Power point presentation on TM-1,3-ButaDiene Complex

Structure , preparation and reactions of transition metal 1,3-butadiene complex is explained here

1. Transition metal-1,3-Butadiene complex DIOLEFIN ISOLATED DIOLEFIN CONJUGATED DIOLEFIN

2. The conjugated dienes such as 1,3-butadiene act as 4e - donor ligand 1,3-butadiene exists as two conformational isomers which are in rapid and dynamic equilibrium with each other This ligand binds to a metal in a cisoid conformation Cis -isomer Trans-isomer

3. The Dewar- Chatt model, when applied to 1,3-butadiene, predicts that the ligand may bind to metal either as a L 2 donor type, similar to that of an alkene , or as an LX 2 donor type, similar to that of a metallacyclopentene form L 2 binding of 1,3-butadiene is rare, while the LX 2 type binding is more common C 1 C 2 C 3 C 4 C 1 C 2 C 3 C 4

4. An implication of the LX 2 type binding is: shortening of the C 2 -C 3 (1.40Ǻ) distance along with the lengthening of the C 1 -C 2 (1.46Ǻ) and C 3 -C 4 (1.46Ǻ)

5. Methods of preparation Metal butadiene complexes are usually prepared by the same methods used for synthesizing metal- alkene complexes Reaction of metal carbonyl with 1,3-butadiene When a metal carbonyl is treated with 1,3-butadiene, two CO ligands are replaced by the diene molecule forming transition metal-butadiene complex + Fe(CO) 5 High Pressure Fe(CO) 3 + 2CO

6. When a mixture of 1,3-butadiene and Fe(CO) 5 is irradiated by UV light, four CO ligands are replaced by two molecules of 1,3-butadiene forming a bis ( η 4 -butadiene) complex Fe(CO) 5 + 2CH 2 =CH-CH=CH 2 [Fe(CO)( η 4 -C 4 H 6 ) 2 ] UV - 4CO

7. Some other examples are :

8. 2. Reaction of metal halide with 1,3-butadiene: Some 1,3-butadiene complexes can be prepared by the reaction of metal halide with 1,3-butadiene under suitable conditions K 2 [PtCl 4 ] + CH 2 =CH-CH=CH 2 K[PtCl 3 ( η 2 -C 4 H 6 )] In this complex 1,3-butadiene acts as 2e - donor, dihapto ligand - KCl

9. . Metal atom vapour phase synthesis: The condensation of vapour of a metal in the presence of 1,3-butadiene and suitable co- ligand (such as CO,PR 3 etc) produces butadiene complex Cr(g) + 4CO(g) + CH 2 =CH-CH=CH 2 (g) [Cr(CO ) 4 ( η 4 -C 4 H 6 )]

10. S tructure

11. [ Cr( CO ) 4 ( η 4 -C 4 H 6 )]

12. Structure and bonding To begin with bonding let us consider hybridisation and molecular orbitals of butadiene molecule 1,3-butadiene is a planar molecule with sp 2 hybridisation on each carbon atom Each carbon atoms form 3 sigma bonds using hybrid orbitals

13. After C-H and C-C bond formation, each carbon atom has one singly filled 2p orbital which is perpendicular to the molecular plane for π -bonding Four 2p orbitals take part in linear combination to form four π -MOs

14. The MOs of the 1,3-butadiene ligand comprises of two filled Ψ 1 (HOMO-1) and Ψ 2 (HOMO) orbitals and two empty Ψ 3 (LUMO) and Ψ 4 (LUMO+1) orbitals

15. Appropriate combinations of four π MOs of 1,3-butadiene and metal based orbitals can be used to construct MOs of transition metal-Butadiene complex

16. Ψ 1 and Ψ 2 π MOs of 1,3-butadiene are involved in the formation of butadiene → M σ and σ bond respectively Similarly, Ψ 3 and Ψ 4 π MOs of 1,3-butadiene are involved in the formation of M → butadiene π and δ bond respectively (back donation)

17. The frontier orbitals of the butadiene, ψ 2 (HOMO) and ψ 3 (LUMO) are expected to be the most important in bonding to the metal. In a metal−butadiene interaction the ligand to metal σ−donation occurs from the filled Ψ 2 orbital of the 1,3−butadiene ligand while the metal to ligand π−back donation occurs on to the empty Ψ 3 orbital of the 1,3−butadiene ligand

18. The molecular orbital diagram shows that both the depletion of electron density in ψ 2 by σ donation to the metal and population of ψ 3 by back donation from the metal lengthens terminal C-C bond and shortens internal C-C bond. Binding to a metal usually depletes the ligand HOMO and fills the ligand LUMO.

