Synthesis, structure, and electrochemistry of pyridinecarboxamide cobalt(III) complexes; the effect of bridge substituents on the redox properties

Two series of complexes of the types trans-[Co-III(Mebpb)(amine)(2)]ClO4 [Mebpb(2-) = N,N-bis(pyridine-2carboxamido)-4-methylbenzene dianion, and annine = pyrrolidine (prldn) (1a), piperidine (pprdn) (2a), morpholine (mrpln) (3a), benzylamine (bzlan) (4a)), and trans-[Co-III(cbpb)(amine)(2)]X {cbpb(2-) = N,N-bis(pyridine-2-carboxamido)-4-chlorobenzene dianion, and arnine = pyrrolidine (prldn), X = PF6 (1b), piperidine (pprdn), X = PF6 (2b), morpholine (mrpln), X = ClO4 (3b), benzylamine (bzlan), X = PF6 (4b)) have been synthesized and characterized by elemental analyses, IR, UV-Vis, and H-1 NMR spectroscopy. The crystal structure of la has been determined by X-ray diffraction. The electrochemical behavior of these complexes, with the goal of evaluating the effect of axial ligation and equatorial substitution on the redox properties, is also reported. The reduction potential of Co-III, ranging from -0.53 V for (1a) to -0.31 V for (3a) and from -0.48 V for (1b) to -0.22 V for (3b) show a relatively good correlation with the sigma-donor ability of the axial ligands. The methyl and chloro substituents of the equatorial ligand have a considerable effect on the redox potentials of the central cobalt ion and the ligand-centered redox processes. (C) 2008 Elsevier Ltd. All rights reserved.