Ruthenium Derivatives of in Situ Generated Redox-Active 1,2-Dinitrosobenzene and 2‑Nitrosoanilido. Diverse Structural and Electronic Forms

The article describes one-pot synthesis and structural elucidation of tc-[RuII(pap)2(L•–)]­ClO4 [1]­ClO4 and tc-[RuII(pap)2(L′–)]­ClO4 [2]­ClO4, which were obtained from tc-[RuII(pap)2(EtOH)2]­(ClO4)2 and benzofuroxan (L = 1,2-dinitrosobenzene, an intermediate tautomeric form of the biologically active benzofuroxan, L′– = 2-nitrosoanilido, pap = 2-phenylazopyridine, tc = trans and cis corresponding to pyridine and azo nitrogen donors of pap, respectively). The same reaction with the newly synthesized and structurally characterized metal precursor cc-RuII(2,6-dichloropap)2Cl2, however, affords isomeric ct-[RuII(2,6-dichloropap)2(L•–)]+ (3a+) and tc-[RuII(2,6-dichloropap)2(L•–)]+ (3b+) (cc, ct, and tc with respect to pyridine and azo nitrogens of 2,6-dichloropap) with the structural authentication of elusive ct-isomeric form of {Ru­(pap)2} family. The impact of trans or cis orientation of the nitroso group of L/L′ with respect to the NN (azo) function of pap in the complexes was reflected in the relative lengthening or shortening of the latter distance, respectively. The redox-sensitive bond parameters of 1+ and 3+ reveal the intermediate radical form of L•–, while 2+ involves in situ generated L′–. The multiple redox processes of the complexes in CH3CN are analyzed via experimental and density functional theory (DFT) and time-dependent DFT calculations. One-electron oxidation of the electron paramagnetic resonance-active radical species (1+ and 3+) leads to [RuII(pap)2(L)]2+ involving fully oxidized L0 in 12+ and 32+; the same in 2+ results in a radical species [RuII(pap)2(L′•)]2+ (22+). Successive reductions in each case are either associated with pap or L/L′–-based orbitals, revealing a competitive scenario relating to their π-accepting features. The isolated or electrochemically generated radical species either by oxidation or reduction exhibits near-IR transitions in each case, attributing diverse electronic structures of the complexes in accessible redox states

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)

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^–) (<b>1</b>; blue solid) and diamagnetic Ru^II(acac)₂(L) (<b>2</b>; red solid), which have been characterized by standard analytical, spectroscopic, and structural analysis. Crystal structures establish that the usual <i>trans</i> configurated and twisted monodeprotonated HL^– and unprecedented <i>cis</i> 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 <b>1</b>. The specific role of the electron-rich {Ru­(acac)₂} metal fragment in stabilizing the <i>cis</i>-configuration of the electron-deficient L in <b>2</b> has been pointed out. Both <b>1</b> and <b>2</b> exhibit reversible one-electron oxidation and successive three reductions with varying <i>K</i>_c (comproportionation constant) values in the range of 10¹⁸–10⁶. The potentials for the redox processes of <b>2</b> are positively shifted with respect to those of <b>1</b>. The involvement of the metal or HL^–/L or mixed metal-HL^–/L-based orbitals in the accessible redox processes of <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> 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 <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> (<i>n</i> = +1, 0, −1, −2, −3)

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^–) (<b>1</b>; blue solid) and diamagnetic Ru^II(acac)₂(L) (<b>2</b>; red solid), which have been characterized by standard analytical, spectroscopic, and structural analysis. Crystal structures establish that the usual <i>trans</i> configurated and twisted monodeprotonated HL^– and unprecedented <i>cis</i> 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 <b>1</b>. The specific role of the electron-rich {Ru­(acac)₂} metal fragment in stabilizing the <i>cis</i>-configuration of the electron-deficient L in <b>2</b> has been pointed out. Both <b>1</b> and <b>2</b> exhibit reversible one-electron oxidation and successive three reductions with varying <i>K</i>_c (comproportionation constant) values in the range of 10¹⁸–10⁶. The potentials for the redox processes of <b>2</b> are positively shifted with respect to those of <b>1</b>. The involvement of the metal or HL^–/L or mixed metal-HL^–/L-based orbitals in the accessible redox processes of <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> 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 <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> (<i>n</i> = +1, 0, −1, −2, −3)

