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

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

The reaction of R-benzofuroxan (R = H, Me, Cl) with the metal precursor [Ru­(Cl)­(H)­(CO)­(PPh₃)₃] (<b>A</b>) or [Ru­(Cl)­(H)­(CH₃CN)­(CO)­(PPh₃)₂] (<b>B</b>) in CH₃CN at 298 K resulted in the intermediate complex [Ru­(Cl)­(L¹)­(CH₃CN)­(CO)­(PPh₃)₂] (L¹ = monodentate 2-nitroanilido) (<b>1</b>, pink), which however underwent slow transformation to the final product [Ru­(Cl)­(L²)­(CO)­(PPh₃)₂] (L² = bidentate 2-nitroanilido) (<b>2</b>, green). On the contrary, the same reaction in refluxing CH₃CN directly yielded <b>2</b> without any tractable intermediate <b>1</b>. Structural characterization of the intermediates <b>1a</b>–<b>1c</b> and the corresponding final products <b>2a</b>–<b>2c</b> (R = H, Me, Cl) authenticated their identities, revealing ruthenium-hydride mediated unsymmetrical cleavage of benzofuroxan to hydrogen bonded monodentate 2-nitroanilido (L¹) in the former and bidentate 2-nitroanilido (L²) in the latter. The spectrophotometric monitoring of the transformations of <b>B</b> → <b>1</b> as well as <b>1</b> → <b>2</b> with time and temperature established the first order rate process with associatively activated pathway for both cases. Both <b>1</b> and <b>2</b> exhibited one reversible oxidation and an irreversible reduction within ±1.5 V versus saturated calomel reference electrode in CH₃CN with slight variation in potential based on substituents in the benzofuroxan framework (R = H, Me, Cl). Spectroscopic (electron paramagnetic resonance and UV–vis) and density functional theory calculations collectively suggested varying contribution of metal based orbitals along with the ligand in the singly occupied molecular orbital of <b>1</b>⁺ or <b>2</b>⁺, ascertaining the noninnocent potential of the in situ generated (L¹) or (L²)

Revelation of Varying Coordination Modes and Noninnocence of Deprotonated 2,2′-Bipyridine-3,3′-diol in {Os(bpy)₂} Frameworks

The reaction of 2,2′-bipyridine-3,3′-diol (H₂L) and <i>cis</i>-Os^II(bpy)₂­Cl₂ (bpy = 2,2′-bipyridine) results in isomeric forms of [Os^II(bpy)₂(HL^–)]­ClO₄, [<b>1</b>]­ClO₄ and [<b>2</b>]­ClO₄, because of the varying binding modes of partially deprotonated HL^–. The identities of isomeric [<b>1</b>]­ClO₄ and [<b>2</b>]­ClO₄ have been authenticated by their single crystal X-ray structures. The ambidentate HL^– in [<b>2</b>]­ClO₄ develops the usual N,N bonded five-membered chelate with a strong O–H···O hydrogen bonded situation (O–H···O angle: 160.78°) at its back face. The isomer [<b>1</b>]­ClO₄ however represents the monoanionic O^–,N coordinating mode of HL^–, leading to a six-membered chelate with the moderately strong O–H···N hydrogen bonding interaction (O–H···N angle: 148.87°) at its backbone. The isomeric [<b>1</b>]­ClO₄ and [<b>2</b>]­ClO₄ also exhibit distinctive spectral, electrochemical, electronic structural, and hydrogen bonding features. The p<i>K</i>_a values for [<b>1</b>]­ClO₄ and [<b>2</b>]­ClO₄ have been estimated to be 0.73 and <0.2, respectively, thereby revealing the varying hydrogen bonding interaction profiles of O–H···N and O–H···O involving the coordinated HL^–. The O–H···O group of HL^– in <b>2</b>⁺ remains invariant in the basic region (pH 7–12), while deprotonation of O–H···N group of HL^– in <b>1</b>⁺ estimates the p<i>K</i>_b value of 11.55. This indeed has facilitated the activation of the exposed O–H···N function in [<b>1</b>]­ClO₄ by the second {Os^II(bpy)₂} unit to yield the L^2– bridged [(bpy)₂Os^II(μ-L^2–)­Os^II(bpy)₂]­(ClO₄)₂ ([<b>3</b>]­(ClO₄)₂). However, the O–H···O function in [<b>2</b>]­ClO₄ fails to react with {Os^II(bpy)₂}. The crystal structure of [<b>3</b>]­(ClO₄)₂ establishes the symmetric N,O^–/O^–,N bridging mode of L^2–. On the other hand, the doubly deprotonated L′^2– (H₂L′ = 2,2′-biphenol) generates structurally characterized twisted seven-membered O^–,O^– bonded chelate (torsion angle >50°) in paramagnetic [Os^III(bpy)₂­(L′^2–)]­ClO₄ ([<b>4</b>]­ClO₄). The electronic structural aspects of the complexes reveal the noninnocent potential of the coordinated HL^–, L^2–, and L′^2–. The <i>K</i>_c value of 49 for <b>3</b>³⁺ reveals a class I mixed-valent Os^IIOs^III state

