Near-Infrared Light Activated Release of Nitric Oxide from Designed Photoactive Manganese Nitrosyls: Strategy, Design, and Potential as NO Donors

Two new manganese complexes derived from the pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide, PaPy2QH, where H is dissociable proton), namely, [Mn(PaPy2Q)(NO)]ClO4 (2) and [Mn(PaPy2Q)(OH)]ClO4 (3), have been synthesized and structurally characterized. The
Mn(III) complex [Mn(PaPy2Q)(OH)]ClO4 (3), though insensitive to dioxygen, reacts with nitric oxide (NO) to
afford the nitrosyl complex [Mn(PaPy2Q)(NO)]ClO4 (2) via reductive nitrosylation. This diamagnetic {Mn-NO}6 nitrosyl exhibits νNO at 1725 cm-1 and is highly soluble in water, with λmax at 500 and 670 nm. Exposure
of solutions of 2 to near-infrared (NIR) light (810 nm, 4 mW) results in bleaching of the maroon solution
and detection of free NO by an NO-sensitive electrode. The quantum yield of 2 (Φ = 0.694 ± 0.010, λirr =
550 nm, H2O) is much enhanced over the first generation {Mn-NO}6 nitrosyl derived from analogous
polypyridine ligand, namely, [Mn(PaPy3)(NO)]ClO4 (1, Φ = 0.385 ± 0.010, λirr = 550 nm, H2O), reported
by this group in a previous account. Although quite active in the visible range (500−600 nm), 1 exhibits
very little photoactivity under NIR light. Both 1 and 2 have been incorporated into sol−gel (SG) matrices
to obtain nitrosyl−polymer composites 1·SG and 2·SG. The NO-donating capacities of the polyurethane-coated hybrid materials 1·HM and 2·HM have been determined. 2·HM has been used to transfer NO to
reduced myoglobin with 780 nm light. The various strategies for synthesizing photosensitive metal nitrosyls
have been discussed to establish the merits of the present approach. The results of the present study
confirm that proper ligand design is a very effective way to isolate photoactive manganese nitrosyls that
could be used to deliver NO to biological targets under the control of NIR light.