Radical and Non-Radical States of the [Os(PIQ)] Core (PIQ = 9,10-Phenanthreneiminoquinone): Iminosemiquinone to Iminoquinone Conversion Promoted o-Metalation Reaction

Complexes of Metal Halides with Unreduced o-(Imino)quinones

A series of complexes of metal halides with unreduced quinone-type ligands have been synthesized and characterized in detail. The 3,6-di-tert-butyl-o-benzoquinone (1) and 4,6-di-tert-butyl-N-aryl-substituted o-iminobenzoquinones (2-5) (aryl is 2,6-dimethylphenyl in 2, 2-methyl-6-ethylphenyl in 3, 2,6-diethylphenyl in 4, and 2,6-diisopropylphenyl in 5) were used to obtain the molecular complexes with metal 12 group halides as well as with indium(III) iodide. The molecular structures of five complexes, bearing an unreduced form of redox-active ligand, have been established by single-crystal X-ray analysis. The spectral data, electrochemical measurements, and DFT calculations indicate the significant transformations of the molecular orbitals of 1-5 upon complexation with Lewis acids. The reduction potentials of o-(imino)quinones in complexes with metal halides shift into the anodic region versus uncoordinated ones. The choice of metal halide allows varying the shift magnitude up to 1.7 V in 2·CdI₂. The change of the oxidizing ability of the 1-5 upon coordination with Lewis acids enables the oxidation of mercury and ferrocene, infeasible for free ligands.

Mixed-valence di-ruthenium(II,III) complexes of redox non-innocent N-aryl-o-phenylenediamine derivatives

A mononuclear ruthenium(ii), [(L1IQ)Ru2+(PPh3)2Cl2]·CHCl3 (1·CHCl3), a di-ruthenium(ii,ii), [(L2IQ)2Ru24+Cl4(PPh3)2] (2) and a mixed-valence di-ruthenium(ii,iii), [(L3IQ)Ru25+Cl5(PPh3)2]·MeOH (3·MeOH) complex, where L1IQ, L2IQ and L3IQ are, respectively, o-diiminobenzoquinone forms of redox non-innocent N-(5-nitropyridyl)-o-phenylenediamine (L1H2), N-(2,4-dinitrophenyl)-o-phenylenediamine (L2H2) and N-(3-nitropyridyl)-o-phenylenediamine (L3H2) derivatives, were successfully isolated. The molecular and electronic structures of 1·CHCl3, 2 and 3·MeOH were confirmed by single-crystal X-ray crystallography, EPR, UV-Vis-NIR spectroscopies and density functional theory (DFT) calculations. Both 1·CHCl3 and 2 exhibited reversible anodic waves due to the Ru(iii)/Ru(ii) redox couple, while the cyclic voltammogram of 3·MeOH displayed two successive cathodic waves due to ruthenium(iii)/ruthenium(ii) and (L3IQ/L3ISQ) redox couples. EPR spectroscopy and DFT calculations inferred that 1+ is a ruthenium(iii) complex of L1IQ, [(L1IQ)Ru3+(PPh3)2Cl2]+, 2+ is a mixed-valence di-ruthenium(ii,iii) complex of L2IQ, [(L2IQ)2Ru25+Cl4(PPh3)2]+ and 3- is a di-ruthenium(ii,ii) complex, [(L3IQ)Ru24+Cl5(PPh3)2]-, while 32- is a di-ruthenium(ii,ii) complex of L3ISQ, [(L3ISQ)Ru24+Cl5(PPh3)2]2-, where L3ISQ is the o-diiminobenzosemiquinonate anion radical form of the L3H2 with a significant contribution of the mixed-valence di-ruthenium(iii,ii) form, [(L3AM)Ru25+Cl5(PPh3)2]2- (L3AM is the di-amido form of the L3H2 ligand). The spin density obtained from the Mulliken spin population analyses of the broken-symmetry (BS) DFT calculations was localized on Ru(2) in 3, while it was delocalized over both Ru(1) and the o-phenylenediamine fragment in 32-. The inter-valence charge-transfer (IVCT) transitions of 2+ and 3 were relatively weaker and occurred at 1680 and 1685 nm, respectively, while 32- exhibited a broader NIR absorption band at 1500-2000 nm. The calculated electronic-coupling parameters (Hab) using the Mulliken-Hush equation for 3 and 2+ ion, respectively, were 62 and 103 cm-1, defining these as Robin-Day class II mixed-valence systems.

