Kinetic studies of thermal dissociation of carbon monoxide ligands from manganese tri- and tetra-carbonyl derivatives containing the bulky dipiperidylmethane ligand, CH2Pip2

Abstract

The group 7 tricarbonyl complexes, fac-[MBr(CO)3(CH2Pip2)] (CH2Pip2 = dipiperidylmethane, M = Mn (1), and M = Re (2)), have been synthesized from the reaction of [MBr(CO)5] and the bulky CH2Pip2 ligand. Complex 1 was found to react with AgBF4 in the presence of carbon monoxide to provide a stable cationic manganese tetracarbonyl complex [Mn(CO)4(CH2Pip2)][BF4] (3). Complex 3 was shown to dissociate CO in acetonitrile to provide the solvate, [Mn(CH3CN)(CO)3(CH2Pip2)][BF4], with a rate constant of 10.6 × 10−4 s−1 at 25 °C. A similar H2O solvated complex was provided upon CO dissociation from complex 3 in water/dichloromethane. Loss of an additional CO ligand occurs on a slower timescale as revealed by further CO exchange with 13CO. This enhanced loss of CO in the [Mn(CO)4(CH2Pip2)]+ cation compared to other analog derivatives such as [Mn(CO)4(phen)]+ is ascribed to the sterically encumbering CH2Pip2 ligand which weakens the MnCO bond towards dissociation.