Synthesis, structural studies and electrocatalytic water oxidation properties of molecular copper complexes: The influence of N-alkylation of ligand and coordination number

In this study, we report the synthesis, structural studies, and electrocatalytic water oxidation properties of three novel Cu complexes featuring 2-pyridylmethyl-substituted 1,4,7-triazacyclononane ligands. Structural characterizations reveal the penta-coordination structure of complex [Cu(Py₂tcan)](ClO₄)₂ (1, Py₂tcan = 1,4-bis(2-picolyl)-1,4,7-triazacyclononane) and complex [Cu(MePy₂tcan)](ClO₄)₂ (2, MePy₂tcan = 1-methyl-4,7-bis(2-picolyl)-1,4,7-triazacyclononane), as well as the six-coordination structure of complex [Cu(Py₃tcan)](ClO₄)₂ (3, Py₃tcan = 1,4,7-tris(2-picolyl)-1,4,7-triazacyclononane). Systematic electrochemical analysis reveals that 1 and 2 exhibit high catalytic efficiency for water oxidation. Mechanistic studies suggest that the secondary amine coordination in 1 significantly reduces the catalytic overpotential by enhancing the σ-donor effect, thereby facilitating Cu center oxidation and enhancing the catalytic cycle. While 2 featuring two pyridine groups and three tertiary amine groups in its coordination environment performs higher catalytic overpotential than 1 because of the poor σ-donor properties of its N–CH₃ structure. Besides, 3 shows much lower catalytic activity, underscoring the necessity of an unsaturated coordination geometry for efficient water oxidation. Compared to its analogous Ni complex [Ni(Py₃tcan)]²⁺ (4) and Fe complex [Fe(Py₃tcan)]²⁺, 3 shows higher onset overpotential and lower catalytic activity, indicating that even when stabilized by the same Py₃tacn ligand, the Cu center possesses inferior intrinsic catalytic activity for water oxidation relative to Ni and Fe centers. Nevertheless, the catalytic performance of such Cu complex can be enhanced through rational modulation of the ligand structure and coordination number.