A Diruthenium Complex of an N-Pyridyl-o-aminophenol Derivative: A Route to Mixed Valency

An N-pyridyl-o-aminophenol derivative that stabilises mixed-valence states of ruthenium ions is disclosed. A diruthenium complex, [(L_IQ ⁰ )Ru₂ Cl₅ ]⋅MeOH (1⋅MeOH) is successfully isolated, in which L_IQ ⁰ is the o-iminobenzoquinone form of 2-[(3-nitropyridin-2-yl)amino]phenol (L_AP H₂ ). In 1, L_IQ ⁰ oriented towards one ruthenium centre is a non-innocent NO-donor redox ligand, whereas another oriented towards another ruthenium centre is an innocent pyridine-donor redox ligand. Complex 1 is a diruthenium(II,III) mixed-valence complex, [Ru^II (L_IQ ⁰ )(μ-Cl)₂ Ru^III ], with a minor contribution from the diruthenium(III,III) state. [Ru^III (L_ISQ ^.- )(μ-Cl)₂ Ru^III ] contains L_ISQ ^.- , which is the o-iminobenzosemiquinonate anion radical form of the ligand. Complexes 1^- and 1⁺ are diruthenium(II,II), [Ru^II (L_IQ ⁰ )(μ-Cl)₂ Ru^II ], and diruthenium(III,III), [Ru^III (L_IQ ⁰ )(μ-Cl)₂ Ru^III ], complexes, respectively, of L_IQ ⁰ . Complex 1^2- is a diruthenium(II,II) complex of the o-iminobenzosemiquinonate anion radical (L_ISQ ^.- ), [Ru^II (L_ISQ ^.- )(μ-Cl)₂ Ru^II ], with a minor contribution from the diruthenium(III,II) form, [Ru^III (L_AP ^2- )(μ-Cl)₂ Ru^II ]. Complex 1²⁺ is a diruthenium(III,IV) mixed-valence complex of L_IQ ⁰ , [Ru^III (L_IQ ⁰ )(μ-Cl)₂ Ru^IV ]. Complexes 1 and 1²⁺ exhibit inter-valence charge-transfer transitions at λ=1300 and 1370 nm, respectively.

Orthometalated N-(Benzophenoxazine)-o-aminophenol: Phenolato versus Phenoxyl States

The molecular and electronic structures of the orthometalated ruthenium(III) and osmium(III) complexes of N-(benzophenoxazine)-o-aminophenol (^OXLH₂) that exhibits versatile redox activities are reported. The redox chemistry of ^OXLH₂ is remarkably different from that of N-(aryl)-o-aminophenol (^APLH₂). The study established that ^OXLH₂ is redox noninnocent and is a precursor of a phenoxyl radical. One of the C-H bonds of ^OXLH₂ is activated by ions, and ^OXLH₂ reveals three different redox states as dianionic phenolato (^OXL^2-), monoanionic phenoxyl radical (^OXL^•-), and zwitterionic phenoxium cation (^OXL^±) states. The reaction of ^OXLH₂ with [Ru^II(PPh₃)₃Cl₂] in boiling toluene in air affords an orthometalated ^OXL^2- complex of ruthenium(III), trans-[(^OXL^2-)Ru^III(PPh₃)₂(Cl)] (1), whereas the similar reaction with [Os^II(PPh₃)₃Br₂] yields an orthometalated ^OXL^•- complex, cis-[(^OXL^•-)Os^III(PPh₃)Br₂] (2). 1 and 2 exhibit ligand-based reversible redox waves due to ^OXL^•-/^OXL^2-, ^OXL^±/^OXL^•-, and M^III/M^II couples. The 1 ⁺ ion is a ^OXL^•- complex of ruthenium(III). 2 ^- exhibits temperature-dependent valence tautomerism due to [Os^II(^OXL^•-) ↔ Os^III(^OXL^2-)] equilibrium. 2 ^2- is a ^OXL^2- complex of osmium(II), while 1 ²⁺ and 2 ⁺ are ^OXL^± complexes of metal(III). 2 is an oxidant and effective catalyst for oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone, and the turnover number is 119.7 h^-1. The UV-vis-NIR absorption spectrum of 1 displays an NIR band at 800 nm due to an intra-ligand-charge-transfer transition, which is absent in 2 incorporating a ^OXL^•- radical. The molecular and electronic structures of 1 and 2 and their oxidized and reduced analogues were confirmed by single-crystal X-ray crystallography, variable-temperature electron paramagnetic resonance spectroscopy, spectroelectrochemical measurements, and density functional theory calculations.

