Redox properties of cobalt(II) complexes with azole-pyridines

We report nine new homoleptic octahedral cobalt(II) complexes [Co(N^N)3]2+ and [Co(N^N^N)2]2+ with bidentate or tridentate azole-pyridine ligands (where azole is 1,2,4-triazole or a substituted pyrazole). The complexes are air- and moisture-stable solids of pale pink to pale brown color that exhibit quasi-reversible redox processes in polar organic solvents. By changing azole heterocycle and by adding methyl groups, we tune redox potentials of the complexes from 0.05 to 0.5 V for the Co(III/II) couple (its cyclic voltammogram depends on the electrode history) and from −1.08 to −1.57 V for the Co(II/I) couple to give a redox gap of 1.37–1.82 V. Replacing a pyrazole with electron-deficient 1,2,4-triazole strongly positively shifts redox potentials of the complexes; in contrast, adding electron-donor methyl groups shifts the potentials negatively.