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) (1 = bluish-green) and Ru(III)(acac)2(L(-)) (2 = 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 2 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 2 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 2 and one-electron-oxidized 1(+) display ruthenium(III)-based anisotropic axial EPR in CH3CN at 77 K with ⟨g⟩/Δg of 2.136/0.488 and 2.084/0.364, respectively (⟨g⟩ = {1/3(g1(2) + g2(2) + g3(2))}(1/2) and Δg = g1 - g3). The multiple electron-transfer processes of 1 and 2 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 1(n) (n = +1, 0, -1, -2) and 2(n) (n = 0, -1, -2) in the UV and visible regions, respectively, has been assessed by TD-DFT calculations in each redox state. The pKa values of 1 and 2 incorporating two and one NH protons of 6.5 (N3H, pKa1)/8.16 (N1H, pKa2) and 8.43 (N1H, pKa1), respectively, are estimated by monitoring their spectral changes as a function of pH in CH3CN-H2O (1:1). 1 and 2 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) (1) → Ru(II)(acac)2(L(-)) (2(-)) and Ru(III)(acac)2(L(-)) (2) → Ru(III)(acac)2(L) (1(+)).