Spectroscopic, electrochemical, and computational aspects of the charge distribution in Ru(acac)2(R-o-benzoquinonediimine) complexes

Diverse Coordination Modes and Bidirectional Noninnocence of Pyridyl-β-diketonate on Ruthenium Platforms as a Function of Coligands

The article demonstrated diverse binding modes of deprotonated 1,3-di(2-pyridinyl)-1,3-propanedione (HL) (κ2-[O,O]-, κ2-[N,O]-, and μ-bis-κ2-[N,O]-) on selective ruthenium platforms: Ru(acac)2 (dimeric [1]ClO4), Ru(bpy)2 (monomeric [2]ClO4), Ru(pap)2 (isomeric monomeric [3]ClO4/[4]ClO4, dimeric [5](ClO4)3), and Ru(PPh3)2(CO) (monomeric 6, isomeric dimeric [7]ClO4/[8]ClO4) (acac = acetylacetonate, bpy = 2,2'-bipyridine, pap = 2-phenylazopyridine). Structural authentication of the complexes revealed (i) diverse binding mode of L- including its unprecedented bridging mode in [8]ClO4, (ii) varying degrees of nonplanarity of L-, and (iii) development of 1D polymeric chains or dimeric/tetrameric forms via intermolecular π-π interactions. The preferential binding feature of L- in the complexes could also be corroborated by their calculated relative energies. The analysis of the multiredox steps of the complexes suggested severe mixing of metal-ligand frontier orbitals, which in effect pinpointed the involvement of L- in both the oxidative and reductive processes along the redox chain, suggesting its bidirectional noninnocence under the present coordination situations. Though α-diketone or β-diketiminate was reported to activate O2 on the selective Ru(acac)2 platform, the inability of analogous β-diketonate-derived [1]ClO4 could be attributed to its calculated greater HOMO-LUMO energy gap, which disfavored electron exchange at the metal(RuIII)-ligand(L-) interface to introduce the required unpaired spin at the ligand backbone toward the 3O2 activation event.

Fine Tuning between Radical versus Nonradical States of Azoheteroarenes on Selective Osmium Platforms

The article highlights the cooperative impact of azoheteroarenes [abbt: 2,2'-azobis(benzothiazole), L1-L3; bmpd: (E)-1,2-bis(1-methyl-1H-pyrazole-3-yl) diazene, L4] and coligands [bpy: 2,2'-bipyridine; pap: 2-phenylazopyridine] in tuning radical (N-N•-) versus nonradical (N═N0) states of L on selective OsII-platforms in structurally/spectroscopically characterized monomeric [1]ClO4-[6]ClO4 and [1](ClO4)2-[2](ClO4)2/[7](ClO4)2-[8](ClO4)2, respectively. The preferred syn-configuration of L in the complexes prevented obtaining ligand bridged dimeric species. It revealed that {Os(bpy)2} facilitated the stabilization of both nonradical ([1](ClO4)2-[2](ClO4)2) and radical ([1]ClO4-[2]ClO4) states of L1/L2, while it delivered exclusively the radical form for L3 in [3]ClO4. In contrast, {Os(pap)2} generated radical states of L1-L3 in [4]ClO4-[6]ClO4, respectively, without any alteration of the redox state of OsII and azo (N═N0) function of the pap coligand. The neutral state of L4 was, however, ascertained in [7](ClO4)2 or [8](ClO4)2 irrespective of the nature of the metal fragment {Os(bpy)2} or {Os(pap)2}, respectively. Switching between radical and nonradical forms of L in the complexes as a function L and coligand could be addressed based on their relative FMO (frontier molecular orbital) energies. Multiple close redox steps of the complexes extended a competitive electron transfer scenario between the redox active components including metal/L/bpy/pap, leading to delicate electronic forms in each case.

Superior electrocatalytic hydrogen evolution activity of a triply bridged diruthenium(II) complex on a carbon cloth support

This article describes the structural authentication of a unique triply bridged [1](ClO4)2 and monomeric [2]ClO4/[3]ClO4. Electrochemical HER on a carbon cloth support demonstrated the superior performance of [1](ClO4)2 with high TON (>105) and its long-term stability. The primary kinetic isotope effect of [1](ClO4)2 revealed the involvement of PCET in the rate-determining step.

Sequential Oxygenation of Bis(β-diketiminate) on a Selective Diruthenium Platform

