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

Abstract

Four complexes of the type [Rh(CO)Cl( η 2- P,O-L)]( 1a,1b) and [Rh(CO)Cl( η 1 -P-L) 2]( 2a,2b), where L = Ph 2PC 6H 4-2-OCH 3( a) and Ph 2PC 6H 4-2-CH 2OCH 3( b), have been synthesized by the reaction of [Rh(CO) 2Cl] 2 with appropriate mol equivalents of the ligands in CH 2Cl 2. 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 CH 3I to afford Rh(III)-acyl complexes of the type [RhCl(COCH 3)I(L)]( 3a,3b) and [RhCl(COCH 3)I(L) 2]( 4a, 4b). The kinetic data for the OA reactions with CH 3I 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) 2I 2] − (TOF = 464–1000 h −1) under similar experimental conditions. The catalytic activities vary in the order as 1a > 1b > 2a > 2b.