Electronic tuning of Cu···Br semi-coordinative interactions, mixed-valence communication, and biological evaluation in SNS-supported dicopper(II) dimers

A series of SNS-supported halide-bridged dicopper(II) dimers [Cu(L)(Br)] 2 were synthesized, where L = 2(C 6 H 5 S)C 6 H 4 N=NC(COCH 3 )-SC 6 H 4 Y (Y = Cl (1), OCH 3 (2), CH 3 (3), F (4), NO 2 (5)), to explore how different substituents affect semi-coordinative Cu···Br interactions and electronic communication in mixed-valence systems. These complexes were characterized using elemental analysis, FTIR, UV–Vis spectroscopy, single-crystal X-ray diffraction, cyclic voltammetry, and thermogravimetric analysis. X-ray crystallography of the dimers (1-5) showed distorted square-pyramidal Cu(II) centers connected via μ 2 -bromide bridges, with substituent-dependent differences in Cu···Br semi-coordination. Cyclic voltammetry revealed two quasi-reversible one-electron reductions related to the Cu(II)/Cu(I) and Cu(I)/Cu(I) couples. The redox separation, with the calculated comproportionation constants ranging from 3.44 × 10 2 to 3.70 × 10 4 , and the stability of the mixed-valence state varied systematically with the ligand substituent Y. Correlations among Hammett constants, Cu···Br distances, and electrochemical data demonstrated a clear structure–property relationship. Additionally, these complexes showed DNA-binding activity and moderate anticancer effects against breast (MCF-7), lung (HCT116), and colon (A549) cancer cells, as well as antimicrobial activity against S. aureus , E. coli , K. pneumoniae , and ESBL-positive K. pneumoniae , compared to free ligands.