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Metal complexes of ferrocenylphosphines : catalytic properties of some rhodium complexes

University of British Columbia

A series of achiral and chiral ferrocenylphosphines have been prepared with special emphasis on those containing bulky tert-butyl groups on phosphorus (I and V were previously known): [See Thesis for Diagrams] They form a number of transition metal complexes such as [Rh(L-L)-(NBD)]ClO₄ (L-L = I-VII; NBD = norbornadiene), M(L-L)X₂ (M = Pd, Ni; L- L = I - IV; X = CI, Br), M(L-L)(CO) ₄ (M = Cr, Mo; L-L = I), and Fe[sub x](L-L)(CO) [sub y](x = 1 , y = 3, x = 2, y = 8, L-L =1). All these ligands and their metal complexes have been fully characterized by analytical and spectroscopic techniques. In a number of cases these results are confirmed by X-ray analyses. The configuration of VII proved to be, for example, (S,S) with regard to central and planar chirality rather than the expected (S,R) found for V and VI. The achiral Rh(I) complexes [Rh(L-L)(NBD)]ClO₄ (L-L = I-IV) are efficient catalyst precursors for the hydrogenation of a range of olefins (1 atm H₂, 30°C). The presence of bulky tert-butyl groups enhances reaction rates except when bulky olefins are the substrates. The chiral trisphosphine Rh(I) complex [Rh(L-L)(NBD)] ClO₄ (L-L = VII) is a very efficient catalyst precursor for the asymmetric hydrogenation of acylamino-cinnamic, acylaminoacrylic, and (E)-α-methylcinnamic acids, giving 91, 95, and 61% e.e., respectively. The chiral Rh(I) complex, where L-L = VI, is a relatively poor catalyst for asymmetric hydrogenation as compared with the other two complexes (L-L = V , VII). Here again the presence of tert-butyl groups increases the reaction rates, and the rates become greater as the number of tert-butyl groups increases. These results and other comparative hydrogenation studies are discussed and rationalized in terms of the steric (including ligand conformation) and electronic effects of the substi-tuents on the phosphorus atom(s). The reaction of H₂ with the hydrogenation catalyst precursor [Rh(L-L)-(NBD)] ClO₄ (L-L = II) in MeOH results in crystals which have the structure [(L-L)(H)Rh(μ-H)3Rh(H)(L-L)] ClO₄. When L-L = IV, the same reaction results in a similar rhodium hydride although the positions of the hydrogen atoms are not well established. A number of other hydrides, some fluxional, are also obtained in various solvents from the catalyst precursors [Rh(L-L)(NBD)]- ClO₄ (L-L = I-VII). Where possible these have been characterized on the basis of their NMR spectra. The implication of these results with respect to the mechanisms of catalytic hydrogenation is discussed.

Text

2010-06-12

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http://hdl.handle.net/2429/25596

Chemistry

University of British Columbia

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