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

We report nine new homoleptic octahedral cobalt(II) complexes Co((NN)-N-boolean AND)(3) and Co((NNN)-N-boolean AND-N-boolean AND)(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 quasireversible 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. (C) 2013 Elsevier B.V. All rights reserved.