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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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High Catalytic Activity of Vanadium Complexes in Alkane Oxidations with Hydrogen Peroxide: An Effect of 8‑Hydroxyquinoline Derivatives as Noninnocent Ligands Izabela Gryca,† Katarzyna Czerwińska,† Barbara Machura,*,† Anna Chrobok,‡ Lidia S. Shul’pina,§ Maxim L. Kuznetsov,*,∥ Dmytro S. Nesterov,∥ Yuriy N. Kozlov,⊥,# Armando J. L. Pombeiro,∥ Ivetta A. Varyan,⊥ and Georgiy B. Shul’pin*,⊥,# †

Department of Crystallography, Institute of Chemistry, University of Silesia, 9th Szkolna Street, 40-006 Katowice, Poland Department of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland § Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Ulitsa Vavilova, 28, 119991 Moscow, Russia # Semenov Institute of Chemical Physics, Russian Academy of Sciences, Ulitsa Kosygina, dom 4, Moscow, Russia ∥ Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal ⊥ Plekhanov Russian University of Economics, Stremyannyi pereulok, dom 36, Moscow 117997, Russia ‡

S Supporting Information *

ABSTRACT: Five monomeric oxovanadium(V) complexes [VO(OMe)(N∩O)2] with the nitro or halogen substituted quinolin-8-olate ligands were synthesized and characterized using Fourier transform infrared, 1H and 13C NMR, high-resolution mass spectrometry−electrospray ionization as well as X-ray diffraction and UV−vis spectroscopy. These complexes exhibit high catalytic activity toward oxidation of inert alkanes to alkyl hydroperoxides by H2O2 in aqueous acetonitrile with the yield of oxygenate products up to 39% and turnover number 1780 for 1 h. The experimental kinetic study, the C6D12 and 18O2 labeled experiments, and density functional theory (DFT) calculations allowed to propose the reaction mechanism, which includes the formation of HO· radicals as active oxidizing species. The mechanism of the HO· formation appears to be different from those usually accepted for the Fenton or Fenton-like systems. The activation of H2O2 toward homolysis occurs upon simple coordination of hydrogen peroxide to the metal center of the catalyst molecule and does not require the change of the metal oxidation state and formation of the HOO· radical. Such an activation is associated with the redox-active nature of the quinolin-8olate ligands. The experimentally determined activation energy for the oxidation of cyclohexane with complex [VO(OCH3)(5Cl-quin)2] (quin = quinolin-8-olate) is 23 ± 3 kcal/mol correlating well with the estimate obtained from the DFT calculations.

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[V2O3]n+ (n = 2−4), and [V2O4]2+. All these moieties exhibit especially strong affinity toward N,O-donor ligands, and the electronic property of the metal center in the resulting complexes can be tuned by use of organic ligands of different basicity or introduction of electron-withdrawing or electrondonating groups in the framework of organic ligand. Their facile interconversion in the presence of suitable ligand environment, type of solvent, and pH of the reaction medium, also creates an attractive area of contemporary research for inorganic chemists.7 Earlier, some of us discovered and studied in detail a system that oxidizes saturated and aromatic hydrocarbons with hydrogen peroxide in acetonitrile solution in air at 20−50 °C.8 This efficient system consists of simple vanadate (nBu4N)VO3 and pyrazine-2-carboxylic acid (PCA; see ref 9 for a review of the accelerating effect of PCA). Vanadate anion in the

INTRODUCTION A widespread interest in vanadium coordination chemistry is attributed to the importance of these compounds due to biochemical, pharmacological, and catalytic activity.1 It has been evidenced that several groups of organisms in the marine and terrestrial environment accumulate vanadium and employ it in life processes.2 In recent years, the stimulation and inhibition of many enzymes by vanadium compounds as well as the active role of this metal in enzymes, such as vanadium-dependent nitrogenases and haloperoxidases, has been well-recognized.3 Also, the pharmacological potential of vanadium compounds has been successfully exploited in the treatment of type I and type II diabetes as well as cancer.4 Most importantly, vanadium compounds have been documented to act as effective catalysts for the syntheses of polyolefin materials5 as well as for the oxidation, oxidative halogenation, and sulfoxidation of a variety of organic substrates.6 The most studied and active vanadium compounds are associated with such cores as [VO]2+, [VO]3+, [VO2]+, © XXXX American Chemical Society

Received: October 23, 2017

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DOI: 10.1021/acs.inorgchem.7b02684 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for 1−5 (1·1/2CHCl3) empirical formula formula weight temperature [K] wavelength [Å] crystal system space group unit cell dimensions [Å, deg]

C39H27Cl7N4O8V2 1029.67 295.0(2) 0.710 73 triclinic P1̅ a = 12.2116(5) b = 13.9935(5) c = 14.3515(5) α = 113.404(3) β = 104.994(3) γ = 93.524(3) volume [Å3] 2136.39(15) Z 2 density (calculated) [mg/m3] 1.601 absorption coefficient 0.931 [mm−1] F(000) 1036 crystal size [mm] 0.224 × 0.188 × 0.116 θ range for data collection 3.98 to 27.86 [deg] index ranges −11 ≤ h ≤ 14 −16 ≤ k ≤ 16 −17 ≤ l ≤ 17 reflections collected 16 143 independent reflections 7571 (Rint = 0.026) completeness to 2θ = 50° [%] 99.8 max and min transmission 0.667 and 1.000 data/restraints/parameters 7571/0/543 goodness-of-fit on F2 1.079 final R indices [I > 2σ(I)] R1 = 0.0597 wR2 = 0.1628 R indices (all data) R1 = 0.0799 wR2 = 0.1746 largest diff peak and hole 0.92 and −0.61 [e Å−3] CCDC no. 1559829

(2·1/2CHCl3)

(3)

C39H27Cl3N8O16V2 1071.91 295.0(2) 0.710 73 triclinic P1̅ a = 12.7036(7) b = 13.8491(5) c = 14.1716(6) α = 112.581(4) β = 103.658(4) γ = 94.781(4) 2194.54(19) 2 1.622 0.690

(4)

(5)

C19H11Cl4N2O4V 524.04 295.0(2) 0.710 73 monoclinic P21/c a = 8.9361(8) b = 15.5069(12) c = 14.8480(12)

C19H11Cl2I2N2O4V 706.94 295.0(1) 0.710 73 monoclinic P21/c a = 9.2269(4) b = 15.8424(7) c = 15.0565(7)

C19H11I4N2O4V 889.84 295.0(2) 0.710 73 monoclinic P21/c a = 9.4038(7) b = 15.7241(10) c = 15.8518(12)

β = 95.597(7)

β = 95.380(4)

β = 95.538(7)

2047.7(3) 4 1.700 1.037

2191.21(17) 4 2.143 3.543

2333.0(3) 4 2.533 5.746

1084 0.222 × 0.162 × 0.087 3.92 to 26.89

1048 0.142 × 0.094 × 0.018 3.41 to 25.04

1336 0.235 × 0.152 × 0.066 3.34 to 25.24

1624 0.110 × 0.080 × 0.058 3.53 to 26.19

−14 ≤ h ≤ 15 −15 ≤ k ≤ 16 −16 ≤ l ≤ 16 16 097 7758 (Rint = 0.030) 99.7 0.643 and 1.000 7758/0/615 1.066 R1 = 0.0708 wR2 = 0.2115 R1 = 0.0932 wR2 = 0.2299 1.26 and −0.84