This article demonstrated the redox-noninnocent phenylene-linked bis(β-diketiminate) (L2-)-bridged first example of isomeric diruthenium(III)-acac species (acac = acetylacetonate) and its ability to activate dioxygen. The coordination of deprotonated L2- to the {Ru(acac)2} in bis(bidentate) mode led to isomeric {(acac)2RuIII}2(μ-L2-) (S = 1, 1-trans/1-cis, green). 1 displayed Ru(III)-based anisotropic EPR in CH3CN but without the resolution of the forbidden (ΔMs = 2) g1/2 signal at 77 K. 1-cis, however, slowly transformed to the energetically favored 1-trans form. 1 underwent two-step oxygenation at the Cβ sites of L2- to form the β-diketiminate/α-ketodiimine (L'-)-bridged mixed valent (acac)2RuIII(μ-L'-)RuII(acac)2 (2, S = 1/2, pink) followed by bis(α-ketodiimine) (L″)-bridged isovalent (acac)2RuII(μ-L″)RuII(acac)2 (3, S = 0, red). The role of O2 toward 1 → 2/3 was corroborated by 18O2 labeling experiment. Redox steps of 1-3 varied as a function of isomeric identity, bridge, and metal oxidation state. The calculated MOs and Mulliken spin densities attributed to the noninnocence of L2-, L'-, and L″ in the respective complexes. Spectrophotometric monitoring of 1 → 2 revealed pseudo-first-order rate constants (105k s-1) of 1.8 (303 K), 3.5 (313 K), 7.7 (323 K), and 17.0 (333 K) and ΔH⧧/ΔS⧧/ΔG⧧ of 14.3 kcal mol-1/-33.1 cal mol-1 K-1/24.2 kcal mol-1 (298 K), respectively. Moreover, characterization of the short-lived blue intermediate obtained during the conversion of 1 → 2/3 upon exposure to O2 supported its valence tautomeric form (VT1, RuIII-L2--RuIII ↔ RuIII-L•--RuII, S = 1), which in effect facilitated oxygen activation at the ligand backbone.

Metal-ligand synergy driven functionalisation of alkylene linked bis(aldimine) on a diruthenium(II) platform. Cyclisation versus oxygenation

This article addresses the impact of metal-ligand redox cooperativity on the functionalisation of coordinated ligands. It demonstrates the structure-reactivity correlation of bis(aldimine) derived bis-bidentate L (Py-CHN-(CH2)n-NCH-Py, with n = 2 (L1), 3 (L2), 4 (L3)) as a function of the conformation (syn/anti) of its alkylene linker as well as the overall structural form (cis/trans) of (acac)2RuII(μ-L)RuII(acac)2 complex moieties (1-5) possessing an electron-rich acetylacetonate (acac) co-ligand. A systematic variation of the bridging alkylene unit of L in RuII/RuII-derived 1-5 led to the following reactivity/redox events, which were validated through structural, spectroscopic, electrochemical and theoretical evaluations: (i) Cyclisation of the ethylene linked (syn conformation) bis-aldimine unit of L1 via C-C coupling yielded pyrazine bridged (acac)2RuII(μ-L1')RuII(acac)2, 1a, while the corresponding anti-form (ethylene linker) of the metal-bound L1 in 2 ((acac)2RuII(μ-L1)RuII(acac)2) led to oxygenation at the ligand backbone (bis-aldimine (L) → bis(carboxamido) (L'')) via O2 activation to generate RuIIIRuIII-derived (acac)2RuIII(μ-L1''2-)RuIII(acac)2 (2a). (ii) Consequently, propylene and butylene linked L2 and L3 bridged between two {Ru(acac)2} units in 3 and 4/5 underwent oxygenation of L to L'' to yield diruthenium(III) complexes 3a and 4a/5a, respectively. (iii) In contrast, analogous L bridged oxidised [(acac)2RuIII(μ-L)RuIII(acac)2](ClO4)2 ([2](ClO4)2-[5](ClO4)2) and [{(PPh3)2(CO)(H)RuII}2(μ-L)](ClO4)2 ([6](ClO4)2-[8](ClO4)2) involving electron poor co-ligands failed to undergo the oxygenation of L irrespective of its n value, reemphasising the effective role of redox interplay between RuII and L particularly in the presence of an electron-rich acac co-ligand in the functionalisation of the latter in 1a-5a.

The isomer-sensitive electrochemical HER of ruthenium(II)-hydrido complexes involving redox-active azoheteroaromatics

The article deals with the development of isomeric ruthenium(II)-hydrido complexes [RuII(H)(L1)(PPh3)2(CO)]ClO4 ([1a]ClO4-[1b]ClO4)/[RuII(H)(L2)(PPh3)2(CO)]ClO4 ([2a]ClO4-[2b]ClO4) involving azo coupled L1 [L1: (E)-1,2-bis(1-methyl-1H-pyrazol-3-yl)diazene]/L2 [L2: (E)-1,2-bis(4-iodo-1-methyl-1H-pyrazol-3-yl)diazene], respectively. Structural evaluation of the complexes affirmed the syn conformation of the coordinated/uncoordinated pyrazole groups of L and its unperturbed neutral azo (NN) state. Isomeric forms in [1a]ClO4/[1b]ClO4 or [2a]ClO4/[2b]ClO4 differed with respect to the cis and trans orientations of the coordinated CO and N(azo) donor of L, respectively. It also demonstrated the formation of intermolecular hydrogen-bonded dimeric or 1D-polymeric chains in [1a]ClO4/[2b]ClO4 or [1b]ClO4, respectively. Successive two-electron reductions of the complexes varied to an appreciable extent as a function of the heterocycles connected to L. The involvement of the azo function of L towards the reductions ([NN]0 → [NN]˙- → [NN]2-) was supported by the DFT calculated MOs and Mulliken spin density at the paramagnetic state, which was further validated by the radical EPR profile of the first reduced (S = 1/2) state. Isomeric [1a]ClO4/[1b]ClO4 or [2a]ClO4/[2b]ClO4 immobilised on the carbon cloth support underwent various electrochemical acidic HERs (hydrogen evolution reactions) with TOF/10-1 s-1: [1a]ClO4 (0.83) > [1b]ClO4 (0.68) > [2a]ClO4 (0.50) > [2b]ClO4 (0.37).

