Kinetics and thermodynamics of C—Cl bond activation by [Ir(CO)2Cl2]−

Abstract

AbstractKinetic measurements are reported for the oxidative addition reactions of methyl chloride and acetyl chloride with [Ir(CO)2Cl2]−. At 40°C the second‐order rate constant for MeCOCl addition is estimated to be nearly 40 000 times larger than that for MeCl addition. The Ir(III) products, [Ir(CO)2Cl3R]− (R = Me, COMe) have been isolated and characterised by spectroscopy and x‐ray crystallography. In the absence of excess organic chloride, both Ir(III) complexes undergo reductive elimination of RCl. Kinetic measurements show these reactions to be first order in the Ir(III) complex with elimination of MeCOCl estimated to be ca 7000 times faster than MeCl elimination at 40°C. Combination of activation parameters for the forward and reverse reactions allows calculation of thermodynamic parameters for oxidative addition. Both MeCl and MeCOCl additions are exothermic (by 44 and 68 kJ mol−1, respectively) but disfavoured entropically. The trends are predicted satisfactorily by ab initio and DFT computational methods. The results for MeCl addition to [Ir(CO)2Cl2]− are compared with data for MeI addition to [Ir(CO)2I2]−. Kinetic data are also reported for carbonylation of [Ir(CO)2Cl3Me]− into [Ir(CO)2Cl3(COMe)]− under mild conditions in PhCl–MeOH. It is concluded that the low activity of iridium–chloride carbonylation catalysts is due primarily to the relatively slow reaction of [Ir(CO)2Cl2]− with MeCl. Copyright © 2004 John Wiley & Sons, Ltd.