Molecular and Electronic Structures of Ruthenium ... - ACS Publications

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Molecular and Electronic Structures of Ruthenium Complexes Containing an ONS-Coordinated Open-Shell π Radical and an Oxidative Aromatic Ring Cleavage Reaction Suvendu Maity, Suman Kundu, Sandip Mondal, Sachinath Bera, and Prasanta Ghosh* Department of Chemistry, R. K. Mission Residential College, Narendrapur, Kolkata 103, India S Supporting Information *

ABSTRACT: The coordination chemistry of 2,4-di-tert-butyl-6-[(2-mercaptophenyl)amino]phenol (LONSH3), which was isolated as a diaryl disulfide form, (LONSH2)2, with a Ru ion is disclosed. It was established that the trianionic LONS3− is redox-noninnocent and undergoes oxidation to either a closed-shell singlet (CSS), LONS−, or an open-shell π-radical state, LONS•2−, and the reactivities of the [RuII(LONS•2−)] and [RuII(LONS−)] states are different. The reaction of (LONSH2)2 with [Ru(PPh3)3Cl2] in toluene in the presence of PPh3 affords a ruthenium complex of the type trans-[Ru(LONS)(PPh3)2Cl] (1), while the similar reaction with [Ru(PPh3)3(H)(CO)Cl] yields a LONS•2− complex of ruthenium(II) of the type trans-[RuII(LONS•2−)(PPh3)2(CO)] (2). 1 is a resonance hybrid of the [RuII(LONS−)Cl] and [RuIII(LONS•2−)Cl] states. It is established that 2 incorporating an open-shell π-radical state, [RuII(LONS•2−)(CO)], reacts with an in situ generated superoxide ion and promotes an oxidative aromatic ring cleavage reaction, yielding a α-N-arylimino-ω-ketocarboxylate (LNS2−) complex of the type [RuII(LNS2−)(PPh3)(CO)]2 (4), while 1 having a CSS state, [RuII(LONS−)Cl], is inert in similar conditions. Notably, 2 does not react with O2 molecule but reacts with KO2 in the presence of excess PPh3, affording 4. The redox reaction of (LONSH2)2 with [Ru(PPh3)3Cl2] in ethanol in air is different, leading to the oxidation of LONS to a quinone sulfoxide derivative (LONSO0) as in cis-[RuII(LONSO0)(PPh3)Cl2] (3), via 1 as an intermediate. The molecular and electronic structures of 1−4 were established by single-crystal X-ray crystallography, electron paramagnetic resonance spectroscopy, electrochemical measurements, and density functional theory calculations. 1+ is a resonance hybrid of [RuIII(LONS−)(PPh3)2Cl ↔ RuIV(LONS•2−)(PPh3)2Cl]+ states, 2− is a LONS3− complex of ruthenium(II), [RuII(LONS3−)(PPh3)2(CO)]−, and 2+ is a ruthenium(II) complex of LONS− of the type [RuII(LONS−)(PPh3)2(CO)]+, where 35% diradical character of the LONS− ligand was predicted.

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cobalt complex of the type [Co(LONO•2−)(LONO−)2], which was considered to be an effective molecule in modeling a memory storage device,6 are surges to the development of the coordination chemistry of the parallel systems. Recently, Heyduk and co-workers notably expanded the area by reporting the syntheses and coordination complexes of bis[(2−3,5-dimethylphenyl)amino-4-methoxyphenyl]amine (LNNNH3) and bis(3,5-di-tertbutylphenolato)amide (LSNSH3).8 The existence of the tridentate π radicals of the types LNNN•2− and LSNS•2− coordinated to transition-metal ions was precisely authenticated. In this project, the chemistry of 2,4-di-tert-butyl-6-[(2-mercaptophenyl)amino]phenol (LONSH3), a tridentate o-aminophenol

INTRODUCTION Open-shell π-radical complexes of transition-metal ions belong to a class that is significant from fundamental, biological, and catalysis perspectives.1 In this context, the growth of the chemistry of various bidentate semiquinonate π radicals is noticeable. However, the coordination complexes of tridentate π radicals are limited in scope, and the well-known systems are aminophenol derivatives2−6 and diiminopyridines.7 The reported aminophenol derivative that affords a tridentate dianionic π radical (LONO•2−) is bis(3,5-di-tert-butyl-2-phenol)amine (LONOH3), as shown in Scheme 1.2 Pierpont et al.3 and Wieghardt et al.4 elucidated molecular and electronic structures of numerous LONO•2− complexes of transition-metal ions. The catalytic activity of the LONO•2− complexes was established in some cases.5 The valence tautomerism and spin-cross-over phenomenon of a © 2017 American Chemical Society

Received: November 30, 2016 Published: March 6, 2017 3363

DOI: 10.1021/acs.inorgchem.6b02862 Inorg. Chem. 2017, 56, 3363−3376

Article

Inorganic Chemistry Scheme 1. Redox States of LONOH3

extraamino-ol oxidative aromatic ring cleavage reaction, affording an α-N-arylimino-ω-ketocarboxylate coordinated to a RuII ion as in [RuII(LNS2−)(PPh3)(CO)]2 [4; LNSH2 = 2(2E,3E)-4(tert-butyl)-2-[(2-mercaptophenyl)imino]-7,7-dimethyl-6-oxooct-3-enoic acid], as depicted in Chart 2, while 1 with a closedshell-singlet (CSS) state, [Ru(LONS)Cl], does not exhibit any cleavage reaction. However, the oxidation of [Ru(LONS)] to [RuII(LONSO0)], yielding cis-[RuII(LONSO0)(PPh3)Cl2] (3), where LONSO0 is a quinone sulfoxide derivative, was authenticated in a different reaction. The molecular and electronic structures of these complexes were established by electron paramagnetic resonance (EPR) spectroscopy, single-crystal X-ray structure determinations of 1, 3·C2H5OH, and 4, spectroelectrochemical measurements, and density functional theory (DFT) calculations.

derivative, was explored. It was disclosed that LONSH3 is redoxnoninnocent and furnishes one-electron-oxidized ONS-coordinated π-radical (LONS•2−) and two-electron-oxidized LONS− complexes with a RuII ion. The noteworthy observation is that the uncoupled “[RuII(LONS•2−)]” state reacts with an in situ generated superoxide ion and promotes an extraamino-ol aromatic ring cleavage reaction. The iron(II) semiquinone radical was considered to be a reactive intermediate of the extradiol and extraamino-ol (see Chart 1) dioxygenase activities,9 and the first Chart 1. Extraamino-ol and Intraamino-ol Aromatic Ring Cleavage Reactions

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EXPERIMENTAL SECTION

Materials and Physical Measurements. Reagents or analyticalgrade materials were obtained from commercial suppliers and used without further purification. Spectroscopic-grade solvents were used for spectroscopic and electrochemical measurements. The C, H, and N contents of the compounds were obtained from a PerkinElmer 2400 series II elemental analyzer. IR spectra of the samples were measured from 4000 to 400 cm−1 with KBr pellets at room temperature on a PerkinElmer Spectrum RX 1 Fourier transform infrared (FT-IR) spectrophotometer. 1H NMR spectra in the CDCl3 solvent were obtained at 296 K on a Bruker DPX 300 MHz spectrometer.