−9 ≤ h ≤ 10 −15 ≤ k ≤ 18 −12 ≤ l ≤ 17 8588 3618 (Rint = 0.068) 99.8 0.740 and 1.000 3618/0/273 1.011 R1 = 0.0771 wR2 = 0.1835 R1 = 0.1621 wR2 = 0.2209 1.32 and −0.44



1559828

1559825

1559826

1559827

the classic (n-Bu4N)VO3 + PCA system (see below about this system) as well as with other vanadium-containing catalytic systems. Theoretical density functional theory (DFT) methods were applied to elucidate the reaction mechanism, which appears to be different from the conventional mechanisms usually proposed for the Fenton or Fenton-like systems.

absence of PCA is almost inactive in the oxidation. The reaction mechanism has been studied by various methods.10 Arenes are oxidized to phenols, whereas alkanes, RH, are transformed into corresponding alkyl hydroperoxides, ROOH, which, in the course of reaction, slowly decompose to produce a mixture of alcohol and ketone. All peroxidic compounds including H2O2 and ROOH can be easily reduced with PPh3 to H2O and ROH. The present work concerns the monomeric oxovanadium(V) complexes 1−6 of the type [VO(OMe)(N∩O)2] incorporating 8-hydroxyquinoline (quinH) and its derivatives (5-Cl-quinH, 5NO2-quinH, 5,7-Cl2-quinH, 5,7-Cl,I-quinH, and 5,7-I2-quinH). The main research objective was to examine the impact of the nitro and the halogen substituents on the catalytic activity of [VO(OMe)(N∩O)2]. The related complex [VO(O-iPr)(quin)2] has been shown to efficiently catalyze the oxidation of benzylic, allylic, and propargylic alcohols with air in the presence of NEt3 (ref 11). Complex [H3O][VO2(quin)2] was used in the bromination reactions with phenol red or xylene cyanole as substrate under acidic conditions, and it exhibited high activity with a catalytic turnover of ∼315 mol product per mol of complex per hour.12 Here we study the catalytic potential of complexes [VO(OMe)(N∩O)2] (1−6) in the oxidation of alkanes with H2O2 and compare their activity with

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EXPERIMENTAL SECTION

Materials. The reagents for the synthesis were commercially available and used without further purification. The compound [VO(OCH3)(quin)2] (6) was prepared as previously described by Blair et al.13 Since the complexes are stable toward air and moisture, the syntheses and all operations were performed under open air conditions. Syntheses of Complexes 1−5. VO(acac)2 (0.26 g, 1 mmol) suspended in hot methanol (20 mL) was added to the corresponding 8-hydroxyquinoline derivative (2 mmol) dissolved in acetic acid (30 mL). The resulting solution was refluxed for 2 h, and after several days dark purple (1, 3, 4, and 5) or dark red (2) crystalline solid of the vanadium(V) complex was filtered. The crystals suitable for X-ray analysis were obtained by recrystallization from MeOH/CHCl3 (1:1 v/v). [VO(OCH3)(5-Cl-quin)2]·1/2CHCl3 (1·1/2CHCl3). Yield 60%. Highresolution mass spectrometry (HRMS) electrospray ionization (ESI): calcd for C18H10N2O3Cl2V 422.9508, found 422.9505. IR (KBr, cm−1): B


Article

Inorganic Chemistry

atoms were placed in calculated positions and refined with riding constraints: d(C−H) = 0.93 Å, Uiso(H) = 1.2 Ueq(C) (for aromatic) and d(C−H) = 0.96 Å, Uiso(H) = 1.5 Ueq(C) (for methyl). The methyl groups were allowed to rotate about their local threefold axis. Details of the crystallographic data collection, structural determination, and refinement for 1−5 are given in Table 1, whereas selected bond lengths and angles for them are listed in Table S1. Crystallographic data for 1−5 were deposited with the Cambridge Crystallographic Data Center, CCDC 1559825−1559829. Additional crystallographic information is available in the Supporting Information. Physical Measurements. The NMR spectra were recorded (295 K) on a Bruker Advance 400 MHz NMR spectrometer using CDCl3 or deuterated dimethyl sulfoxide (DMSO-d6) as the solvent and tetramethylsilane (TMS) as an internal standard (Figures S1 and S2). HRMS analyses were performed on a Waters Xevo G2 Q-TOF mass spectrometer (Waters Corporation) equipped with an ESI source operating in both positive- and negative-ion modes (Figure S3). Fullscan MS data were collected from 100 to 1000 Da in positive and negative ion modes with a scan time of 0.5 s. To ensure accurate mass measurements, data were collected in centroid mode and mass was corrected during acquisition using leucine enkephalin solution as an external reference (Lock-Spray), which generated reference ion at m/z 556.2771 Da ([M + H]+) in positive ESI mode and at m/z 554.2615 Da ([M−H]−) in negative-ion mode. The accurate mass and composition for the molecular ion adducts were calculated using MassLynx software (Waters) incorporated with the instrument. The IR spectra were recorded on a Nicolet iS5 spectrophotometer in the spectral range of 4000−400 cm−1 with the samples in form of KBr pellets (Figure S4). The electronic spectra were obtained using Nicolet Evolution 220 in the range of 240−1000 nm in chloroform, DMSO, or methanol (Figures S5−S7 and Table S2). UV−vis spectroscopy was also used to study the stability of the V(V) complexes in chloroform and methanol. The concentration of 1−5 in the final samples was 2.5 × 10−5 M, and the resulting solutions were monitored by collecting a spectrum once every 4 h over 24 h at room temperature. Oxidation of Alkanes. Catalyst 1−5 was introduced into the reaction mixture in the form of solid powder. Acetonitrile was used as a solvent. The alkane was then added, and the reaction started when hydrogen peroxide was introduced in one portion. (Caution! The combination of air or molecular oxygen and H2O2 with organic compounds at elevated temperatures may be explosive.) The reactions after addition of nitromethane as a standard compound were analyzed by gas chromatography (GC). In accord with the previously reported procedure,15 the samples obtained in the alkane oxidation were typically analyzed twice (before and after their treatment with PPh3) by GC (instruments: (i) the chromatograph-3700 constructed at Nesmeyanov Institute of Organoelement Compounds; fused silica capillary column FFAP/OV-101 20/80 w/w, 30 m × 0.2 μm × 0.3 μm; argon as a carrier gas and (ii) the PerkinElmer Clarus 500 gas chromatograph, equipped by a polar capillary column, SGE BP-20; 30 m × 0.32 mm × 25 μm, and a flame ionization detector (FID)). This method (the comparison of chromatograms of the same sample obtained before and after addition of PPh3), which was proposed by one of us earlier,15 allows us to estimate the real concentration of an alkyl hydroperoxide, ketone (aldehyde), and alcohol present in the reaction solution. Addition of solid PPh3 to the aliquot taken from the reaction mixture immediately quenches the reaction. In experiments without the addition of PPh3, the reaction in aliquot was quenched by its rapid cooling to the room temperature. The typical time frame between the consecutive points analyzed was 20 min, 1 h, or 2 h. Samples for the analysis with and without addition of PPh3 were taken simultaneously. Attribution of peaks was made by comparison with chromatograms of authentic samples and by GC-MS. In our kinetic studies described below, we measured concentrations of cyclohexanone and cyclohexanol only after reduction of the reaction mixture with PPh3, because in this case we measure precisely concentration of a sum of the oxygenates. Blank experiments with cyclohexane showed that, in the absence of a catalyst, products were formed in negligible concentrations.

