The impact of metal-to-ligand charge transfer towards the redox noninnocence of 2,2'-azobis(benzothiazole) (abbt) has been highlighted on coordination to {Ru II (acac) 2 } (acac = 2,4-pentanedionato). It led to the authentication of a series of mononuclear and dinuclear complexes incorporating variable oxidation states of abbt (abbt 0/ C À/2À ). Mononuclear 1 was identified as [Ru III (abbtC À )], a MLCT excited state of [Ru II (abbt)]. Dinuclear 2 was however recognized as two discrete redox isomers: (i) radical bridged mixed-valent meso-[Ru 2.5 (m-abbtC À )Ru 2.5 ] (2a) and (ii) dianionic ligand bridged isovalent meso-[Ru III (m-abbt 2À )Ru III ] (2b), demonstrating unprecedented structural confirmation of valence tautomerism in azo-based ligand systems. A crystal structure of [2]ClO 4 validated the formation of [Ru III (m-abbtC À ) Ru III ]ClO 4 . Analysis of electronic structural forms of 1 and 2 in accessible redox states via spectroelectrochemistry and DFT revealed their electron reservoir feature.

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The paper deals with the electronic impact of ancillary ligands on the varying redox features of azobis(benzothiazole) (abbt) in the newly introduced mononuclear ruthenium complexes [Ru(pap) 2 (abbt)] n (1 n ) and [Ru(bpy) 2 (abbt)] n (2 n ), where pap = 2phenylazopyridine and bpy = 2,2′-bipyridine. In this regard, the complexes [Ru II (pap) 2 (abbt •− )]ClO 4 ([1]ClO 4 ), [Ru II (pap) 2 (abbt 0 )](ClO 4 ) 2 ([1](ClO 4 ) 2 ), [Ru II (bpy) 2 (abbt 0 )](ClO 4 ) 2 ([2](ClO 4 ) 2 ), and [Ru II (bpy) 2 (abbt •− )]ClO 4 ([2]ClO 4 ) were structurally and spectroscopically characterized. Unambiguous assignments of the aforestated radical and nonradical forms of abbt in 1 + /2 + and 1 2+ /2 2+ , respectively, were made primarily based on their redox-sensitive azo (NN) bond distances as well as by their characteristic electron paramagnetic resonance (EPR)/NMR signatures. Although the radical form of abbt •− was isolated as an exclusive product in the case of strongly π-acidic pap-derived 1 + , the corresponding moderately π-acidic bpy ancillary ligand primarily delivered an oxidized form of abbt 0 in 2 2+ , along with the radical form in 2 + as a minor (<10%) component. The oxidized abbt 0 -derived [1](ClO 4 ) 2 was, however, obtained via the chemical oxidation of [1]ClO 4 . Both 1 + and 2 2+ displayed multiple closed by reversible redox processes (one oxidation O1 and four successive reductions R1−R4) within the potential window of ±2.0 V versus saturated calomel electrode. The involvement of metal-, ligand-, or metal/ligand-based frontier molecular orbitals along the redox chain was assigned based on the combined experimental (structure, EPR, and spectroelectrochemisry) and theoretical [density functional theory (DFT): molecular orbitals, Mulliken spin densities/time-dependent DFT] investigations. It revealed primarily ligand (abbt/pap or bpy)-based redox activities, keeping the metal ion as a simple spectator. Moreover, frontier molecular orbital analysis corroborated the initial isolation of the radical and nonradical species for the pap-derived 1 + and bpy-derived 2 2+ as well as facile reduction of pap and abbt in 1 + and 2 + , respectively.

