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 (1–3) 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 GC-1–GC-3 were prepared with films of 1–3 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 1 and 2. The charge-transfer resistances for GC-1–GC-3 were determined to be 270, 160, and 630 Ω, respectively. Characterization of the modified surfaces for GC-1 and GC-2 by scanning electron microscopy and Raman spectroscopy revealed similar crystalline coatings before HER that changed to surface-modified crystallites after conditioning. The surface of GC-3 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 1 and 2 pack as columns of interacting complexes in the crystallographic a direction with short interplanar spacings between 3.37 and 3.54 Å. Complex 3 packs as columns of isolated molecules in the crystallographic b direction with long-range interplanar spacings of 9.40 Å.