Tetracoordinated 15-Electron Ruthenium(I) in a Discrete Triruthenium Framework

This paper highlights the unique case of a tetracoordinated Ru(I) (15-electron) component in a structurally characterized discrete triruthenium setup, [(acac)2RuIIIL1(μ-RuI)L1RuII (acac)2](ClO4)2 ([3](ClO4)2, where acac = acetylacetonate; S = 1), which was formed along with the monomeric [(acac)2RuIII(L1)] ([1]ClO4; S = 1/2) and dimeric [{(acac)2RuIII}2(μ-L1)](ClO4)2 ([2](ClO4)2; S = 1) counterparts upon interaction of {Ru(acac)2} and L1 = 3,3'-dipyridin-2-yl-1,1'-bis(imidazo[1,5-a]pyridinyl).

Unique Metal-Ligand Interplay in Directing Discrete and Polymeric Derivatives of Isomeric Azole-Carboxylate. Varying Electronic Form, C-C Coupling, and Receptor Feature

This article dealt with the ruthenium and osmium derivatives of isomeric 1H-indazole-3-carboxylic acid/2H-indazole-3-carboxylic acid (H₂L1) and 1H-benzimidazole-2-carboxylic acid (H₂L2) along with the π-acidic bpy (bpy = 2,2'-bipyridine) and pap (pap = 2-phenylazopyridine) co-ligands. It thus extended structurally authenticated monomeric ([(bpy)₂Ru^II(HL1^-)]ClO₄ [1]ClO₄, (pap)₂Ru^II(L1^2-) 2, (bpy)₂Os^II(L1^2-) 3, (pap)₂Os^II(L1^2-) 4, (bpy)₂Ru^II(L2^2-) 5, (bpy)₂Os^II(L2^2-) 8, and (pap)₂Os^II(L2^2-) 9) and dimeric ([(bpy)₂Ru^II(μ-L2^2-)Ru^II(bpy)₂](ClO₄)₂ [6](ClO₄)₂) complexes. It also described modified L2'^2- (L2'^2- = 2,2'-bisbenzimidazolate)-bridged [(pap)₂Ru^II(μ-L2'^2-)Ru^II(pap)₂](ClO₄)₂ [7](ClO₄)₂, where L2'^2- was developed selectively with the {Ru(pap)₂} metal fragment via in situ intermolecular C-C coupling of the two units of decarboxylated benzimidazolate. Moreover, chemical oxidation (Os^II to Os^III) of (bpy)₂Os^II(L1^2-) 3 (E⁰ = 0.11 V versus SCE) and (bpy)₂Os^II(L2^2-) 8 (E⁰ = 0.12 V versus SCE) by AgClO₄ yielded unprecedented Os^III-Ag^I derived polymeric {[(bpy)₂Os^III-L1^2--Ag^I(CH₃CN)](ClO₄)₂}_n {[10](ClO₄)₂}_n and dimeric [(bpy)₂Os^III-L2^2--Ag^I(CH₃CN)](ClO₄)₂ [11](ClO₄)₂ complexes as a function of trans and cis orientations of the active N2 donor with special reference to the carboxylate O2 of L^2-, respectively. Microscopic (FE-SEM, TEM-EDX, and AFM) and DLS experiments suggested a homogeneously dispersed hollow spherical shaped morphology of {[10](ClO₄)₂}_n with an average particle size of 200-400 nm as well as its non-dissociative feature in the aprotic medium. Experimental (structure, spectroscopy, and electrochemistry) and theoretical (DFT/TD-DFT) explorations revealed a redox non-innocent feature of L^2- in the present coordination situations and the selective anion sensing (X = F^-, CN^-, and OAc^-) event of [1]ClO₄ involving a free NH group at the backface of HL1^-, which proceeded via the NH···X hydrogen bonding interaction.

Ruthenium Azobis(benzothiazole): Electronic Structure and Impact of Substituents on the Electrocatalytic Single-Site Water Oxidation Process

