Synthesis, reactivities and catalytic carbonylation of rhodium(I) carbonyl complexes containing isomeric acetylpyridine ligands

Abstract

[Rh(CO)2Cl]2 reacts with two mole equivalent of 2-acetylpyridine (a), 3-acetylpyridine (b) and 4-acetylpyridine (c) to afford chelate [Rh(CO)Cl(η2-N∩O)] (1a) and non-chelate [Rh(CO) 2Cl(η1-N∼O)] (1b, 1c) complexes, where, N∩O = a, N∼O = b, c. Oxidative addition (OA) of 1a-1c with CH3I and C2H5I yields penta coordinate rhodium(III) complexes, [Rh(COR)ClI(η2-N∩O)] {R = -CH3 (2a); -C 2H5 (3a)} and [Rh(COR)(CO)ClI(η1-N∼O)] {R = -CH3 (2b, 2c); -C2H5 (3b, 3c)}. Kinetic study for the reaction of 1a-1c with CH3I indicates a pseudo-first order reaction. The catalytic activity of 1a-1c for the carbonylation of methanol to acetic acid and its ester was evaluated at different initial CO pressures 5, 10 and 20 bar at ∼25 °C and higher turn over numbers (TON = 1581-1654) were obtained compared to commercial Monsanto's species [Rh(CO) 2I2]- (TON = 1000) under the reaction conditions: temperature = 130 ± 1 °C, pressure = 15-32 bar, rpm = 450, time = 1 h and catalyst: substrate = 1: 1900. © 2011 Elsevier Ltd. All rights reserved.