Dinuclear PhotoCORMs: Dioxygen-Assisted Carbon Monoxide Uncaging from Long-Wavelength-Absorbing Metal–Metal-Bonded Carbonyl Complexes

We
describe a new strategy for triggering the photochemical release of
caged carbon monoxide (CO) in aerobic media using long-wavelength
visible and near-infrared (NIR) light. The dinuclear rhenium–manganese
carbonyl complexes (CO)5ReMn­(CO)3(L), where
L = phenanthroline (1), bipyridine (2),
biquinoline (3), or phenanthrolinecarboxaldehyde (4), each show a strong metal–metal-bond-to-ligand (σMM → πL*) charge-transfer absorption
band at longer wavelengths. Photolysis with deep-red (1 and 2) or NIR (3 and 4) light
leads to homolytic cleavage of the Re–Mn bonds to give mononuclear
metal radicals. In the absence of trapping agents, these radicals
primarily recombine to reform dinuclear complexes. In oxygenated media,
however, the radicals react with dioxygen to form species much more
labile toward CO release via secondary thermal and/or photochemical
reactions. Conjugation of 4, with an amine-terminated
poly­(ethylene glycol) oligomer, gives a water-soluble derivative with
similar photochemistry. In this context, we discuss the potential
applications of these dinuclear complexes as visible/NIR-light-photoactivated
CO-releasing moieties (photoCORMs).