iron(II) o-iminobenzosemiquinonate anion-radical complex was isolated and characterized by Fiedler et al.10 In this context, ruthenium(II) complexes of LONS•2− are significant. In this particular study, ruthenium complexes of the types trans-[Ru(LONS)(PPh3)2Cl] (1) and trans-[RuII(LONS•2−)(PPh3)2(CO)] (2) were successfully isolated. 1 is a delocalized state of the [RuII(LONS−)Cl] and [RuIII(LONS•2−)Cl] states. 2 with the open-shell state, [RuII(LONS•2−)(CO)], undergoes an Chart 2. Isolated Ligand and Ruthenium Complexes

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of the C2H5OH solvent under high vacuum. ESI-MS (positive ion, CH3OH): m/z 822 for [3]+. Anal. Calcd for C40H45Cl2NO3PRuS: C, 58.39; H, 5.51; N, 1.70. Found: C, 58.11; H, 5.48; N, 1.70. 1H NMR (300 MHz, CDCl3): δ 7.68 (s, 1H), 7.65 (d, 1H), 7.64 (s, 1H), 7.54 (t, 1H), 7.47−7.44 (m, PPh3), 7.19 (d, 1H), 7.11 (t, 1H), 1.57 (s, 2tBu), 1.40 (q, 2H of OEt), 1.24 (t, 3H of OEt). IR (KBr, cm−1): ν 3456(m), 3053(m), 2925(m), 2959(m), 2865(m), 1585(s), 1436(s), 1364(m), 1298(m), 1173(m), 1119(s), 1028(m), 722(vs), 693(s), 541(s). [RuII(LNS2−)(PPh3)(CO)]2 (4). To a solution of (LONSH2)2 (32 mg, 0.1 mmol) in moist toluene (30 mL) were added excess PPh3 (131 mg, 0.5 mmol) and [RuII(PPh3)3(CO)(H)Cl] (95 mg, 0.1 mmol), and the reaction mixture was refluxed for 15 min in an argon environment. The solution turned green, and it was cooled to 296 K. Upon exposure to air, it turned deep red. To this red solution was added n-heptane (20 mL), and the mixture was allowed to evaporate slowly in air; after 3−4 days, light-brown crystals of 4 separated out. The mixture was filtered, and the crystals, which were used for the X-ray diffraction study and spectroscopic and electrochemical measurements, were dried in air and collected. Yield: 38 mg (∼53% with respect to ruthenium). ESI-MS (positive ion, CH3OH). Calcd for [4-PPh3]+: m/z 1241.1. Anal. Calcd for C78H80N2O8P2Ru2S2: C, 62.30; H, 5.50; N, 1.86. Found: C, 61.25; H, 5.12; N, 1.62. 1H NMR (300 MHz, CDCl3): δ 8.10 (t, 2H), 7.90 (d, 2H), 7.72 (d, 2H), 7.50−7.28 (m, 2PPh3), 7.01 (t, 2H), 6.29 (s, 2HC), 3.51 (s, 2−CH2), 1.27 (s, 2tBu), 1.29 (s, 2tBu). IR (KBr, cm−1): ν 3055(m), 2953(m), 1958(s), 1702(m), 1483(s), 1438(vs), 1311(m), 1154(vs), 1120(vs), 720(vs), 542(vs). Single-Crystal X-ray Structure Determinations of the Complexes. Single crystals of 1, 3·C2H5OH, and 4 were picked up with nylon loops and mounted on a Bruker AXS D8 QUEST ECO diffractometer equipped with a Mo-target rotating-anode X-ray source and a graphite monochromator (Mo Kα, λ = 0.71073 Å). The final cell constants were obtained from least-squares fits of all measured reflections. Intensity data were corrected for absorption using the intensities of redundant reflections. The structures were readily solved by direct methods and subsequent difference Fourier techniques. The crystallographic data are listed in Table 1. The Siemens SHELXS-97 software package was used for solution, and SHELXL-97 was used for refinement.11 All non-H atoms were refined anisotropically. H atoms were placed at the calculated positions and refined as riding atoms with isotropic displacement parameters. The B level errors of the structures are mainly due to the solvent molecules and the bulky tert-butyl substituents on the ligand backbone. DFT Calculations. All calculations reported in this Article were done with the Gaussian 03W12 program package supported by GaussView 4.1. The DFT13 and time-dependent DFT (TD-DFT)14 calculations were performed at the level of the Becke three-parameter hybrid functional with the nonlocal correlation functional of Lee−Yang−Parr (B3LYP).15 Gas-phase geometries of trans-[Ru(LONS′)(PMe3)2(Cl)] (1′), trans[Ru(L ONS ′)(PMe 3) 2(CO)] (t-2′), cis-[Ru(LONS′)(PMe3 )2 (CO)] (c-2′), cis-[Ru(LONSO)(PMe3)Cl2] (3′), monoanionic [Ru(peroxoLONS′-OO)(PMe3)2(CO)]− (A′), neutral [Ru(dioxetane-LONS′-OO)(PMe3)2(CO)] (B′), a mononuclear cleaved carboxylate complex, [RuII(LNS2−)(PMe3)2(CO)] (4′), and [Ru(peroxo-LONS′-OO)(PMe3)(CO)] (C′) were optimized using Pulay’s Direct Inversion16 in the Iterative Subspace (DIIS), “tight” convergent self-consistent-field procedure,17 ignoring symmetry. 1′, B′, C′, 4′, and t-2′+ were optimized with a singlet spin state, while t-2′ and t-2′ were optimized with a doublet spin state. 1′ and 3′ are also optimized with a triplet spin state. The stabilities of the CSS solutions of 1′, 3′, and t-2′+ were analyzed. The relative energies of the intermediates are summarized in Table S5. In all calculations, a LANL2DZ basis set along with the corresponding effective core potential was used for ruthenium metal.18 The valence double-ζ basis set 6-31G19 was used for the H atom. For C, N, Cl, P, and non-H atoms, the valence double-ζ with diffuse and polarization functions, 6-31+G* as the basis set,20 was employed for all calculations. The percentage contributions of metal and ligands to the frontier orbitals were calculated using the GaussSum program package.21 The 60 lowest singlet excitation energies on the optimized geometry of 1′ (with a singlet spin state) in CH2Cl2 using the CPCM model22 were calculated by the TD-DFT method.