The present article deals with the structurally and spectroelectrochemically characterized newer class of ruthenium-azoheteroarenes [Ru^II(Ph-trpy)(Cl)(L)]ClO₄, [1]ClO₄-[3]ClO₄ (Ph-trpy: 4'-phenyl-2,2':6',2″-terpyridine; L1: 2,2'-azobis(benzothiazole) ([1]ClO₄); L2: 2,2'-azobis(6-methylbenzothiazole) ([2]ClO₄); L3: 2,2'-azobis(6-chlorobenzothiazole) ([3]ClO₄)). A collective consideration of experimental (i.e., structural and spectroelectrochemical) and theoretical (DFT calculations) results of [1]ClO₄-[3]ClO₄ established selective stabilization of (i) the unperturbed azo (N═N)⁰ function of L, (ii) the exclusive presence of the isomeric form involving the N(azo) donor of L trans to Cl, and (iii) the presence of extended, hydrogen-bonded trimeric units in the asymmetric unit of [2]ClO₄ (CH---O) via the involvement of ClO₄^- anions. The detailed electrochemical studies revealed metal-based oxidation of [Ru^II(Ph-trpy)(Cl)(L)]⁺ (1⁺-3⁺) to [Ru^III(Ph-trpy)(Cl)(L)]²⁺ (1²⁺-3²⁺); however, the electronic form of the first reduced state (1-3) could be better represented by its mixed Ru^II(Ph-trpy)(Cl)(L^•-)/Ru^III(Ph-trpy)(Cl)(L^2-) state. Both native (1⁺-3⁺) and reduced (1-3) states exhibited weak lower energy transitions within the range of 1000-1200 nm. Further, [1]ClO₄-[3]ClO₄ delivered an electrochemical OER (oxygen evolution reaction) process in alkaline medium on immobilizing them to a carbon cloth support, which divulged an amplified water oxidation feature for [2]ClO₄ due to the presence of electron-donating methyl groups in the L2 backbone. The faster OER kinetics and high catalytic stability of [2]ClO₄ could also be rationalized by its lowest Tafel slope (85 mV dec^-1) and choronoamperometric experiment (stable up to 12 h), respectively, along with high Faradic efficiency (∼97%). A comparison of [2]ClO₄ with the reported analogous ruthenium complexes furnished its excellent intrinsic water oxidation activity.

Inner-Sphere Electron Transfer Induced Reversible Electron Reservoir Feature of Azoheteroarene Bridged Diruthenium Frameworks

This article demonstrates the stabilization of ground- and redox-induced metal-to-ligand charge transfer excited states on coordination of azo-coupled 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)₂Ru^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 L4^•- bridged isovalent higher analogue [(acac)₂Ru^III(μ-L4^•-)Ru^III(acac)₂]ClO₄ in diastereomeric forms, rac-[2a]ClO₄/meso-[2b]ClO₄. On 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)₂Ru^II(μ-L4)Ag^I(μ-L4)Ru^II(acac)₂]ClO₄ ([4]ClO₄) involving unreduced L4 via the reaction of 1 and AgClO₄ 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.

On the Question of S-S Bond Cleavage of 2,2'-Dithiodipyridine on Selective Ru and Os Platforms. MLCT or Hydride or Solvent Mediated Event

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: Ru^II(acac)₂, [Ru^IICl₂(PPh₃)₃], [Ru^IIHCl(CO)(PPh₃)₃], [Ru^II(H)₂(CO)(PPh₃)₃], [Ru^II(bpy)₂Cl₂], [Ru^II(pap)₂Cl₂], [Os^II(bpy)₂Cl₂], and [Os^II(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) (Ru^II → σ* of DTDP)-driven metal oxidation (Ru^II → Ru^III) process in the case of relatively electron-rich metal fragments {Ru^II(acac)₂} or Ru^IICl₂ 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.

Bidirectional noninnocence of hinge-like deprotonated bis-lawsone on selective ruthenium platform: a function of varying ancillary ligands

The present work aimed to obtain discrete heavier metal complexes of unperturbed deprotonated bis-lawsone (hinge-like H₂L = 2,2'-bis(3-hydroxy-1,4-napthoquinone). This is primarily due to its limited examples with lighter metal ions (Co, Zn, and Ga) and the fact that our earlier approach with the osmium ion facilitated its functionalisation. Herein, we demonstrated the successful synthesis and structural characterisation of L^2--derived diruthenium [(bpy)₂Ru^II(μ-L^2-)Ru^II(bpy)₂](ClO₄)₂ [1](ClO₄)₂ (S = 0), (acac)₂Ru^III(μ-L^2-)Ru^III(acac)₂2 (S = 1) and monoruthenium (pap)₂Ru(L^2-) 3 (S = 0) derivatives (bpy = 2,2'-bipyridine, acac = acetylacetonate, and pap = 2-phenylazopyridine). The crystal structures established that (i) O,O^-/O,O^- donating five-membered bis-bidentate and O^-,O^- donating seven-membered bidentate chelating modes of deprotonated L^2- in rac (ΔΔ/ΛΛ) diastereomeric [1](ClO₄)₂, 2 and 3, respectively. (ii) The L^2- bridging unit in [1](ClO₄)₂, 2 and 3 underwent twisting its two naphthoquinone rings with respect to the ring connecting C-C bond by 73.01°, 62.15° and 59.12°, respectively. (iii) Intermolecular π-π interactions (∼3.5 Å) between the neighbouring molecules. The paramagnetic complex 2 (S = 1) with two non-interacting Ru(III) (S = 1/2) ions exhibited weak antiferromagnetic coupling only at very low temperatures. In agreement with the magnetic results, 2 displayed typical Ru^III-based anisotropic EPR in CH₃CN (<g>/Δg: 2.314/0.564) but without any forbidden g_1/2 signal at 120 K. The complexes exhibited multiple redox processes in CH₃CN in the experimental potential window of ± 2.0 V versus SCE. The analysis of the redox steps via a combined experimental and theoretical (DFT/TD-DFT) approach revealed the involvement of L^2- to varying extents in both the oxidative and reductive processes as a consequence of its bidirectional redox non-innocent feature. The mixing of the frontier orbitals of the metal ion and L^2- due to their closeness in energy indeed led to the resonating electronic form in certain redox states instead of any precise electronic structural state.

