Factors contributing to one-electron metalloradical activation of ethene and carbon monoxide illustrated by reactions of Co(II), Rh(II), and Ir(II) porphyrins

Metallo-porphyrin complexes of Co(II), Rh(II), and Ir(II) are used as prototype metal-centered radicals in examining the factors that contribute to obtaining one-electron activated ethene and CO substrate adducts [M(CH2CH2)]radical dot and [M(CO)]radical dot that subsequently react on to produce complexes with reduced substrate units including M–CH2CH2–M, M–(CH2)4–M, M–C(O)–M, M–C(O)–C(O)–M, and M–C(O)H. Cobalt(II) and rhodium(II) complexes of the form [(por)M(CH2CH2)]radical dot and [(por)M(CO)]radical dot occur as primarily metal-centered radicals and the iridium analogs are porphyrin anion radical complexes ((por)radical dot−IrIII(CH2CH2), (por)radical dot−IrIII(CO)). Relatively small (por)Co–C bond dissociation enthalpies preclude forming any reduced substrate species. Rhodium porphyrins produce a complete set of reduced and coupled ethene and CO complexes, but iridium porphyrins only give ethene reduction and coupling products (por)Ir–CH2CH2–Ir(por) and (por)Ir–(CH2)4–Ir(por). Thermodynamic criteria and analysis of substrate reactions are used to guide interpretations of the observed reactivity.