Photo-, Electro-, Catalysts Based on Schiff Base Ni^II Complexes for Formate/Hydrogen Generation and Their Potential Biomedical Applications

Transition metal complexes, particularly those derived from Schiff bases, are of considerable interest due to their multifunctional biological and photocatalytic properties, especially in formate production and hydrogen evolution. In this study, two nickel(II) complexes, [Ni(L¹)(pyS)] (1) and [Ni(L¹)(MBT)] (2), were synthesized and systematically evaluated for their photocatalytic and biological performances. Various organic dyes were employed as photosensitizers, while amines or bases served as sacrificial electron donors for visible-light-driven (> 420 nm) CO₂ reduction and hydrogen generation. The photocatalytic conditions were optimized by varying the pH (7–12.5), solvent ratios, and catalyst concentrations (2 mM). Density functional theory (DFT) studies revealed that complex 2 has a lower activation energy barrier (0.025 eV) for CO₂-to-formate conversion than complex 1 (0.043 eV). Both Ni(II) complexes demonstrated strong interactions in DNA/BSA binding studies, notable cytotoxicity toward HCT 116 (human colorectal carcinoma) cells, minimal toxicity toward NIH-3T3 (mouse embryonic fibroblast) cells, effective DPPH radical scavenging activity, and pronounced antimicrobial effects. Complexes 1 and 2 exhibited IC₅₀ values of 16.55 ± 0.05 μM and 13.99 ± 0.05 μM, respectively, comparable to cisplatin (IC₅₀ = 7.05 ± 0.05 μM). These findings highlight the dual potential of the synthesized Ni(II) complexes as efficient photocatalysts and promising candidates for biomedical applications.