2900(w), 2801(w), 1600(w), 1573(m), 1495(s), 1462(s), 1400(w), 1369(s), 1310(s), 1256(w), 1153(w), 1080(m), 955(s), 824(m), 783(m), 763(s), 754(s), 676(w), 628(m), 606(m), 596(m), 546(s), 460(m). 1H NMR (400 MHz, CDCl3) δ: 8.64 (d, J = 4.3 Hz, 1H), 8.50 (d, J = 4.0 Hz, 1H), 8.43 (d, J = 8.4 Hz, 1H), 8.37 (d, J = 8.4 Hz, 1H), 7.60 (d, J = 8.3 Hz, 2H), 7.39 (dd, J = 8.3, 4.8 Hz, 1H), 7.34 (dd, J = 8.4, 4.5 Hz, 1H), 7.14 (dd, J = 8.3, 3.0 Hz, 2H), 5.60 (s, 3H). 13C NMR (100 MHz, CDCl3) δ: 163.27, 162.11, 146.68, 146.41, 136.07, 135.04, 129.98, 129.79, 128.64, 127.10, 122.93, 120.53, 117.77, 111.72, 110.50, 78.03. UV−vis (CHCl3, λmax, nm (ε, dm3·mol−1·cm−1)): 245 (50 100), 327 (5800), 398 (4800), 497 (3200). [VO(OCH3)(5-NO2-quin)2]·1/2CHCl3 (2·1/2CHCl3). Yield 41%. HRMS (ESI): calcd for C18H10N4O7V 444.9989, found 444.9987. IR (KBr, cm−1): 2899(w), 2800(w), 1607(m), 1568(m), 1499(s), 1465(s), 1378(m), 1294(s), 1274(s), 1192(m), 1143(m), 1093(s), 1053(s), 999(m), 965(s), 845(w), 816(w), 791(m), 752(s), 739(m), 664(w), 626(m), 491(s), 410(w). 1H NMR (400 MHz, CDCl3) δ: 9.45 (d, J = 8.8 Hz, 1H), 9.31 (d, J = 8.7 Hz, 1H), 8.77 (d, J = 8.9 Hz, 1H), 8.73−8.69 (m, 2H), 8.50 (d, J = 3.7 Hz, 1H), 7.62 (dd, J = 8.6, 4.7 Hz, 1H), 7.54 (dd, J = 8.6, 4.4 Hz, 1H), 7.21 (dd, J = 8.8, 3.2 Hz, 2H), 5.75 (s, 3H). 13C NMR (100 MHz, CDCl3) δ: 170.73, 168.81, 147.11, 146.67, 140.37, 138.84, 137.43, 136.16, 135.56, 133.86, 132.10, 130.78, 125.77, 125.59, 124.51, 123.76, 110.65, 109.58, 80.13. UV−vis (CHCl3, λmax, nm (ε, dm3·mol−1·cm−1)): 246 (42 800), 300 (12 100), 364 (17 300), 403 (21 700), 467 (7500). [VO(OCH3)(5,7-Cl2-quin)2] (3). Yield 83%. HRMS (ESI): calcd for C18H8N2O3Cl4V 490.8724, found 490.8729. IR (KBr, cm−1): 2900(w), 2805(w), 1596(w), 1567(m), 1489(s), 1456(s), 1396(m), 1375(s), 1365(s), 1293(w), 1236(m), 1200(m), 1144(m), 1109(m), 1050(s), 973(m), 962(s), 896(m), 870(w), 810(m), 764(s), 668(m), 612(m), 597(m), 516(m), 497(s), 478(m). 1H NMR (400 MHz, CDCl3) δ: 8.62 (d, J = 4.2 Hz, 1H), 8.46 (d, J = 4.3 Hz, 1H), 8.43 (d, J = 8.4 Hz, 1H), 8.36 (d, J = 8.4 Hz, 1H), 7.68 (s, 2H), 7.44 (dd, J = 8.3, 4.7 Hz, 1H), 7.38 (dd, J = 8.3, 4.4 Hz, 1H), 5.69 (s, 3H). 13C NMR (100 MHz, CDCl3) δ: 166.04, 157.75, 147.76, 147.43, 141.89, 141.77, 141.76, 136.36, 135.32, 130.44, 129.52, 122.88, 120.51, 117.93, 78.94. UV−vis (CHCl3, λmax, nm (ε, dm3·mol−1·cm−1)): 251 (55 700), 336 (6500), 405 (6000), 497 (3900). [VO(OCH3)(5,7-Cl,I-quin)2] (4). Yield 90%. HRMS (ESI): calcd for C18H8N2O3Cl2VI2 674.7449, found 674.7441. IR (KBr, cm−1): 2905(w), 2797(w), 1575(m), 1557(m), 1484(s), 1447(s), 1393(m), 1372(s), 1360(s), 1232(m), 1220(m), 1135(w), 1104(m), 1048(s), 969(s), 961(s), 858(m), 809(w), 781(w), 759(s), 711(w), 660(m), 608(m), 575(m), 513(w), 485(s), 467(w). 1H NMR (400 MHz, CDCl3) δ: 8.57 (s, 1H), 8.46−8.30 (m, 3H), 7.95 (d, J = 6.8 Hz, 2H), 7.46 (s, 1H), 7.40 (s, 1H), 5.63 (s, 3H). 13C NMR (100 MHz, CDCl3) δ: 162.98, 162.19, 147.63, 147.24, 139.10, 137.97, 136.94, 136.32, 136.13, 135.29, 127.29, 126.80, 123.14, 120.79, 118.08, 78.49. UV−vis (CHCl3, λmax, nm (ε, dm3·mol−1·cm−1)): 259 (63 400), 341 (8100), 405 (6800), 492 (4900). [VO(OCH3)(5,7-I2-quin)2] (5). Yield 74%. HRMS (ESI): calcd for C18H8N2O3VI4 858.6147, found 858.6153. IR (KBr, cm−1): 2897(w), 2799(w), 1551(m), 1474(s), 1446(s), 1387(m), 1364(s), 1354(s), 1278(w), 1240(m), 1135(w), 1104(m), 1407(s), 964(s), 930(m), 855(m), 808(m), 780(m), 754(s), 657(s), 606(m), 567(s), 511(w), 471(s). 1H NMR (400 MHz, CDCl3) δ: 8.52 (d, J = 4.8 Hz, 1H), 8.41 (s, 1H), 8.37 (s, 1H), 8.32 (dd, J = 4.1, 1.4 Hz, 1H), 8.23 (dd, J = 8.4, 1.0 Hz, 1H), 8.16 (dd, J = 8.5, 1.2 Hz, 1H), 7.43 (dd, J = 8.5, 4.8 Hz, 1H), 7.36 (dd, J = 8.5, 4.6 Hz, 1H), 5.63 (s, 3H). UV−vis (CHCl3, λmax, nm (ε, dm3·mol−1·cm−1)): 261 (63 800), 341 (9200), 414 (8400), 493 (6000). X-ray Crystal Structure Determination. The X-ray diffraction data for complexes 1−5 were collected using Oxford Diffraction fourcircle diffractometer Gemini A Ultra with Atlas CCD detector using graphite monochromated Mo Kα radiation (λ = 0.710 73 Å) at room temperature. Diffraction data collection, cell refinement, and data reduction were performed using the CrysAlisPro software.14a The structures were solved by the direct methods using SHELXS and refined by full-matrix least-squares on F2 using SHELXL-2014.14b All the non-hydrogen atoms were refined anisotropically, and hydrogen C


