Synthesis, structure, spectral and electrochemical properties, and catalytic use of cobalt(III)-Oxo cubane clusters

Olive-green cobalt(III) complexes having the general formula Co4O4(O2CMe)(4)L-4 (1) where L = py (1a), 4-Mepy (1b), 4-Etpy (1c), and 4-CNpy (1d) have been prepared by the H2O2 oxidation of a mixture of Co2+, MeCO2-, and pyridine or 4-substituted pyridines in a 1:21 molar ratio in methanol. Spectroscopic and X-ray crystallographic studies show that these complexes contain a tetrameric cubane-like core [CO4(mu(3)-O)(4)](4+) where the four cobalt atoms form an approximate tetrahedron with edge lengths of similar to 2.75 A. Each cobalt in the crystallographically determined structure Of CO4(mu(3)-O)(4)(mu-O2CMe)(4)L-4 in 1a center dot NaCIO4 center dot 3.5H(2)O and 1b center dot 3H(2)O is hexacoordinate. Infrared spectra of the complexes show characteristic bands near 700, 635, and 580 cm(-1) due to the central cubane-like core. H-1 NMR spectra of the complexes show that the dissolved species are essentially diamagnetic and also that the complexes maintain their integrity in solution. UV-vis spectra of the green solutions have been interpreted in terms of ligand-field and charge-transfer bands. The electrochemical behavior of the complexes studied by cyclic and differential pulse voltammetric techniques indicates that the [(Co-III)(4)(mu(3)-O)(4)](4+) core present in the complexes undergoes a reversible one-electron oxidation to the [(Co-III)(3)Co-IV(mu(3)-O)(4)](5+) core with an E-1/2 value below 1 V. This suggests that these complexes of cobalt may be suitable as catalysts for the oxidation of organic compounds. Preliminary investigations indicate that la has a role to play in the cobalt-catalyzed aerobic oxidation of neat ethylbenzene and p-xylene.