Rhodium Diamidobenzene Complexes: A Tale of Different Substituents on the Diamidobenzene Ligand

Diamidobenzene ligands are a prominent class of redox-active ligands owing to their electron reservoir behaviour, as well as the possibility of tuning the steric and the electronic properties of such ligands through the substituents on the N-atoms of the ligands. In this contribution, we present Rh(iii) complexes with four differently substituted diamidobenzene ligands. By using a combination of crystallography, NMR spectroscopy, electrochemistry, UV-vis-NIR/EPR spectroelectrochemistry, and quantum chemical calculations we show that the substituents on the ligands have a profound influence on the bonding, donor, electrochemical and spectroscopic properties of the Rh complexes. We present, for the first time, design strategies for the isolation of mononuclear Rh(ii) metallates whose redox potentials span across more than 850 mV. These Rh(ii) metallates undergo typical metalloradical reactivity such as activation of O₂ and C–Cl bond activations. Additionally, we also show that the substituents on the ligands dictate the one versus two electron nature of the oxidation steps of the Rh complexes. Furthermore, the oxidative reactivity of the metal complexes with a [CH₃]⁺ source leads to the isolation of a unprecedented, homobimetallic, heterovalent complex featuring a novel π-bonded rhodio-o-diiminoquionone. Our results thus reveal several new potentials of the diamidobenzene ligand class in organometallic reactivity and small molecule activation with potential relevance for catalysis.

Diamidobenzene ligands are versatile platforms in organometallic Rh-chemistry. They allow the isolation of tunable mononuclear ate-complexes, and the formation of a unprecedented homobimetallic, heterovalent complex.

Radical versus Nonradical States of Azobis(benzothiazole) as a Function of Ancillary Ligands on Selective Ruthenium Platforms

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 (1ⁿ) and [Ru(bpy)₂(abbt)]ⁿ (2ⁿ), where pap = 2-phenylazopyridine and bpy = 2,2'-bipyridine. In this regard, the complexes [Ru^II(pap)₂(abbt^•-)]ClO₄ ([1]ClO₄), [Ru^II(pap)₂(abbt⁰)](ClO₄)₂ ([1](ClO₄)₂), [Ru^II(bpy)₂(abbt⁰)](ClO₄)₂ ([2](ClO₄)₂), and [Ru^II(bpy)₂(abbt^•-)]ClO₄ ([2]ClO₄) were structurally and spectroscopically characterized. Unambiguous assignments of the aforestated radical and nonradical forms of abbt in 1⁺/2⁺ and 1²⁺/2²⁺, 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 abbt⁰ in 2²⁺, along with the radical form in 2⁺ as a minor (<10%) component. The oxidized abbt⁰-derived [1](ClO₄)₂ was, however, obtained via the chemical oxidation of [1]ClO₄. Both 1⁺ and 2²⁺ 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 2²⁺ as well as facile reduction of pap and abbt in 1⁺ and 2⁺, respectively.

Chiroptical switching behavior of heteroleptic ruthenium complexes bearing acetylacetonato and tropolonato ligands

Four types of tris-chelate ruthenium complexes bearing acetylacetonato (acac) and tropolonato (trop) ligands were synthesized and optically resolved into Δ and Λ isomers: [Ru(acac)₃] (Ru-0), [Ru(acac)₂(trop)] (Ru-1), [Ru(acac)(trop)₂] (Ru-2), and [Ru(trop)₃] (Ru-3). Chiral HPLC chromatograms, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) of the four ruthenium complexes were systematically investigated. As a result, the absolute configurations of the newly prepared enantiomeric complexes Ru-2 and Ru-3 were determined. For the case of Ru-2, its absolute configuration was also confirmed by single crystal X-ray diffraction analysis. The ECD changes upon chemical oxidation were further investigated for the four complexes. An ECD change in enantiomeric Ru-1 was observed upon oxidation, but the oxidized species soon returned to the neutral state within a few minutes. Enantiomers of Ru-3 also showed explicit ECD changes upon oxidation. Further, the lifetime of the oxidized state was the longest among the four investigated complexes, whereas they racemized in solution at room temperature. In contrast, the enantiomers of heteroleptic complexes (Ru-1 and Ru-2) concurrently exhibited ECD changes, relatively long lifetime of the oxidized state, and nil or quite slow racemization behavior. The coexistence of acac and trop ligands was key to making the competing factors compatible in the resultant ruthenium complexes.

Ruthenium-Benzothiadiazole Building Block Derived Dynamic Heterometallic Ru-Ag Coordination Polymer and Its Enhanced Water-Splitting Feature