Electrospray ionization mass spectrometry (ESI-MS) was performed on a Micromass Q-TOF mass spectrometer. Electronic absorption spectra in solution were obtained on a PerkinElmer Lambda 750 spectrophotometer in the range 3300−175 nm. The X-band EPR spectra were measured on a Magnettech GmbH MiniScope MS400 spectrometer (equipped with a TC H03 temperature controller), where the microwave frequency was measured with an FC400 frequency counter. The EPR spectra were simulated using EasySpin software. The electroanalytical instrument BASi Epsilon-EC for cyclic voltammetric experiments in CH2Cl2 solutions containing 0.2 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte was used. The BASi platinum working electrode, platinum auxiliary electrode, and Ag/AgCl reference electrode were used for the measurements. The redox potential data are referenced versus a ferrocenium/ferrocene (Fc+/Fc) couple. A BASi SEC-C thin-layer quartz glass spectroelectrochemical cell kit (light path length of 1 mm) with a platinum gauze working electrode and a SEC-C platinum counter electrode was used for spectroelectrochemistry measurements. Syntheses. 6,6′-[[Disulfanediylbis(2,1-phenylene)]bis(azanediyl)]bis(2,4-di-tert-butylphenol) [(LONSH2)2]. To a solution of 3,5-di-tert-butylcatechol (1.1 g, 5 mmol) and triethylamine (0.3 mL) in n-hexane (150 mL) was added o-aminothiophenol (1.0 g, 5 mmol). The resulting mixture was stirred in air for 3 days. A reddish oily solution was obtained, which was poured on a basic alumina column. Elution with a mixture of chloroform and n-hexane solvents (1:5) afforded a pure (LONSH2)2 ligand. Yield: 1.52 g (∼92%). Anal. Calcd for C40H52N2O2S2: C, 73.13; H, 7.98; N, 4.26. Found: C, 72.79; H, 7.94; N, 4.24. EI-MS. Calcd for [(LONS)2 + Na]+: m/z 679. 1H NMR (300 MHz, CDCl3): δ 7.31 (s, 2H), 7.23 (t, 1H), 7.21 (t, 1H), 7.18 (s, 2H), 6.72 (d, 1H), 6.18 (d, 1H), 6.13 (s, 2H), 6.30 (br, 4H), 5.08 (s, 2H), 1.37 (s, 18H), 1.25 (s, 18H). IR (KBr, cm−1): ν 3435(m), 3058(s), 2957(s), 1587(s), 1477(s), 1421(s), 1310(m), 1221(m), 1024(s), 738(s), 690(s). trans-[Ru(LONS)(PPh3)2Cl] (1). To a solution of (LONSH2)2 (32 mg, 0.1 mmol) in moist toluene (30 mL) were added excess PPh3 (131 mg, 0.5 mmol) and [RuII(PPh3)3Cl2] (95 mg, 0.1 mmol), and the reaction mixture was refluxed for 30 min in an argon environment. The solution turned green. The reaction mixture was cooled to 296 K. The solution upon exposure to air turned violet immediately. It was allowed to evaporate slowly in air, and deep-black crystals of 1 separated out, which were collected upon filtration and dried in air. Yield: 65 mg (∼66% with respect to ruthenium). Single crystals for X-ray diffraction analyses were collected from this crop. ESI-MS (positive ion, CH3OH). Calcd for [1]+: m/z 987.1. Anal. Calcd for C56H54ClNOP2RuS: C, 68.11; H, 5.51; N, 1.42. Found: C, 67.81; H, 5.49; N, 1.42. 1H NMR (300 MHz, CDCl3): δ 8.01 (d, 1H), 7.69 (m, PPh3), 7.58 (s, 1H), 7.57 (s, 1H), 7.50 (m, PPh3), 7.40 (t, 1H), 7.20 (d, 1H), 7.10 (t, 1H), 1.29 (s, tBu), 1.26 (s, t Bu). IR (KBr, cm−1): ν 3046(m), 2955(m), 1572(m), 1481(m), 1432(s), 1152(s), 1096(m), 693(vs), 516(vs). trans-[RuII(LONS•2−)(PPh3)2(CO)] (2). To a solution of (LONSH2)2 (32 mg, 0.1 mmol) in moist toluene (30 mL) were added excess PPh3 (131 mg, 0.5 mmol) and [RuII(PPh3)3(CO)HCl] (95 mg, 0.1 mmol), and the reaction mixture was refluxed for 20−30 min in air. The red solution was evaporated under vacuum to obtain a solid of 2, which was washed with diethyl ether and n-hexane. Yield: 82 mg (∼84% with respect to ruthenium). The compound was collected for spectroscopic and electrochemical measurements. ESI-MS. Calcd for [2]+ (positive ion, CH3OH): m/z 980.1. Anal. Calcd for C57H54NO2P2RuS: C, 69.85; H, 5.55; N, 1.43. Found: C, 69.51; H, 5.53; N, 1.43. IR (KBr, cm−1): ν 3052(m), 2953(m), 1945(s), 1582(m), 1497(m), 1433(s), 1089(m), 1176(m), 1080(m),742(s), 694(s), 518(s). cis-[RuII(LONSO0)(PPh3)Cl2] (3). To a solution of (LONSH2)2 (32 mg, 0.1 mmol) in ethanol (20 mL) were added excess PPh3 (131 mg, 0.5 mmol) and [RuII(PPh3)3Cl2] (95 mg, 0.1 mmol), and the reaction mixture was refluxed for 6 h in air. The solution turned violet and then green. The reaction mixture was cooled to 296 K and filtered. Complex 3 as a residue was dried in air and collected. Yield: 52 mg (∼63% with respect to ruthenium). It was further recrystallized by diffusing n-hexane to the dichloromethane solution (with a few drops of ethanol) of the crude product at room temperature, and single crystals of 3·C2H5OH grew. All of the analytical data of 3 were collected after evaporation 3365


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Inorganic Chemistry

as shown in Scheme 2) of (LONSH2)2 with ruthenium precursors were performed. Reaction 1 is a reaction of (LONSH2)2 with [RuII(PPh3)3Cl2] in boiling moist toluene under argon in the presence of excess triphenylphosphine (PPh3). Exposure of this reaction solution in air yielded the crystals of 1. In this reaction, PPh3 is a two-electron reducing agent and cleaves the S−S bond of (LONSH2)2, producing OPPh3 in the presence of moisture.23 The formation of OPPh3 was detected by ESI-MS. In the presence of O2, the LONS3− state undergoes oxidation, yielding violet 1. Reaction 2 is a reaction of (LONSH2)2 with a carbonyl precursor, [RuII(PPh3)3(CO)(H)Cl], in boiling toluene. In contrast to Reaction 1, exposure of the solution mixture obtained from Reaction 2 in air (at 296 K) promotes an aromatic ring cleavage reaction, generating 4. Diffusion of n-heptane into this solution in air generated crystals of 4 in moderate yield. From this reaction, 2, which is an intermediate of this conversion and a LONS•2− π-radical complex of ruthenium(II), was isolated after evaporation of the solvent and characterized. Reaction 3 is a reaction of [RuII(PPh3)3Cl2] with (LONSH2)2 in boiling ethanol in the presence of PPh3 in air for a longer period (6 h), which generated 3, where LONS is oxidized to a quinone sulfoxide (LONSO0). Similar precursors were used for Reactions 1 and 3, but Reaction 3 was performed in ethanol, which promotes a different redox reaction, furnishing a different product, and no aromatic ring cleavage was noted. Initially, the reaction mixture turned violet (similar to Reaction 1), but with time, it turned green, depositing 3. The intermediate violet solution was analyzed by ESI-MS, and the signal at m/z 988, corresponding to that of 1, was detected (see Figure S1), inferring that conversion occurs via 1. The conversion of 1 → 3 occurs via the