Ru-Complex Framework toward Aerobic Oxidative Transformations of β‑Diketiminate and α‑Ketodiimine

The impact of the {Ru­(acac)₂} (acac^– = acetylacetonate) framework on the transformations of C–H and C–H/C–C bonds of coordinated β-diketiminate and ketodiimine scaffolds, respectively, has been addressed. It includes the following transformations involving {Ru­(acac)₂} coordinated β-diketiminate in <b>1</b> and ketodiimine in <b>2</b> with the simultaneous change in metal oxidation state: (i) insertion of oxygen into the C­(sp²)–H bond of β-diketiminate in <b>1</b>, leading to the metalated ketodiimine in <b>2</b> and (ii) Bronsted acid (CH₃COOH) assisted cleavage of unstrained C­(sp²)–C­(sp²)/CN bonds of chelated ketodiimine (<b>2</b>) with the concomitant formation of intramolecular C–N bond in <b>3</b>, as well as insertion of oxygen into the C­(sp³)–H bond of <b>2</b> to yield −CHO function in <b>4</b> (−CH₃ → −CHO). The aforesaid transformation processes have been authenticated via structural elucidation of representative complexes and spectroscopic and electrochemical investigations

Unsymmetric (μ-oxido)/(μ-pyrazolato) and Symmetric (μ-pyrazolato)₂ Bridged Diosmium Frameworks: Electronic Structure and Magnetic Properties

