Fluxional Isomerism in organometallic compounds.pptx

Fluxional isomerism is one of the property exhibited by the unsaturated organometallic compounds

1. F L UX IO NAL ISO M E R IS M 1

2. Fluxional behaviour is characteristic of certain classes of organometallic compound No molecule can be rigid => it shows vibration The atoms execute more or less harmonic vibrations about their equilibrium positions, but in other respects the structures may be considered rigid molecule Vibration => change in bond length and bond angle 2

3. Most molecules => well defined nuclear configuration Molecular vibrations carry a molecule from one type of configuration into another Molecules => more than one thermally accessible structures , which under suitable conditions (T) are interconvertible from one to another fairly rapidly. Phenomenon: Fluxional behaviour 3

4. If the process occur at a rate such that they can be detected by atleast some physical or chemical method the molecule => stereochemically non rigid ( Muetterties, 1965 ) 1. In some cases- non equivalence of 2 or more structures results in the molecules with different instantaneous identities, process of interconversion -> known in organic molecules for a long time 4

5. Ex: Tautomerism of β-diketones in keto and enol form 2. In other cases- 2 or more configurations are equivalent (in structure, bonding and energy content). Molecules: ( Doering & Roth, 1963 ) Process: Fluxionality O O R 1 R O H O R 1 R Tautomerism of beta-diketones 5

6. When two or more configurations are chemically non equivalent => isomers/tautomers When two or more configurations are chemically equivalent => Fluxional molecules 6

7. Another term “ stereochemically non rigidity” ( Muetterties, 1965 ) for fluxional molecules F.A.Cotton, a prominent worker in this field, suggested (1968) that the term “stereochemically non rigidity” be reserved as a general term for molecules of both the types, i.e., tuatomeric and fluxional, which undergo rapid reversible intramolecular rearrangements 7

8. History of Fluxionalism The development of the concept of ‘fluxionality’ in molecules has an interesting history The historical development of the concept is presented in a little more detail and examples of some simple inorganic molecules like PF 5 is included for elucidating the concept before p resenting the features of a few ‘fluxional’ organometallic compounds The NMR non-equivalence of the fluxional atoms in PF 5 - reported 8

9. by Gutosvky and cowerkers in 1953, but the accompanying phenomenon was understood after the availability of commercial n.m.r. spectrometers 9

10. Fluxional behaviour first detected in PF 5 and Fe(CO) 5 In 19 F and 13 C n.m.r. spectra respectively of these two molecules, only one resonance peak was observed instead of two in the ratio of 2 : 3 (i.e., ratio of axial to equatorial sites) 10

11. Thoeretical consideration-concluded that when two nuclei , the resonance frequencies, of which differ by (ʋ 1 -ʋ 2 )= Δʋ/sec., change site at a frequency higher than (ʋ 1 +ʋ 2 )= Δʋ/sec., then only one resonance peak is observed at the mean (ʋ 1 +ʋ 2 )= Δʋ/2 position of the two frequencies characteristic of two types of sites, between which interchange is occurring 11

12. In both PF 5 and Fe(CO) 5 the dissociative or bimolecular mechanism appears to be ruled out in view of the persistence of 31 P- 19 F and 57 Fe- 13 C coupling The available experimental evidence favour a 2- for -2 process in which axial ligands exchange simultaneously with two equatorial ones Probable mechanism -exchange- by Berry ( 1960 ) 12

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15. The exchange process involves change from triagonal pyramid (tbp) to square planar (sp) intermediate by simultaneous closing of the F 2 PF 3 angle from 180 and opening of the F 4 PF 5 angle from 120 so that both attain the midway value, giving rise to the sqaure set of atoms F 2 , F 3 , F 4 , F 5 all equivalent to one another 15

16. The sp configuration can return to the tbp configuration or go a step further to attain the configuration in which the molecules have effectively rotated round the F 1 P axis through 90 In view of this apparent rotation, Berry mechanism is termed ‘pseudorotation’ So in this Berry rotation it is possible to rotate all the 5 carbonyls in Fe(CO) 5 compound 16

