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Reactions of Odd-Electron Cobaltacycles:  Characterization of a Persistent 17-Electron Anionic Intermediate in Electron-Transfer-Catalyzed (ETC) Substitution Reactions

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journal contribution
posted on 07.07.1999, 00:00 authored by Bernadette T. Donovan-Merkert, Philip H. Rieger, William E. Geiger
The cobaltafluorene complex Cp(PPh3)CoC12H8 (1) undergoes an electrochemically irreversible one-electron reduction in THF (Ep,c = ca. −2.56 V vs ferrocene) with release of PPh3 to give a persistent 17-electron Co(II) monoanion, 2-, which was characterized by electrochemistry and by ESR spectroscopy. The unpaired spin density in 2- is highly delocalized, with the majority being located in the cyclopentadienyl ring rather than the cobaltacyclic fragment. Reoxidation of 2- in the presence of L (L = phosphines, phosphites) forms Cp(L)CoC12H8 in high yield. Slow electron-transfer-catalyzed (ETC) substitution processes are found when 1 is electrolyzed in the presence of P(OMe)3, with the efficiency of the catalysis being dependent on both the relative and absolute concentrations of 1 and P(OMe)3. The substitution reaction is accounted for by a model in which the anion 2- is in equilibrium with the 19-electron adducts [Cp(L)CoC12H8]-, where L = THF, PPh3, P(OMe)3. Further reduction of 2- is possible in a reversible one-electron reduction (E1/2 = −2.80 V) to an 18-electron dianion that is stable in solution. The dianion 22- was characterized by 1H NMR spectroscopy and shown to have Cs or higher symmetry.