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

The
reaction of alloxazine (L) and RuII(acac)2(CH3CN)2 (acac– = acetylacetonate)
in refluxing methanol leads to the simultaneous formation of RuII(acac)2(L) (1 = bluish-green) and
RuIII(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 {RuIII(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(g12 + g22 + g32)}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•– → L2–) or L– (L– → L•2– → L3–) besides metal-based (RuII/RuIII) redox process. The origin of the ligand as well
as mixed metal–ligand-based multiple electronic transitions
of 1n (n = +1, 0, −1, −2) and 2n (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: RuII(acac)2(L) (1) → RuII(acac)2(L–) (2–) and RuIII(acac)2(L–) (2) →
RuIII(acac)2(L) (1+).