Addition of alkyl radicals to transition-metal-coordinated CO: Calculation of the reaction of [Ru(CO)₅] and related complexes and relevance to alkane carbonylation

Electronic structure methods have been used to study the transition state and products of the reaction between alkyl radicals and CO coordinated in transition-metal complexes. At the B3LYP DFT level, methyl addition to a carbonyl of [Ru(CO)₅] or [Ru(CO)₃(dmpe)] is calculated to be about 6 kcal/mol more exothermic than addition to free CO. In contrast, methyl addition to [Mo(CO)₆] is 12 kcal/mol less exothermic than addition to CO, while the reaction enthalpy of methyl addition to [Pd(CO) ₄] is comparable to that of free CO. Related results are obtained at the CCSD-T level and for the reactions of the cyclohexyl radical. The transition state for these reactions is characterized by significant distortion of the geometry of the reactant complex, which include lengthening and bending of the M-CO bond, but with negligible C-C bond formation. Accordingly, the activation energy for addition to coordinated carbonyls is 2-10 kcal/mol greater than that of addition to free CO. Additional calculations were also carried out on representative unsaturated metal carbonyls. The calculated results afford an understanding of the mechanism of previously reported photochemical alkane carbonylation systems utilizing d⁸-ML₅ metal carbonyls as cocatalysts. In particular, it is strongly indicated that such systems operate via direct attack by an alkyl radical at a CO ligand, a reaction that has not previously been proposed.