Transition-metal Complexes of Pyrrole Pigments. XIII. Reduction and Coordination Behaviors of the Cobalt(II) Complexes of 1,19-Disubstituted Tetradehydrocorrins as Demonstrated by the Presence of Various Bases

Abstract

Abstract The interaction of various bases with the cobalt(II) complexes of 1,19-disubstituted tetradehydrocorrins (Co(II)–TDHC) was investigated. The parent cobalt(II) complexes were shown to undergo ready reduction at the nuclear cobalt(II) with the unsolvated hydroxide ion as well as with bulky triethylamine. A relatively large positive reduction potential of Co(II)–TDHC is responsible for these novel electron transfer reactions. Less hindered amines (B), on the other hand, failed to show such electron transfer reactions with the cobalt(II) complexes and Co(II)–TDHC became the pentacoordinated complex by taking up an amine base in the axial position: Co(II)–TDHC+B\ightleftharpoonsB–Co(II)–TDHC. For the interaction of 4-substituted pyridines with the cobalt(II) complex, the general behavior of the equilibrium constants was interpreted in terms of the amine basicity and the Hammett equation by referring to the corresponding behavior of the porphyrin complexes. The weaker coordination tendency of some aliphatic amines and hindered pyridines toward Co(II)–TDHC was attributed to their electronic and steric effects in the coordination process. The intrinsic correlation between electronic and stereochemical characteristics pertaining to the tetradehydrocorrin skeleton and the nature of nuclear cobalt(II) was discussed on the basis of these observations.