Magnetic study and DFT analysis of a doubly carboxylato-bridged Co(<scp>ii</scp>) derivative anchored with a ‘scorpionate’ precursor as a potential electrocatalyst for heterogeneous H₂ evolution

Abstract

A new doubly carboxylato-bridged Co(II) dinuclear complex, [Co(bdtbpza)(NCS)]₂ (1), was obtained in a satisfactory yield by employing a 'scorpionate'-type precursor, <b>bdtbpza</b> {bis-(3,5-di-<i>tert</i>-butylpyrazol-1-yl)acetate}, and was then structurally characterized. Single-crystal X-ray diffraction analysis revealed that, in 1, each Co(II) is penta-coordinated, leading to a distorted trigonal-bipyramidal geometry within the coordination environment of N₃O₂. Weak antiferromagnetic coupling within the Co(II) ions in 1 was found based on the isotropic spin Hamiltonian <i>H</i> = -<i>J</i>(<i>S</i>₁·<i>S</i>₂) for the <i>S</i>_<i>i</i> = 3/2 system. For evaluating the spin density distribution and the mechanism for the magnetic exchange coupling, DFT analysis was performed, with the calculated result agreeing the experimental magnetic data. A study into electrochemical H₂ evolution, involving cyclic voltammetry (CV), controlled potential electrolysis (CPE), and gas chromatographic (GC) analyses of the graphite electrode modified with the cobalt complex in a neutral aqueous solution revealed the high catalytic activity of the complex with a low overpotential toward H₂O reduction. The faradaic efficiency of the catalyst was found to be 83.7% and the di-cobalt catalyst-modified electrode displayed quite an interesting H₂-evolution activity compared with that of bare electrodes. These results are encouraging for the future potential application of 1 in water splitting.