ChemInform Abstract: Heats of Reaction of Cp*(PMe3)Ir(R)(H) (R: C6H5, C6H11, and H) with HCl, CCl4, CBr4, and CH3I. A Solution Thermochemical Study of the C‐H Insertion Reaction.

Abstract

A number of coordinatively unsaturated transition-metal complexes have been found to insert into carbon-hydrogen bonds, leading to stable hydrido alkyl metal complexes as shown in eq 1. In such reactions, the sum of the M-H and M-C bond M + R-H ..-->.. R-M-H dissociation energies in R-M-H must be comparable to, or larger than, the energy of the alkane C-H bond which is broken. This is the driving force for C-H activation and must be on the order of 110 kcal/mol for reaction to occur. Values have been obtained for some first-row transition-metal-alkyl (=30 kcal/mol) and metal-hydride(=60 kcal/mol) bond strengths, but less information has been available for the second- and third-row metals. The authors now wish to report thermochemical studies which provide a precise value of the difference between the metal-carbon and metal-hydrogen bond energies in Cp*(PMe/sub 3/)Ir(H)(R) complexes and reasonable estimates for the absolute values of these and several other Ir-X bonds (X = H, Cl, Br, I, C/sub 6/H/sub 11/, and /sub 6/CH/sub 5/).