This work evaluated the switchable binding profile of the biochemically relevant and redox non-innocent C-organonitroso (ArNO) moiety with the selective {Ru­(acac)2} (acac = acetylacetonate) metal fragment as a function of external stimuli, including the solvent medium (EtOH versus toluene) and aryl substituents (C6H5, p-OMe-C6H4, and p-Cl-C6H4) in the framework of ArNO. In this context, the reaction of ArNO (Ar = C6H5 or p-OMe-C6H4) with the metal precursor RuII(acac)2(CH3CN)2 in polar protic EtOH led to the formation of monomeric [RuII(acac)2(ArNOo)2] (1a or 1b) with η1-N-bonded terminal ArNOo and double-ArNOo-bridged dimeric [(acac)2RuII(μ-ArNOo)2RuII(acac)2], 2a or 2b, respectively. On the other hand, the use of p-Cl-substituted ArNO selectively yielded the corresponding dimeric 2c. However, the use of nonpolar toluene resulted in monomeric 1 irrespective of the nature of aryl substituents in ArNO. Molecular identities, including the redox state of ArNOo in 1 and 2, were authenticated by their single-crystal X-ray structures as well as by solution spectral features. Though monomeric 1 exhibited reversible one-electron oxidation and reduction processes, leading to the electron paramagnetic resonance active [RuIII(acac)2(ArNOo)2]+ (1 +; S = 1/2) and [RuII(acac)2(ArNO•–)­(ArNOo)]− (1 •–; S = 1/2), respectively, redox states of dimeric 2 were found to be unstable on the electrolysis time scale. Interestingly, monomeric 1 underwent transformation to dimeric 2 in the presence of a strong reducing agent, hydrazine hydrate, and the reverse process, i.e., conversion of dimeric 2 to 1, took place under the influence of external coordinating agent ArNO. The detailed experimental exploration, including kinetic investigations related to 1 → 2 and 2 → 1 transformations, revealed that the electronic aspects of ArNO (redox non-innocence of ArNOo/•–, π-accepting and coordinating features of ArNOo) had facilitated its switchable binding event in combination with the {Ru­(acac)2} metal fragment.

The paper documents redox-triggered C–C coupling of acyclic N,N′-bis­(2-pyridylmethylene)­ethylenediamine (BPE) to yield 2,3-bis­(2-pyridyl)­pyrazine (DPP) upon coordination to an electron-rich {Ru­(acac)2} (acac = acetylacetonate) unit. This led to DPP-bridged [{Ru­(acac)2}2(DPP)]0/+ (2 and [2]­ClO4) along with the unperturbed BPE-bridged [{Ru­(acac)2}2(BPE)] (1). On the contrary, electron-poor {Ru­(Cl)­(H)­(CO)­(PPh3)3} yielded BPE-bridged [3]­(ClO4)2 as an exclusive product. Synergistic metal (Ru)–ligand (BPE) redox participation toward chemical noninnocence of the Schiff base ligand and DPP-mediated electronic communication in RuIIRuIII-derived [2]­ClO4 are addressed.

This article deals with the S–S bond scission of the model substrate 2,2′-dithiodipyridine (DTDP) in the presence of a selective set of metal precursors: RuII(acac)2, [RuIICl2(PPh3)3], [RuIIHCl­(CO)­(PPh3)3], [RuII(H)2(CO)­(PPh3)3], [RuII(bpy)2Cl2], [RuII(pap)2Cl2], [OsII(bpy)2Cl2], and [OsII(pap)2Cl2] (acac, acetylacetonate; bpy, 2,2′-bipyridine; pap, 2-phenylazopyridine). This led to the eventual formation of the corresponding mononuclear complexes containing the cleaved pyridine-2-thiolate unit in 1–4/[5]­ClO4–[8]­ClO4. The formation of the complexes was ascertained by their single-crystal X-ray structures, which also established sterically constrained four-membered chelate (average N1–M–S1 angle of 67.89°) originated from the in situ-generated pyridine-2-thiolate unit. Ruthenium­(III)-derived one-electron paramagnetic complexes 1–2 (S = 1/2, magnetic moment/B.M. = 1.82 (1)/1.81­(2)) exhibited metal-based anisotropic electron paramagnetic resonance (EPR) (Δg: 1/2 = 0.64/0.93, ⟨g⟩: 1/2 = 2.173/2.189) and a broad 1H nuclear magnetic resonance (NMR) signature due to the contact shift effect. The spectroelectrochemical and electronic structural aspects of the complexes were analyzed experimentally in combination with theoretical calculations of density functional theory (DFT and TD-DFT). The unperturbed feature of DTDP even in refluxing ethanol over a period of 10 h can be attributed to the active participation of the metal fragments in facilitating S–S bond cleavage in 1–4/[5]­ClO4–[8]­ClO4. It also revealed the following three probable pathways toward S–S bond cleavage of DTDP as a function of metal precursors: (i) the metal-to-ligand charge-transfer (MLCT) (RuII → σ* of DTDP)-driven metal oxidation (RuII → RuIII) process in the case of relatively electron-rich metal fragments {RuII(acac)2} or RuIICl2 in 1 or 2, respectively; (ii) metal hydride-assisted formation of 3 or 4 with the concomitant generation of H2; and (iii) S–S bond reduction with the simultaneous oxidation of the solvent benzyl alcohol to benzaldehyde.

