Chelate and trans effect of P,O donor phosphine ligands on rhodium catalyzed carbonylation of methanol

Article abstract of DOI:10.1016/j.molcata.2011.10.023

Four complexes of the type [Rh(CO)Cl(η²-P,O-L)](1a,1b) and [Rh(CO)Cl(η¹-P-L)₂](2a,2b), where L = Ph ₂PC₆H₄-2-OCH₃(a) and Ph ₂PC₆H₄-2-CH₂OCH₃(b), have been synthesized by the reaction of [Rh(CO)₂Cl]₂ with appropriate mol equivalents of the ligands in CH₂Cl₂. The complexes show single intense ν(CO) bands in the range 1965-1989 cm ^-1 indicating the presence of terminal carbonyl groups. All the complexes have been characterized by elemental analyses, mass spectrometry, IR and multinuclear NMR (¹H, ³¹P and ¹³C) spectroscopy, and the molecular structure of the ligand b is determined by single crystal X-ray diffraction. The complexes undergo oxidative addition (OA) with excess CH₃I to afford Rh(III)-acyl complexes of the type [RhCl(COCH₃)I(L)](3a,3b) and [RhCl(COCH₃)I(L) ₂](4a,4b). The kinetic data for the OA reactions with CH₃I indicate a first order reaction and also exhibit that the rate of OA for the chelate complexes (1a and 1b) is higher than those of trans-complexes (2a and 2b). The catalytic efficiencies of 1a, 1b, 2a and 2b in carbonylation of methanol exhibit higher Turn Over Frequency (TOF) 689-1808 h^-1 than the well-known Monsanto's species [Rh(CO)₂I₂]^- (TOF = 464-1000 h^-1) under similar experimental conditions. The catalytic activities vary in the order as 1a > 1b > 2a > 2b.