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-[CoIII(Mebpb)(amine)2]ClO4 {Mebpb2− = N,N-bis(pyridine-2-carboxamido)-4-methylbenzene dianion, and amine = pyrrolidine (prldn) (1a), piperidine (pprdn) (2a), morpholine (mrpln) (3a), benzylamine (bzlan) (4a)}, and trans-[CoIII(cbpb)(amine)2]X {cbpb2− = N,N-bis(pyridine-2-carboxamido)-4-chlorobenzene dianion, and amine = 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 1H NMR spectroscopy. The crystal structure of 1a 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 CoIII, 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 σ-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.

Co(III) complexes of two substituted pyridine carboxamide ligands, Mebpb and cbpb, with two axial amine ligands have been synthesized and structurally characterized. The effect of methyl and chloro substitution on the redox properties, and the sensitivity of the CoIII–CoII redox potentials to the donor strength of the axial ligands have been investigated. The irreversible Co(III/II) redox process becomes quasi-reversible in the presence of excess amine. The two redox waves, resulting from the combination of CoII–CoI couple and the redox couple corresponding to one of the pyridyl rings, overlap in [Co(Mebpb)(amine)2]+, but are resolved in [Co(cbpb)(amine)2]+ complexes.Figure optionsDownload as PowerPoint slide