Investigation of light-triggered carbon monoxide release from two manganese photoCORMs by IR, UV–Vis and EPR spectroscopy

Water-soluble complexes containing the fac-[MnI(CO)3]-moiety have been identified as candidates for the photoactivated release of molecular carbon monoxide, a potential therapeutic agent. Here, we employed a combination of spectroscopic methods (UV–Vis, IR and EPR) to reveal details of the processes following CO release. Studying the two manganese(I) complexes [Mn(CO)3(tpm)](PF6) (1, tpm = tris(pyrazolyl)methane) and [Mn(CO)3(bpzaa)] (2, bpzaa = bis(pyrazolyl)acetic acid), we find that the manganese centers lose their carbonyl ligands in a stepwise manner. First, manganese dicarbonyl intermediates are formed after the release of one CO per Mn. The last two carbonyl ligands are then liberated in a reaction accompanied by manganese oxidation to MnII and beyond, as clearly detected by EPR spectroscopy. Ultimately, μ-O–MnIII-compounds appear to be the most likely final reaction products. The spectroscopic methodology used here thus enables us to present a much more detailed picture of the light-triggered CO releasing reactions of fac-[MnI(CO)3]-complexes than previously accessible by the use of the classical myoglobin assay alone. The additional information gained in this way on the CO release process as well as the fate of the manganese precursors seems essential to us for a potential pharmaceutical application of manganese photoCORMs.