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) sub(2)(4-PyCOO)} sub(2)Zn(DMF) sub(2)] (1) and [{ClCo(dmgH) sub(2)(4-PyCOO)} sub(2)Cd(H sub(2)O) sub(3)(DMF)].4H sub(2)O (2) bridged by isonicotinate-Zn(II) and Cd(II) moieties. These complexes were synthesized upon reaction of a monomeric chlorocobaloxime [ClCo(dmgH) sub(2)(4-PyCOOH)] with Zn(NO sub(3))2.6H sub(2)O and Cd(OAc) sub(2).2H sub(2)O in a methanol/DMF solvent mixture. Both complexes are fully characterized by UV-Visible, FT-IR, and NMR (sup(1)H and sup(13)C{sup(1)H}) 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 (ZnO sub(4)) 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 (CdO sub(6)), 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 sup(-)) 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/H sub(2)O (95:5) solution using 0.1 M TBAPF sub(6) 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 sup(III)/ sup(II) and Co sup(II)/Co sup(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.