Manganese Carbonyls Bearing Tripodal Polypyridine Ligands as Photoactive Carbon Monoxide-Releasing Molecules

The recently discovered cytoprotective action of CO has raised interest in exogenous CO-releasing materials (CORMs) such as metal carbonyls (CO complexes of transition metals). To achieve control on CO delivery with metal carbonyls, we synthesized and characterized three Mn­(I) carbonyls, namely, [Mn­(tpa)­(CO)₃]­ClO₄ [<b>1</b>, where tpa = tris­(2-pyridyl)­amine], [Mn­(dpa)­(CO)₃]Br [<b>2</b>, where dpa = <i>N</i>,<i>N</i>-bis­(2-pyridylmethyl)­amine], and [Mn­(pqa)­(CO)₃]­ClO₄ [<b>3</b>, where pqa = (2-pyridylmethyl)­(2-quinolylmethyl)­amine], by crystallography and various spectroscopic techniques. All three carbonyls are sensitive to light and release CO when illuminated with low-power UV (5–10 mW) and visible (λ > 350 nm, ∼100 mW) light. The sensitivity of <b>1–3</b> to light has been assessed with respect to the number of pyridine groups in their ligand frames. When a pyridine ring is replaced with quinoline, extended conjugation in the ligand frame increases the absorptivity and makes the resulting carbonyl <b>3</b> more sensitive to visible light. These photosensitive CORMs (photoCORMs) have been employed to deliver CO to myoglobin under the control of light. The superior stability of <b>3</b> in aqueous media makes it a photoCORM suitable for inducing vasorelaxation in mouse aortic muscle rings