A kinetic investigation of the oxidative addition reactions of the dimeric Bu4N[Ir2(μ-Dcbp)(CO)2(PCy3) 2] complex with iodomethane

Article abstract of DOI:10.1016/j.jorganchem.2010.10.049

The IR and UV/visible kinetic results of the oxidative addition of iodomethane to Bu₄N[Ir₂(μ-Dcbp)(CO) ₂(PCy₃)₂] (Dcbp = 3,5-dicarboxylatepyrazolate anion) showed three time separable reactions. The first, very fast reaction corresponds to the a Ir(I)-Ir(III) alkyl species formation within 10 ^-3 s. The second, relative fast reaction corresponds to Ir(III)-Ir(III) alkyl formation with a rate constant of 3.25(4) × 10 ^-2 M^-1 s^-1 while the third and slowest reaction corresponds to Ir(III)-Ir(III) acyl formation with a rate constant of 1.42 × 10^-5 s^-1. The IR data clearly show the existence of a number of equilibria with the formation of an Ir(I)-Ir(III) alkyl product which then react to form the Ir(III)-Ir(III) which then slowly react to form the Ir(III)-Ir(III) acyl product. A solvent study indicated increased oxidative addition activity in the presence of polar solvents, which is indicative of a polar transition state. The large negative entropy of activation for the Ir(III)-Ir(III) alkyl formation step (k₂) of -178(23) JK^-1 mol^-1 is indicative of an associative process. DFT calculations successfully identified the stereochemistry of the starting complex, [Ir ₂(μ-Dcbp)(CO)₂(PCy₃)₂] ^- as well as that of the Ir-alkyl and acyl isomers. A reaction pathway, using the IR data and DFT calculations, is proposed for the reaction.