Ankita Das

The Electron-Rich {Ru(acac)2} Directed Varying Configuration of the Deprotonated Indigo and Evidence for Its Bidirectional Noninnocence

This article highlights the hitherto unexplored varying binding modes of the deprotonated natural dye indigo (H2L) and its bidirectional noninnocent potential. The reaction of H2L with the selective metal precursor Ru(II)(acac)2(CH3CN)2 (acac(-) = acetylacetonate) leads to the simultaneous formation of paramagnetic Ru(III)(acac)2(HL(-)) (1; blue solid) and diamagnetic Ru(II)(acac)2(L) (2; red solid), which have been characterized by standard analytical, spectroscopic, and structural analysis. Crystal structures establish that the usual trans configurated and twisted monodeprotonated HL(-) and unprecedented cis configurated nearly planar dehydroindigo (L) bind to the {Ru(acac)2} metal fragment via the N(-),O and N,N donors, forming six- and five-membered chelates, respectively. It also reveals the existence of intramolecular N-H···O hydrogen-bonding interaction between the NH proton and C═O group at the back face of the coordinated HL(-), in addition to an intermolecular N-H···O hydrogen bonding between the NH proton of HL(-) of Molecule B and oxygen atom of the nearby acac of the second molecule (Molecule A) in the asymmetric unit of 1. The specific role of the electron-rich {Ru(acac)2} metal fragment in stabilizing the cis-configuration of the electron-deficient L in 2 has been pointed out. Both 1 and 2 exhibit reversible one-electron oxidation and successive three reductions with varying Kc (comproportionation constant) values in the range of 10(18)-10(6). The potentials for the redox processes of 2 are positively shifted with respect to those of 1. The involvement of the metal or HL(-)/L or mixed metal-HL(-)/L-based orbitals in the accessible redox processes of 1(n) and 2(n) has been analyzed by spectroelectrochemistry, EPR at the paramagnetic states, and DFT calculated MO compositions/spin density distributions. The collective consideration of the experimental results and DFT/TD-DFT data has ascertained the participation of both the metal fragment {Ru(acac)2} and the HL(-)/L in the redox processes, which in effect result in mixed electronic structural forms of 1(n) and 2(n) (n = +1, 0, -1, -2, -3).

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

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(+)).

Light-Induced Proton-Coupled Electron Transfer Inside a Nanocage

Triggering proton-coupled electron-transfer (PCET) reactions with light in a nanoconfined host environment would bring about temporal control on the reactive pathways via kinetic stabilization of intermediates. Using a water-soluble octahedral Pd6L4 molecular cage as a host, we show that optical pumping of host-guest charge transfer (CT) states lead to generation of kinetically stable phenoxyl radical of the incarcerated 4-hydroxy-diphenylamine (1-OH). Femtosecond broadband transient absorption studies reveal that CT excitation initiates the proton movement from the 1-OH radical cation to a solvent water molecule in ~890 fs, faster than the time scale for bulk solvation. Our work illustrates that optical host-guest CT excitations can drive solvent-coupled ultrafast PCET reactions inside nanocages and if optimally tuned should provide a novel paradigm for visible-light photocatalysis.

Tunable Electrochemical and Catalytic Features of BIAN- and BIAO-Derived Ruthenium Complexes

This article deals with a class of ruthenium-BIAN-derived complexes, [Ru(II)(tpm)(R-BIAN)Cl]ClO4 (tpm = tris(1-pyrazolyl)methane, R-BIAN = bis(arylimino)acenaphthene, R = 4-OMe ([1a]ClO4), 4-F ([1b]ClO4), 4-Cl ([1c]ClO4), 4-NO2 ([1d]ClO4)) and [Ru(II)(tpm)(OMe-BIAN)H2O](2+) ([3a](ClO4)2). The R-BIAN framework with R = H, however, leads to the selective formation of partially hydrolyzed BIAO ([N-(phenyl)imino]acenapthenone)-derived complex [Ru(II)(tpm)(BIAO)Cl]ClO4 ([2]ClO4). The redox-sensitive bond parameters involving -N═C-C═N- or -N═C-C═O of BIAN or BIAO in the crystals of representative [1a]ClO4, [3a](PF6)2, or [2]ClO4 establish its unreduced form. The chloro derivatives 1a(+)-1d(+) and 2(+) exhibit one oxidation and successive reduction processes in CH3CN within the potential limit of ±2.0 V versus SCE, and the redox potentials follow the order 1a(+) < 1b(+) < 1c(+) < 1d(+) ≈ 2(+). The electronic structural aspects of 1a(n)-1d(n) and 2(n) (n = +2, +1, 0, -1, -2, -3) have been assessed by UV-vis and EPR spectroelectrochemistry, DFT-calculated MO compositions, and Mulliken spin density distributions in paramagnetic intermediate states which reveal metal-based (Ru(II) → Ru(III)) oxidation and primarily BIAN- or BIAO-based successive reduction processes. The aqua complex 3a(2+) undergoes two proton-coupled redox processes at 0.56 and 0.85 V versus SCE in phosphate buffer (pH 7) corresponding to {Ru(II)-H2O}/{Ru(III)-OH} and {Ru(III)-OH}/{Ru(IV)═O}, respectively. The chloro (1a(+)-1d(+)) and aqua (3a(2+)) derivatives are found to be equally active in functioning as efficient precatalysts toward the epoxidation of a wide variety of alkenes in the presence of PhI(OAc)2 as oxidant in CH2Cl2 at 298 K, though the analogous 2(+) remains virtually inactive. The detailed experimental analysis with the representative precatalyst 1a(+) suggests the involvement of the active {Ru(IV)═O} species in the catalytic cycle, and the reaction proceeds through the radical mechanism, as also supported by the DFT calculations.

On discounted AR–AT semi-Markov games and its complementarity formulations

In this paper, we introduce a class of two-person finite discounted AR–AT (Additive Reward–Additive Transition) semi-Markov games (SMGs). We provide counterexamples to show that AR–AT and AR–AT–PT (Additive Reward–Additive Transition Probability and Time) SMGs do not satisfy the ordered field property. Some results on AR–AT–AITT (Additive Reward–Additive Transition and Action Independent Transition Time) and AR–AIT–ATT (Additive Reward–Action Independent Transition and Additive Transition Time) games are obtained in this paper. For the zero-sum games, we prove the ordered field property and the existence of pure stationary optimals for the players. Moreover, such games are formulated as a vertical linear complementarity problem (VLCP) and have been solved by Cottle-Dantzig’s algorithm under a mild assumption. We illustrate that the nonzero-sum case of such games do not necessarily have pure stationary equilibria. However, there exists a stationary equilibria which has at most two pure actions in each state for each player.