The present article deals with the structurally characterized unsymmetric oxido/pyrazolato-bridged [(bpy)₂Os^III(μ-oxido)­(μ-pz)­Os^III­(bpy)₂]­(ClO₄)₃ ([<b>1</b>]­(ClO₄)₃) and symmetric dipyrazolato-bridged [(bpy)₂Os^II(μ-pz)₂­Os^II(bpy)₂]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂) (pz = pyrazolato, bpy = 2,2′-bipyridine) complexes with the Os···Os separations of 3.484 and 4.172 Å, respectively. The anti-ferromagnetically coupled Os^III centers [<i>E</i>(<i>S</i> = 1)-<i>E</i>(BS­(1,1) <i>S</i> = 0) = 322.504 cm^–1] in <b>1</b>³⁺ and diamagnetic (<i>S</i> = 0) <b>2</b>²⁺ exhibit well-resolved ¹H NMR resonances. [<b>1</b>]­(ClO₄)₃ shows temperature- and magnetic field-dependent paramagnetism at low magnetic field and diamagnetism at high magnetic field. <b>1</b>³⁺ and <b>2</b>²⁺ display successive metal-based oxidation processes involving the intermediate mixed-valent states and isovalent congeners: Os^IVOs^IV (<b>1</b>⁵⁺)→Os^IIIOs^IV (<b>1</b>⁴⁺)⇌Os^IIIOs^III (<b>1</b>³⁺)⇌Os^IIIOs^II (<b>1</b>²⁺) and Os^IIIOs^III (<b>2</b>⁴⁺)→Os^IIOs^III (<b>2</b>³⁺)⇌Os^IIOs^II (<b>2</b>²⁺) as well as bpy-centered reductions. The effect of π donor O^2– and σ/π-donating pz^– in <b>1</b>³⁺ and <b>2</b>²⁺, respectively, leads to varying oxidation state of the metal ions in the isolated complexes: Os^IIIOs^III versus Os^IIOs^II. UV–visible–near-IR–electron paramagnetic resonance spectro-electrochemistry and density functional theory (DFT)/time-dependent DFT calculations collectively reveal overlapping of the metal- and ligand (pz, O, bpy)-based frontier orbitals in the delocalized mixed-valent states in <b>1</b>⁴⁺ and <b>1</b>²⁺ with comproportionation constant (<i>K</i>_c) value > 1 × 10¹⁴ as well as in isovalent <b>1</b>³⁺, resulting in mixed metal/ligand to metal/ligand near-IR transitions in all the three states. The mixed-valent Os^IIOs^III state in <b>2</b>³⁺ exhibits high <i>K</i>_c value of 1 × 10²² corresponding to a strong electrochemical coupling situation. However, closeness of the bandwidth (Δν_1/2, 4861 cm^–1) of broad and weak intervalence charge transfer transition of <b>2</b>³⁺ at 1360 nm (ε/M^–1 cm^–1: 490) with the calculated Δν_1/2 of 4121 cm^–1 based on the Hush formula as well as spin-density distributions of Os1: 0.811/0.799, Os2: 0.045/0042, and pz: 0.162/0.173 in <i>meso</i> and <i>rac</i> diastereomeric forms, respectively, attribute its localized class II state

Ancillary Ligand Control of Electronic Structure in o-Benzoquinonediimine-Ruthenium Complex Redox Series: Structures, Electron Paramagnetic Resonance (EPR), and Ultraviolet−Visible−Near-Infrared (UV-vis-NIR) Spectroelectrochemistry

The compounds Ru­(acac)₂(Q) (<b>1</b>), [Ru­(bpy)₂(Q)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), and [Ru­(pap)₂(Q)]­PF₆ ([<b>3</b>]­PF₆), containing Q = <i>N,N</i>′-diphenyl-<i>o</i>-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac^–), π-accepting 2,2^/-bipyridine (bpy), or strongly <i>π-</i>accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system <b>1</b>⁺–<b>1</b>^– prefers trivalent ruthenium with corresponding oxidation states Q⁰–Q^2– of the noninnocent ligand, the series <b>2</b>²⁺–<b>2</b>⁰ and <b>3</b>²⁺–<b>3</b>^– retain Ru^II. The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in <i>o</i>-quinonediimine ligands [Kalinina et al., <i>Inorg. Chem</i>. <b>2008</b>, <i>47</i>, 10110] and to the discussion of the most appropriate oxidation state formulation Ru^II(Q⁰) or Ru^III(Q^• –)

Ancillary Ligand Control of Electronic Structure in o-Benzoquinonediimine-Ruthenium Complex Redox Series: Structures, Electron Paramagnetic Resonance (EPR), and Ultraviolet−Visible−Near-Infrared (UV-vis-NIR) Spectroelectrochemistry