17. Can not observe unless labelled atom/s used CO stretching frequency- different in the two positions Single 13 C peak in the n.m.r spectrum further support the fluxionality (expected 2 for tbp (2/3) sp (1/4) ) X-ray crystallography- tbp Fe(CO) 5 17

18. Techniques used to study Different spectroscopic tools : microwave, IR and NMR spectroscopy NMR technique ; maximum utility For a number of systems, speed of rearrangement is fast at ambient temperature => n.m.r technique not able to detect the change But at lower temperature rate of rearrangement slow => detectable by n.m.r spectroscopy 18

19. In some fast techniques: electronic and vibrational spectroscopy and gas phase electron diffraction, the act of observing the molecule completes in ,10-11sec Rate of rearrangement not fast enough to influence these observations. Techniques are not suitable to detect fluxional behaviour Suitable for tuatomeric or isomeric molecule detection 19

20. By proper choice of temperature, many rearrangements can be controlled so that they are slow enough at lower temperatures to allow signals due to different fluxional species appearing distinctly rapid enough at higher temperatures for the signals from the different configurations or environments to be averaged into a single line at a mean position 20

21. Objective to study fluxional moleccules to establish the structure of the molecule to study the pathway of rearrangements If a nucleus is exchanging between two molecular configurations A and B, then we have equilibrium A n A B n B 21

22. When the rate of interconversion of A and B is slow, two separate signals representing two non-equivalent sites (due to 2 seperate species A and B) are observable Due to rapid nucleus migration from one site (A) to another (B), the n.m.r spectrum exhibits one signal In intermediate rates of exchange broad signals are observed 22

23. 23 Hg(η 1 -C 5 H 5 ) 2 -only singlet Fe(η 1 -C 5 H 5 )(CO) 2 (η 5 -C 5 H 5 ) - two singlets for 2 Cp rings at RT Cu (η 1 -C 5 H 5 )P(C 2 H 5 ) 3 - one singlet for all 5 ring C’s Appearance of single resonance -all protons in σ -bonded Cp- rapid shifting of Metal-Carbon bond among the five possible positions

24. Ti(η 1 -C 5 H 5 ) 2 (η 5 -C 5 H 5 ) 2 which give rise in its n.m.r spectrum near the lower temperature (-27 ° C ) limit two equal intensity signals which collapse , on raising the temperature (to 6 2 ° C ) of recording the spectrum, into one signal with no coupling present The temperature at which the 2 separate signals just merge into one is called the ‘ coalescence temperature ’ 24

25. Carbonyl Hopping Observed in bridged metal carbonyls In this process, exchange of bridged and terminal carbonyls Involves opening up of the bridge- to terminal carbonyl- closure of terminal to - bidged carbonyl exchange is very fast The solution 13 C NMR spectrum of exhibits one resonance even as low as 123 K showing that the molecule is fluxional. 25

26. The process can be described in terms of exchange of terminal and bridging CO ligands, or by considering the tilting of the Fe 2 -unit within a shell of CO ligands. This illustrates that the CO term–CO bridge exchange is a low-energy process. 26

27. When solid Co 2 (CO) 8 is dissolved in hexane, the IR spectrum changes. The spectrum of the solid contains bands assigned to terminal and bridging CO ligands, but in hexane, only absorptions due to terminal carbonyls are seen. This is explained by the equilibrium in the scheme and solid state 13 C NMR spectroscopic data show that terminal–bridge CO exchange occurs even in solid Co 2 (CO) 8 . 27

28. Fluxionality in cyclopentadiene complex (CO) 4 Fe 2 (η 5 -C 5 H 5 ) 2 This shows two separate Cp resonances for cis- and trans- CO bridged isomers in the proton NMR below - 50 ° C But 1 resonance at RT showing the fast exchange 28 -E x ist in cis - and trans forms in solution - 2 types of Cp-rings ( 2 η 5 - Cp rings in cis-isomer equivalent but different from 2 equivalent η 5 - Cp rings in trans ) in equilibrium mi x ture

29. 29 * 1 H-NMR spectrum at low temperature (-70 ° C) --- 2 signals * with increase of temperature - 2 peaks collapse..... ultimately at 28 ° C ---- merge into single peak => at -70 ° C, cis-trans isomerisation- slow- NMR technique- distinguish 2 isomers (chemical environments of Cp rings) => at 28 ° C, process- fast - can not distinguish the isomers- it represents their time averaged structure. 13 C-NMR- indicates cis-trans interconversion -accompanied with scrambling/hopping of the terminal and bridging CO groups.

