Exploring the effect of a pendent amine group poised over the secondary coordination sphere of a cobalt complex on the electrocatalytic hydrogen evolution reaction

Abstract

A Co^III complex (2) of a bispyridine-dioxime ligand (H₂L^NMe2) containing a tertiary amine group in the proximity of the Co center is synthesized and characterized. One of the oxime protons of the ligand is deprotonated, and the amine group remains protonated in the solid-state structure of the Co^II complex (2a). The acid–base properties of 2 showed pK_a values of 5.9, 8.4, and 9.6, which are assigned to the dissociation of two consecutive oxime protons and amine protons, respectively. The electrocatalytic proton reduction of 2 was investigated in an aqueous phosphate buffer solution (PBS), revealing a catalytic hydrogen evolution reaction (HER) at an E_cat/2 of −1.01 V vs. the SHE, with an overpotential of 673 mV and a k_obs value of 2.6 × 10³ s⁻¹ at pH 7. For comparison, the HER of the Co complex (1) lacking the tert-amine group at the secondary sphere was investigated in PBS, which showed a k_obs of 1.3 × 10³ s⁻¹ and an overpotential of 577 mV. At pH 4, however, 2 revealed a ∼3 times higher k_obs value than 1, which suggests that the protonated amine group likely works as a proton relay site. Notably, no significant change in the reaction rate was observed at different pH values for 1, implying that oxime protons may not be involved in the intramolecular proton-coupled electron transfer reaction in the HER. The k_obs values for Co complexes at pH 7.0 are significantly higher than those of the [Co(dmgH)₂(pyridine)(Cl)] complex, implying that the primary coordination sphere around 1 or 2 enhances the HER and offers better catalyst stability in acidic buffer solutions.