Electrochemical, Theoretical and Structural Investigations on the "Butterfly" Co4(CO)8L2(RC2R) (L=CO, PPh3, R=H, Et, Ph) Cluster

The butterfly clusters Co₄(CO)₁₀(RC₂R) (R = H, Et, Ph) undergo two consecutive le reduction processes, the former being fully chemically and electrochemically reversible and the latter followed by a moderately fast chemical complication. The disubstituted cluster Co₄(CO)₈(PPh₃)₂(HC₂H) has been synthesized and its solution- and solid-state structure determined by multinuclear NMR spectroscopy and single-crystal X-ray diffraction: Monoclinic, P2₁/n (No. 14), a = 11.491 (5) Å, b = 24.178 (8) Å, c = 15.998 (5) Å, β = 105.84 (3)°, V = 4275.9 (9) ų, Z = 4, R_F = 0.061. The substitution of PPh₃ for CO has occurred regiospecifically on both wing-tip cobalt atoms. The electrochemistry is qualitatively similar to that of the parent cluster, the reduction potentials being shifted toward more negative values. The strong influence of the substitution of PPh₃ for CO and the weak influence of the alkyne substituents on the reduction potentials suggest that the LUMO is metal core in character. This hypothesis is corroborated by ESR results on the radical monoanion and by theoretical calculations within the Fenske-Hall scheme.