Synthesis and assessment of CO-release capacity of manganese carbonyl complexes derived from rigid α-diimine ligands of varied complexity.

Four manganese carbonyl complexes of the type [MnBr(CO)3(NˆN)] (NˆN = α-diimine ligands) namely [MnBr(CO)3(bpy)] (1), [MnBr(CO)3(phen)] (2), [MnBr(CO)3(dafo)] (3) and [MnBr(CO)3(pyzphen)] (4) (where bpy = bipyridine, phen = 1,10-phenanthroline, dafo = 4,5-diazafluoren-9-one and pyzphen = pyrazino[2,3-f][1,10]-phenanthroline) have been synthesized and structurally characterized. These four complexes containing the fac-[Mn(CO)3] motif release CO upon illumination with low power visible and UV light. The CO release rates and the absorption maxima of the complexes are however very similar despite systematic increase in structural complexity in the rigid α-diimine ligand frames. This is quite in contrary to manganese carbonyl complexes derived from α-diimine ligands in which at least one of the imine functions is not part of the rigid ring systems. Results of this study will provide help in the future design of ligand frames suitable for the syntheses of photoCORMs to deliver CO to biological targets under the control of light.