Article

Inorganic Chemistry Scheme 1. General Scheme of Syntheses of 1−6

Catalytic Reactions under 18O2 Atmosphere. The reactions were typically performed under N2 atmosphere in thermostated Schlenk tube under vigorous stirring. The reagents were introduced in the same order as for a normal catalytic reaction. After addition of H2O2 the Schlenk tube was closed with the septum, and the mixture was immediately frozen with liquid nitrogen; the gas atmosphere was pumped and filled with N2 a few times to remove air. The frozen mixture was left to warm under vacuum (to degasify) until becoming liquid, and the above procedure was repeated. Finally, the Schlenk tube was filled with 18O2 through the septum, and the reaction mixture was heated at 40 °C with a possibility of gas to escape to compensate the excessive pressure. Gas Chromatography and Mass Spectrometry. A PerkinElmer Clarus 500 gas chromatograph, equipped a polar capillary column (SGE BP-20; 30 m × 0.32 mm × 25 μm) and an FID detector, was used for analyses of the reaction products of alkanes oxidation. The following GC conditions were used: 100 °C (1 min), 100−160 °C (10°/min), 160 °C (1 min), 8 min total run time; 200 °C injector temperature. A PerkinElmer Clarus 600 gas chromatograph, equipped with two nonpolar capillary columns (SGE BPX5; 30 m × 0.32 mm × 25 μm), one having an electron impact (EI) MS detector and the other one with an FID detector, was used for detailed analyses of the reaction mixtures. The following GC conditions were used: 50 °C (3 min), 50−120 °C (8°/min), 120−300 °C (35°/min), 300 °C (3.11 min), 20 min total run time; 200 °C injector temperature. Helium was used as the carrier gas (constant 14 psi pressure and constant 1 mL min−1 flow for Clarus 500 and Clarus 600 devices, respectively). All EI mass spectra were recorded with 70 eV energy. The 16O/18O compositions of the oxygenated products were determined by the relative abundances of m/z mass peaks 57/59 for cyclohexanol, 90/ 100 for cyclohexanone, and at 116/118/120 for cyclohexyl hydroperoxide. Computational Details. The full geometry optimization of all structures and transition states (TSs) was performed at the DFT level of theory by using the M06 functional16a with the help of the Gaussian 09 program package.16b No symmetry operations were applied. The geometry optimization was performed by using a relativistic Stuttgart pseudopotential, which describes 10 core electrons (MDF10) and the appropriate contracted basis set (8s7p6d1f)/[6s5p3d1f]17 for the vanadium atoms and the 6-31+G* basis set for other atoms. Singlepoint calculations were performed on the basis of the equilibrium geometries found by using the 6-311+G** basis set for nonmetal atoms. The stability test was performed using the keyword STABLE in Gaussian 09, and the reoptimization of wave functions was performed when it was necessary to achieve a stable solution. The Hessian matrix was calculated analytically for the optimized structures to prove the location of correct minima (no imaginary frequencies) or saddle points (only one imaginary frequency) and to estimate the thermodynamic parameters, with the latter calculated at 25 °C. The nature of all transition states was investigated by analysis of the vectors associated with the imaginary frequency and by the calculations of the intrinsic reaction coordinates (IRC) by using the method developed by Gonzalez and Schlegel.18

The total energies corrected for solvent effects Es were estimated at the single-point calculations on the basis of gas-phase geometries at the CPCM-M06/6-311+G**//gas-M06/6-31+G* level of theory using the polarizable continuum model in the CPCM version19 with CH3CN as solvent. The United Atom topological model UAKS was applied for the molecular cavity, and dispersion, cavitation, and repulsion terms were taken into account. The entropic term in CH3CN solution (Ss) was calculated according to the procedure described by Wertz20a and Cooper and Ziegler20b using eqs 1−4: ΔS1 = R ln(V s m,liq /Vm,gas)

ΔS2 = R ln(V °m /V ,s

α = [S°

s

m,liq )

,s

liq

− (S°

(1) (2) ,s

gas

+ ΔS1)]/[S°

gas

+ ΔS1]

(3)

Ss = Sg + ΔSsol = Sg + [ΔS1 + α(Sg + ΔS1) + ΔS2] = Sg + [(− 12.21 cal/mol·K) − 0.23(Sg − 12.21 cal/mol·K) + 5.87 cal/mol·K]

(4)

where Sg is the gas-phase entropy of solute, ΔSsol is the solvation entropy, Sliqo,s, Sgaso,s, and Vm,liqs are the standard entropies and molar volume of the solvent in the liquid or gas phases (149.62 and 245.48 J/ mol·K and 52.16 mL/mol, respectively, for CH3CN), Vm,gas is the molar volume of the ideal gas at 25 °C (24 450 mL/mol), and Vmo is the molar volume of the solution that corresponds to the standard conditions (1000 mL/mol). The enthalpies and Gibbs free energies in solution (Hs and Gs, respectively) were estimated using the expressions 5 and 6. Hs = Es(6‐311+G**) + Hg(6‐31+G*) − Eg (6‐311+G**) (5)

Gs = Hs − TSs

(6)

where Es and Eg are the total energies in solution and the gas phase, and Hg is the gas-phase enthalpy calculated at the corresponding level. Gibbs free energies in solution are discussed in this work if not stated otherwise. For the proton transfer reactions assisted by a solvent (water) molecule, explicit consideration of the second shell H2O molecule is essential for the correct estimates of the activation energies. Therefore, for the initial species of these steps, one water molecule was added to the second shell. For the key complexes and intermediates, all possible geometrical isomers were calculated, and the most stable ones were further considered.