30. The trans compound gives much faster exchange between bridging and terminal COs as shown by the greater initial broadening seen for the trans isomer by 13 C NMR. This is because only the nonbridged form of the trans compound, has a choice of COs for re-forming the bridge. 30

31. If the starred COs were originally bridging, the compound can choose the unstarred pair to re-form the new bridge. This means that a terminal CO in the trans compound has a 50% chance of changing into a bridging carbonyl after one exchange event (going to the open form and back again to the trans form). In the open form of the cis compound, there is only one pair of trans COs, and the same ones that opened up also have to re-form the bridge unless a rotation takes place, which is thought to be slow. 31

32. This means that a terminal CO in the cis compound has a low chance of changing into a bridging carbonyl in any one exchange event. Another example is: CpFe(CO) 2 (η 1 -C 5 H 5 ) which shows only two sharp proton resonances at room temperature , one for the η 5 -C 5 H 5 , and one for the η 1 - C 5 H 5 . But 4 signals at low temperature The iron atom is migrating around the η 1 -C 5 H 5 ring at a sufficient rate to average all the proton resonances from the η 1 -C 5 H 5 ring. (1,2- metal shift) 32

33. i.e., movement of a metal atom around the cyclopentadienyl ring from one carbon to the carbon α to it This movement averages proton on η 1 -cyclopentadiene molecule => only one signal for η 1 - Cp M H * M H * 1 , 2- s h i f t M H * H * 1,3-shift M 33

34. On going to lower temperature , separate resonances can be distinguished for the three different types of proton in the static η 1 -C 5 H 5 group => 3 signals (1:2:2) . If we warm the sample from the low-temperature limit, there will be a different degree of initial broadening of the different proton resonances of the η 1 -C 5 H 5 group if the fluxionality involves 1,2 - metal shifts rather than 1,3 -metal shifts. 34

35. This is because the H C ’s are next to one another and so a 1,2 shift (which is indistinguishable from a 1,5 shift) will mean that one of the H C ’s will still end up in an H C site after a 1,2 shift. On the other hand, all the H B ’s will end up in non-H B sites. 35 M H * M H * 1 , 2- s h i f t

36. The exchange rate for H C ’s will therefore appear to be one-half of the exchange rate for H B ’s, and the resonance for H C will show less initial broadening. Experimentally, it is found that a 1,2 shift is taking place . 36

37. Metal hopping differ from carbonyl hopping in a way that here ligand system is stationary, metal is moving from one pi system to another 37

38. Fluxional behaviour in Olefins Simple double bonded systems can show variety of interesting fluxional behaviour because of the rotation of the metal with respect to the olefin In a square planar system - Zies’s salt - olefin is perpendicular to the plane of the metal ligand system isomerisation => all atoms come into the same plane ( high energy state) then come back to the ground state 38

39. In solution, alkene complexes are often fluxional, with rotation of a Barrel type (occurring as shown in figure) The model compound in the figure contains a mirror plane passing through M, L and L’. CH 3 H 3 C CH 3 H 3 C 39

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41. The limiting low-temperature 1 H spectrum shows one resonance for H 1 and H 4 , and another due to H 2 and H 3 , i.e. a static picture of the molecule. On raising the temperature, the molecule gains sufficient energy for the alkene to rotate and the limiting high temperature spectrum contains one resonance since H 1 , H 2 , H 3 and H 4 become equivalent on the NMR timescale. 41

