pH-modulated Activation of Pendant Amine Leading to Rapid Electrocatalytic H2 Production by a Molecular Copper Complex in Acidic Water

Abstract

A modular multidentate ligand scaffold is crafted by strategically incorporating three pyridines (NPy) and three imines along with a pendant tertiary amine (Ntert) around a mononuclear copper centre. This unique design leads to the generation of a molecular copper complex C1 with a dynamically adaptive coordination environment, where the multiple proton and electron movements can be accommodated. Complex C1 demonstrates rapid hydrogen generation from water across a wide pH range (pH 1.0–7.0), with a markedly enhanced catalytic performance under acidic conditions. At pH 1.0, C1 achieves high turnover numbers (TONs) of 1014 ± 10 within 1 hour and 2980 ± 20 over 3 hours. In operando spectroelectrochemical investigations, in conjunction with density functional theory (DFT) calculations, reveal a unique pH-dependent structural flexibility of the ligand scaffold around the Cu centre in C1. In near-neutral to slightly acidic media (pH 3.0-7.0), the protonation of an NPy group (pKa1~ 11.6) following its cleavage from Cu linkage provides the primary protonation site, which is essential for Cu-complex driven H2 production catalysis. The Ntert group (pKa2~ 2.8), positioned in the outer coordination sphere of Cu, gets involved in highly acidic conditions (pH < 3.0). Here, this pendant amine acts as the initial protonation site and alters the course of the catalysis by unleashing an energetically downhill reaction pathway consisting of spontaneous electron and proton transfer steps. This pH-specific participation of the pendant Ntert functionality is key for the escalated HER activity by C1 under strongly acidic conditions, which is rarely observed for Cu-based molecular complexes. Complementary surface and solution-phase analyses confirm the molecular integrity of the complex, supporting a homogeneous catalytic mechanism operative throughout the hydrogen evolution process.