A pentadentate nitrogen-rich copper electrocatalyst for water reduction with pH-dependent molecular mechanisms

The new pentadentate 3d⁹ complex [Cu^II(L^N2Py3)](PF₆)₂ (1) based on a nitrogen-rich framework acts as an electrocatalyst toward dihydrogen production from water. This species is active at pHs 7 and 2.5 yielding respective TON_3h values of 1670 and 3900. Comparison of the molecular structure of 1 with that of the reduced [Cu^I(L^N2Py3)]PF₆ (2) evidences elongated Cu-N bond lengths resulting from an increased electron density around the 3d¹⁰ Cu^I center. The absence of nanoparticulate formation indicates that molecular mechanisms prevail at both pHs. Furthermore, experimental and DFT data support that distinct mechanisms are operative: while the metal center plays a key role at pH 7, one dangling pyridine moiety gets protonated at pH 2.5 and becomes actively involved in a relay mechanism. In both cases the Cu^III-H^- intermediate seems to be bypassed by PCET processes.