting anion with a metal halide. 277

278

Nomenclature for Heteroborates Vertices of closo - nido - and arachno-polyhedra are given numbers based by convention on planar projections of polyhedral structures. The numbering is by zones (planes) perpendicular to the major axis. Interior vertices on the projection are numbered first, then peripheral ones. This corresponds to numbering apical vertices with lower numbers. The numbering proceeds clockwise starting from the twelve o’clock position or at the first position clockwise. The location of heteroatoms can be specified by numbers. 279

PB 11 H 12 is phospha-closo-dodecaborane (12). There is no need to specify the position of P since all icosahedral vertices are equivalent. 280

General organizational scheme for the neutral boron hydrides, the closo -polyhedral, hydroborate ions, and the carboranes Given the molecular formula, a method is needed for predicting the probable structural classification of a boron compound i.e. either a closo -, nido - or arachno -. As described earlier on, the (BH) p H q symbolism is best for structural purposes and these scheme has been extended by Wade and Rudolph. The formula of any neutral borane , hydroborate or carborane can be written as: Where the number of vertices of the polyhedral fragment is ( a+p ) = n and the qHs are involved in a BHB or extra BH T ’s. 281

Assuming that the number of electron pairs contributed by CH is 3/2 The number of framework electrons is given by : 3/2a + p +1/2(q + c) = 3a + 2p + q + c =2n + a + q + c Since n = a + p, 2n = number of vertices When, a + q + c = 2 closo a + q + c = 4 nido a + q + c = 6 arachno 282

Eg . [B 6 H 6 ] 2- (BH) 6 2- = (6x3e -s ) + (6x1e - ) + 2e -s = 26e -s 6B-H = (6x2e -s ) = -12e -s 2n + a + q + c = 14e -s 2n = -12e -s Closo a + q + c = 2e -s B 5 H 11 (BH) 5 H 6 = (5x3e -s ) + (11x1e - ) = 26e -s 5B-H = (5x2e -s ) = -10e -s 2n + a + q + c = 16e -s 2n = -10e -s Arachno a + q + c = 6e -s 283

Closo-1,2-B 10 C 2 H 12 can be converted to the nido ion 7,8- B 9 C 2 H 12 - by reacting with KOH in EtOH . The formula of this ion 7,8-B 9 C 2 H 12 - anion may be written as [(CH) 2 (BH) 9 H] - if we compare with [(BH) 11 H] 3- . The [(CH) 2 (BH) 9 H] - anion compared with [(BH) 9 H] 3- with 2CH + unit replacing 2BH units and with a proton H + stitching up part of the opened face. 284

Here the a + q + c = 4 since it is a nido Deprotonation of 7, 8-B 9 C 2 H 12 - and this is a carborane isoelectronic with the (BH) 11 4- On the other hand protonation of the nido-7, 8-B 9 C 2 H 12 - ion gives a neutral compound nido 7, 8-B 9 C 2 H 13 . 285

Pyrolysis of the molecule nido - 7, 8-B 9 C 2 H 13 in solution can give a closo - polyhedron- 2, 3-B 9 C 2 H 11 . 9B  9x3e- = 27e- 9B  9 x3e- = 27e- 9B  9 x3e- = 27e- 9H  9 x1e- = 9e- 11H  11 x1e- = 11e- 12H  12 x1e- = 12e- 1S 2+  1 x4e- = 4e- 1S 2+  1 x4e- = 4e- 1S 2+  1 x4e- = 4e- = 40e- = 42e- charge =1x1e - = 1e- = 44e - 9BH  9 x2e- = -18e- 9BH  9 x2e- = -18e- 9BH  9 x2e- = -18e- = 22e- = 24e- = 26e- 10 vertices = -20e- 10 vertices = -20e- 10 vertices = -20e- a + q + c = 2e- a + q + c = 4e- a + q + c = 6e- Closo nido arachno 286

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Nano Technology “There’s plenty of room at the bottom, the principles of physics, as far as I can see, do not speak against the possibility of maneuvering things atom by atom, put the atoms down where the chemist says, and you make the substance...”, - Richard Feynman ; 1959

The Nano particles affects many properties such as: Melting point Boiling point Band gap Optical properties Electrical properties Magnetic properties Even the structure of materials changes with respect to Size

The properties of materials can be different at the Nanoscale for two main reasons: 1. Nanomaterials have a relatively larger surface area when compared to the same mass of material produced in a larger form. Nano particles can make materials more chemically reactive and affect their strength or electrical properties. 2. Quantum effects can begin to dominate the behaviour of matter at the Nanoscale

