Electrochemical and spectral studies of binary and mixed-ligand copper(II) and cobalt(II) complexes with 2,6-dipicolinic acid and bathophenanthroline chelating ligand in aqueous medium

Abstract

• Preparation of binary copper(II) and cobalt(II) complexes with 2,6-dipicolinic acid in aqueous medium(1: 2 and 1: 10 molar ratios). • Preparation of mixed ligand copper(II) and cobalt(II) complexes with 2,6-dipicolinic acid and bathophenanthroline in aqueous medium(in 1: 1: 1 and 1: 2: 1 molar ratios). • Cyclic voltammetric studies of binary and mixed ligand copper(II) /cobalt complexes in aqueous medium(in 1: 2, 1: 10, 1: 1:1, 1: 2: 1 molar ratios), containing 0.2 M NaClO4. • UV-Visible spectroscopy of binary and mixed ligand copper(II)/cobalt(II) complexes(1: 2, 1: 10, 1: 1: 1 and 1: 2: 1 molar ratios) in aqueous medium. In this paper, we reported the preparation of four copper(II) and cobalt(II) complexes viz; [Cu(dipic) n (H 2 O) n ] 1, [Cu(dipic) n (bathophen) n ].nH 2 O 2, [Co(dipic) n (H 2 O) n ] 3, and [Co(dipic) n (bathophen) n ].nH 2 O 4 (where, n is the number of ligands attached and it could be 1 or 2), in different molar ratios; 1:2, 1:10, 1:1:1 and 1:2:1 in aqueous medium. All these complexes ( 1–4 ) were characterized by cyclic voltammetry and electronic absorption spectroscopy in different molar ratios as mentioned above. The 2,6-dipicolinic acid (dipic) and 4,7-diphenyl-2,9-dimethyl-1,10-phenanthroline (bathophen) used as primary and secondary ligands, respectively. The electrochemical behavior of binary copper(II) complex 1 with 2,6-dipicolinic acid in 1:2 and 1:10 molar ratios were showed a quasi-reversible single electron transfer redox couple Cu 2+/+ from +1000 mV to -200 mV potential limit (at 25 mVs −1 to 300 mVs −1 ) with easier reduction potential with increasing scan rate/concentration of ligand in binary complex 1 , containing 0.2 M NaClO 4 as a supporting electrolyte at glassy carbon working electrode. The redox behavior of mixed-ligand complex 2 with 2,6-dipicolinic acid and bathophen (in 1:1:1 and 1:2:1 molar ratios) shows similar pattern to that of complex 1 . The complex 2 showed an additional anodic peak potential (Epa’) due to oxidation of complex chemical species present in solution which is assigned to Cu 0/+2 , while during cathodic scaning it does not show any cathodic peak potential (Epc’), corresponding to (Epa’). The cyclic voltammetric studies of complexes 3 and 4 have also been done in aqueous medium. The observations of complexes 3 and 4 were totally different to that of the complexes 1 and 2 . The anodic scanning was started from +0.0 mV to -1500 mV potential limit (25 mVs −1 to 300 mVs −1 ), showed two pairs of redox couples and which are assigned to Co 2+/3+ and Co 2+/+ , respectively (in 1:1:1 and 1:2:1 molar ratios), quasi-reversible single electron transfer redox couples. The electrochemical behavior of complex 4 (in 1:1:1 and 1:2:1 molar ratios) showed similar behavior to that of binary complex 3 but different electrode potentials, confirmed to complexation. The UV-Vsibile spectroscopy of the binary and mixed ligand complexes ( 1–4 ) has also been done in aqueous medium (different molar ratios). The results of CV and UV-Visible spectral studies indicate that the geometry around the central metal ions is octahedral, in aqueous medium.