Towards Cardiolite-Inspired Carbon Monoxide Releasing Molecules - Reactivity of d4, d5 Rhenium and d6 Manganese Carbonyl Complexes with Isocyanide Ligands

Carbon monoxide releasing molecules (CORMs) are investigated widely in synthetic and medicinal chemistry owing to the potential therapeutic applications of the CO gas. Organometallic carbonyl complexes are best suited to play the role of CO carriers as they allow the exogenous release of CO under controlled conditions, and the toxicity of the gas can be overcome. With the long-term goal of developing CORMs with similar properties to those of the sesta-methoxyisobutylisonitrile (sesta-mibi) (99)mTc complex (Cardiolite), we have studied the reactivity of isocyanide ligands towards 16- and 17-electron cis-[Re(CO)(2)Br-4](-/2-) species and the [Mn(CO)(5)Br] complex. Six different isocyanide ancillary ligands (CNR), including mibi, were selected for this study. Their reactions with cis-dicarbonyl Re-III and Re-II complexes were accompanied by two-and one-electron reduction of the metal center and resulted in the formation of stable cis-mer-[Re(CO)(2)(CNR)(3)Br] species, whereas the same reactions with [Mn(CO)(5)Br] gave fac-[Mn(CO)(3)(CNR)(2)Br] compounds. All of the complexes were fully characterized, and single-crystal X-ray diffraction structure determinations were performed for selected species. In addition, unique monocarbonyl complexes were obtained from the reactions of cis-[Re(CO)(2)Br-4](-) (1) with tert-butyl isocyanide and cis-[Re(CO)(2)Br-4](2-) (2) with mibi. The species, a heptacoordinate Re-III and a hexacoordinate Re-II complex, respectively, were also characterized structurally. The CO-releasing profiles, the cytotoxic effects against 3T3 fibroblast cells, and the antibacterial properties of the compounds were also investigated.