Ruthenium Derivatives of in Situ Generated Redox-Active 1,2-Dinitrosobenzene and 2‑Nitrosoanilido. Diverse Structural and Electronic Forms

The article describes one-pot synthesis and structural elucidation of <i>tc</i>-[Ru^II(pap)₂(L^•–)]­ClO₄ [<b>1</b>]­ClO₄ and <i>tc</i>-[Ru^II(pap)₂(L′^–)]­ClO₄ [<b>2</b>]­ClO₄, which were obtained from <i>tc</i>-[Ru^II(pap)₂(EtOH)₂]­(ClO₄)₂ and benzofuroxan (L = 1,2-dinitrosobenzene, an intermediate tautomeric form of the biologically active benzofuroxan, L′^– = 2-nitrosoanilido, pap = 2-phenylazopyridine, <i>tc</i> = <i>trans</i> and <i>cis</i> corresponding to pyridine and azo nitrogen donors of pap, respectively). The same reaction with the newly synthesized and structurally characterized metal precursor <i>cc</i>-Ru^II(2,6-dichloropap)₂Cl₂, however, affords isomeric <i>ct</i>-[Ru^II(2,6-dichloropap)₂(L^•–)]⁺ (<b>3a</b>⁺) and <i>tc</i>-[Ru^II(2,6-dichloropap)₂(L^•–)]^<b>+</b> (<b>3b</b>⁺) (<i>cc</i>, <i>ct</i>, and <i>tc</i> with respect to pyridine and azo nitrogens of 2,6-dichloropap) with the structural authentication of elusive <i>ct</i>-isomeric form of {Ru­(pap)₂} family. The impact of <i>trans</i> or <i>cis</i> orientation of the nitroso group of L/L′ with respect to the NN (azo) function of pap in the complexes was reflected in the relative lengthening or shortening of the latter distance, respectively. The redox-sensitive bond parameters of <b>1</b>⁺ and <b>3</b>⁺ reveal the intermediate radical form of L^•–, while <b>2</b>⁺ involves in situ generated L′^–. The multiple redox processes of the complexes in CH₃CN are analyzed via experimental and density functional theory (DFT) and time-dependent DFT calculations. One-electron oxidation of the electron paramagnetic resonance-active radical species (<b>1</b>⁺ and <b>3</b>⁺) leads to [Ru^II(pap)₂(L)]²⁺ involving fully oxidized L⁰ in <b>1</b>²⁺ and <b>3</b>²⁺; the same in <b>2</b>⁺ results in a radical species [Ru^II(pap)₂(L′^•)]²⁺ (<b>2</b>²⁺). Successive reductions in each case are either associated with pap or L/L′^–-based orbitals, revealing a competitive scenario relating to their π-accepting features. The isolated or electrochemically generated radical species either by oxidation or reduction exhibits near-IR transitions in each case, attributing diverse electronic structures of the complexes in accessible redox states

Host–Guest Feature of DPPP Bridged Arene–Ruthenium Clip Derived Molecular Rectangle

