Revelation of Varying Bonding Motif of Alloxazine, a Flavin Analogue, in Selected Ruthenium(II/III) Frameworks

Abstract

The reaction of alloxazine (L) and Ru^II(acac)₂(CH₃CN)₂ (acac^– = acetylacetonate) in refluxing methanol leads to the simultaneous formation of Ru^II(acac)₂(L) (<b>1</b> = bluish-green) and Ru^III(acac)₂(L^–) (<b>2</b> = red) encompassing a usual neutral α-iminoketo chelating form of L and an unprecedented monodeprotonated α-iminoenolato chelating form of L^–, respectively. The crystal structure of <b>2</b> establishes that N5,O4^– donors of L^– result in a nearly planar five-membered chelate with the {Ru^III(acac)₂⁺} metal fragment. The packing diagram of <b>2</b> further reveals its hydrogen-bonded dimeric form as well as π–π interactions between the nearly planar tricyclic rings of coordinated alloxazine ligands in nearby molecules. The paramagnetic <b>2</b> and one-electron-oxidized <b>1</b>⁺ display ruthenium­(III)-based anisotropic axial EPR in CH₃CN at 77 K with ⟨<i>g</i>⟩/Δ<i>g</i> of 2.136/0.488 and 2.084/0.364, respectively (⟨<i>g</i>⟩ = {1/3­(<i>g</i>₁² + <i>g</i>₂² + <i>g</i>₃²)}^1/2 and Δ<i>g</i> = <i>g</i>₁ – <i>g</i>₃). The multiple electron-transfer processes of <b>1</b> and <b>2</b> in CH₃CN have been analyzed by DFT-calculated MO compositions and Mulliken spin density distributions at the paramagnetic states, which suggest successive two-electron uptake by the π-system of the heterocyclic ring of L (L → L^•– → L^2–) or L^– (L^– → L^•2– → L^3–) besides metal-based (Ru^II/Ru^III) redox process. The origin of the ligand as well as mixed metal–ligand-based multiple electronic transitions of <b>1</b>^<i>n</i> (<i>n</i> = +1, 0, −1, −2) and <b>2</b>^<i>n</i> (<i>n</i> = 0, −1, −2) in the UV and visible regions, respectively, has been assessed by TD-DFT calculations in each redox state. The p<i>K</i>_a values of <b>1</b> and <b>2</b> incorporating two and one NH protons of 6.5 (N3H, p<i>K</i>_a1)/8.16 (N1H, p<i>K</i>_a2) and 8.43 (N1H, p<i>K</i>_a1), respectively, are estimated by monitoring their spectral changes as a function of pH in CH₃CN–H₂O (1:1). <b>1</b> and <b>2</b> in CH₃CN also participate in proton-driven internal reorganizations involving the coordinated alloxazine moiety, i.e., transformation of an α-iminoketo chelating form to an α-iminoenolato chelating form and the reverse process without any electron-transfer step: Ru^II(acac)₂(L) (<b>1</b>) → Ru^II(acac)₂(L^–) (<b>2</b>^–) and Ru^III(acac)₂(L^–) (<b>2</b>) → Ru^III(acac)₂(L) (<b>1</b>⁺)