Nanoscale materials are divided into three categories , Zero dimension – length , breadth and heights are confined at single point. (for example, Nano dots) One dimension – It has only one parameter either length (or) breadth (or) height (example: very thin surface coatings) Two dimensions- it has only length and breadth (for example, nanowires and nanotubes)

What do you mean by Nano Particles ? Nano Particles are the particles of size between 1 nm to 100 nm Nanometer - One billionth (10 -9 ) of a meter • The size of Hydrogen atom 0.04 nm The size of Proteins ~ 1-20 nm Feature size of computer chips 180 nm Diameter of human hair ~ 10 µm At the nanoscale, the physical, chemical, and biological properties of materials differ in fundamental and valuable ways from the properties of individual atoms and molecules or bulk matter 1 nm is only three to five atoms wide. ~40,000 times smaller than the width of an average human hair

Nano particles are of interest because of the new properties (such as chemical reactivity and optical behaviour ) that they exhibit compared with larger particles of the same materials. For example, titanium dioxide and zinc oxide become transparent at the nanoscale and have found application in sunscreens. Nanoparticles have a range of potential applications: In the short-term application such as in cosmetics, textiles and paints . In the longer term applications such as drug delivery where they could be to used deliver drugs to a specific site in the body. Nanoparticles can also be arranged into layers on surfaces, providing a large surface area and hence enhanced activity, relevant to a range of potential applications such as catalysts. Why Nano Particles ?

• Examples - Carbon Nanotubes - Proteins, DNA - Single electron transistors AFM Image of DNA Carbon Nanotubes

Nanotechnology deals with the creation of USEFUL materials, devices and systems using the particles of nanometer length scale and exploitation of NOVEL properties (physical, chemical, biological) at that length scale

Various Nanomaterials and Nanotechnologies • Nanoparticles • Nanocapsules • Nanofibers • Nanowires • Fullerenes (carbon 60) • Nanotubes • Nanosprings • Nanobelts • Quantum dots • Nanofluides Based on the size and shape, the Nano materials are classified as follows

Quantum well It is a two dimensional system The electron can move in two directions and restricted in one direction. Quantum Wire It is a one-dimensional system The electron can move in one direction and restricted in two directions. Quantum dot It is a zero dimensional system The electron movement was restricted in entire three dimensions Why called Quantum ? Because, the electronic property is quantized The spatial distance is very very small

substrate Semiconductor growth (single layer)

Quantum wire Quantum wires are ultra fine wires or linear arrays of Nano dots, formed by self-assembly They can be made from a wide range of materials such as Semiconductor Nanowires made of silicon, gallium nitride and indium phosphide. Nanowires have potential applications in In high-density data storage, either as magnetic read heads or as patterned storage media In electronic and opto-electronic Nanodevices, for metallic interconnects of quantum devices and Nanodevices. Nanowires can be prepared by growth techniques such as Chemical Vapour deposition (CVD) Electroplating

We need two dimension to calculate area of conducting material, but not present in quantum wire In quantum wire, Two dimensions are reduced and one dimension remains large Therefore, the electrical resistivity of quantum wire can be calculated using conventional formula as follows,

General properties of Nanowire Diameter – 10s of nanometers Single crystal formation -- common crystallographic orientation along the nanowire axis Minimal defects within wire Minimal irregularities within nanowire arrays Some example of Nanowire

Magnetic nanowires Example: Cobalt, gold, copper and cobalt-copper nanowire arrays Important for storage device applications Electrochemical deposition is the fabrication technique <20 nm diameter nanowire arrays can be fabricated by electrochemical deposition Cobalt nanowires on Si substrate

In quantum dot all the three dimensions are reduced to zero Quantum dot

Dimension Variation 

Properties of Nano Materials

The melting point decreases dramatically as the particle size gets below 5 nm Melting Point

Band gap and surface Area The total surface area (or) the number of surface atom increases with reducing size of the particles The band gap increases with reducing the size of the particles

Size-Dependent Properties of semiconductor and magnetic materials • For semiconductors such as ZnO , CdS , and Si, the bandgap changes with size - Bandgap is the energy needed to promote an electron from the valence band to the conduction band - When the bandgaps lie in the visible spectrum, changing bandgap with size means a change in colour • For magnetic materials such as Fe, Co, Ni, Fe 3 O 4 , etc., magnetic properties are size dependent - The ‘coercive force’ (or magnetic memory) needed to reverse an internal magnetic field within the particle is size dependent - The strength of a particle’s internal magnetic field can be size dependent

Colour

Synthesis of Nanomaterials Bottom-up Approach (1) Wet-chemical methods. Molecular beam epitaxy (MBE), Sputtering, liquid metal ion sources, (2) vapour-phase methods. Top-Down Synthesis Processes Electron beam lithography Reactive-ion etching wet chemical etching, Focused ion or laser Etching. Dry etching. Reactive ion etching (RIE). Focused ion beam (FIB)