The development of DPPP2– (H2DPPP = 2,5-dihydro-3,6-di-2-pyridylpyrrolo­(3,4-c)­pyrrole-1,4-dione) bridged (NN∩NN) diruthenium complexes [(Cym)­(X)­RuII(μ-dppp)­RuII(X)­(Cym)] (Cym = para-cymene and X = OTf– (1), SCN– (2), N3– (3), NO2–(4)) are considered as the probable molecular clips for the construction of metallarectangle. Crystal structures of 2–4 established anticonfiguration with respect to monodentate SCN–, N3– and NO2– groups, respectively. Though molecular clips 2–4 failed to provide the desired metallarectangle in combination with the 4,4′-bipyridine spacer, 1 with the labile OTf groups facilitated to achieve the metallarectangle 5. The crystal structure of 5 revealed that two twisted 4,4′-bipyridine spacers bridged between the two units of dimeric 1 in symmetric fashion, which in effect led to the newer class of molecular rectangle 5 with a hydrophobic cavity size of the cationic host of 8.32 × 11.11 Å2. Furthermore, the host–guest interaction potential of 5 with special reference to the guest molecule, pyrene, was explored. The crystal structure of the resultant molecule 6 ascertained the partial encapsulation of two pyrene molecules inside the hydrophobic cavity of 5, due to the twisted 4,4′-bipyridine spacer units between the two ruthenium clips. It also attributed a noncovalent CH−π interaction involving protons of pyrene and the π-electron cloud of 4,4′-bipyridine as well as a weak interaction between pyrene protons and the pendant CO group of DPPP. Encapsulation of the guest molecule (pyrene) inside the cavity of the metallarectangle was also monitored by following the quenching of florescent intensity of pyrene on addition of 5

Mixed-Valent Ru^IIIRu^IV Configuration in an Oxido–Carboxylato-Bridged Diastereomeric Pair

An unprecedented diastereomeric pair [meso, ΔΛ (1); rac, ΔΔ/ΛΛ (2)] involving a doubly oxido–carboxylato-bridged mixed-valent RuIIIRuIV (d5d4, S = 1/2) state in [(acac)2RuIII(μ-O)­(μ-CH3COO)­RuIV(acac)2] (acac = acetylacetonate) was structurally characterized. 1n and 2n (n = +, 0, −) display comparable spectroelectrochemical features for the accessible redox states

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^–) (<b>1</b>; blue solid) and diamagnetic Ru^II(acac)₂(L) (<b>2</b>; red solid), which have been characterized by standard analytical, spectroscopic, and structural analysis. Crystal structures establish that the usual <i>trans</i> configurated and twisted monodeprotonated HL^– and unprecedented <i>cis</i> 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 <b>1</b>. The specific role of the electron-rich {Ru­(acac)₂} metal fragment in stabilizing the <i>cis</i>-configuration of the electron-deficient L in <b>2</b> has been pointed out. Both <b>1</b> and <b>2</b> exhibit reversible one-electron oxidation and successive three reductions with varying <i>K</i>_c (comproportionation constant) values in the range of 10¹⁸–10⁶. The potentials for the redox processes of <b>2</b> are positively shifted with respect to those of <b>1</b>. The involvement of the metal or HL^–/L or mixed metal-HL^–/L-based orbitals in the accessible redox processes of <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> 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 <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> (<i>n</i> = +1, 0, −1, −2, −3)

Example of Highly Stereoregulated Ruthenium Amidine Complex Formation: Synthesis, Crystal Structures, and Spectral and Redox Properties of the Complexes [Ru^II(trpy){NC₅H₄CHNN(C₆H₅)C(CH₃)NH}](ClO₄)₂ (<b>1</b>) and [Ru^II(trpy)(NC₅H₄CHNNHC₆H₅)Cl]ClO₄ (<b>2</b>) (trpy = 2,2‘:6‘,2‘ ‘-Terpyridine)

