Spectroscopic interplay of Co(II) and Pb(II) Salamo complexes: From coordination-induced electronic modulation to antibacterial activity.

The interplay between coordination-induced electronic modulation and the resulting physicochemical properties of metal complexes is fundamental to coordination chemistry. Herein, we report a comprehensive spectroscopic and theoretical investigation of two novel metal complexes derived from a single-armed Salamo-type ligand H3L: dinuclear [Co2(L)(μ2-OAc)]·EtOH (1) and tetranuclear [Pb4(L)2(μ4-O)]·2CHCl3 (2). FT-IR, UV-Vis, and fluorescence spectroscopies, together with DFT calculations, unequivocally confirmed metal-ligand coordination and revealed significant electronic restructuring upon complexation. Key spectral signatures-including the disappearance of the OH stretch, redshifts in CN and π → π* transitions, and the emergence of new metal-involved charge-transfer bands-provided direct evidence of ligand-to-metal binding. Notably, the complexes 1 exhibited opposite fluorescence responses: quenching in 1 and enhancement in 2, which were correlated with their DFT-derived HOMO-LUMO gaps (ΔE = 0.972 eV for 1; 2.167 eV for 2) and frontier orbital distributions. Hirshfeld surface and IRI analyses further visualized the non-covalent interactions stabilizing the crystal packing. Importantly, these coordination-induced electronic features translated into distinct biological activities: both complexes showed concentration-dependent antibacterial effects against Escherichia coli and Staphylococcus aureus, with the Pb(II) complex 2 exhibiting superior efficacy. This work establishes a clear spectroscopic-electronic-activity relationship, demonstrating how systematic spectral analysis can guide the rational design of functional coordination compounds.