Because of their small size, nanoscale devices can readily interact with biomolecules on both the surface of cells and inside of cells. By gaining access to so many areas of the body, they have the potential to detect disease and the deliver treatment . 1. Nanotechnology Applications in Medicine Nanoparticles can deliver drugs directly to diseased cells in the body. Nanomedicine is the medical use of molecular-sized particles to deliver drugs, heat, light or other substances to specific cells in the human body. Applications of Nano Materials

Quantum dot - that identify the location of cancer cells in the body. Nano Particles - that deliver chemotherapy drugs directly to cancer cells to minimize damage to healthy cells. Nanoshells - that concentrate the heat from infrared light to destroy cancer cells with minimal damage to surrounding healthy cells. Nanotubes - used in broken bones to provide a structure for new bone material to grow.

Nano shells as Cancer Therapy Nano shells are injected into cancer area and they recognize cancer cells. Then by applying near-infrared light, the heat generated by the light-absorbing Nano shells has successfully killed tumor cells while leaving neighboring cells intact.

Nano sized sensing wires are laid down across a micro fluidic channel. As particles flow through the micro fluidic channel, the Nanowire sensors pick up the molecular identifications of these particles and can immediately relay this information through a connection of electrodes to the outside world. These Nanodevices are man-made constructs made with carbon, silicon Nanowire . They can detect the presence of altered genes associated with cancer and may help researchers pinpoint the exact location of those changes Nanowires – used as medical sensor

Past Shared computing thousands of people sharing a mainframe computer Present Personal computing Future Ubiquitous computing thousands of computers sharing each and everyone of us; computers embedded in walls, chairs, clothing, light switches, cars….; characterized by the connection of things in the world with computation. 2. Nano Computing Technology

3. Sunscreens and Cosmetics Nanosized titanium dioxide and zinc oxide are currently used in some sunscreens, as they absorb and reflect ultraviolet (UV) rays. Nanosized iron oxide is present in some lipsticks as a pigment. 4. Fuel Cells The potential use of nano-engineered membranes to intensify catalytic processes could enable higher-efficiency, small-scale fuel cells. 5. Displays Nanocrystalline zinc selenide, zinc sulphide , cadmium sulphide and lead telluride are candidates for the next generation of light-emitting phosphors. CNTs are being investigated for low voltage field-emission displays; their strength, sharpness, conductivity and inertness make them potentially very efficient and long-lasting emitters.

6. Batteries With the growth in portable electronic equipment (mobile phones, navigation devices, laptop computers, remote sensors), there is great demand for lightweight, high-energy density batteries. Nanocrystalline materials are candidates for separator plates in batteries because of their foam-like (aerogel) structure, which can hold considerably more energy than conventional ones. Nickel–metal hydride batteries made of nanocrystalline nickel and metal hydrides are envisioned to require less frequent recharging and to last longer because of their large grain boundary (surface) area. 7. Catalysts In general, nanoparticles have a high surface area, and hence provide higher catalytic activity.

8. Magnetic Nano Materials applications It has been shown that magnets made of nanocrystalline yttrium–samarium–cobalt grains possess unusual magnetic properties due to their extremely large grain interface area (high coercivity can be obtained because magnetization flips cannot easily propagate past the grain boundaries). This could lead to applications in motors, analytical instruments like magnetic resonance imaging (MRI), used widely in hospitals, and microsensors. Nanoscale-fabricated magnetic materials also have applications in data storage. Devices such as computer hard disks storage capacity is increased with Magnetic Nano materials

. Unfortunately, in some cases, the biomedical metal alloys may wear out within the lifetime of the patient. But Nano materials increases the life time of the implant materials. Nanocrystalline zirconium oxide (zirconia) is hard, wear resistant, bio-corrosion resistant and bio-compatible. It therefore presents an attractive alternative material for implants. Nanocrystalline silicon carbide is a candidate material for artificial heart valves primarily because of its low weight, high strength and inertness. 9. Medical Implantation 10. Water purification Nano-engineered membranes could potentially lead to more energy-efficient water purification processes, notably in desalination process.

11. Military Battle Suits Enhanced nanomaterials form the basis of a state-of- the-art ‘battle suit’ that is being developed. A short-term development is likely to be energy-absorbing materials that will withstand blast waves; longer-term are those that incorporate sensors to detect or respond to chemical and biological weapons (for example, responsive nanopores that ‘close’ upon detection of a biological agent).

CHEM 351-2023.inorganic chemistry for universityey

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CHEM 351-2023.inorganic chemistry for universityey

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