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RESULTS AND DISCUSSION Syntheses. The ligand LONSH3 was isolated in a dimeric form, abbreviated as (LONSH2)2. Three different redox reactions (classified as Reactions 1, Reaction 2, and Reaction 3, Table 1. Crystallographic Data of 1, 3·C2H5OH, and 4 formula fw cryst color cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å) Z T (K) reflns collected (2θmax) ρcalcd (g cm−3) unique reflns reflns [I > 2σ(I)] μ (mm−1) λ (Å) F(000) R1a/GOFb wR2c [I > 2σ(I)] no. of param/restraints residual density (e Å−3)

1

3·C2H5OH

4

C56H54ClNP2RuS 987.52 black orthorhombic P212121 10.9598(3) 19.6153(6) 22.1528(7) 90 90 90 4762.4(2) 4 100(2) 56.74 1.378 77599 10420 0.530 0.71073 1880 0.0481/0.894 0.1009 575/0 1.008/−0.685

C42H50Cl2NO4PRuS 867.83 green monoclinic P21/n 13.517(5) 19.123(7) 16.340(6) 90 101.904(5) 90 4133(3) 4 296(2) 48.1 1.395 25783 4921 0.639 0.71073 1800 0.0440/1.033 0.1061 478/5 0.994/−0.594

C84H86N2O8P2Ru2S2 1579.74 red triclinic P1̅ 12.1337(8) 12.7972(8) 14.3906(10) 83.291(4) 81.820(4) 66.357(4) 2021.7(2) 1 291(2) 52.76 1.298 21379 4634 0.519 0.71073 818 0.0869/1.030 0.1919 442/6 2.041/−0.802

R1 = ∑||Fo| − |Fc||/∑|Fo|. bGOF = {∑[w(Fo2 − Fc2)2]/(n − p)}1/2. cwR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2 where w = 1/[σ2(Fo2) + (aP)2 + bP], with P = (Fo2 + 2Fc2)/3. a

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Inorganic Chemistry Scheme 2

Figure 1. (a) Molecular geometry of 1 in crystals (40% thermal ellipsoids; H atoms are omitted for clarity). (b) X-band EPR spectrum of CH2Cl2 frozen glass of 1+ at 115 K. (c) Spin density of 1′+ (Ru, 0.59; Cl, 0.03; S, 0.26; O, 0.12; N, −0.06) obtained from Mulliken spin-population analyses (yellow, α spin; red, β spin).

dissociation of one of the PPh3 ligands and oxidation of the thiolato function to −SO(OEt). Details of the syntheses and analytical and spectral data are outlined in the Experimental Section. Molecular and Electronic Structures of the Complexes. The molecular structures of the complexes were confirmed by the single-crystal X-ray structure determinations of 1, 3·C2H5OH, and 4. The crystallographic data are summarized in Table 1. The electronic structures of the complexes were elucidated by the single-crystal X-ray bond parameters, EPR spectroscopy, and DFT calculations. The EPR measurement parameters are summarized in Table S1. DFT calculations were performed on the truncated models using PMe3 as ligands. 1 crystallizes in the P212121 space group. The molecular structure in crystals and the atom-labeling scheme of 1 are illustrated in Figure 1. The selected bond parameters are summarized in Table 2. In 1, LONS is a redox-noninnocent pincer ligand and two PPh3 ligands coordinate trans to each other. The C−N, C−O, and C−S bond lengths are significant to analyze the oxidation level of the LONS ligand. 1 can be defined by three tautomeric electronic states: [RuIV(LONS3−)], [RuIII(LONS•2−)], and [RuII(LONS−)]. LONS•2− is a resonance hybrid of o-iminobenzosemiquinonate and o- iminothiobenzosemiquinonate anion radicals, while LONS− is a resonance hybrid of o-iminobenzoquinone and o-iminothiobenzoquinone states, as illustrated in Chart 3.

Table 2. Selected Experimental Bond Lengths (Å) of 1, 3·C2H5OH, and 4 bond

1

3·C2H5OH

4

Ru−O Ru−N Ru−S Ru−P (avg) Ru−Cl C−O C−N (phenol ring) C−N (thiol ring) C−S average C−N/O average C−N/S

2.167(4) 1.941(4) 2.312(2) 2.379(2) 2.427(2) 1.302(6) 1.395(6) 1.391(6) 1.704(5) 1.348(6) 1.547(6)

2.090(3) 1.968(3) 2.186(2) 2.355(2) 2.409(2) 1.276(5) 1.346(5) 1.421(5) 1.773(4) 1.311(5) 1.597(5)

2.094(5) 2.073(6) 2.356(2) 2.328(2) 1.269(2) 1.317(9) 1.422(1) 1.755(8)

The C−O and C−S bond distances of LONS•2− are expected to be intermediate between the dianionic and monoanionic forms of the corresponding O,O- and S,S-donor ligands because of complete delocalization of the unpaired electron. Similarly, the corresponding bond lengths of LONS− are intermediate between the neutral and monoanionic forms of the related bidentate ligands, as shown in Chart 4. Thus, the expected approximate C−O and C−S lengths of LONS•2− are 1.32 ± 0.01 and 1.75 ± 0.01 Å, while those of the LONS− state are 1.27 ± 0.01 and 1.70 ± 0.01 Å. 3367