This article demonstrates the stabilization of ground-and redoxinduced metal-to-ligand charge transfer excited states on coordination of azocoupled bmpd(L4) [bmpd = (E)-1,2-bis(1-methyl-1H-pyrazol-3-yl)diazene; L4 = −N�N−] to the electron-rich {Ru(acac) 2 } (acac = acetylacetonate) unit in mononuclear Ru II (acac) 2 (L4) (1) and diastereomeric dinuclear (acac) 2 Ru 2.5 (μ-L4 •− )Ru 2.5 (acac) 2 [rac, ΔΔ/ΛΛ (2a)/meso, ΔΛ (2b)] complexes, respectively. It also develops further one-step intramolecular electron transfer induced L4On the contrary, under identical reaction conditions electronically and sterically permuted bimpd [L5, (E)-1,2-bis(4-iodo-1-methyl-1H-pyrazol-3-yl)diazene)] delivered mononuclear Ru II (acac) 2 (L5) (3) as an exclusive product. Further, the generation of unprecedented heterotrinuclear complex [(acac) 2 Ru II (μ-L4)Ag I (μ-L4)Ru II (acac) 2 ]ClO 4 ([4]ClO 4 ) involving unreduced L4 via the reaction of 1 and AgClO 4 revealed the absence of any inner-sphere electron transfer (IET) as in precursor 1, which in turn reaffirmed an IET (at the interface of electron-rich Ru(acac) 2 and acceptor L4) mediated stabilization of 2. Structural authentication of the complexes with special reference to the tunable azo distance (N�N, N−N •− , N−N 2− ) of L and their spectro-electrochemical events in accessible redox states including the reversible electron reservoir feature of 2 → 2 + /2 + → 2 were evaluated in conjunction with density functional theory/time-dependent density functional theory calculations. The varying extent of IET as a function of heteroaromatics appended to the azo group of L (L1 = abpy = 2,2′-azobipyridine, L2 = abbt = 2,2′-azobis(benzothiazole), L3 = abim = azobis(1-methylbenzimidazole), L4 and L5, Schemes 1 & 2) in the Ru(acac) 2 -derived respective molecular setup has been addressed.

The unexplored 'actor' behavior of redox-active bis(aldimine) congener, p-phenylene-bis(picoline)aldimine (L1), towards dioxygen activation and subsequent functionalization of its backbone was demonstrated on coordination with {Ru(acac) 2 } (acac = acetylacetonate). Reaction under aerobic condition led to the one-pot generation of dinuclear complexes with unperturbed L1, imino-carboxamido (L2 À ), and bis(carboxamido) (L3 2À )-bridged isovalent {Ru II (μ-L1)Ru II }, 1/ {Ru III (μ-L3 2À )Ru III }, 3 and mixed-valent {Ru II (μ-L2 À )Ru III }, 2.Authentication of the complexes along with the redox noninnocence behavior of their bridge have been validated through structure, spectroelectrochemistry and DFT calculations. Kinetic and isotope labelling experiments together with DFT analyzed transition states justified the consideration of redox shuttling at metal/L1 interface for 3 O 2 activation despite of the closed shell configuration of 1 (S = 0) to give carboxamido derived 2/3. Synergistic effort of redox-active metal and ligand is used as a protagonist in numerous biological [1] and catalytic [2] processes. [3] Complex with ligand-centered redox event facilitates shuttling of electron at the metal-ligand interface (M n L p $ M n + 1 L pÀ 1 ) through the effective overlapping of their suitably oriented frontier molecular orbitals and that results in valence tautomerism [4,5] or dynamic resonance. [4,6] This in effect promotes diverse reactivity profile of such ligand systems to manifest their "actor" behavior. [7] In line with this, the present article highlights a unique example of ligand-centered oxygenation for the stoichiometric conversion of bis(aldimine) to bis(carboxamido) on diruthenium platform based on dynamic resonance induced dioxygen activation (Scheme 1). Although metal ion mediated functionalization of imine to carboxamido in presence of exogenous reagents such as H 2 O, H 2 O 2 , acid, Ce IV , [8] are well explored, only a few reports by Manuel and Rohde, [9a] de Bruin and co-workers [9b] (M = Ni, Ir, Rh, Scheme 1) have demonstrated oxygen atom transfer from molecular oxygen to the metal bound imine backbone, which led to an occasional scenario of carboxamido formation.In this context, the present work delineates stepwise oxygenation of the two aldimine functions of p-phenylene-[a