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

Abstract

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 pi-accepting pyrazine group to the sigma-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 underwent reaction with Rh2(OAc)4 and ruthenium(II) phthalocyanine[bis(benzonitrile)] to form tetrametallic and trimetallic complexes through coordination bridging by the cyanide nitrogen lone pair. These complexes represent two quite different structural types for cyanide-bridged polymetallics. Complex has a relatively long (2.192 A) cyanide N-to-Rh bond, and the CN-Rh bond angle (157.6 degrees) is strongly distorted from linear. In contrast, complex has a substantially shortened cyanide N-to-Ru bond (2.017 A) and an almost linear arrangement along the entire bridging axis of the molecule.