An electron-transfer induced migratory insertion reaction originating from a 19-electron cobaltacyclic anion

The cyclopentadienyl cobaltafluorenyl carbonyl complex Cp(CO)CoC12H8 (3) is reduced in THF in a one-electron irreversible process. The reduction products include the fluorenone radical anion, Fl−, which is proposed to arise through migratory insertion of the CO ligand into the metallacyclic CoC bond of the 19-electron complex 3−. Electrochemical and IR analyses show that bulk electrolysis under N2 gives 1/3 equiv. each of Fl− and CpCo(CO)2 and 2/3 equiv. of a 17-electron anion [CpCoC12H8]− (2−) Under CO the yield of Fl− is quantitative. The insertion product is also formed when the reduction of the analogous phosphine complex Cp(PPh3)CoC12H8 is performed under CO. An equilibrium between 17- and 19-electron compounds is postulated to account for the synthetic and voltammetric observations. When the cobaltacycle Cp(PPh3)CoC4Ph4 (5) is reduced under carbon monoxide, the CO-insertion product is the π-cyclopentadienone complex CpCo(η4-C4Ph4O) (8).