Effect of Stacking Interactions on the Translation of Structurally Related Bis(thiosemicarbazonato)nickel(II) HER Catalysts to Modified Electrode Surfaces

Abstract

A series of crystalline nickel(II) complexes (<b>1</b>-<b>3</b>) based on inexpensive bis(thiosemicarbazone) ligands diacetylbis(4-methyl-3-thiosemicarbazone) (H₂ATSM), diacetylbis(4,4-dimethyl-3-thiosemicarbazone) (H₂ATSDM), and diacetylbis[4-(2,2,2-trifluoroethyl)-3-thiosemicarbazone] (H₂ATSM-F₆) were synthesized and characterized by single-crystal X-ray diffraction and NMR, UV-visible, and Fourier transform infrared spectroscopies. Modified electrodes <b>GC-1</b>-<b>GC-3</b> were prepared with films of <b>1</b>-<b>3</b> deposited on glassy carbon and evaluated as potential hydrogen evolution reaction (HER) catalysts. HER studies in 0.5 M aqueous H₂SO₄ (10 mA cm^-2) revealed dramatic shifts in the overpotential from 0.740 to 0.450 V after extended cycling for <b>1</b> and <b>2</b>. The charge-transfer resistances for <b>GC-1</b>-<b>GC-3</b> were determined to be 270, 160, and 630 Ω, respectively. Characterization of the modified surfaces for <b>GC-1</b> and <b>GC-2</b> by scanning electron microscopy and Raman spectroscopy revealed similar crystalline coatings before HER that changed to surface-modified crystallites after conditioning. The surface of <b>GC-3</b> had an initial glasslike appearance before HER that delaminated after HER. The differences in the surface morphology and the effect of conditioning are correlated with crystal-packing effects. Complexes <b>1</b> and <b>2</b> pack as columns of interacting complexes in the crystallographic <i>a</i> direction with short interplanar spacings between 3.37 and 3.54 Å. Complex <b>3</b> packs as columns of isolated molecules in the crystallographic <i>b</i> direction with long-range interplanar spacings of 9.40 Å.