Subtle ligand effects in Cu(I) thiocyanate complexes: from molecular structure to interfacial electron transfer

Two new mononuclear tetrahedral copper(I) thiocyanate complexes, [Cu(NCS)(PPh₃)(bq)] (1) and [Cu(NCS)(PPh₂py)(bq)] (2) (PPh₃ = triphenylphosphine, PPh₂py = diphenyl(2-pyridyl)phosphine, bq = 2,2'-biquinoline), were synthesized and structurally characterized in order to investigate the effect of subtle ligand modification on redox behaviour and interfacial electron transfer. Replacement of the triphenylphosphine ligand with the mixed phosphine-pyridine donor PPh₂py introduces a distinct electronic perturbation at the Cu(I) center, while preserving the overall coordination geometry. Single-crystal X-ray diffraction analysis revealed that both Cu(I) complexes crystallize in the triclinic P1̄ space group and adopt four-coordinate geometries with N₃P donor sets composed of bidentate biquinoline, monodentate phosphine ligands and terminal N-coordinated thiocyanate anions. The calculated τ₄ values of 0.89 for 1 and 0.85 for 2 indicated slightly distorted tetrahedral coordination environments around the Cu(I) centres. Electrochemical impedance spectroscopy confirmed the improved interfacial electron transfer after immobilization of the complexes on glassy carbon electrodes (GCE), as the charge-transfer resistance (R_ct) decreased from 14 754 Ω for the bare GCE to 4773.8 Ω for GCE modified with complex 1 (GCE-1) and 6141.8 Ω for GCE modified with complex 2 (GCE-2), indicating that ligand substitution strongly influences electron transfer at the electrode interface. Cyclic voltammetry (CV) studies in the ferri/ferrocyanide redox probe revealed distinct interfacial behaviours, with GCE-1 showing a predominantly diffusion-controlled response and an increased electroactive surface area of 0.0349 cm², compared with 0.0149 cm² for the bare GCE, whereas GCE-2 exhibited adsorption-controlled behaviour with an estimated electroactive surface coverage of 2.35 × 10^-9 mol cm^-2. The combined structural and electrochemical data establish a direct correlation between ligand design and electrochemical response, showing that even minimal modifications at the molecular level can significantly impact macroscopic electron transfer processes. These findings provide new insight into the role of ligand environment in tuning the redox behaviour of Cu(I) systems and contribute to the rational design of functional coordination compounds for electrochemical applications.