Isonicotinate-Zn(ii)/Cd(ii) bridged dicobaloximes: synthesis, characterization and electrocatalytic proton reduction studies

Abstract

Herein, we present the synthesis of two new dicobaloxime complexes, [{ClCo(dmgH)₂(4-PyCOO)}₂Zn(DMF)₂] (1) and [{ClCo(dmgH)₂(4-PyCOO)}₂Cd(H₂O)₃(DMF)].4H₂O (2) bridged by isonicotinate-Zn(II) and Cd(II) moieties. These complexes were synthesized upon reaction of a monomeric chlorocobaloxime [ClCo(dmgH)₂(4-PyCOOH)] with Zn(NO₃)₂·6H₂O and Cd(OAc)₂·2H₂O in a methanol/DMF solvent mixture. Both complexes are fully characterized by UV-Visible, FT-IR, and NMR (¹H and ¹³C{¹H}) spectral studies. The solid-state structures are also determined by single-crystal X-ray crystallography. In complex 1, Zn (II) metal ions reside within a four coordinated distorted tetrahedral geometry (ZnO₄) formed by two oxygen atoms of isonicotinate connected to cobaloxime units and two oxygen atoms of DMF molecules. In complex 2, the Cd(II) metal ion exhibited distorted octahedral geometry (CdO₆), with two oxygen atoms of isonicotinate that connect to cobaloxime units, one DMF, and three water molecules. The Co(III) metal center of cobaloxime units in both complexes 1 and 2 displayed distorted octahedral geometry with two dmgH units in the equatorial plane whereas chloride ion (Cl⁻) and the nitrogen atom of isonicotinate occupy the axial coordination sites. The redox behaviour of both complexes was studied by cyclic voltammetry at variable scan rates in deoxygenated DMF/H₂O (95 : 5) solution using 0.1 M TBAPF₆ as the supporting electrolyte and a glassy carbon (GC) electrode as the working electrode. Both complexes exhibited similar redox properties and two redox couples Co^III/^II and Co^II/Co^I are observed in the reductive scan. Furthermore, complexes are investigated as electrocatalysts for proton reduction in the presence of acetic acid (AcOH) and complex 1 exhibited impressive electrocatalytic activity compared to complex 2 and monomer. The stability study indicated the retention of molecular structural integrity during HER electrocatalytic experiments.