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(+)).

Probing the Noninnocent π-Bonding Influence of N-Carboxyamidoquinolate Ligands on the Light Harvesting and Redox Properties of Ruthenium Polypyridyl Complexes

“…The combination of controlled potential electrolysis with EPR spectroscopy is a very powerful tool to probe the frontier orbital character of transition metal complexes . EPR spectroscopy of paramagnetic transition metal complexes is capable of quantifying spin-density at the metal center via axial or rhombic g -anisotropy or by deviation of the isotropic g- factor ⟨ g ⟩ from that of the free electron g- factor ( g e = 2.0023) . The in situ generated one-electron oxidized [Ru­(bpy) 2 (R-CAQN)] 2+ complexes are hypothesized to have a singly occupied valence molecular orbital where the electron–hole is delocalized across the Ru­(dπ)–CAQN­(π) manifold due to the noninnocent nature of this system (Scheme ).…”

“…38 EPR spectroscopy of paramagnetic transition metal complexes is capable of quantifying spin-density at the metal center via axial or rhombic g-anisotropy or by deviation of the isotropic g-factor ⟨g⟩ from that of the free electron g-factor (g e = 2.0023). 39 The in situ generated one-electron oxidized [Ru(bpy) 2 (R-CAQN)] 2+ complexes are hypothesized to have a singly occupied valence molecular orbital where the electron−hole is delocalized across the Ru(dπ)−CAQN(π) manifold due to the noninnocent nature of this system (Scheme 2). The oneelectron oxidized species are EPR silent at room temperature due to a large Ru spin−orbital coupling constant (ξ ∼ 1000 cm −1 ) giving rise to fast relaxation times.…”

Ruthenium-Hydride Mediated Unsymmetrical Cleavage of Benzofuroxan to 2-Nitroanilido with Varying Coordination Mode

“…The resurgence of research interest in developing transition metal complexes of redox active ligands from the broader perspective of assessing valence and spin distributions at the metal–ligand interface (i.e., delicate electronic structural aspects) including their potential applications in cooperative catalysis has spurred the designing of a plethora of molecular frameworks with a wide variety of noninnocent ligand systems …”

Ruthenium(III)-Catalyzed β-Alkylation of Secondary Alcohols with Primary Alcohols

“…Unexpectedly, Ru­(III) complex D also acted as an efficient catalyst for the transfer hydrogenation of ketones . As compared to the extensively investigated Ru­(II) complexes, Ru­(III) complexes have been paid much less attention . In the latter case, Ru­(III) complexes were reported as the catalysts for C–H activation, heterocycle synthesis, and transfer hydrogenation of ketones .…”