Reduction–oxidation properties of organotransition-metal complexes. Part 23. Pentaphenylcyclopentadienyl carbonyl complexes of cobalt and rhodium

Abstract

The reaction of [Co₂(CO)₈] or [{RhCl(CO)₂}₂] with C₅Ph₅Br and zinc dust in tetrahydrofuran (thf) gives [M(CO)₂(η-C₅Ph₅)](1, M = Co or Rh); in the case of rhodium, the intermediate [RhBr(CO)₂(η-C₅Ph₅)][ZnCl₃] is isolable if the reaction is carried out in benzene. In the presence of [Fe(η-C₅H₅)₂]⁺, (1, M = Rh) undergoes oxidatively-induced carbonyl substitution with ligands, L; for L = P(OPh)₃, the formation of [M(CO)L(η-C₅Ph₅)][2, M = Rh, L = P(OPh)₃] is catalytic in the oxidant. The complexes (2) are also prepared via(a) the substitution of [RhBr(CO)₂(η-C₅Ph₅)][ZnCl₃] with PPh₃ or AsPh₃ and subsequent reduction by [N(PPh₃)₂][Mn(CO)₅], (b)[Co(η-C₅H₅)₂] reduction of [MI₂(CO)(η-C₅Ph₅)](3, M = Co) in the presence of P(OMe)₃ or P(OPh)₃, or (c)[Co(η-C₅H₅)₂] reduction of [RhBr₂{P(OMe)₃}(η-C₅Ph₅)](5) in the presence of CO. Complexes (3, M = Co or Rh) are prepared from (1) and iodine in toluene–hexane at 0 °C; the rhodium complex (3, M = Rh) is reduced by [Co(η-C₅H₅)₂] to give [{Rh(µ-CO)(η-C₅Ph₅)}₂]. Compound (5) results from the addition of P(OMe)₃ to [{RhBr₂(η-C₅Ph₅)}₂](4), which is the product of the reaction between (1, M = Rh) and bromine in CH₂Cl₂. The compounds (2) undergo two one-electron oxidations at a platinum electrode in CH₂Cl₂; the first is reversible and the second is irreversible. The monocations [2⁺, M = Rh; L = PPh₃, P(OPh)₃, or AsPh₃] have been characterised in solution by i.r. and e.s.r. spectroscopy. Low-temperature e.s.r spectroscopy has shown that the radical cation [Rh(CO)(PPh₃)(η-C₅Me₅)]⁺ is stable at –196 °C.