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RESULTS AND DISCUSSION Syntheses and Structures of the Complexes. The vanadium(V) complexes were obtained by reaction of [VIVO(acac)2] with 2 equiv of the corresponding 8-hydroxyquinoline derivative in open air (Scheme 1). As previously reported,21 during this reaction, vanadium(IV) undergoes oxidation by D


Article

Inorganic Chemistry

Figure 1. Perspective views of the asymmetric entities of the representative structures 1 (a) and 3 (b).

for the C−O stretching of the quinolin-8-olate occur at ∼1290−1360 cm−1. The 1H NMR spectra of 1−5 (Figure S1) reveal two sets of 1 H NMR signals for the attached ligands, in accordance with their inequivalent coordination geometry. The distinctive signals corresponding to the alkyl protons of methoxido group occur in the range of 5.60−5.75 ppm. Description of Structures. Perspective views of the asymmetric entities of the representative structures 1·1/2CHCl3 and 3 are shown in Figure 1. Identical atom numbering is adopted for the remaining compounds 2·1/2CHCl3, 4, 5, and the drawings of their molecular structures are included in Supporting Information (Figures S8−S10). Compounds 1 and 2 are isomorphous, and they crystallize in the triclinic space group P1̅, with chloroform and two independent vanadium(V) complex molecules in the asymmetric unit (Figure 1a). Two V(V) complexes display very similar structural parameters, that is, bond lengths and angles, as shown in Supporting Information (Table S1). The asymmetric units of the isomorphous compounds 3−5

molecular oxygen, and the acetylacetonato ligands are exchanged by the corresponding quinolin-8-olate ions to give [VO(OCH3)(5-Cl-quin)2]·1/2CHCl3 (1·1/2CHCl3), [VO(OCH3 )(5-NO 2 -quin) 2]·1/2CHCl 3 (2·1/2CHCl3 ), [VO(OCH3)(5,7-Cl2-quin)2] (3), [VO(OCH3)(5,7-Cl,I-quin)2] (4), [VO(OCH3)(5,7-I2-quin)2] (5), and [VO(OCH3)(quin)2] (6). The solvent CH3OH supplies the −OCH3 donor group and in this way stabilizes vanadium in the +5 oxidation state.22 Three of them, 1, 5, and 6, have been reported previously,13,23 but only 6 was synthesized in this way, and its structure was confirmed by X-ray analysis.13 All the quinolin-8-olate vanadium(V) complexes exhibit strong ν(VO) band in the range of 960−970 cm−1, whereas the region of 750−770 cm−1 is assigned to V−OCH3 fragment. The characteristic bands assignable to ν(CC) and ν(CN) vibration of coordinated quinolin-8-olate ions occur in the range of 1610−1550 cm−1. Compared to the free ligands, they are slightly shifted toward lower wavenumbers, indicating V−N bond formation. The band appearing in the 1045−1055 cm−1 region is attributed to ν(O−CH3), while characteristic bands E


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Inorganic Chemistry crystallizing in the monoclinic space group P21/c comprise only one molecule of the oxovanadium complex (Figure 1b). In all examined compounds, the vanadium(V) ion is sixcoordinate in a distorted [N2O4] octahedral geometry defined by two quinolin-8-olate bidentate ligands with the two N atoms bound trans to the oxo and methoxy groups. An angular distortion of [VO(OCH3)(N∩O)2] is attributed to the multiple bonding oxo ligand and the rigidity and geometrical constraints issued from the occurrence of five-member chelate rings of the quinolin-8-olate bidentate ligand. In the reported complexes, the bite angles of the chelating ligands fall from 77.09(11)° in 1 to 75.61(12)° in 2, and the dihedral angle between the leastsquares planes formed by the organic ligands is 89.50° and 89.77° in 1, 86.51° and 88.27° in 2, 88.50° in 3, 88.65° in 4, and 89.84° in 5. The vanadium atom is displaced from the plane formed by three oxygen and nitrogen atoms by 0.2562(15) Å and 0.2634(16) Å for 1, 0.2466(16) Å and 0.2483(17) Å for 2, 0.2618(26) Å for 3, 0.2702(24) Å for 4, and 0.2415(34) for 5. The repulsion exerted by the VO unit is clearly visible in increasing the angles O(1)−V(1)−O(2) and O(1)−V(1)−O(3) beyond 90° (in the range from 100.89° to 103.2(2)°). The VO bond lengths fall in the range of 1.603(4)−1.667(6) Å, which is generally typical of mononuclear complexes of vanadium(V).13,22,24,25 The longest VO distance has been reported for complex 5. The elongation of the VO bond in 5 is accompanied by the shortening of the V(1)−N(2) distance [2.250(6) Å] that indicates the electron density delocalization in the core [OV−quin] facilitated by the C(5)−I(1)···Cg(4)s [s = 1 − x, 2 − y, −z] interactions. The planar structure of quinolin-8-olate skeleton and presence of halogen or nitro substituents provide both geometric and electronic conditions to enable stacking π···π and X···π interactions in the reported crystal structures (Figure S11 and Tables S3−S5). As previously found in the related V(V) oxo complexes,13b,22b,24,25 the vanadium oxygen bond lengths follow the order: oxido < methoxy < phenolate (Table S1). A noticeable elongation of V(1)−N(2) in comparison with V(1)−N(1) in 1−4 is consistent with a stronger trans influence of the oxido group than the alkoxido one. Catalyzed Oxidation of Alkanes with H2O2 to the Corresponding Alkyl Hydroperoxides. We found that alkanes can be oxidized in acetonitrile solution to the corresponding alkyl hydroperoxides by hydrogen peroxide in air in the presence of catalytic amounts of complexes 1−6. The alkyl hydroperoxides are relatively stable in the solution and can be easily reduced by PPh3 to the corresponding alcohols. For the oxidation of cyclohexane, the chromatograms obtained after the reduction with PPh3 are different from those obtained for the unreduced samples (compare graphs A and B in Figure 2) indicating that cyclohexyl hydroperoxide is formed in this reaction.15 The initial reaction rates and kinetic curves of the oxygenate accumulation are similar for the different vanadium complexes 1−5 (see Figures 2 and S12 as examples). Kinetic curves are also similar to those obtained previously for the vanadate-PCA catalytic system. The maximum yields of the oxygenate products for 1−5 are also comparable and attain 33 ± 4% based on cyclohexane after 6 h (Table S6). Thus, it may be concluded that the nature of a substituent in the ligand does not significantly affect the efficiency of the catalyst. Further, the kinetic features of the reaction under study are discussed in detail for catalyst 1. Under optimal conditions ([1]0 = 2 × 10−4 M; [cyclohexane]0 = 0.46 M; [H2O2]0 = 2 M; 50 °C, 1 h) the yield was

Figure 2. Oxidation of cyclohexane (0.46 M) with H2O2 (50%, 2 M) catalyzed by complex 5 (2 × 10−4 M) at 40 °C. Concentrations of cyclohexanol (curves 1, red ●) and cyclohexanone (curves 2, blue ■) were measured both before (A) and after (B) reduction with PPh3.

