Lecture 9 CH202 CH324 PDF

LECTURE 9 PART 1 Topic 8: Applications of organo-transition metal compounds Catalysis (the future) organometallic reagents provide a rich source of catalysts for organic transformations two industrially important catalytic cycles will be discussed one in h...

LECTURE 9 PART 1 Topic 8: Applications of organo-transition metal compounds Catalysis (the future) organometallic reagents provide a rich source of catalysts for organic transformations two industrially important catalytic cycles will be discussed one in heterogeneous catalysis, the other in homogeneous catalysis 1. Olefin (alkene) polymerisation utilising Ziegler- Natta catalysis this process uses Ti(+3) supported on a surface or doped into a lattice the substrate (solution) and catalyst (solid) are in different (hetero) phases therefore this process is an example of heterogeneous catalysis TiCl4 on MgCl2 support - vacant site reforms ethyl migrates -complex Explanatory Notes on Olefin Ziegler Natta Catalysis catalyst made by adding small amount of TiCl4 into solid MgCl2 (doping) and grinding together Et3Al co-catalyst replaces one chloride with an ethyl group, reducing Ti(+4) to Ti(+3) loss of Et2AlCl creates a vacant coordination site on Ti vacant site is filled by ethene molecule bonded via a -interaction Ethyl group transfers to the olefin to form a coordinated Bu group and a new vacant site this is subsequently occupied by more olefin and the butyl group is transferred to form a hexyl group the cycle repeats itself. The polymer increases its size by 2 (–CH2–) units each time finally, termination occurs when acid is added to give the desired polymer  important to realise that Ti doped into the lattice is the catalyst (or propagation site) and the Mg is the support overall reaction is: Annual production: Polyethylene 100 million tonnes Polypropylene 56 million tonnes  The Nobel Prize in Chemistry 1963 Karl Ziegler 1898 - 1973 Giulio Natta 1903 -1979 Born: Germany Born: Italy Max-Planck-Institute for Carbon Research, Institute of Technology, Milan, Italy Mülheim/Ruhr, Germany Prize motivation: "for their discoveries in the field of the chemistry and technology of high polymers" END OF LECTURE 9 PART 1 Essential Learning: Understanding the fundamental steps in Ziegler-Natta Catalysis LECTURE 9 PART 2 Topic 8: Applications of organo-transition metal compounds continued 2. Hydrogenation of olefins using Wilkinson’s catalyst [Rh(PPh3)3Cl] example of homogeneous catalysis where catalysts and substrate are in the same (homo) phase, that is, both dissolved in solution  H2, catalyst Alkene Alkane oxidative addition stereochemistry critical H’s must be cis to each other Rh(+1) d8 square planar Rh(+3) d6 octahedral reductive elimination alkene insertion Notes on Olefin Hydrogenation Catalysis Rh(+1) triad Co/Rh/Ir Group 9; square planar d 8 adding H2 forms a di-hydride (H-) oxidising metal to Rh(+3) oxidative addition this is now d6 with a propensity for octahedral geometry thus 2 extra coordination sites become available and are occupied by the H- ligands phosphine ligands become labile, exchanging with the olefin in solution (strong trans effect of hydride ion) coordinated H- ions transferred to olefin to give an alkane Rh(+3) reduced back to square planar Rh(+1) recombining with the phosphine in solution thus re-forming the catalyst the catalytic cycle is complete Application of Olefin Hydrogenation Catalysis If a chiral phosphine is employed then the product will be chiral (a structure where its mirror image is not superimposable on itself) This chiral version forms the basis of the production of L-DOPA, the organic drug used to treat Parkinson’s disease also known as L-3,4-dihydroxyphenylalanine Topic 9: Fundamental Metallocene Chemistry (η5-Cp)2Fe obeys 18e− rule ferrocene Fe 3d6 4s2 = 8 air stable (C5H5) 2 × 5 = 10 orange solid 18 e− Rich chemistry; Redox properties; Ligand properties The “Godfather”* of metallocenes numerous niche applications: fuel additive (diesel; high-grade lead-free fuels); magnetic and conducting materials; molecular switches; biosensors; anti-cancer agents; and many others (*The Godfather, 1972, starring Marlon Brando and Al Pacino)  Other 3d transition metal parent metallocenes Number of valence electrons all air-sensitive to varying degree (unlike ferrocene which is air stable) Manganocene is pyrophoric and dimorphic (exists as two structural forms): at room temperature it’s a polymer; at high temperature it’s a molecular sandwich easy to derivatise the C-H frame of ferrocene due to its 18 e− stability BuLi lithiation CH3COCl Friedel-Crafts + AlCl3 acylation excess I2 oxidation Exercise Example Exercise Example 3 9 Account for the following facts: (a) Titanocene, (C5H5)2Ti does not exist (b) (C5H5)2Co is a powerful reducing agent (c) (C5H5)2V has a magnetic moment eff of 3.87 B.M. 6 marks END OF LECTURE 9 PART 2 Essential Learning: Following the redox chemistry underpinning hydrogenation reactions with Wilkinson’s catalyst