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

The cobaltafluorene complex Cp(PPh₃)CoC₁₂H₈ (1) undergoes an electrochemically irreversible one-electron reduction in THF (E_p,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 (E_1/2 = −2.80 V) to an 18-electron dianion that is stable in solution. The dianion 2^2- was characterized by ¹H NMR spectroscopy and shown to have C_s or higher symmetry.