A Redox-Active Cascade Precursor: Isolation of a Zwitterionic Triphenylphosphonio-Hydrazyl Radical and an Indazolo-Indazole Derivative

A redox-active [ML] unit (M = Co^II and Mn^II; LH₂ = N'-(1,4-dioxo-1,4-dihydronaphthalen-2-yl)benzohydrazide) defined as a cascade precursor that undergoes a multicomponent redox reaction comprising of a C-N bond formation, tautomerization, oxidation, C-C coupling, demetalation, and affording 6,14-dibenzoylbenzo[f]benzo[5,6]indazolo[3a,3-c]indazole-5,8,13,16-tetraone (^IndL₂) is reported. Conversion of LH₂ → ^IndL₂ in air is overall a (6H⁺+6e) oxidation reaction, and it opens a route for the syntheses of bioactive diarylindazolo[3a,3-c]indazole derivatives. The reaction occurs via a radical coupling reaction, and the radical intermediate was isolated as a triphenylphosphonio adduct. In presence of PPh₃ the [ML] unit promotes a reaction that involves a C-P bond formation, tautomerization, and oxidation to yield a stable zwitterionic triphenylphosphonio-hydrazyl radical (^PPh3L^±•). Conversion of LH₂ → ^PPh3L^±• is a (3H⁺+3e) oxidation reaction. To authenticate the [ML] unit, in addition to the ^IndL₂, a zinc(II) complex, [(L₃)Zn^II(H₂O)Cl]·2MeOH (1·2MeOH), was successfully isolated (L₃H = a pyridazine derivative of 1,4 naphthoquinone) from a reaction of LH₂ with hydrated ZnCl₂. Conversion of 3LH₂ → 1 is also a multicomponent (6H⁺+6e) oxidation reaction promoted by zinc(II) ion via a radical intermediate. Facile oxidation of [L^2-] to [L^•-] that was considered as an intermediate of these conversions was confirmed by isolating a 1,4 naphthoquinone-benzhydrazyl radical (LH^•) complex, [(LH^•)Zn^II(H₂O)Cl₂] (2H^•). The intermediates of LH₂ → ^IndL₂, LH₂ → ^PPh3L^±•, and 3LH₂ → 1 conversions were analyzed by electrospray ionization mass spectroscopy. The molecular and electronic structures of ^PPh3L^±•, ^IndL₂, 1·2MeOH, and 2H^• were confirmed by single-crystal X-ray crystallography, electron paramagnetic resonance spectroscopy, and density functional theory calculations.

Ni(II) Complexes of the Redox-Active Bis(2-aminophenyl)dipyrrin: Structural, Spectroscopic, and Theoretical Characterization of Three Members of an Electron Transfer Series

The sterically hindered bis(2-aminophenyl)dipyrrin ligand H₃^NL was prepared. X-ray diffraction discloses a bifurcated hydrogen bonding network involving the dipyrrin and one aniline ring. The reaction of H₃^NL with one equivalent of nickel(II) in the air produces a paramagnetic neutral complex, which absorbs intensively in the Vis-NIR region. Its electron paramagnetic resonance spectrum displays resonances at g₁ = 2.033, g₂ = 2.008, and g₃ = 1.962 that are reminiscent of an (S = 1/2) system having a predominant organic radical character. Both the structural investigation (X-ray diffraction) and density functional theory calculations on [Ni^II(^NL^•)] points to an unprecedented mixed "pyrrolyl-anilinyl" radical character. The neutral complex [Ni^II(^NL^•)] exhibits both a reversible oxidation wave at -0.28 V vs Fc⁺/Fc and a reversible reduction wave at -0.91 V. The anion was found to be highly air-sensitive, but could be prepared by reduction with cobaltocene and structurally characterized. It comprises a Ni(II) ion coordinated to a closed-shell trianionic ligand and hence can be formulated as [Ni^II(^NL)]^-. The cation was generated by reacting [Ni^II(^NL^•)] with one equivalent of silver hexafluoroantimonate. By X-ray diffraction we established that it contains an oxidized, closed-shell ligand coordinated to a nickel(II) ion. We found that a reliable hallmark for both the oxidation state of the ligand and the extent of delocalization within the series is the bond connecting the dipyrrin and the aniline, which ranges between 1.391 Å (cation) and 1.449 Å (anion). The cation and anion exhibit a rich Vis-NIR spectrum, despite their nonradical nature. The low energy bands correspond to ligand-based electronic excitations. Hence, the HOMO-LUMO gap is small, and the redox processes in the electron transfer series are exclusively ligand-centered.