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

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, [Ir2(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.