Cobalt(III) complexes using in-plane tetradentate pyridinecarboxamide ligands and two monodentate axial ligands: spectroelectrochemical correlation

Abstract

A series of diamagnetic cobalt(III) complexes of bpb and bpc ligands [H₂bpb = 1,2-bis(2-pyridinecarboxamido)benzene; H₂bpc = 4,5-dichloro-1,2-bis(2-pyridinecarboxamido) benzene] with axial ligands (Cl^-, N₃^-, SCN^-, NO₂^- or MeCO₂^-) has been synthesized. The trans geometry has been shown by ¹H NMR spectroscopy. The brown or green crystalline complexes display dominant ligand-to-metal charge-transfer transitions at ca 400 nm, while in the low-energy region ligand field transitions are observed. From an analysis of the d-d transition(s) ligand field parameters of the in-plane and axial ligands have been determined. In acetonitrile solution the complexes exhibit an irreversible Co^III-Co^II couple [E_pc -0.42 to -1.18 V vs saturated calomel electrode (S.C.E.)] and a quasi-reversible Co^II-Co^I couple (E_f -1.12 to - 1.27 V vs S.C.E.). When X = SCN^- this couple is irreversible (E_pc = -1.40 V vs S.C.E.). These complexes display an additional quasi-reversible oxidative response (0.69-0.92 V vs S.C.E.) of primarily ligand oxidation origin. When X = NO₂^- and SCN^- this couple is irreversible [E_pa = 0.88 V (NO₂^-); E_pa = 1.00 V (SCN^-)]. A linear spectroelectrochemical correlation has been obtained between the ligand field strength of the axial ligands and the cathodic peak potential for the Co^III-Co^II couple.