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 [1, where tpa = tris­(2-pyridyl)­amine], [Mn­(dpa)­(CO)3]Br [2, where dpa = N,N-bis­(2-pyridylmethyl)­amine], and [Mn­(pqa)­(CO)3]­ClO4 [3, 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 1–3 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 3 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 3 in aqueous media makes it a photoCORM suitable for
inducing vasorelaxation in mouse aortic muscle rings.