39%. This is a very high value taking into account inertness of alkanes. Usually in the metal-catalyzed alkane oxidation with hydrogen peroxide yields of products are not higher than 10− 20%. Unlike the case of the alkane oxidation catalyzed by the simple vanadate, the reactions in the presence of complexes 1− 6 do not require the addition of PCA. Moreover, when PCA was added, the initial reaction rate was halved. Dependence of the initial rate W0 of the cyclohexane oxidation on temperature of the process (Figure 3) allowed us estimating the efficient activation energy Ea = 23 ± 3 kcal/mol. This value nicely correlates with the activation energy calculated by the DFT methods (see below). The dependence of the cyclohexane hydroperoxidation initial rate on the initial concentration of cyclohexane is presented in Figure 4. The mode of this dependence is in accord with our assumption that the limiting stage of cyclohexane oxidation is the interaction between cyclohexane and an intermediate species X generated from H2O2 under the action of vanadium complex 1. Species X interacts also with another component of the system under investigation, namely, with acetonitrile solvent. The simplest kinetic scheme can be presented by three equations: H 2O2 + catalyst 1 → X (rate Wi ) X + RH → ROOH (constant kRH)

X + MeCN → products (constant kMeCN)

Assuming quasi-stationary concentration of species X, the analysis of this scheme leads to the equation W0 =

F

d[ROOH] = dt 1+

Wi kMeCN[MeCN] kRH[RH] DOI: 10.1021/acs.inorgchem.7b02684 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

equation allowed us to measure values Wi and kMeCN[MeCN]/ kRH: Wi = 1.3 × 10−5 M s−1 and kMeCN[MeCN]/kRH = 0.10 M. Species X can interact with three components of the reaction mixture, that is, catalyst 1, H2O2, and MeCN. The rate of cyclohexane oxidation depends on [1] and [H2O2]. However, the ratio of the cyclohexane oxidation rates does not practically depend on the initial concentrations [1] or [H2O2]. For example, at [1] = 2 × 10−4 M and [H2O2] = 2 M, the ratio of oxidation rates at [RH] = 0.12 and 0.3 M is 0.73. At [1] = 5 × 10−4 M and the same other conditions, the rate ratio is 0.8. At concentrations [1] = 2 × 10−4 M and [H2O2] = 1 M, the ratio of oxidation rates at [RH] = 0.12 and 0.3 M is 0.7. These results allowed us the conclusion that cyclohexane competes with acetonitrile solvent for the oxidizing species. The value of parameter kMeCN[MeCN]/kRH = 0.10 M is in agreement with the assumption that the oxidizing species X is hydroxyl radical (Table S7).26 Experiments on the oxidations of n-heptane [positional selectivity C(1)/C(2)/C(3)/C(4) = 1:7.2:7.4:6.6], methylcyclohexane (bond selectivity, i.e., the ratio of reactivities of the primary, secondary, and tertiary C−H bonds 1:2:3 = 1:5:16.6), and cis-1,2-dimethylcyclohexane (stereoselectivity, i.e., the ratio trans-alcohol/cis-alcohol = 0.8) with the 1/H2O2 system additionally confirmed that the oxidation with H2O2 occurs with the participation of the hydroxyl radicals.26 We obtained additional information about the nature of the oxidizing species in experiments with some traps of free radicals. Addition of 2,6-bis(tert-butyl)-4-methylphenol (the inhibitor of free-radical chain reactions) does not substantially affect the reaction rate. However, in the presence of inhibitors of alkyl radicals, that is, CCl4 or CCl3Br (1 M) or CBr4 (0.01 M), the reaction rate is reduced ca. 10 times. In these cases, cyclohexyl chloride or cyclohexyl bromide is produced instead of cyclohexanol and cyclohexanone. The addition of a trap for oxygen-centered radicals (Ph2NH) suppresses the cyclohexane oxidation. Catalytic Reactions under 18O2 Atmosphere. Chromatograms recorded in the course of oxidation of cyclohexane (0.46 M) with H2O2 (2 M), catalyzed by complex 1 (2 × 10−4 M), revealed that the cyclohexyl hydroperoxide (Cy−OOH) is a main reaction product (Figure 5). Direct detection of Cy− OOH was possible due to the use of a low-polar GC column (SGE BPX5), instead of commonly used polar columns, according to recent observations.26b,27a Apart from cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide plus an

Figure 3. Oxidation of cyclohexane (0.46 M) with H2O2 (50%, 3 M) catalyzed by complex 1 (2 × 10−4 M) at different temperatures. Concentrations of cyclohexanol and cyclohexanone were measured after reduction with PPh3.

Figure 4. Oxidation of cyclohexane (RH) with H2O2 (50%, 2 M) catalyzed by complex 1 (2 × 10−4 M) at different concentrations of cyclohexane. Concentrations of cyclohexanol and cyclohexanone were measured after reduction with PPh3.

The analysis of experimental data (see Supporting Information) under conditions shown in Figures 4 and S12 using this

Figure 5. Fragments of the chromatograms showing the reaction products recorded before (top) and after (bottom) addition of the solid PPh3 to reaction sample (15 min reaction time). G


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Inorganic Chemistry

confirming that the latter is an intermediate toward alcohol. This is also in agreement with the observation that the 18O incorporation into cyclohexanol is only slightly affected by the addition of PPh3 to the reaction samples. In contrast, cyclohexanone revealed notable amounts of labeled oxygen only before the PPh3 addition (Figure 7). The negligible levels of Cy18O after addition of PPh3 can be explained by a rapid oxygen exchange between cyclohexanone and excess of nonlabeled water present in reaction mixture (as the hydrogen peroxide used was 50% aqueous).27b The most uncommon feature of the present catalytic system is the observation of a secondary peak at chromatograms (at ca. 10.7 min retention time), presumably attributable to a peroxide compound (Figure 8). This peak exhibits a gradual growth with the reaction time and disappears after reduction of the samples with PPh3 (Figures S14 and S15). No peaks other than those of cyclohexanol and cyclohexanone are observed at the chromatograms recorded after the treatment with PPh3 (Figure S15) confirming that the final yields can be calculated as sums of these two main products. The EI mass spectrum of the secondary peak shows signals up to 117 m/z (Figure S16) and was recognized by the massspectral library as acetone peroxide. However, the presence of signals at 91 m/z (absent in the reference spectrum of acetone peroxide)28 suggests that this assignment is not correct, although the structures of compounds may be similar. Both cyclohexyl hydroperoxide and second unknown compound were found to be relatively stable in solution. No significant changes in chromatographic pattern were observed after keeping the sample at −20 °C for 3 d (Figure 8). When the same reaction sample was then diluted (up to 30%) with D2O, the mass spectrum of Cy−OOH revealed expected +1 shift due to exchange of the −OOH proton with D+ from heavy water (Figure S17). The mass spectrum of the unrecognized peroxide compound was also changed (a new peak at 76 m/z appeared) but less pronouncedly (Figure S16, bottom). Finally, the presence of water caused partial decomposition of cyclohexyl hydroperoxide (in particular, to form cyclohexanone, which is almost absent before addition of D2O) and almost complete decomposition of a second peak (Figures 7 and 8). Theoretical Mechanistic Study. Mechanisms of catalytic reactions with the participation of vanadium compounds have been discussed in a number of works1a,g,h,2c,e,4b,29 including our previous publications.8−10 In the present work, we based our mechanistic considerations on the experimental kinetic model, selectivity parameters, effect of radical traps, and DFT calculations. With aim to elucidate the intimate details of the reaction mechanism and to uncover the driving forces of the process, the rate-limiting step of the whole reaction of alkane oxidation (i.e., the generation of the HO· radicals from H2O2) catalyzed by 1 was extensively investigated by theoretical (DFT) methods. Mechanism Based on the Simple Coordination of H2O2 (Mechanism I). The calculated O−O bond homolytic dissociation energy in free hydrogen peroxide in CH3CN solution is 45.1 kcal/mol in terms of Gibbs free energy (the calculated gas-phase bond dissociation enthalpy 48.6 kcal/mol is in good agreement with the experimental value of 48.75 ± 0.005 kcal/mol).30a This value is sufficiently high to keep H2O2 inactive toward the oxidation of alkanes in the absence of a catalyst. Therefore, the principal role of the catalyst should be an activation of H2O2 toward the homolytic O−O bond cleavage. The first step of such an activation is the coordination

