ORGANOTRANSITION-METAL METALLACARBORANES .35. ELECTROCHEMISTRY, ESR, AND CORRELATED NMR-SPECTROSCOPY OF PARAMAGNETIC MONONUCLEAR AND DINUCLEAR CP(ASTERISK)COC(2)B(4) CLUSTERS

The redox behavior of the cobaltacarborane sandwich complexes CPCo[(M e(3)Si)(2)C2B4H4] (1), CpCo[(Me(3)Si)C2B4H5] (2), {Cp*Co[Me(3)Si)C2B4 H4]CH2}2C6H4 (4), CpCo(Et(2)C(2)B(4)H(4)) (5), and Cp*Co(Me(2)C(2)B(4 )H(4)) (6) was investigated via cyclic voltammetry, proton NMR spectro scopy, and ESR spectra of paramagnetic anionic species. The reductions of the neutral diamagnetic monocobalt complexes 1 and 2 to their mono anions and of the dicobalt species 4 to the dianion were studied by re cording in each case a series of NMR and ESR spectra during intermitte nt exposure to a potassium mirror until complete conversion to the ani on was achieved. The NMR chemical shift correlations so obtained allow complete assignment of the paramagnetic NMR signals and afford inform ation on the effects of reduction in different regions of the molecule s. The NMR and ESR data indicate that reduction takes place primarily at cobalt. In general, as probed by H-1 NMR spectroscopy, the effects of reduction are strongest for the Cp protons and are weaker for thos e further from the paramagnetic metal center. The methyl protons in Si Me(3) groups attached to the carborane are more strongly affected than are the methyl protons of carborane-bound ethyl groups. These finding s are consistent with the ESR spectra of the paramagnetic anions, whic h show smaller cobalt hyperfine coupling constants for the silyl-subst ituted species than for the ethyl derivatives.