Visible Light-Activated CO Release and ¹O₂ Photosensitizer Formation with Ru(II),Mn(I) Complexes

Two diimine-bridged Ru(II),Mn(I) complexes with a [(bpy)₂Ru(BL)Mn(CO)₃Br]²⁺ architecture, where bpy = 2,2'-bipyridine and BL = 2,3-bis(2-pyridyl)pyrazine (dpp; Ru(dpp)Mn) or 2,2'-bipyrimidine (bpm; Ru(bpm)Mn), were designed to both dissociate multiple equivalents of CO and produce ¹O₂ when irradiated with visible light. Analysis of the complexes by Fourier transform infrared (FTIR) spectroscopy and cyclic voltammetry suggest a stronger π-accepting ability for bpm compared to that of dpp. Both complexes absorb light throughout the UV and visible regions with lowest energy absorption bands comprising overlapping Ru(dπ)→BL(π*) and Mn(dπ)→BL(π*) singlet metal-to-ligand charge transfer (¹MLCT) and Br(p)→dpp(π*) singlet halide-to-ligand charge transfer (¹XLCT) transitions. This lowest energy band is centered at 510 nm (ε = 12 000 M^-1cm^-1) for Ru(dpp)Mn and 553 nm (ε = 3240 M^-1cm^-1) for Ru(bpm)Mn, and the absorption band extends to nearly 700 nm in each case. Irradiation with visible light (both 470 and 627 nm) releases all three CO ligands, as observed by a combination of UV-vis, FTIR, and gas chromatography. The exchange of the first CO ligand with a solvent molecule occurs more efficiently for Ru(dpp)Mn (Φ₄₇₀ = 0.22 ± 0.03 in H₂O; 0.37 ± 0.06 in CH₃CN) than for Ru(bpm)Mn (Φ₄₇₀ = 0.049 ± 0.008 in H₂O and 0.16 ± 0.03 in CH₃CN), and the CO dissociation efficiency is unaffected by irradiation wavelength. The differences between Ru(dpp)Mn and Ru(bpm)Mn are proposed to result from the variation in electron density distribution across each formally reduced BL in the Mn(dπ)→BL(π*) ¹MLCT excited state based on the nature of BL. Exhaustive photolysis causes the decomplexation of oxidized Mn(II), and the resulting [(bpy)₂Ru(BL)]²⁺ complexes produce ¹O₂ with quantum yields (Φ_Δ) of 0.37 ± 0.03 and 0.16 ± 0.01 for Ru(dpp) and Ru(bpm), respectively, with 460 nm irradiation. This bimetallic architecture presents the opportunity to use visible light to codeliver both CO and ¹O₂, both of which have biological relevance in photoactivated therapeutics, with spatiotemporal control.