Cyclic voltammetry and spectroelectrochemical study of nickel and cobalt diphenyltetraazaannulene complexes

The absorption spectra, cyclic voltammetry and spectroelectrochemistry of [NiIIDPTAA] and [CoIIDPTAA] (DPTAA = 6,13-diphenyldibenzo[b,i][1,4,8,11]tetraaza[14]annulene) complexes in DMF are reported in detail. The ligand oxidation is observed for [NiIIDPTAA] at +0.70 V vs. SCE whereas Ni2+/+ occurs at −1.60 V. For [CoIIDPTAA], a ligand oxidation redox couple is seen at +0.56 V while the Co2+/+ and Co2+/3+ redox couples appear at −1.21 and +0.24 V, respectively. All observed redox couples are assigned to reversible one-electron processes on account of peak separations and scan-rate dependency. These processes were further investigated by spectroelectrochemistry for [CoIIDPTAA]. For [CoIIDPTAA], axial ligation of pyridine was found to shift the Co2+/3+ redox couple more negative, while the ligand oxidation was shifted to more positive potentials. From a spectrophotometric titration of [CoIIDPTAA] with pyridine an equilibrium constant, Kf, was determined for the binding of pyridine to [CoIIDPTAA]. This was found to be 10.2 dm3 mol−1, slightly lower than that of [CoIITAA], indicating the influence of the phenyl groups. From this value and shifts in the Co2+/3+ redox couple upon ligation, an equilibrium constant for the binding of pyridine to [CoIIIDPTAA], Kf′, was found to be 5.06 × 106 dm3 mol−1.