Visible light photoactivatable CO releasing manganese (I) tricarbonyl complexes: Experimental and DFT studies

The reaction between [Mn(CO)₅Br] and antibacterial drugs (Nifuroxazide (NIF) and Nitazoxanide (NTZ)) afforded fac-[MnBr(CO)₃(NIF)] (1) and fac-[MnBr(CO)₃(NTZ)₂] (2) complexes, which were characterized by different analytical and spectral tools. The photo-induced CO releasing properties and the ability of the metal carbonyls to drive CO to the myoglobin solution were investigated. The green-light (525 nm) illumination of the DMSO solution of 1 for 20 min gave rise to the maximum amount of CO. Compared with the DMSO solution of 1, the rate of CO release from the 80% aqueous-DMSO mixed media is slower, and the plateau is reached upon the illumination for 140 min. On the contrast, a clear isosbestic point is developed during the storing of the DMSO solution of 2 for 6 h, which reflects its instability. To drive CO from 2 to myoglobin solution, the energy of the green light was not sufficient. However, the grown of the bands corresponding to the formation of carboxy-myoglobin species is stopped after 50 min of the irradiation at 468 nm and 36 min at 410 nm. The electronic structures and the related electronic transitions have been calculated by TD-DFT/B3LYP (or CAM-B3LYP)/LANL2DZ methods. The spectra calculated by the hybrid exchange-correlation functional CAM-B3LYP with a long range correction term are more closed to the experimental findings. The remarkable toxicity of 2 against the tested microbes, with respect to the free drug, may be attributed to the change of the lipophilicity of free NTZ upon the complex formation as well as the releasing of CO upon the incubation in dark.