This article deals with the development of the unprecedented redox-mediated heterometallic coordination polymer {[Ru^III(acac)₂(μ-bis-η¹-N,η¹-N-BTD)₂Ag^I(ClO₄)]ClO₄}_n (3) via the oxidation of the monomeric building block cis-[Ru^II(acac)₂(η¹-N-BTD)₂] (1) by AgClO₄ (BTD = exodentate 2,1,3-benzothiadiazole, acac = acetylacetonate). Monomeric cis-[Ru^II(acac)₂(η¹-N-BTD)₂] (1) and [Ru^II(acac)₂(η¹-N-BTD)(CH₃CN)] (2) were simultaneously obtained from the electron-deficient BTD heterocycle and the electron-rich metal precursor Ru^II(acac)₂(CH₃CN)₂ in refluxing CH₃CN. Molecular identities of 1-3 were authenticated by their single-crystal X-ray structures as well as by solution spectral features. These results also reflected the elusive trigonal-planar geometry of the Ag ion in Ru-Ag-derived polymeric 3. Ru(III) (S = 1/2)-derived 3 displayed metal-based anisotropic EPR with ⟨g⟩/Δg = 2.12/0.56 and paramagnetically shifted ¹H NMR. Spectroelectrochemistry in combination with DFT/TD-DFT calculations of 1ⁿ and 2ⁿ (n = 1+, 0, 1-) determined a metal-based (Ru^II/Ru^III) oxidation and BTD-based reduction (BTD/BTD^•-). The drastic decrease in the emission intensity and quantum yield but insignificant change in the lifetime of 3 with respect to 1 could be addressed in terms of static quenching and/or a paramagnetism-induced phenomenon. A homogeneously dispersed dumbbell-shaped morphology and the particle diameter of 3 were established by microscopic (TEM-EDX/SEM) and DLS analysis, respectively. Moreover, the dynamic nature of polymeric 3 was highlighted by its degradation to the η¹-N-BTD coordinated monomeric fragment 1, which could also be followed spectrophotometrically in polar protic EtOH. Interestingly, both monomeric 1 and polymeric 3 exhibited efficient electrocatalytic activity toward water oxidation processes (OER, HER) on immobilization on an FTO support, which also divulged the better intrinsic water oxidation activity of 3 in comparison to 1.

Variable electronic structure and spin distribution in bis(2,2'-bipyridine)-metal complexes (M = Ru or Os) of 4,5-dioxido- and 4,5-diimido-pyrene

The odd-electron compounds [M(bpy)₂(L¹)](ClO₄) M = Ru ([1](ClO₄)) or Os ([2](ClO₄)), and the even-electron species [M(bpy)₂(H₂L²)](ClO₄)₂, M = Ru ([3](ClO₄)₂) or Os ([4](ClO₄)₂) were obtained from pyrene-4,5-dione, L¹, or 4,5-diaminopyrene, H₄L², and were characterised structurally, electrochemically and spectroscopically. Experimental and computational analysis (TD-DFT) revealed rather different electronic structures and spin distributions of the paramagnetic monocations 1⁺-4⁺. EPR investigations and electronic absorption studies exhibit increasing metal contributions to the singly occupied MO along the series 1⁺ < 3⁺ < 4⁺ < 2⁺, illustrated by g value and long-wavelength absorbance. In addition to variations of the metal (Ru,Os) and the donor atoms (O,NH) the extension of the π system of the semiquinone-type ligand has a large effect on the electronic structure of the paramagnetic cations.

On the non-innocence and reactive versus non-reactive nature of α-diketones in a set of diruthenium frameworks

Slightly modified ligand designs in diruthenium setups have major impacts on the reactivity/stability of coordination complexes. The 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione bridge is also potentially redox non-innocent.

Noninnocence of the deprotonated 1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine bridge in diruthenium frameworks - a function of co-ligands

The article deals with the sensitive electronic forms in accessible redox states of structurally and spectroscopically authenticated deprotonated 1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine (H2LR, R = H) or 1,2-bis((3,5-dimethyl-1H-pyrrol-2-yl)methylene)hydrazine (H2LR, R = Me), a BODIPY analogue bridged diruthenium complex as a function of varying ancillary ligands. It involved rac-(acac)2RuIII(μ-LR 2-)RuIII(acac)21a, R = H; 1b, R = Me (S = 1, acac = acetylacetonate), rac-[(bpy)2RuII(μ-L2-)RuII(bpy)2](ClO4)2 [2](ClO4)2 (S = 0, bpy = 2,2'-bipyridine) and diastereomeric [(pap)2RuII(μ-L2-)RuII(pap)2](ClO4)2meso-[3a](ClO4)2/rac-[3b](ClO4)2 (S = 0, pap = phenylazopyridine). The crystal structure established the linkage of the conjugated -C5[double bond, length as m-dash]N2-N3[double bond, length as m-dash]C6- central unit with the two terminal deprotonated pyrrole units of coordinated L2-. The bridging L2- in 1a, 1b, [2](ClO4)2, [3b](ClO4)2 and [3a](ClO4)2 was slightly twisted and planar with torsional angles of 41.54°, 42.91°, 37.38°, 35.33° and 0°, respectively, with regard to the central N2-N3 bond. The extent of twisting of the bridge followed an inverse relationship with the RuRu separation: 4.935/4.934 Å 1a/1b < 5.141 Å [2](ClO4)2 < 5.201 Å [3b](ClO4)2 < 5.351 Å [3a](ClO4)2. This is also attributed to the intermolecular ππ/CHπ interactions between the nearby aromatic rings of L and bpy or pap in [2](ClO4)2 or [3](ClO4)2, respectively. The multiple redox steps of the complexes varied appreciably based on the σ-donating (acac) and π-acidic (bpy, pap) characteristics of the ancillary ligands. Experimental (structure, EPR) and theoretical (DFT) evaluation pertaining to the electronic forms of 1n, 2n and 3n demonstrated the preferential involvement of L based frontier orbitals in electron transfer processes even in combination with the redox facile ruthenium ion. This in turn highlighted its redox non-innocent feature as in the case of well-documented metal coordinated quinonoid, formazanate, diimine (bpy), azo (pap) and β-diketiminate functions.

