Palladium Complexes of the Heterodiphosphine o-C6H4(CH2PtBu2)(CH2PPh2) Are Highly Selective and Robust Catalysts for the Hydromethoxycarbonylation of Ethene
journal contributionposted on 24.05.2010 by Tamara Fanjul, Graham Eastham, Natalie Fey, Alex Hamilton, A. Guy Orpen, Paul G. Pringle, Mark Waugh
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The coordination chemistry and ethene hydromethoxycarbonylation catalysis with the diphosphine o-C6H4(CH2PtBu2)(CH2PPh2) (L3) is reported and the results compared with the analogous chemistry of the symmetrical diphosphines o-C6H4(CH2PtBu2)2 (L1) and o-C6H4(CH2PPh2)2 (L2). Palladium-catalyzed ethene hydromethoxycarbonylation studies under the commercial catalytic conditions are reported. The results obtained using L1−3 as supporting ligands show that the catalysts derived from L3 and L1 have similar activity and selectivity for methyl propanoate (MeP). In addition, the Pd−L3 catalyst has much greater longevity than the Pd−L1 catalyst. Treatment of the appropriate [Pt(X)(Y)(cod)] with L3 gave [PtCl2(L3)] (3), [Pt(CH3)2(L3)] (6), and [PtCl(CH3)(L3)] (9). At equilibrium, complex 9 is a 90:1 mixture of geometric isomers 9a (with CH3 trans to the tBu2P) and 9b (with Cl trans to the tBu2P). The fluxionality of complex 3, detected by 1H NMR, is interpreted in terms of the conformation of the seven-membered chelate. The complexes [Pt(CH3)(PMe3)(L3)]Cl (10b) and [PtH(PPh3)(L3)]Cl (12b) are formed as essentially single isomers with CH3/H trans to the Ph2P group. The palladium complexes [PdCl2(L3)] (13), [PdCl(CH3)(L3)] (14a/14b), and [PdH(PCy3)(L3)]BF4 (15b) have been made by similar methods to their platinum analogues. The factors controlling the relative isomer stabilities are explored experimentally and computationally. The complexes [PtCl2(L4)] (16) and [PtCl(CH3)(L4)] (17a/17b) where L4 = o-C6H4(CH2PnBu2)(CH2PPh2) are reported, and the geometric isomers of 17 are almost isoenergetic. The crystal structures of 3, 14a, 15b, and 16 have been determined by X-ray crystallography. DFT calculations on complexes of the type [Pt(X)(Y)(L3)] gave only small calculated differences in energy between the geometrical isomers (0−4 kcal mol−1), which are consistent with the experimental observations. It is suggested that repulsive intramolecular H···H interactions (between the Pt−CH3 and PC(CH3)3 groups) determine which isomer predominates. The reasons for the favorable catalytic properties of the Pd−L3 catalyst are probed by 13CO reactions with the model complexes 9a/9b and 14a/14b, and the structures of the resulting acyl complexes are assigned on the basis of 13C and 31P NMR and IR spectroscopy. From these studies, it is suggested that the reason for the Pd−L3 catalyst resembling the Pd−L1 catalyst in terms of selectivity is that the crucial acyl intermediates are similar.