unidentified one (see below), no other products were detected by GC-MS techniques. EI mass spectrum of deuterated cyclohexyl hydroperoxide resulted from the oxidation of C6D12 (under the same conditions) shows an expected +11 m/z shift of molecular ion peak, in a full agreement with the formula C6D11−OOH (Figure S13). A commonly accepted mechanism of the Cy−OOH formation is the hydroxyl radical attack to alkane with the subsequent capture of dioxygen.15b Hence, the incorporation of the labeled oxygen from 18O2 into oxygenates (first of all, alcohols) is a common indicator of the presence of alkyl radicals.26b,27b−d The same catalytic test was performed under the atmosphere of 18O2, and the chromatograms were recorded before and after addition of PPh3. The degree of the 18O incorporation into Cy−OOH with time is shown in Figure 6. The incorporation proceeds mostly in a double-labeling mode as should be expected if an 18O2 molecule is captured by the Cy· radical.

Figure 6. Incorporation of 18O from 18O2 into cyclohexyl hydroperoxide (Cy−OOH) to give single-labeled (red ●) and doubly labeled (black ■) species.

The maximum relative amount of Cy−18O18OH (50.7%) is observed at the beginning of the reaction (15 min reaction time, Figure 6), while only 24.5% of Cy−18O18OH was observed after 3 h. Such a decay can be explained by a catalytic decomposition of H216O2 with formation of 16O2 (in this way changing the 18O2 atmosphere to the mixed 18O2/16O2 one) reacting with alkyl radicals to form the nonlabeled product. The degree of the 18O incorporation into cyclohexanol (Figure 7) resembles that for the cyclohexyl hydroperoxide, thus

Figure 7. Incorporation of 18O from 18O2 into cyclohexanol and cyclohexanone measured before and after addition of PPh3. H


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Inorganic Chemistry

Figure 8. Fragments of chromatograms showing the reaction products recorded after 2 h reaction time (A), after keeping the same sample at −20 °C for 3 d (B) and following addition of D2O to the same sample (C). The peaks at ca. 10.7 min were not assigned.

Scheme 2. Most Favorable Mechanism I of the HO· Generation Involving One H2O2 Moleculea

a

Relative ΔGs values are indicated (in kcal/mol).

stabilized by addition of one electron to give the OH− anion. In other words, the activation of H2O2 toward homolysis requires the presence of an efficient reducing agent, which could reduce HO· to the more stable species. In the classic Fenton chemistry, the metal center of a catalyst plays the role of such reducing agent:

of H2O2 to a metal center. Since the coordination sphere of V is saturated in complex 1, the ligation of H2O2 requires the liberation of one coordination place. This can occur upon the cleavage of one of the V−N bonds to give the coordinatively unsaturated species 7 (Scheme 2). The cleavage of the V−N bond in the trans position to the methoxy ligand is by 1.9 kcal/ mol less favorable than the cleavage of the V−N bond trans to the oxo ligand. This step is endoergonic by 9.4 kcal/mol as the following coordination of H2O2 to give 8 (ΔGs = 9.5 kcal/mol). Very surprisingly, the energy of the HO−OH bond homolytic cleavage in 8 is very low being only 4.2 kcal/mol. In the following paragraphs we explain this curious finding. Upon the homolytic O−O bond cleavage in H2O2, two highly unstable HO· radicals are formed. To activate H2O2 toward this process, at least one of the HO· radicals should be

[MmLn]p + + H 2O2 → [M(m + 1)Ln](p + 1) + + HO· + OH−

where m is the oxidation state of the metal. Recently, it was found by some of us that not only the metal center but also some redox-active ligands (e.g., OOH−) may exhibit the same role in the activation of H2O2. It was found that hydrogen peroxide becomes highly activated in complexes [M(H2O)n(OOH)(H2O2)]p+ (M = Al, Ga, In, Sc, Y, La, Be, Zn, Cd, Bi) bearing the OOH− ligand, the HO−OH bond cleavage I


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Inorganic Chemistry

Mechanism I Involving Two H2O2 Molecules. In the previous works, 30b−d we demonstrated that two H 2 O 2 molecules participated in Mechanism I for the [M(H2O)n]p+ catalysts (Al, Ga, In, Sc, Y, La, Be, Zn, Cd, Bi)one H2O2 molecule is a direct source of HO·, while another H2O2 is a precursor of the redox-active OOH− ligand. Hence, the possibility of the involvement of the second H2O2 molecule in the case of catalyst 1 was also analyzed in this work. Several possible mechanisms of this type were calculated. However, all of them were found to be less favorable than Mechanism I discussed above (the corresponding activation barriers are 32.5−36.5 kcal/mol; see Supporting Information for details). Conventional (Less Favorable) Fenton-like Mechanism (Mechanism II). For the HO· radical generation catalyzed by complexes of metals in the highest oxidation state, the Fentonlike mechanism is generally accepted.10b,31 It includes the generation of the HOO· radical at the first step that is accompanied by the reduction of the metal. The HO· radical is then formed upon reduction of another H2O2 molecule by the reduced catalyst, which recovers its initial oxidation state at this second step:

energy being reduced by 24.1−40.2 kcal/mol in comparison with free H2O2.30b−d Analysis of the electronic structures indicated that upon the HO−OH bond rupture, reduction of the metal-bound HO· ligand and the oxidation of the OOH− ligand occur (Scheme 3). Since the HOO· radical is more stable Scheme 3. HO−OH Bond Cleavage and Intramolecular Electron Transfer in Complexesa [M(H2O)n(OOH)(H2O2)]p+

a

Oxidation state of the oxygen atoms is indicated.

than HO·, the resulting complex [M(H2O)n(OOH)·(OH)]p+ is significantly stabilized by such intramolecular electron transfer. Because of the involvement of the OOH− ligand in the radical generation process, the oxidation state of the metal remains unchanged during the whole reaction. This effect explained why complexes of some metals that have only one stable nonzero oxidation state (Al, Ga, In, Sc, Y, La, Be, Zn, Cd) display a significant Fenton-like catalytic activity. Theoretical calculations performed in this work indicate that the mechanism of the H2O2 activation in complex 8 is very similar. Indeed, as a result of the HO−OH bond rupture in 8, the HO· ligand bound to V is reduced to OH− (Figure 9). The

