Consequently, aqueous cyclic voltammetry was performed on the series of [PY(CO2)4]4− metal complexes at pH 7 in 0.1 M KClO4 electrolyte solution to examine the impact of the tetraanionic donor on the redox potentials. The Zn2+ complex exhibited no redox activity from −1.5 to 1.5 V versus Ag/AgCl, confirming that [PY(CO2)4]4− is not electrochemically active within this range. As shown in Figure 4, quasireversible 3+/2+
redox couples were observed for the Fe2+ and Ni2+ complexes with E1/2 values of −0.16 and 1.24 V, respectively, though neither of these redox events exhibited pH dependence from pH = 7−10. In the case of the Co2+ complex, an irreversible oxidation occurred at 0.74 V and was followed by an irreversible reduction at −0.30 V on the return scan. This cathodic feature was only observed after initial oxidation of [(PY(CO2)4)Co(H2O)]2−. In an e ffort to identify the oxidized species, we also performed cyclic voltammetry on the low-spin complex [(PY(CO2)4)Co(OH)]2−, which is formed cleanly via chemical oxidation of [(PY(CO2)4)Co(H2O)]2− with H2O2.36