Synthesis of phenanthroline-based ligand and its UV activable tetracarbonyl photoCORMs based on chromium, molybdenum, and tungsten as cytotoxic and antimicrobial agents

A phenanthroline-based ligand L1 ((((1,10-phenanthrolin-5-yl)imino)methyl)-6-methoxyphenol) was synthesized and reacted with chromium, molybdenum, and tungsten hexacarbonyl to form ultraviolet (UV) activable photoCORMs C1, M1, and T1, respectively. All compounds were characterized via IR, ¹H-NMR, ¹³C-NMR, and CHN elemental analyses, and the X-ray crystal structures of L1 and M1 were determined. The half-life of the photoCORMs decreased with an increasing concentration and the rate of CO release increased in the order of T1 < M1 < C1, with C1 having the shortest half-life (7.4 seconds at 60 µM). Both M1 and T1 displayed remarkable stability in 10% DMSO-PBS solution over 48 hours in the dark, whereas C1 showed signs of decomposition. In the absence of UV, all compounds exhibited appreciable cytotoxicity against human-derived colorectal adenocarcinoma HCT 116, with T1 being the most cytotoxic (IC₅₀ 5.49 µM). The cytotoxicity of the photoCORMs were elevated substantially after brief exposure to UV light (365 nm), with that of C1 having the most drastic improvement (IC₅₀ from 9.98 to 1.24 µM). Moreover, all compounds were selective towards HCT 116 while being relatively less toxic against normal human colon fibroblast CCD-18Co. Furthermore, all compounds were generally not active in the bactericidal and growth inhibitory effects against the tested bacteria and fungi when UV light is absent, except for L1 that exhibited notable activity against B. cereus, B. subtilis, E. coli, K. pneumoniae, and S. flexneri (MIC ≤ 12.5 mg/ml). In contrast, the photoCORMs showed significant improvement in the growth inhibitory effect against several microbes in the presence of UV light, viz., B. cereus, B. subtilis, methicillin-sensitive S. aureus (ATCC 29213 and 33591 strain), and S. flexneri, while no notable improvement in the bactericidal effect was shown.