REDUCTION-OXIDATION PROPERTIES OF ORGANOTRANSITION-METAL COMPLEXES .32. ONE-ELECTRON OXIDATION OF [CO(ETA-4-COT)-(ETA-C5ME5)] (COT=CYCLOOCTATETRAENE) - REDOX-INDUCED POLYCYCLIC RING TRANSFORMATIONS OF THE RESULTING DIMER, AND CRYSTAL-STRUCTURE OF [CO2(ETA-4-ETA'4-C16H16)(ETA-C5ME5)2]

The complex [Co(eta-4-cot)(eta-C5Me5)] 4 (cot = cyclooctatetraene) undergoes irreversible one-electron oxidation in CH2Cl2 at a platinum electrode; chemical oxidation with [Fe(eta-C5H5)2]PF6 gives the dimeric dication [Co2(eta-5:eta-'5-C16H16)(eta-C5Me5)2][PF6]2 5 as a diastereomeric mixture. The reduction of 5 with two equivalents of [Co(eta-C5H5)2] gives three asymmetric isomers [Co2(sigma,eta-3:eta-'4-C16H16)(eta-C5Me5)2] 8, [Co2(sigma,eta-3:eta-'4-C16H16)(eta-C5Me5)2] 9 and [Co2(eta-4:eta-'4-C16H16)(eta-C5Me5)2] 10. Complex 8 is thermally isomerised to 10 which has been structurally characterised by a single crystal X-ray diffraction study. The two cobalt atoms, each carrying an eta-C5Me5 ligand, are bonded to opposite sides of a tetracyclic C16H16 ligand which contains fused cyclooctatriene, cyclopentane, cyclopropane and cycloheptadiene rings; the cobalt atoms are bound via eta-4-1,3- and eta-4-1,5-diene interactions involving the C7 and C8 rings respectively. Complex 10 is oxidised by [Fe(eta-C5H5)2]PF6, to give [Co2(eta-2,eta-3:eta-'5-C16H16)(eta-C5Me5)2][PF6]2 13, and undergoes partial decomplexation with Me3NO.2H2O to yield the monometallic compound [Co(eta-4-C16H16)(eta-C5Me5)] 15. Stepwise rather than concerted mechanisms are proposed for the overall two-electron reduction of 5 and two-electron oxidation of 10.