[MmLn]p + + H 2O2 → [M(m − 1)Ln](p − 1) + + HOO· + H+

[M(m − 1)Ln](p − 1) + + H 2O2 → [MmLn]p + + HO· + OH−

where m is the oxidation state of the metal. This mechanism was found to be feasible for several catalytic systems based on V(V)10g,32a and Re(VII).10d,32b The possibility of its realization for catalyst 1 was investigated in this work. This pathway starts with the coordination of H2O2 to vanadium (1 + H2O2 → 8). Since hydrogen peroxide and the oxo ligand in 8 are in the mutual trans position, the proton transfer to the oxo ligand may occur only in a stepwise manner via the sequence of steps 8 → 10 → 11 (Scheme 4). Elimination of HOO· in 11 gives complex 12. Coordination of the second H2O2 molecule, stepwise H-transfer from H2O2 to the OH ligand, and O−OH bond cleavage in 15 lead to HO· and complex 16. Initial catalytic form is regenerated after water elimination in 16. Alternatively, the HO· radical may be formed upon the HO−OH cleavage in 13. Analysis of the calculated energies shows that most of the Htransfer and radical elimination steps have significant activation barriers. As a result, the overall activation barrier of the whole process is very high (>60 kcal/mol). Thus, the calculations indicate that this mechanism is highly unfavorable in the case of catalyst 1. Several alternative mechanisms of this type were also analyzed in detail. However, all of them appeared to be less favorable than the most plausible mechanism shown in Scheme 2 (the corresponding activation barriers are 36.5−48.7 kcal/ mol; see Supporting Information for details).

Figure 9. HO−OH bond cleavage and intramolecular electron transfer in complex 8 and spin density distribution in [9]L·.

reducing species in this case is one of the 5-chloroquinolin-8olate ligands, which, upon oxidation, is decoordinated and goes to the second coordination sphere. In the resulting complex [V(O)(OMe)(OH)(L)]L· [9]L·, the unpaired electron is delocalized among the atoms of the uncoordinated 5-Cl-quin ligand. Thus, ability of the 5-Cl-quin ligand to be easily oxidized and signif icant delocalization of spin density in [9]L· are the factors that determine tremendous activation of H2O2 in 8. It is important that, as a result of the HO−OH bond cleavage in 8, the oxidation state of vanadium remains the same (+V). The calculated overall Gibbs free energy of activation and activation enthalpy for the HO· generation along this mechanism are 23.1 and 23.5 kcal/mol, respectively. The latter value is in good agreement with experimentally determined activation enthalpy for catalyst 1 (23 ± 3 kcal/mol, this work, see above). The oxidized quinolin-8-olate ligand can be reduced by another H2O2 molecule to give the HOO· radical and 8hydroxyquinoline (ΔGs = 7.4 kcal/mol), which protonates and substitutes the OH− ligand in 9 restoring the initial catalyst and completing the catalytic cycle (Scheme 2).

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FINAL REMARKS Vanadium complexes are promising catalytic systems, some of them exhibiting significant catalytic activity toward oxidation of inert alkanes in the presence of hydrogen peroxide as an ecologically benign oxidant. In this work, it was found that five new oxovanadium(V) complexes [VO(OMe)(N∩O)2] with the 8-hydroxyquinoline derived ligands not only efficiently catalyze the alkane oxidation with H2O2 but also demonstrate a type of the H2O2 activation that is different from the common Fenton or Fenton-like mechanisms. Such an activation is associated J


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Inorganic Chemistry Scheme 4. Conventional Fenton-like Mechanisma

a

The ΔGs values are indicated relative to 1 + H2O2 (in kcal/mol).

The calculations show that the mechanism of the HO· generation without a direct electronic involvement of the metal center developed previously for nontransition metal complex catalysts30b−d is also operating at least for some transition-metal complexes traditionally used in the Fenton chemistry. Some redox-active carbohydrazone-type ligands are believed to operate (on the basis of electrochemical studies and DFT calculations) in other oxidovanadium(V) catalysts for the microwave-assisted oxidation of cyclohexane with tert-butylhydroperoxide.6m Moreover, the redox activity of other N,O ligands (i.e., N-oxyiminodicarboxylates) was found recently in the reaction of water oxidation with Ce4+ catalyzed by amavadin and homologues.33

with the redox-active nature of the quinolin-8-olates, and it does not require a change of the metal oxidation state and formation of the HOO· radical in the course of reaction. Complexes [VO(OMe)(N∩O)2] were synthesized upon reaction of [VIVO(acac)2] with the nitro or halogen substituted 8-hydroxyquinoline and then characterized by Fourier transform infrared (FT-IR), 1H and 13C NMR, HRMS-ESI, as well as X-ray diffraction and UV−vis spectroscopy. The yield of oxygenates attains 39%, and turnover number (TON) is 1730. The experimental kinetic study, experiments with the labeled C6D12 and under 18O2 atmosphere, and theoretical DFT calculations indicated that the oxidation apparently involves generation of hydroxyl radicals that then react with alkane to give alkyl radicals R·, the most favorable mechanism being shown in Scheme 2. Upon reaction with molecular oxygen, alkyl radicals produce alkyl hydroperoxide, which is decomposed to the corresponding alcohol and ketone. It is usually accepted that the generation of the HO· radical from H2O2 catalyzed by transition-metal complexes occurs via Fenton or Fenton-like mechanism, which requires a change of the metal oxidation state during the reaction. In this mechanism, the metal plays the role of a reducing agent that stabilizes one of the formed HO· radicals, reducing it to the OH− anion. However, theoretical DFT calculations indicate that the mechanism of such type is not feasible for the catalyst 1, because the proton transfer and the HOO· elimination steps appear to be very energy demanding in overall. Instead, it was unexpectedly found that a simple coordination of the hydrogen peroxide to the metal center in 1 tremendously activates H2O2 toward the homolytic O−O bond cleavage. The main factors of such an activation are (i) ability of the 5-chloroquinolin-8-olate ligand to be easily oxidized playing the same role as a transition metal does in the classical Fenton chemistry and (ii) delocalization of spin electron density in a product of the HO−OH bond rupture among the oxidized 5-chloroquinolin8-olate moiety. These results suggest that, namely, the redoxactive 5-Cl-quin ligand in the catalyst molecule but not the metal center plays the crucial role in the generation of HO· radicals from H2O2 and, therefore, in the oxidation of alkanes.

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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.7b02684. X-ray crystallographic checkcif files for complexes 1−5. Description of less favorable reaction mechanisms, experimental bond lengths and angles, hydrogen bonds, stacking interactions, ESI mass spectra, FT-IR, UV−vis, NMR, XPRD spectra of compounds, GC-MS chromatograms, and EI mass spectra of reaction products, X-ray molecular structures and crystal packings, figures with details of the 18O and 2H labeled experiments, calculated energies and atomic coordinates of the equilibrium structures (PDF) Accession Codes

CCDC 1559825−1559829 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. K


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (B.M.) *E-mail: [email protected]. (M.L.K.) *E-mail: [email protected]. (G.B.S.) ORCID

Anna Chrobok: 0000-0001-7176-7100 Maxim L. Kuznetsov: 0000-0001-5729-6189 Dmytro S. Nesterov: 0000-0002-1095-6888 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partially supported by the Russian Foundation for Basic Research (Grant No. 16-03-00254) and the Fundaçaõ para a Ciência e a Tecnologia (FCT), Portugal (Project Nos. UID/QUI/00100/2013 and PTDC/QEQ-QIN/ 3967/2014, fellowship SFRH/BPD/99533/2014 (D.S.N.)).

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