Synthesis and Properties of Cobalt(III) Complexes of 4-Substituted Pyridine-Capped Dioxocyclams

Cobalt(III) acetate and cyanide complexes of a series of 5,12-dioxocyclams capped across the 1,8-position by 4-substituted pyridines or pyrazine were synthesized and fully characterized. Both the spectroscopic and structural parameters for these complexes were remarkably insensitive to the electronic nature of the capping group, which ranged from the π-accepting pyrazine group to the σ-donating 4-[(dimethylamino)phenyl]pyridyl group. All of the complexes underwent an irreversible, one-electron reduction [Co(III) → Co(II)] at potentials ranging from −0.95 V vs saturated calomel electrode (SCE) for the pyrazine-capped cobalt acetate complex to −1.36 V vs SCE for the pyridine-capped cobalt cyanide complexes. Pyridine-capped cobalt(III) cyanide complex 6a underwent reaction with Rh₂(OAc)₄ and ruthenium(II) phthalocyanine[bis(benzonitrile)] to form tetrametallic (7) and trimetallic (8) complexes through coordination bridging by the cyanide nitrogen lone pair. These complexes represent two quite different structural types for cyanide-bridged polymetallics. Complex 7 has a relatively long (2.192 Å) cyanide N-to-Rh bond, and the C⋮NRh bond angle (157.6°) is strongly distorted from linear. In contrast, complex 8 has a substantially shortened cyanide N-to-Ru bond (2.017 Å) and an almost linear arrangement along the entire bridging axis of the molecule.