Electrochemical and electron spin resonance studies of dichloro(1,2,4-triazole)copper(II) and dichlorobis(1,2,4-triazole)copper(II) in dimethyl sulphoxide

Abstract

An electrochemical and e.s.r. characterization of [Cu(Hta)Cl₂] and [Cu(Hta)₂Cl₂](Hta = 1,2,4-triazole) was carried out in dimethyl sulphoxide. Molar conductance data indicate that one Cl^– ion is dissociated from each species which exist as 1:1 electrolytes in Me₂SO. Both complexes undergo two one-electron reductions in Me₂SO. The first is reversible and occurs at 0.33 and 0.34 V vs. saturated calomel electrode (s.c.e.) respectively. The second reduction is irreversible and occurs in either one or two steps both of which result in deposition of copper metal onto the platinum electrode surface. These reactions occur at E_p values between –0.54 and –0.66 V depending upon the scan rate. In situ e.s.r. spectra recorded during bulk controlled-potential reduction of [Cu(Hta)Cl₂] indicate that the singly reduced species undergoes a dissociation of one Hta ligand which then binds to unreduced [Cu(Hta)Cl₂] to form higher-co-ordination-number complexes of the type [Cu(Hta)_nCl]⁺ where n= 3 or 4. The frozen-solution e.s.r. spectrum of [Cu(Hta)Cl₂] in Me₂SO has an anisotropic signal at g_∥= 2.377 with A_∥^Cu= 128 G and g_⊥= 2.090. No superhyperfine structure in g_⊥ is observed. These e.s.r. parameters are similar to those of CuCl₂ in Me₂SO suggesting that the Hta ligands of [Cu(Hta)Cl₂] are only weakly bonded to the Cu^II in solution. The spectrum of [Cu(Hta)₂Cl₂] exhibits a decreased value of g_∥(2.297) and an increased value of A_∥^Cu(166 G) with respect to [Cu(Hta)Cl₂], consistent with the presence of a stronger equatorial ligand field in the former complex. An overall electroreduction mechanism for [Cu(Hta)Cl₂] and [Cu(Hta)₂Cl₂] is proposed on the basis of the voltammetric and e.s.r. data and comparisons are made with data for CuCl₂ under similar solution conditions.