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

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 = Ph2PC6H4-2-OCH3(a) and Ph2PC6H4-2-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.

Figure optionsDownload high-quality image (113 K)Download as PowerPoint slideHighlights▶ Synthesis of rhodium carbonyl complexes of P,O donor ortho-substituted triphenylphosphine ligands. ▶ Oxidative addition of CH3I to square planar Rh(I) complexes. ▶ Kinetic studies for the oxidative addition reactions of CH3I. ▶ Carbonylation of methanol to acetic acid and methyl acetate. ▶ The highest Turn Over Frequency is 1808 h−1.