The compounds Ru­(acac)₂(Q) (<b>1</b>), [Ru­(bpy)₂(Q)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), and [Ru­(pap)₂(Q)]­PF₆ ([<b>3</b>]­PF₆), containing Q = <i>N,N</i>′-diphenyl-<i>o</i>-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac^–), π-accepting 2,2^/-bipyridine (bpy), or strongly <i>π-</i>accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system <b>1</b>⁺–<b>1</b>^– prefers trivalent ruthenium with corresponding oxidation states Q⁰–Q^2– of the noninnocent ligand, the series <b>2</b>²⁺–<b>2</b>⁰ and <b>3</b>²⁺–<b>3</b>^– retain Ru^II. The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in <i>o</i>-quinonediimine ligands [Kalinina et al., <i>Inorg. Chem</i>. <b>2008</b>, <i>47</i>, 10110] and to the discussion of the most appropriate oxidation state formulation Ru^II(Q⁰) or Ru^III(Q^• –)

Significant Influence of Coligands Toward Varying Coordination Modes of 2,2′-Bipyridine-3,3′-diol in Ruthenium Complexes

The varying coordination modes of the ambidentate ligand 2,2′-bipyridine-3,3′-diol (H₂L) in a set of ruthenium complexes were demonstrated with special reference to the electronic features of the coligands, including σ-donating acac^– (= acetylacetonate) in Ru^III(acac)₂(HL^–) (<b>1</b>), strongly π-accepting pap (= 2-phenylazopyridine) in Ru^II(pap)₂(L^2–) (<b>2</b>)/[(pap)₂Ru^II(μ-L^2–)­Ru^II­(pap)₂]­(ClO₄)₂ ([<b>4</b>]­(ClO₄)₂), and reported moderately π-accepting bpy (= 2,2′-bypiridine) in [Ru^II(bpy)₂­(HL^–)]­PF₆ ([<b>5</b>]­PF₆)/[(bpy)₂Ru­(μ-L^2–)­Ru­(bpy)₂]­(PF₆)₂ ([<b>7</b>]­(PF₆)₂). The single-crystal X-ray structures reveal that, in paramagnetic and electron paramagnetic resonance active <b>1</b> and reported diamagnetic [<b>5</b>]­PF₆, nearly planar monoanionic HL^– coordinates to the metal ion via the <i>N</i>,<i>N</i> donors forming a five-membered chelate ring with hydrogen-bonded O–H···O function at the backbone of the ligand framework, as has also been reported in other metal complexes. However, structurally characterized diamagnetic <b>2</b> represents O^–,O^– bonded seven-membered chelate of fully deprotonated but twisted L^2–. The nonplanarity of the coordinated L^2– in <b>2</b> does not permit the second metal fragment {Ru­(pap)₂} or {Ru­(bpy)₂} or {Ru­(acac)₂} to bind with the available N,N donors at the back face of L^2–. Further, the deprotonated form of the model ligand 2,2′-biphenol (H₂L′) yields Ru^II(pap)₂(L′^2–) (<b>3</b>); its crystal structure establishes the expected O^–,O^– bonded seven-membered chelate of nonplanar L′^2– as in reported Ru^II(bpy)₂(L′^2–) (<b>6</b>), although {Ru­(acac)₂} metal precursor altogether fails to react with H₂L′. All attempts to make diruthenium complex from {Ru­(acac)₂} and H₂L failed; however, the corresponding {Ru­(pap)₂²⁺} derived dimeric [<b>4</b>]­(ClO₄)₂ was structurally characterized. It establishes the symmetric N,O^–/N,O^– bridging mode of nonplanar L^2– as in reported [<b>7</b>]­(PF₆)₂. Besides structural and spectroscopic characterization of the newly developed complexes, the ligand (HL^–, L^2–, L′^2–, pap)-, metal-, or mixed metal–ligand-based accessible redox processes in <b>1</b>^<i>n</i> (<i>n</i> = +2, +1, 0, −1), <b>2</b>^<i>n</i>/<b>3</b>^<i>n</i> (<i>n</i> = +2, +1, 0, −1, −2), and <b>4</b>^<i>n</i> (<i>n</i> = +4, +3, +2, +1, 0, −1) were analyzed in conjunction with density functional theory calculations