42. Rhodium olefin complex, where two ethylenes are coordinated to a RhCp unit (η 5 -Cp)Rh(η 2 -C 2 H 4 ) 2 two alkene proton signals are observed at low temperature Outer protons ---- downfield multiplets Inner protons ----- upfield multiplets - a t low temperature, the rotation of the olefin becomes slow, that it does not affect the observation in the n.m.r spectrum of the two distinct environments for the protons in the molecule H = inner hydrogen H = outer hydrogen 42

43. At high temperature the proton environments become equivalent on the NMR spectroscopic timescale as each alkene ligand rotates about the metal–ligand coordinate bond and separate lines collapse into a broad signal Exchange of inner and outer protons can take place by an alternate mechanism, dissociation of an olefin Recombine- inner can be outer, outer can be inner 43

44. Since 105 Rh is NMR active differentiation of two mechanism possible Dissociate: Rh-H connectivity would loss => loss of Rh-H coupling Rotation of Olefin: Average coupling constant At -40C 2 distinct peaks 2 J (Rh-Hi) = 1.8Hz 2 J (Rh-Ho) = 2.5Hz At RT only one J Average 2 J (Rh-Hi) = 2.1Hz Expect loss of Rh-H coupling 44

45. Fluxionalism in allyl complex Allyl can be η 1 or η 3 η 3 -allyl ligands - non-rigid on -> 1 H n.m.r time-scale Presence of three types of H atoms-confirmed- 1 H NMR , 3 sets of signals with an intensity ratio of 1:2:2 (H c :H b :H a ) With respect to middle H (H c )- 2 H’s occupying cis or syn position and 2 ‘s trans or anti positions At lower temperature , η 3 -allyl ligand static - 3 signals -for 3 types of protons. Separate signals due to terminal H’s (H c & H t ) split into doublet by the middle H-atom (H m ) C H 2 C H M H b H a H 2 C σ, η 2 allyl complex H c C H b H a M H =S yn b H a = Ant i 45

46. η 3 - allyl -----> η 1 -allyl i.e., allyl group => σ -bonded to metal through one of its terminal carbons, Then rotation about C-C single bond and reformation of η 3 -allyl complex lead to cis,trans-proton equilibrium 46 Signal due to H m split by 4 terminal H’s => multiplet A t higher temperature - 2 doublets collapse in to a single doublet - rapid intramolecular rearrangement -causes anti & syn protons to be equivalent => At high T only 2 types of H’s - 4 equivalent terminal proton and 1 middle or non-terminal proton - Rapid scrumbling of H’s through η 3 - η 1 -η 3 process

47. Ring Whizzing Cyclopentadienyl Haptotropy Common in Cp complex Ti(IV) complex, 4 Cp anions interacting with Ti 2 types of Cp ring At relatively high T (62 ° C) -> single peak => equivalence of 4 Cp rings . At this T, mutual interconversion among these ligands so fast that different chemical environments indistinguishable 47

48. 48 At low T (-27°C) - mutual interconversion between η 1 - and η 5 - modes of binding of Cp rings -> slow NMR technique - identify different chemical environments of H’s in η 1 - Cp & η 5 -Cp => as T reduced, single peak splits into 2 peaks of equal intensity In η 5 -Cp all H’s equivalent In η 1 - Cp - 3 types of H’s - non distinguishable at -27°C Metal hopping - mutual interchange among 3 types of H’s in η 1 - Cp => 2 signals (for η 1 - Cp & η 5 -Cp) On further reducing the T ( -80°C ) - mutual interconversion of H’s of η 1 - Cp - slow => At -80°C 1 H-NMR detect H’s of (which are equivalent) of η 5 -Cp and 3 types H’s (in 1:2:2 ratio) in η 1 - Cp => 4 signals

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50. Cp is a ligand which can adjust the electron donation by sliding the metal along the Cp bisecting plane Consider a M atom attached to only one C(η 1 ) As it moves along the arrow an allyl like coordinate is formed (η 3 ) As moved further to the centre of the ring then it is η 5 50

51. Fluxional behaviour distinguished from rearrangement by the fact that the ring system or the carbon framework attached to the metal is not changed. It is the only the arrangement of the ring with respect to the metal or w.r.t the other part of the molecule Energy difference betweene these two forms very small, rapid exchange between these two structural forms 51