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Inorganic Chemistry Chart 3

those obtained from the single-crystal X-ray structure determination of 1. The bond parameters of the CSS state are similar to those obtained experimentally. The calculated C−S length of the CSS state is 1.738 Å and the experimental value is 1.704(5) Å, while that of the triplet state is 1.773 Å. The CSS state exhibits quinoidal distortion of the C−C lengths of the two phenyl rings, as observed in the X-ray structure of 1 (see Table S2). The feature corresponds well to the resonance structures of [RuII(LONS−)Cl] state, as shown in Chart 3. Thus, the calculation authenticated a CSS ground electronic state of 1, with a significant contribution of the [RuII(LONS−)Cl] state. Analyses of the frontier orbitals of the CSS state of 1 revealed that both highest occupied molecular orbital (HOMO; 26% dRu) and lowest unoccupied molecular orbital (LUMO; 23% dRu) are composed of dRu and LONS. In HOMO, the contribution of the S atom is significant (29%), while in LUMO, the contribution of pN is 17%. The frozen-glass EPR spectrum of 1+ obtained from a constant potential coulometric experiment is anisotropic, as depicted in Figure 1b. The axial spectrum is relatively broader, and no hyperfine couplings due to 99,101Ru, 14N, and 31P nuclei are resolved. The simulation gave g values (g1 = 1.988 and g2 = 2.145) that are indicative of a significant contribution of ruthenium to the singly occupied molecular orbital (SOMO), predicting a [RuIII(LONS−)] ground electronic state of 1+. Similar to 1′, the frontier molecular orbitals of 1′+ are composed of both dRu and LONS. The SOMO is composed of 17% dRu, whereas the β-LUMO is composed of 40% dRu. The contribution of the S atom to SOMO is 30%. The spin density obtained from the DFT calculations on [Ru(LONS′)(PMe3)2(Cl)]+ (1′+) with a doublet spin state disperses over both ruthenium (59%) and the ligand backbone (41%), as depicted in Figure 1c. It infers that 1+ is a resonance hybrid of the [RuIII(LONS−) ↔ RuIV(LONS•2−)]+ state. Although the cathodic wave due to the 1/1− couple was irreversible, the electronic state of 1− was investigated by DFT calculations on 1′− with a doublet spin state. Similar to 1′+, in 1′− the spin density scatters on both ruthenium (31%) and the LONS ligand (69%), revealing a resonance state, [RuII(LONS•2−) ↔ RuIII(LONS3−)]−, as a ground electronic state of 1−. It was established that 1− is relatively more stable in a 0.01 M PPh3 solution, of which the EPR spectrum was recorded and is depicted in Figure S2. The isotopic signal at g = 2.003 correlates well with that of the [RuII(LONS•2−)] state of 1−. The ground electronic state of 2 incorporating a neutral CO ligand instead of a chloride is different from that of 1. The fluid-solution and frozen-glass EPR spectra are consistent with the [RuII(LONS•2−)(CO)] state of 2. The isotropic EPR signal of 2 in CH2Cl2 with g = 2.004 predicted that 2 is a LONS•2− anionradical complex of ruthenium(II). The spectrum exhibits hyperfine splitting due to both 14N, I = 1 (AN = 22 G), and 31P, I = 1/2 (AP = 12 G), nuclei, as illustrated in Figure 2a. The CH2Cl2 frozen-glass EPR spectrum of 2 at 115 K is depicted in Figure 2b.

Chart 4

The C−O and C−S lengths in 1, 1.302(6) and 1.704(5) Å, are relatively shorter than those expected for the o-dithiobenzosemiquinonte(1−) and o-iminothiobenzosemiquinonte(1−) radicals coordinated to transition-metal ions.24 The length is similar to the C−S length of a o-iminothiobenzosemiquinone coordinated to ruthenium(II), the C−S length of which is 1.706(2) Å.25 The average C−N/S length in 1, 1.547(6) Å, is also relatively shorter than that [1.576(2) Å] of [LSNS•2−] anion radicals3a,8 coordinated to tungsten(IV) (Chart S1). The features predict a dominant contribution of the [RuII(LONS−)] state to 1. The C−C lengths of both phenyl rings exhibit quinoidal long−short−long−short−long types of distortions. Notably, the C(3)−C(4) and C(5)−C(6) lengths of the phenolato ring are 1.376(8) and 1.377(7) Å, and the C(10)− C(11) and C(12)−C(13) lengths of the thiophenolato ring are 1.376(7) and 1.365(8) Å. The trend corroborates with the oxidation of LONS3− to the LONS•2− or LONS− state (see Chart 3) and excludes the contribution of the [RuIV(LONS3−)] state to 1. The Ru−O, Ru−S, and Ru−N lengths are 2.167(4), 2.312(2), and 1.941(4) Å. The relatively longer Ru−Cl and shorter Ru−P lengths are 2.427(2) and 2.379(2) Å, which correlate well with those of the reported complexes of ruthenium(II).26 It is worth noting that the C−N lengths are similar to those of the LONO•2− state (see Scheme 1 and Chart S1).2c,4 Thus, it is hard to assign unambiguously the electronic state of the LONS ligand from these bond parameters and preferably a resonance state, [RuIII(LONS•2−)] ↔ [RuII(LONS−)], because the ground electronic state of 1 is predicted. The similar feature of the electronic state of a ruthenium complex containing a redox-noninnocent toluene-3,4-dithiolate (tdt) ligand, {[RuII(cyclam)(tdt•−)]+ ↔ [RuIII(cyclam)(tdt2−)]+}, was established precisely by Wieghardt and co-workers.27 DFT calculations at the B3LYP level were employed on [Ru(LONS′)(PMe3)2(Cl)] (1′) to elucidate the electronic state of 1. The gas-phase geometry of 1′ was optimized with the singlet and triplet spin states, and the calculated bond parameters are summarized in Table S2. Analyses authenticated that the CSS solution of 1′ is stable. No perturbation due to the open-shellsinglet (OSS) state was observed, and no broken symmetry BS(↑↓) solution condensing the atomic spin to ruthenium is successfully achieved. The triplet solution is 4.5 kcal mol−1 higher in energy than the CSS solution, and the calculated bond parameters of the triplet state are significantly different from 3368


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Figure 2. X-band EPR spectra of 2 in (a) CH2Cl2 at 296 K (the simulation considered the hyperfine splitting due to the 14N nucleus, producing a three line spectrum; two of the lines further split because of the 31P nucleus) and (b) CH2Cl2 frozen glass at 115 K (experimental, black; simulated, red). (c) Spin density of t-2′ [S, 0.11; N, 0.34; O, 0.08; C(2), 0.11; C(4), 0.12; C(6), 0.08; C(12), 0.13]. (d) Spin density of t-2′+ (Ru, 0.03; S, 0.16; N, −0.03, O, −0.07) obtained from Mulliken spin-population analyses (yellow, α spin; red, β spin).

The spectrum displays hyperfine splitting due to the 14N, I = 1 (AN = 20 G), nucleus only; hyperfine splitting due to the 31P nucleus is not observable here, and the simulated g value, 2.004, correlates well with those of organic radicals.28 The geometry of 2 was investigated by DFT calculations. The gas-phase geometries of both t-2′ and c-2′ were optimized with the doublet spin state. It is observed that the ground-state energy of the cis analogue is 9.6 kcal mol−1 higher than that of the trans analogue. Thus, similar to 1, the trans geometry is assigned to 2. The significant bond parameters of t-2′ are summarized in Table S2 for comparison. The calculated C−S lengths of t-2′ (C−O, 1.306 Å; C−N, 1.379 and 1.383 Å; C−S, 1.762 Å) are notably different from those of 1′ (C−O, 1.291 Å; C−N, 1.397 and 1.385 Å; C−S, 1.738 Å). The spin densities obtained from the Mulliken spinpopulation analyses are shown in Figure 2c. The spin is dominantly localized on the N atom (34%). The rest scatters on the phenolato and thiophenolato rings. The feature is consistent with the oxidation of LONS3− to the semiquinonate anion radical, LONS•2−. No significant localization of the atomic spin on ruthenium is observed in t-2′, and the [RuII(LONS•2−)(CO)] state is assigned to the ground electronic state of 2. The gas-phase geometry of t-2′+ was optimized with the singlet spin state. However, the CSS solution of t-2′+ is unstable because

