Establishing the Electrocatalytic HER Pathway of Indole N-Functionalized S ∩ N ∩ N Pincer Palladium(II) Complexes through Computational Mechanistic Analysis and Enabling Nitroarene Reduction

To establish palladium as a potential substitute for the conventional Pt/C electrocatalyst for the electrochemical hydrogen evolution reaction (HER), a clean energy alternative, a series of stable pincer Pd(II) complexes with tunable indole N-functionalized S∩N∩N ligands are synthesized. Among all of the synthesized distorted square-planar Pd(II) complexes (C1–C4), [(κ3-Me-Ind-S∩N∩N)PdCl], C2, emerged as the most active HER catalyst, achieving a low overpotential of −142 mV (vs RHE) and a Tafel slope of 120 mV dec–1, while maintaining robust and stable operation (10 h) in 0.5 M H2SO4. Comprehensive UV–visible (UV–vis) spectroscopy, Nuclear Magnetic Resonance (NMR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses verified that the palladium(II) complex, C2, retains its structural integrity, thereby emphasizing its potential as a highly stable and efficient HER electrocatalyst. Kinetic and thermodynamic favorability of C2 was established through computational studies, which revealed that the proton binds as a hydride to palladium without chloride displacement being thermodynamically favored, exhibiting the lowest energy barrier of 0.54 kcal mol–1. Notably, the Pd(II)-hydride formed during HER proved synthetically valuable, enabling the efficient reduction of nitroarenes.