Modulating the reduction potential and ligand basicity of Ni(II) HER catalysts with highly conjugated N2S2 ligands

Abstract

Molecular catalysts with N2S2 chelates and earth abundant metals have been used in different applications. With their versatile nature to modulate activity through changes in the ligand framework, they are excellent candidates to generate low-cost and sustainable energy as electrocatalysts for the hydrogen evolution reaction (HER). Herein we demonstrate the modulation of the reduction potential and basicity across a series of six square planar Ni(II) complexes with bis(thiosemicarbazonato) (BTSC), hybrid thiosemicarbazonato-alkylthiocarbamato (TSTC), and bis(alkylthiocarbamato) (BATC) ligands and evaluate their activity as homogenous HER catalysts. The redox active ligands have been designed to modulate the electronic structure of the N2S2Ni core and to systematically alter their respective pKa values. Cyclic voltammograms of all six complexes show two reversible reduction events in acetonitrile that systematically shift to more anodic potentials when the pendent methylthiosemicarbazonato (–NN=C(S)NHMe) groups are substituted with ethylthiocarbamato (–NN=C(S)OEt) groups and/or when the 2,4-butanedione backbone is replaced by 1-phenyl-1,2-propandione. Ligand basicity was evaluated by spectroscopic titration in acetonitrile and found to vary systematically across the series. Within the series of complexes the reduction potentials vary over an ∼ 500 mV range and the ligand basicity over ∼ 7 pKa units. Density functional theory (DFT) computations are consistent with experimental changes in redox potential and pKa values across the series of complexes. The results demonstrate a unique example in which electronic properties of the complexes are modulated via small ligand structure variations. The complexes are stable under acidic conditions or upon reduction, but they degrade upon reduction in the presence of acid limiting their application as homogeneous HER electrocatalysts.