19. The extent of ligand donation or metal back-donation depends on the metal, substituents on the diene and other ligands present An electron-rich metal will tend to populate Ψ 3 ; an electron-poor metal will tend to depopulate Ψ 2 . This is the main reason why binding has such a profound effect on the chemical character of a ligand .

20. The structure of the bound form of a ligand is similar to that of the first excited state of the free ligand because to reach this state we promote an electron from the HOMO to the LUMO, thus partially depleting the former and filling the latter.

21. When the butadiene is excited, one of the electron excited to higher orbital i.e., one of the electron from ψ 2 moves to ψ 3 The internal double bond character, bond length decreases to 1.39 Ǻ and terminal bond is single bond and bond length increases to 1.45 Ǻ As a result, lengthening of terminal C-C bonds as well as shortening of the internal C-C bond can be observed

22. Similarly in metal-butadiene complexes, depopulation of ψ 2 and population of ψ 3 would result in lengthening of terminal C-C bonds and shortening of internal C-C bonds

23. The bonding between metal and butadiene may be represented as follows When the donation from ψ 2 into metal is important, the bonding may be represented by (a) If donation from ψ 1 into metal is important, the bonding may be represented by (c) If ψ 3 interactions are important, then represented by (b) i.e., 1 st excited state of butadiene fragment

24. For ex: [Fe(CO) 3 ( η 4 -C 4 H 6 )]. Its structure and bonding is close to (c) in which π electron cloud of butadiene is delocalized on the entire butadiene moiety [Fe(CO) 3 ( η 4 -C 4 H 6 )]

25. If the interaction of ψ 3 with metal orbital is important, the structure and bonding may be represented by (b) which suggests a shorter C 2 −C 3 distance than two outer C-C distances i.e., C 1 −C 2 and C 3 −C 4 Ex: in [Cp 2 Zr( η 4 -2,3-Me 2 -C 4 H 4 )] C 1 -C 2 and C 3 −C 4 distances are longer than C 2 −C 3 distance

26. Therefore, (b) is the main contributor to the structure and bonding of the compound In this complex, C 1 -C 2 and C 3 -C 4 distances are longer than C 2 -C 3 distance The Zr-C 1 and Zr-C 4 distances are typical of Zr -C sigma bonds

27. At the same time Zr is farther from C 2 and C 3 than C 1 and C 4 2,3-dimethyl butadiene shows extreme LX 2 i.e., (b) bonding pattern Zr is electron rich , it transfers 2 e - s to butadiene via π-back-donation and generates metallacyclopentene

28. Properties: [Fe(CO) 3 ( η 4 -C 4 H 6 )] is a yellow - brown oil which is soluble in most of the organic solvents The coordinated butadiene does not undergo hydrogenation and Diels-Alder reaction

29. Electrophilic addition reaction : the attack by electrophile on the coordinated butadiene is less common. However illustrated by direct protonation and hydrogen abstraction

30. Sometimes, the carbocation electrophile abstracts h y dride ion from the saturated carbon atom adjacent to the olefinic carbon atom extending the conjugation => lead to a change in the hydrocarbon ligand and its mode of bonding

31. Nucleophilic attack : the coordinated butadiene is readil y attacked by a variety of nucleophiles such as CH 3 O - , H - , CN - , R - etc.

32. The addition of nucleophile to butadiene saturates an olefinic carbon atom and thus removes it from conjugation

33. The nucleophile ma y also attack on the metal atom.

34. Displacement reaction: The coordinated 1,3-butadiene molecule can be displaced b y other neutral ligands such as PPh 3