of OSS perturbation. Reoptimization of the solution results in a stable solution where the contribution of the OSS state is 35.1% and both the α and β spins scatter on the LONS fragment. Thus, in 2+, coordination of the diradical singlet state of LONS− is proposed. The spin density of t-2′+ is depicted in Figure 2d. Notably, the α spin is dominantly localized on the thiophenol ring particularly on the S atom, and the β spin is delocalized over the aminophenol ligand. The pattern correlates with a state having both thiyl and o-iminobenzosemiquinonate radicals in the LONS− ion and can be considered to be coordination of the excited state of LONS− to the RuII in 2+.29 On the basis of these calculations, the ground electronic state of 2+ is defined by the [RuII(LONS−)(CO)]+ state. 3·C2H5OH crystallizes in the P21/n space group. The molecular geometry in crystals and the atom-labeling scheme of 3 are illustrated in Figure 3a. The selected bond parameters are summarized in Table S3. In 3, the LONS ligand is oxidized to neutral o-iminobenzoquinone containing a −S(O)OEt function, and it is abbreviated as LONSO0, as given in Scheme 3. Two Cl atoms lie cis to each other. The C−O and C−N lengths of LONSO are significantly different from those of LONS in 1. The C−O and C−N lengths of 3·C2H5OH, 1.276(5) and 1.346(5) Å, are relatively shorter than those observed in the cases of [LONS•2−], [LONO•2−], and [LSNS•2−]. The average C−O and C−N lengths 3369


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Figure 3. (a) Molecular geometry of 3·C2H5OH in crystals (40% thermal ellipsoids; H atoms and C2H5OH are omitted for clarity). (b) X-band EPR spectrum of 3− in CH2Cl2 at 296 K. (c) Spin density of 3′− (Ru, 0.27; O, 0.14; N, 0.22) obtained from Mulliken spin-population analyses (yellow, α spin; red, β spin).

spin state, and the calculated selected bond parameters are summarized in Table S3. The calculated C−O and C−N lengths in 3′− are 1.306 and 1.382 Å, which are relatively longer than those of 3′, inferring a reduction of LONSO to the LONSO•− anion radical. The calculated average C−O/N length of 3′− is 1.344 Å, which is similar to those observed in the cases of LONO•2− coordinated to transition-metal ions.4 However, analysis of the Mulliken spin established that the spin density scatters on both LONSO (71%) and ruthenium (29%) in 3′−, as depicted in Figure 3c. Thus, the calculation predicts a resonance hybrid of the [RuII(LONSO•−)] and [RuIII(LONSO2−)] states as the ground electronic state of 3−. 4 crystallizes in the P1̅ space group. The molecular structure in crystals and the atom-labeling scheme of 4 are illustrated in Figure 4. The selected bond parameters are summarized in Table 2. The RuS2NOPC octahedra are severely distorted. The thiolato function bridges the RuII ion, producing a Ru2S2 rectangle. Two RuII−S lengths are 2.356(2) and 2.493(2) Å. The C(11)−N(1) and C(12)−S(002) lengths are 1.422(1) and 1.755(8) Å. The C(2)−N(1) and C(3)−C(4) lengths, 1.317(9) and 1.328(2) Å, are relatively shorter, while the C(4)−C(5) and C(5)−C(6) lengths are 1.501(16) and 1.513(17) Å. The feature is consistent with the existence of CN and CC functions in LNS2−, as depicted in Figure 4, and is defined as a α-N-aryliminoω-ketocarboxylate coordinated to a RuII ion. The X-ray structure of 4 confirmed the extraamino-ol C−C bond (ortho to the −OH function and meta to the −NH function) cleavage of the LONSH3 ligand. Redox Activities. The redox activities of 1−3 were investigated by cyclic voltammetry in CH2Cl2. The redox potential data referenced to Fc+/Fc couple are summarized in Table 3. The reversibility of the redox waves significantly changes in the PPh3 solution. The 1+/1 redox wave at +0.02 V due to the [RuIII(LONS−)(Cl)]+/[Ru(LONS)(Cl)] redox couple is reversible, while the 1/1− redox wave at −1.23 V due to the [Ru(LONS)(Cl)]/ [RuII(LONS•2−)(Cl)]− redox couple is irreversible, as depicted in Figure 5a(i). However, the cathodic wave is reversible (ic/ia = 1.0) in a 0.01 M PPh3 solution, as illustrated in Figure 5a(ii), inferring that the irreversibility of the cathodic wave is due to PPh3 dissociation from 1−. The cyclic voltammogram of 2 incorporating the CO ligand is different from that of 1, as depicted in Figure 5b. 2 exhibits two anodic waves at +0.05 and +0.38 V respectively assigned to [RuII(LONS−)(CO)]+/[RuII(LONS•2−)(CO)] and

of 3·C2H5OH are also shorter than the C−O and C−N lengths of QSMe•− and QSeMe•−, respectively, in the [Ru(Cym)(QSMe•−)]+ and [Ru(Cym)(QSeMe•−)]+ complexes reported by Kaim et al.30 The average C−O/N length of 3·C2H5OH is 1.311(5) Å, which corresponds well with that of [Fe(LONO−)X2], as given in Chart S1 and cis-[RuII(LSPhIQ)(PPh3)Cl2] (LSPhIQ = 2,4-di-tertbutyl-N-[2-(phenylthio)]phenyl-o-iminobenzoquinone) reported recently.31 The C−C lengths of the phenolato ring exhibit a quinoidal distortion, where the C(3)−C(4) and C(5)−C(6) lengths are 1.355(6) and 1.343(6) Å. Thus, 3 is defined as an o-iminobenzoquinone complex of the type cis-[RuII(LONSO0)(PPh3)Cl2]. The RuII−O, RuII−P, and RuII−N lengths are 2.090(3), 2.355(2), and 1.968(3) Å. DFT calculations were performed on the PMe3 analogue (3′) with the singlet spin state, and the calculated bond parameters are listed in Table S3. Analysis affirmed that the CSS solution of 3′ is stable, and the calculated C−O and C−N bond lengths, 1.279 and 1.351 Å, are similar to those obtained from the X-ray diffraction study of 3·C2H5OH. The energy of the triplet state is 10.4 kcal mol−1 higher than that of the corresponding singlet state. Thus, a CSS ground electronic state, [RuII(LONSO0)], is considered for 3. The CH2Cl2 solution of 3− obtained from a constant potential coulometric experiment exhibits an isotropic EPR spectrum (see Figure 3b) at g = 1.994. The g parameter corroborates well with the existence of a LONSO•− anion radical, as shown in Scheme 3. Scheme 3. LONSO0 Ligand and the Reduced Analogue, LONSO•−

