Theoretical Approaches To Estimating Homolytic Bond Dissociation Energies of Organocopper and Organosilver Compounds

Although organocopper and organosilver compounds are known to decompose by homolytic pathways among others, surprisingly little is known about their bond dissociation energies (BDEs). In order to address this deficiency, the performance of the DFT functionals BLYP, B3LYP, BP86, TPSSTPSS, BHandHLYP, M06L, M06, M06-2X, B97D, and PBEPBE, along with the double hybrids, mPW2-PLYP, B2-PLYP, and the ab initio methods, MP2 and CCSD­(T), have been benchmarked against the thermochemistry for the M–C homolytic BDEs (D₀) of Cu–CH₃ and Ag–CH₃, derived from guided ion beam experiments and CBS limit calculations (D₀(Cu–CH₃) = 223 kJ·mol^–1; D₀(Ag–CH₃) = 169 kJ·mol^–1). Of the tested methods, in terms of chemical accuracy, error margin, and computational expense, M06 and BLYP were found to perform best for homolytic dissociation of methylcopper and methylsilver, compared with the CBS limit gold standard. Thus the M06 functional was used to evaluate the M–C homolytic bond dissociation energies of Cu–R and Ag–R, R = Et, Pr, iPr, tBu, allyl, CH₂Ph, and Ph. It was found that D₀(Ag–R) was always lower (∼50 kJ·mol^–1) than that of D₀(Cu–R). The trends in BDE when changing the R ligand reflected the H–R bond energy trends for the alkyl ligands, while for R = allyl, CH₂Ph, and Ph, some differences in bond energy trends arose. These trends in homolytic bond dissociation energy help rationalize the previously reported (Rijs, N. J.; O’Hair, R. A. J. Organometallics 2010, 29, 2282–2291) fragmentation pathways of the organometallate anions, [CH₃MR]⁻.