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

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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.

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