Thus, 3− is defined as a LONSO•− complex of ruthenium(II) of the type cis-[RuII(LONSO•−)(PPh3)Cl2]−. To justify the assignment, the gas-phase geometry of 3′− was optimized with the doublet 3370


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Figure 4. Molecular geometry of 4 in crystals (40% thermal ellipsoids; H and solvent atoms are omitted for clarity) showing the cleaved ligand (LNS2−) in the inset.

values at 1267.8 and 617.6 nm are respectively due to the pS + dRu → dRu + πLONS* and pS + dRu + pO → dRu + πLONS* transitions. These are defined as mixed-metal−ligand to mixed-metal−ligand charge-transfer transitions. In 2, a similar transition appears at 1425 nm. The corresponding transitions are absent in 3 and 4. The UV−vis−NIR spectrum of 3 displays an absorption band at 793 nm, assigned to the dRu → π* transition of LONSO0. No defined absorption band at NIR is observed in the case of 4. The spectrum of 4 displays a shoulder at 624 nm, which may be due to the dRu → πimine* charge-transfer transition of LNS2−. Aromatic Ring Cleavage Reaction. In the vast coordination chemistry, numerous o-benzosemiquinonate, o-iminobenzosemiquinonate, and o-diiminobenzosemiquinonate anion-radical complexes of transition-metal ions were reported; however, their dioxygenase activity leading to aromatic ring cleavage reaction is rare.32a It is worth noting that the peroxide intermediates with Rh, Ir, and WV ions based on 9,10-phenanthrenesemiquinone and o-amidophenolato fragments, where no aromatic C−C bond cleavage was observed, were reported.32b,c The present study established that the reactions of [Ru(PPh3)3Cl2] and [Ru(PPh3)3(CO)(H)Cl] with (LONSH2)2 in the presence of PPh3 afford different products, and with [Ru(PPh3)3(CO)(H)Cl], an aromatic ring cleavage reaction occurs. It was investigated that the electronic states of 1 and 2, an intermediate of the cleavage reaction, are different. The delocalized CSS state, [RuII(LONS−)(PPh3)2(Cl)] ↔ [RuIII(LONS•2−)(PPh3)2(Cl)], of 1 is relatively inert, while the open-shell π-radical state, [RuII(LONS•2−)(CO)], of 2 reacts further with the in situ produced superoxide ion, promoting an aromatic ring cleavage reaction. The mechanistic aspect of the O2 activity was investigated intensively by Que et al.,33 and following their proposal, the plausible intermediates of the conversion of 2 → 4 are depicted in Scheme 4. Two popular intermediates of these types of reactions are (i) dioxetane, reported by Hayaishi et al.34 in 1955, and (ii) lactone (Criegee rearrangement), reported by Que et al.33 and Bugg et al.35 It is notable that, in 2 → 4 conversion, one of the

Table 3. Redox Potentials of 1−3 Determined by Cyclic Voltammetry in CH2Cl2 at 296 K complex 1 2 3 a

E1/21, V (ΔEc, mV)

E1/22, V (ΔEc, mV)

E1/23, V (ΔEc, mV)

+0.38 (60)

+0.02 (80) +0.05 (60) +0.84a

−1.23b −0.36 (80) −0.62 (90)

Anodic peak potential. separation.

b

E1/24, V (ΔEc, mV)

−1.02b c

Cathodic peak potential. Peak-to-peak

[RuIII(LONS−)(CO)]2+/[RuII(LONS−)(CO)]+ redox couples. Unlike the 1/1− redox wave, the 2/2− redox wave at −0.36 V due to the [RuII(LONS•2−)(CO)]/[RuII(LONS3−)(CO)]− redox couple is reversible, concluding stabilization of the [RuII(LONS3−)] state having CO as a coligand. A notable change of the reversibility of the [RuII(LONS•2−)(CO)]/[RuII(LONS3−)(CO)]− redox wave was observed in a 0.01 M PPh3 solution, as illustrated in Figure 5b(ii). The potential data infer that in air the [RuII(LONS3−)(CO)]− state is not stable and is easily oxidized to the [RuII(LONS•2−)(CO)] state. The cyclic voltammogram of 3 is shown in Figure 5c. 3 exhibits a cathodic wave and a peak respectively at −0.62 and −1.02 V due to LONSO0/LONSO•− (3/3−) and LONSO•−/LONSO2− (3−/32−) redox couples. However, the single 3/3− wave is reversible, as depicted in Figure 5c(ii). The anodic peak of 3 at +0.84 V is irreversible. The electronic state of 3− obtained from a controlled potential coulometric experiment was established by EPR spectroscopy and DFT calculations (vide supra). Electronic Spectra. The UV−vis−near-IR (NIR) absorption spectrum of (LONS)2 is illustrated in Figure S3, and those of 1−4 are shown in Figure 6. The spectral data are summarized in Table 4. 1 exhibits a NIR absorption band at 1580 nm. The origin of the NIR transition of 1 was elucidated by TD-DFT calculations on 1′ (with the singlet spin state) in CH2Cl2 using the CPCM model. The calculated energies of selected transitions with oscillation strengths (f) are listed in Table S4. The λcalc 3371


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Figure 5. Cyclic voltammograms of (a) 1 (i) in CH2Cl2 and (ii) in CH2Cl2 with 0.01 M PPh3, (b) 2 (i) in CH2Cl2 and (ii) in CH2Cl2 with 0.01 M PPh3, and (c) 3 in CH2Cl2 (i) up to −1.2 V and (ii) up to −0.8 V at 296 K. Conditions: 0.2 M [N(n-Bu)4]PF6 supporting electrolyte; scan rate, 100 mV s−1; platinum working electrode.

(after 36 h) of the cleavage reaction is illustrated in Figure 7, which identified the m/z signals corresponding to the peroxide intermediate A (m/z = 1012), in addition to [4-PPh3] (m/z 1241). The analysis proposed the formation of a peroxide derivative, which leads to the aromatic ring cleavage reaction. It is worth noting that 2 does not react with a O2 molecule and no cleavage of 2 in toluene/heptane was detected even after 7 days in air, but 2 reacts with a superoxide ion. The reaction of 2 with KO2 was investigated by MS. The reaction of 2 containing soft phosphorus and sulfur ligands with KO2 is hazardous. The reaction of 2 with KO2 in tetrahydrofuran is faster, and the formation of a LONSO2 adduct was predicted. ESI mass spectral analysis of the reaction mixture obtained just after 10 min detected only a peak at m/z 493 corresponding to the mass of [Ru(LONSO2)CO], indicating dissociation of PPh3 (maybe as OPPh3). However, the ESI mass spectrum recorded in the presence of PPh3 displays a peak at m/z 754 that corresponds to the [Ru(LONSO2)(PPh3)CO] fragment, likely to be C of Scheme 4. Detection of the intermediate C reveals that A → C → D is a possible path of the formation of 4. The study infers that 2 does not react with a O2 molecule but reacts faster with a superoxide ion, supporting the proposal of Scheme 4. Thus, the reaction of 2 with KO2 in the presence of excess PPh3 was performed, and the products were analyzed by MS. The mass spectrum of the reaction mixture (after 5 days), as illustrated in Figure S4, is similar to that of Figure 7, exhibiting an extra m/z peak corresponding to the potassium salt of the peroxide intermediate A (m/z 1053). This strongly suggests that the

Figure 6. UV−vis−NIR absorption spectra of 1 (black), 2 (blue), 3 (red), and 4 (green) in CH2Cl2 at 296 K.