The reaction of Ru(trpy)Cl3 (trpy = 2,2‘:6‘,2‘ ‘-terpyridine) with the pyridine-based imine function NpC5H4−CHNi-NH−C6H5 (L), incorporating an NH spacer between the imine nitrogen (Ni) and the pendant phenyl ring, in ethanol medium followed by chromatographic work up on a neutral alumina column using CH3CN/CH2Cl2 (1:4) as eluent, results in complexes of the types [Ru(trpy)(L‘)](ClO4)2 (1) and [Ru(trpy)(L)Cl]ClO4 (2). Although the identity of the free ligand (L) has been retained in complex 2, the preformed imine-based potentially bidentate ligand (L) has been selectively transformed into a new class of unusual imine−amidine-based tridentate ligand, NpC5H4CHNiN(C6H5)C(CH3)NaH (L‘), in 1. The single-crystal X-ray structures of the free ligand (L) and both complexes 1 and 2 have been determined. In 2, the sixth coordination site, that is, the Cl- function, is cis to the pyridine nitrogen (Np) of L which in turn places the NH spacer away from the RuCl bond, whereas, in 1, the corresponding sixth position, that is, the RuNa (amidine) bond, is trans to the pyridine nitrogen (Np) of L‘. The trans configuration of Na with respect to the Np of L‘ in 1 provides the basis for the selective L → L‘ transformation in 1. The complexes exhibit strong Ru(II) → π* (trpy) MLCT transitions in the visible region and intraligand transitions in the UV region. The lowest energy MLCT band at 510 nm for 2 has been substantially blue-shifted to 478 nm in the case of 1. The reversible Ru(III)−Ru(II) couples for 1 and 2 have been observed at 0.80 and 0.59 V versus SCE, respectively. The complexes are weakly luminescent at 77 K, exhibiting emissions at λmax, 598 nm [quantum yield (Φ) = 0.43 × 10-2 ] and 574 nm (Φ = 0.28 × 10-2 ) for 1 and 2, respectively

Indazole-Derived Mono-/Diruthenium and Heterotrinuclear Complexes: Switchable Binding Mode, Electronic Form, and Anion Sensing Events

The article deals with the newer classes of mononuclear: [(acac)2RuIII(H-Iz)(Iz–)] 1, [(acac)2RuIII(H-Iz)2]ClO4 [1]ClO4/[1′]ClO4, and [(bpy)2RuII(H-Iz)(Iz–)]ClO4 [2]ClO4, mixed-valent unsymmetric dinuclear: [(acac)2RuIII(μ-Iz–)2RuII(bpy)2]ClO4 [3]ClO4, and heterotrinuclear: [(acac)2RuIII(μ-Iz–)2MII(μ-Iz–)2RuIII(acac)2] (M = Co:4a, Ni:4b, Cu:4c, and Zn:4d) complexes (H-Iz = indazole, Iz– = indazolate, acac = acetylacetonate, and bpy = 2,2′-bipyridine). Structural characterization of all the aforestated complexes established their molecular identities including varying binding modes (Na and Nb donors and 1H-indazole versus 2H-indazole) of the heterocyclic H-Iz/Iz– in the complexes. Unlike [1′]ClO4 containing two NH protons at the backface of H-Iz units, the corresponding [1]ClO4 was found to be unstable due to the deprotonation of its positively charged quaternary nitrogen center, and this resulted in the eventual formation of the parent complex 1. A combination of experimental and density functional theory calculations indicated the redox noninnocent feature of Iz– in the complexes along the redox chain. The absence of intervalence charge transfer transition in the near-infrared region of the (Iz–)2-bridged unsymmetric mixed-valent RuIIIRuII state in [3]ClO4 suggested negligible intramolecular electronic coupling corresponding to a class I setup (Robin and Day classification). Heterotrinuclear complexes (4a–4d) exhibited varying spin configurations due to spin–spin interactions between the terminal Ru(III) ions and the central M(II) ion. Though both [3]ClO4 and 4a–4d displayed ligand (Iz–/Iz•)-based oxidation, reductions were preferentially taken place at the bpy and metal (RuIII/RuII) centers, respectively. Unlike 1 or [2]ClO4 containing one free NH proton at the backface of H-Iz, [1′]ClO4 with two H-Iz units could selectively and effectively recognize F–, OAc–, and CN– among the tested anions: F–, OAc–, CN–, Cl–, Br–, I–, SCN–, HSO4–, and Η2PΟ4– in CH3CN via intermolecular NH···anion hydrogen bonding interaction. The difference in the sensing feature between [1′]ClO4 and 1/[2]ClO4 could be rationalized by their pKa values of 8.4 and 11.3/10.8, respectively