Unprecedented metal-metal bonded {Ru4(μ3-O)2} butterfly core in oxido-carboxylato bridged mixed valence cluster-structural elucidation and electronic forms in accessible redox states

Unprecedented metal-metal bonded oxido-carboxylato bridged mixed valence tetraruthenium cluster [(acac)₆Ru(μ₃-O)₂(μ-CH₃COO)₃] 1 (S = 1/2) (acac = acetylacetonate) with {Ru₄(μ₃-O)₂} "butterfly" core has been achieved via the reaction of Ru(acac)₂(CH₃CN)₂ with excess CH₃COONa (Ru : CH₃COONa = 1 : 15) in refluxing EtOH-H₂O (5 : 1). Structural analysis of 1 ascertained a unique Ru-Ru bonded (Ru2-Ru3: 2.5187(6) Å (DFT: 2.560 Å)) {Ru₂(Ru-Ru)(μ₃-O)₂} butterfly core, unlike the reported other Fe₄ or Mn₄ derived "butterfly" core. The connectivity of Ru₄(μ₃-O)₂ core in 1 with three Ru₂(μ-CH₃COO)₃ and two each Ru-(acac)₂/Ru-acac units resulted in interconnected four RuO₆ octahedral entities. The doubly bridged μ₃-O^2- ions of the nearly planar central metal-metal bonded Ru₂O₂ core (Ru2-Ru3, "body" or "hinge") linked to the remaining two "wing-tip" Ru atoms (Ru1 and Ru4). Complex 1 with a S = 1/2 spin state displayed paramagnetically shifted ¹H NMR over a wide chemical shift range in CDCl₃ (δ, 13 to -30 ppm) and a metal based anisotropic EPR (g₁ = 2.17, g₂ = 2.01, g₃ = 1.86; Δg = g₁-g₃ = 0.31 and 〈g〉 = [1/3(g₁² + g₂² + g₃²]^1/2 = 2.01) at 100 K in CH₃CN-toluene. The metal based one-electron reversible oxidation at 0.49 V and reduction at 0.485 V versus SCE of 1 led to the EPR inactive (even at 4 K) spin-coupled Ru^IIIRu^IIIRu^IVRu^IV (S = 0) and Ru^IIIRu^IIIRu^IIIRu^III (S = 0) electronic configurations for 1⁺ and 1^-, respectively. Mixed valence 1 and 1⁺ exhibited low-energy near-infrared (NIR) absorption bands at 1350 nm and 1156 nm, respectively, in CH₃CN. A combined experimental (UV-vis-NIR and EPR spectroelectrochemistry) and theoretical (DFT) analysis indicated a delocalised mixed valence form of 1 ({Ru₂(Ru-Ru)(μ₃-O)₂}).

Tris(tropolonato) ruthenium as a hub for connecting π-conjugated systems

In this study, the intramolecular electronic communication between π-conjugated moieties bridged by the tris-chelate [Ru(trop)₃] (trop = tropolonate) framework has been investigated and compared with [Ru(acac)₃] (acac = acetylacetonate) derivatives. Two types of π-conjugated groups, -C₂SiMe₃ and -C₂Ph, which were each introduced at the 5-position of tropolonate, were found to behave almost independently in the resultant ruthenium complexes. In contrast, the cyclic voltammetry study of [Ru(trop)₃] derivatives showed a clear decrease in ΔE, the difference in the potentials of the two redox couples, with an increase in the number of π-conjugated groups introduced into the [Ru(trop)₃] framework. The lower ΔE values observed in [Ru(trop)₃] derivatives compared to those in [Ru(acac)₃] derivatives illustrated the non-innocent behavior of the tropolonate ligand, i.e., the mixing between the metal- and ligand-based frontier orbitals. This orbital mixing was exemplified in the oxidized [Ru(trop)₃] derivatives, which showed a broad near-infrared (NIR) absorption. The increase in the red shift of the NIR absorption with the increasing number of terminal groups indicated intramolecular electronic communication between the terminal π-conjugated moieties. In contrast, no NIR absorption was observed upon oxidation of [Ru(acac)₃] derivatives with -C₂Ph groups. The lifetimes of the in situ formed [Ru(trop)₃]⁺ and [Ru(acac)₃]⁺ in acetonitrile solution were found to be several hours and minutes, respectively. Density functional theory calculations for [Ru(trop)₃] and [Ru(acac)₃] with terminal -C₂Ph groups demonstrated that the spin densities in their mono-oxidized states were evenly distributed over the entire molecular framework and localized mainly on one ligand, respectively. These results confirmed that the mono-oxidized [Ru(trop)₃]⁺ framework can behave as a hub and induce moderate intramolecular electronic communication between terminal π-conjugated groups.

