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

Abstract

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)ReMn(CO)(L), where L = phenanthroline (1), bipyridine (2), biquinoline (3), or phenanthrolinecarboxaldehyde (4), each show a strong metal-metal-bond-to-ligand (σ → π*) 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).

Correlation of MLCTs of Group 7 fac‐[M(CO)3]+ Complexes (M = Mn, Re) with Bipyridine, Pyridinylpyrazine, Azopyridine, and Pyridin‐2‐ylmethanimine Type Ligands for Rational photoCORM Design

“…fac ‐[Mn(CO) 3 L 2 Br] L 2 = (6‐Methyl‐4,4′‐bis(trifluoromethyl)‐2,2′‐bipyridine) (47): The complex was obtained by a modification of a published procedure . 34 mg (0.13 mmol, 1.5eq) of Mn(CO) 5 Br and 26 mg (0.08 mmol, 1 equiv.)…”

“…fac ‐[Mn(CO) 3 L 2 Br] L 2 = (4‐Methoxy‐6‐methyl‐2,2′‐bipyridine) (49): The L 2 ligand was prepared following the procedure described in literature . The complex was obtained by a modification of a published procedure . 34 mg (0.13 mmol, 1.5eq) of Mn(CO) 5 Br and 16.5 mg (0.08 mmol, 1 equiv.)…”

“…[37] The complex was obtained by a modification of a published procedure. [35] 34 mg (0.13 mmol, 1.5eq) of Mn(CO) 5 Br and 16.5 mg (0.08 mmol, 1 equiv.) of L 2 were refluxed in dry diethyl ether (10 mL) for 4 h under argon atmosphere, until an orange precipitate was formed.…”

Near-Infrared and Visible Photoactivation to Uncage Carbon Monoxide from an Aqueous-Soluble PhotoCORM

“…Thus, various researchers have employed molecular engineering to shift photoCORM lability to longer wavelengths. [14][15][16][17][18][19][20] The "phototherapeutic window" of mammalian tissues is ~750 to 1100 nm in the near infrared (NIR) wavelength region. 21 For this reason there has been considerable interest in developing multi-photon strategies for uncaging small molecule bioregulators, such as NO or CO, with such excitation wavelengths.…”

Near‐IR Photochemistry for Biology: Exploiting the Optical Window of Tissue

“…produced a dinuclear Re‐Mn CORM complex with a strong single‐photon absorbance corresponding to a low‐energy metal–metal‐to‐ligand charge transfer (MMLCT) transition. Mechanistic investigation suggests that this specific MMLCT triggers homolytic cleavage of the metal–metal bond, with the resulting radicals trapped by O 2 to generate key μ ‐peroxo intermediates, which themselves fragment to high‐spin intermediates that eject CO equivalents after subsequent single‐photon NIR photolysis . This dependence on O 2 implies excellent tolerance of physiological oxygenation, as opposed to Kianfar's comparable system, which was quenched by the presence of the triplet diradical O 2 .…”