Table 4. UV−Vis−NIR Spectral Data of (LONSH2)2 and 1−4 in CH2Cl2 at 296 K compound

λmax, nm (ε, ×105 M−1 cm−1)

(LONSH2)2 1 2 3 4

340 (0.25), 278 (0.53) 1580 (0.14), 727 (0.31), 581 (0.64), 501 (0.64), 410 (0.52) 1425 (0.09), 721 (0.10), 656 (0.14), 541 (0.35), 401 (0.97) 1065 (0.02), 793 (0.11), 617 (0.34), 433 (0.24), 407 (0.27) 1020 (0.08), 624 (0.24), 438 (0.48), 360 (0.81)

PPh3 ligands is eliminated, furnishing a thiolato-bridged dimeric product. The cleavage reaction of this study was investigated by ESI-MS. The ESI mass spectrum of the intermediate products Scheme 4. Plausible Paths of 2 → 4 Conversion

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Figure 7. ESI-MS m/z peaks of the intermediates of the cleavage reaction.

Scheme 5

superoxide ion promotes the cleavage reaction. A similar reaction of 1 with KO2 in a toluene solvent in the presence of excess PPh3 did not produce any LONSO2 adduct, but rather a peak at m/z 725 corresponding to the [Ru(LONS)(PPh3)Cl] fragment was detected. This indicates that, in similar conditions, 1 does not react with the superoxide ion. On the basis of the results achieved, it is proposed that the carbonyl precursor reacts with LONS3−, affording the [RuII(LONS3−)CO]− state, which in the presence of air undergoes oxidation to LONS•2−, producing an open-shell state, 2 and superoxide, as depicted in Scheme 5. The existence of 2− was justified by a reversible cathodic 2/2− wave in cyclic voltammetry in CH2Cl2 in the presence of PPh3 [see Figure 5b(ii)]. The superoxide ion reacts with 2, generating an arylperoxo derivative, and promotes the cleavage reaction, as illustrated in Scheme 4. No cleavage reaction of the isolated sample of 2 was established in the presence of a O2 molecule, while 2 reacts with potassium superoxide, yielding 4. The redox reaction of 1 with O2 is completely different, as established by Reaction 3. In toluene, no reaction occurs with 1 and O2 (Reaction 1). Reaction 3 was performed in boiling ethanol. The violet solution obtained initially from Reaction 3 was similar to that of Reaction 1, and the formation of 1 having a [Ru(LONS)Cl] unit as an intermediate of this reaction was authenticated by ESI-MS. However, upon prolonged boiling in ethanol, dissociation of PPh3 and oxidation of the thiolato function to −SO(OEt) occurred. The violet solution turned green due to the formation of 3, which is a dichlororuthenium(II) complex of a sulfoxide derivative. DFT calculations were performed to compare the groundstate energies of the plausible intermediates of the cleavage reaction of 2. The calculations infer that 2 → 4 conversion is thermodynamically favorable. The gas-phase geometries of the benzosemiquinone radical (2′), arylperoxide (A′), dioxetane

(B′), and the mononuclear cleaved carboxylate (4′) intermediates, as illustrated in the Experimental Section of the DFT calculations, were optimized at the B3LYP level of theory using similar basis sets. 2′ was optimized with the doublet spin state, while monoanionic A′, neutral B′, and 4′ were optimized with the singlet spin state. The relative energies of the intermediates are summarized in Table S5. It is observed that each step of the 2′ → A′ → B′ → 4′ conversion is energetically favorable, as shown in Chart S2. The formation of B′ from A′ occurs by a release of ∼1292 kJ mol−1, when the entropy of the reaction remains unchanged. Morokuma et al. predicted similar mechanisms for the oxidative cleavage of the pyrrole ring of Ltryptophan catalyzed by indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase.36

■

CONCLUSION The coordination chemistry of a new redox-noninnocent o-(Narylamino)thiophenol derivative, LONSH3, with a Ru ion is explored. Three ruthenium complexes of the types trans[Ru(LONS)(PPh3)2Cl] (1), trans-[RuII(LONS•2−)(PPh3)2(CO)] (2), and cis-[RuII(LONSO0)(PPh3)Cl2] (3) are reported. 1 is a delocalized state of [RuII(LONS−)(PPh3)2Cl] ↔ [RuIII(LONS•2−)(PPh3)2Cl], where LONS− is a resonance hybrid of o-iminobenzoqunone and o-iminothiobenzoquinone and LONS•2− is a resonance hybrid of o-iminobenzosemiqunonate(1−) and o-iminothiobenzosemiquinonate(1−) anion radicals. The study disclosed the different reactivities of [Ru(LONS)] with different electronic states. It is established that the open-shell π-radical state [RuII(LONS•2−)(CO)] reacts with a superoxide ion and promotes an aromatic ring cleavage reaction, affording a dinuclear complex, [RuII(LNS2−)(PPh3)(CO)]2 (4; LNS2− = α-N-arylimino-ω-keto-carboxylate). 1 having a CSS ground electronic state, [RuII(LONS−)Cl], is inert under similar conditions and does not promote any ring cleavage reaction. 3373


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Inorganic Chemistry However, in a different reaction, oxidation of LONS3− to a neutral quinone sulfoxide derivative (LONSO0) in boiling ethanol in air was authenticated by isolating 3. The different reactivities of LONS coordinated to a Ru ion having different electronic states are noteworthy. Thus, elucidation of the electronic structures of the transition-metal complexes of LONS is a subject of investigation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02862. ESI-MS data, bond parameters of the different states of LONOH3, LNNNH3, and LSNSH3 in complexes, calculated excitation energies and oscillator strengths, energies of the optimized geometries, and gas-phase-optimized coordinates (PDF) X-ray crysatallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-33-2428-7347. Fax: +91-33-2477-3597. ORCID

Prasanta Ghosh: 0000-0002-2925-1802 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Su.M. [Grant 08/531(0004)/2010-EMR-I] and Sa.M. (Grants 11-24/2013/SA-I and 766/FRD-III), respectively are grateful to CSIR and UGC, New Delhi, India, for fellowships. We thank UGC [Grant 43-214/2014(SR)], SERB-DST (Grant EMR/ 2016/005222), and WB-DST, India, for funding.

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REFERENCES

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