Tuning PtII -Based Donor-Acceptor Systems through Ligand Design: Effects on Frontier Orbitals, Redox Potentials, UV/Vis/NIR Absorptions, Electrochromism, and Photocatalysis

Asymmetric platinum donor-acceptor complexes [(pimp)Pt(Q2- )] are presented in this work, in which pimp=[(2,4,6-trimethylphenylimino)methyl]pyridine and Q2- =catecholate-type donor ligands. The properties of the complexes are evaluated as a function of the donor ligands, and correlations are drawn among electrochemical, optical, and theoretical data. Special focus has been put on the spectroelectrochemical investigation of the complexes featuring sulfonyl-substituted phenylendiamide ligands, which show redox-induced linkage isomerism upon oxidation. Time-dependent density functional theory (TD-DFT) as well as electron flux density analysis have been employed to rationalize the optical spectra of the complexes and their reactivity. Compound 1 ([(pimp)Pt(Q2- )] with Q2- =3,5-di-tert-butylcatecholate) was shown to be an efficient photosensitizer for molecular oxygen and was subsequently employed in photochemical cross-dehydrogenative coupling (CDC) reactions. The results thus display new avenues for donor-acceptor systems, including their role as photocatalysts for organic transformations, and the possibility to introduce redox-induced linkage isomerism in these compounds through the use of sulfonamide substituents on the donor ligands.

Chelate rings of different sizes with non-innocent ligands

Redox-active unsaturated chelate ligands can be realised with different ring sizes of the resulting metallacycles. An overview is presented, starting from an exposition of non-innocent behaviour and chelate effects. A systematic approach is used to describe the most familiar situation, the metal complexes of 1,4-hetero-1,3-dienes in established forms (e.g. o-quinone, α-dithiolene, and α-diimine ligands) and with less common combinations of O, S, and N heteroatoms. The different steric and electronic conditions in six-membered chelate ring systems derived from the β-diketonate structure will be discussed with examples of substituted and π extended ligands, including 9-oxidophenalenyl, formazanate, and anions derived from indigo or 9,10-anthraquinone. Four-membered chelate rings existing in at least two ligand-based oxidation states are available through steric and electronic stabilisation in amidinate or triazenide complexes. Three-membered and seven-membered chelate ring situations are discussed briefly as further alternatives.

A coordinatively unsaturated iridium complex with an unsymmetrical redox-active ligand: (spectro)electrochemical and reactivity studies

Metal–ligand cooperativity can be used in iridium complexes with an unsymmetrically substituted redox-active diamidobenzene ligand for bond activation reactions.

Diverse modes of functionalisation of ruthenium coordinated β-ketoiminate analogues

Varying chelation assisted as well as solvent dependent reactivity profiles of isostructural β-ketoiminate analogues explicate their non-spectator behaviour and fractional redox non-innocence.

Impact of {Os(pap)2} in fine-tuning the binding modes and non-innocent potential of deprotonated 2,2'-bipyridine-3,3'-diol

The reaction of ctc-Os(II)(pap)2Cl2 (pap = 2-phenylazopyridine, ctc = cis-trans-cis with respect to chlorides and pyridine/azo nitrogens of pap, respectively) and ambidentate 2,2'-bipyridine-3,3'-diol (H2L) leads to the simultaneous formation of isomeric [Os(II)(pap)2(HL(-))](+) ((2+)/(3+)), seven-membered chelate containing Os(II)(pap)2(L(2-)) (4) and diastereomeric [{Os(II)(pap)2}2(μ-L(2-))](2+) (5a(2+) (meso, ΔΛ)/5b(2+) (rac, ΔΔ/ΛΛ)). The reaction of 2,2'-biphenol (H2L') and ctc-Os(II)(pap)2Cl2 yields Os(II)(pap)2(L'(2-)) (6), an analogue of 4. The identities of the newly designed complexes have been established by different analytical, spectroscopic and X-ray diffraction techniques. (1)H-NMR spectra of the complexes and single crystal X-ray structures of selective derivatives [2]ClO4, [3]ClO4, [5a](ClO4)2, and 6 establish the retention of the tc-configuration of the precursor {Os(pap)2}. In isomeric 2(+) and 3(+), monodeprotonated HL(-) is linked to the {Os(II)(pap)2} fragment through N,N and N,O(-) donors, resulting in nearly planar five- and six-membered chelates with O-HO(-) and O-HN hydrogen bonds at its back face, respectively. The O(-),O(-) donating L'(2-) extends a severely twisted seven-membered chelate with the {Os(pap)2} unit in 6. The N,O(-)/O(-),N donors of deprotonated L(2-) bridge the two {Os(II)(pap)2} units in a symmetric fashion in 5a(2+), forming two moderately twisted six-membered chelates. Though the deprotonation of the O-HN hydrogen bond in (+) by another unit of {Os(II)(pap)2} generates a diastereomeric mixture of 5a(2+) and 5b(2+), attempts to deprotonate the relatively stronger O-H···O(-) hydrogen bond in 2(+) have failed. The isomeric 2(+)/3(+), seven-membered chelate containing 4/6 and diastereomeric 5a(2+)/5b(2+) exhibit distinctive (1)H-NMR and absorption spectra as well as electrochemical responses. The pap (N[double bond, length as m-dash]N) based two successive reductions and the participation of HL(-), L(2-), L'(2-) in the oxidation processes of the respective complexes have been revealed using EPR and DFT calculated MOs and Mulliken spin density plots at the intermediate paramagnetic states.