Stability vs. activity in electrocatalytic hydrogen production and magnetic studies of mononuclear labile Cu(ii) and Ni(ii) complexes

Abstract

In this study, two new mononuclear Schiff base salen-derived complexes, [Cu^II(H₂L^Me)]·1.5H₂O (C1) and [Ni^II(H₂L^Me)]·3H₂O (C2), were prepared from 2,6-bis(hydroxymethyl)-4-methylphenol H₄L and characterised by single crystal X-ray diffractometry, infrared spectroscopy and elemental analysis. Electrochemical studies in DMF with acetic acid as a proton source revealed catalytic hydrogen evolution activity for both complexes, with C1 displaying higher activity, lower overpotential (η = 0.71 V), and a higher TOF_max of 452 029 s⁻¹ compared to C2 (η = 0.89 V, TOF_max = 986 s⁻¹). The stability of the compounds was tested via controlled potential electrolysis (CPE) and rinse tests, indicating partial heterogeneous contributions to catalysis from electrode-deposited species. These deposits were further analysed by SEM and EDX. To rule out the possibility of nanoparticle formation, dynamic light scattering DLS analysis was performed.