High Catalytic Activity of Vanadium Complexes in Alkane Oxidations with Hydrogen Peroxide: An Effect of 8-Hydroxyquinoline Derivatives as Noninnocent Ligands

Article

pubs.acs.org/IC

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

High Catalytic Activity of Vanadium Complexes in Alkane Oxidations

with Hydrogen Peroxide: An Eﬀect of 8‑Hydroxyquinoline

Derivatives as Noninnocent Ligands

Izabela Gryca,^†Katarzyna Czerwinska,^†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 Tec

́

nico, 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 H₂O₂in aqueous acetonitrile with the yield of

oxygenate products up to 39% and turnover number 1780 for 1 h. The experimental

kinetic study, the C₆D₁₂and ¹⁸O₂labeled 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 H₂O₂toward 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-8-

olate ligands. The experimentally determined activation energy for the oxidation of cyclohexane with complex [VO(OCH₃)(5-

Cl-quin)₂] (quin = quinolin-8-olate) is 23 3 kcal/mol correlating well with the estimate obtained from the DFT calculations.

[V₂O₃]ⁿ⁺(n = 2−4), and [V₂O₄]²⁺. All these moieties exhibit

INTRODUCTION

■

especially strong aﬃnity toward N,O-donor ligands, and the

A widespread interest in vanadium coordination chemistry is

electronic property of the metal center in the resulting

complexes can be tuned by use of organic ligands of diﬀerent

attributed to the importance of these compounds due to

biochemical, pharmacological, and catalytic activity.¹It has been

basicity or introduction of electron-withdrawing or electron-

evidenced that several groups of organisms in the marine and

donating groups in the framework of organic ligand. Their facile

terrestrial environment accumulate vanadium and employ it in

life processes.²In recent years, the stimulation and inhibition of

interconversion in the presence of suitable ligand environment,

type of solvent, and pH of the reaction medium, also creates an

many enzymes by vanadium compounds as well as the active

attractive area of contemporary research for inorganic

role of this metal in enzymes, such as vanadium-dependent

chemists.⁷

nitrogenases and haloperoxidases, has been well-recognized.³

Also, the pharmacological potential of vanadium compounds

Earlier, some of us discovered and studied in detail a system

that oxidizes saturated and aromatic hydrocarbons with

has been successfully exploited in the treatment of type I and

type II diabetes as well as cancer.⁴Most importantly, vanadium

compounds have been documented to act as eﬀective catalysts

for the syntheses of polyoleﬁn materials⁵as well as for the

oxidation, oxidative halogenation, and sulfoxidation of a variety

of organic substrates.⁶

hydrogen peroxide in acetonitrile solution in air at 20−50

°C.⁸This eﬃcient system consists of simple vanadate (n-

Bu₄N)VO₃and pyrazine-2-carboxylic acid (PCA; see ref 9 for a

review of the accelerating eﬀect of PCA). Vanadate anion in the

The most studied and active vanadium compounds are

Received: October 23, 2017

associated with such cores as [VO]²⁺, [VO]³⁺, [VO₂]⁺,

A

DOI: 10.1021/acs.inorgchem.7b02684

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 1. Crystal Data and Structure Reﬁnement for 1−5

(1·1/2CHCl₃)

(2·1/2CHCl₃)

(3)

(4)

(5)

empirical formula

formula weight

temperature [K]

wavelength [Å]

crystal system

space group

C₃₉H₂₇Cl₇N₄O₈V₂

1029.67

C₃₉H₂₇Cl₃N₈O₁₆V₂

1071.91

C₁₉H₁₁Cl₄N₂O₄V

524.04

C₁₉H₁₁Cl₂I₂N₂O₄V

706.94

C₁₉H₁₁I₄N₂O₄V

889.84

295.0(2)

295.0(1)

295.0(2)

0.710 73

triclinic

monoclinic

P2₁/c

monoclinic

P2₁/c

monoclinic

P2₁/c

P1

̅

P1

̅

unit cell dimensions [Å, deg] a = 12.2116(5)

b = 13.9935(5)

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

a = 8.9361(8)

b = 15.5069(12)

c = 14.8480(12)

a = 9.2269(4)

b = 15.8424(7)

c = 15.0565(7)

a = 9.4038(7)

b = 15.7241(10)

c = 15.8518(12)

c = 14.3515(5)

α = 113.404(3)

β = 104.994(3)

β = 95.597(7)

β = 95.380(4)

β = 95.538(7)

γ = 93.524(3)

volume [Å³]

2136.39(15)

2

density (calculated) [mg/m³] 1.601

2047.7(3)

4

2191.21(17)

2333.0(3)

4

Z

4

1.622

1.700

1.037

2.143

3.543

2.533

5.746

absorption coeﬃcient

0.931

0.690

[mm⁻¹

]

F(000)

1036

1084

1048

1336

1624

crystal size [mm]

0.224 × 0.188 × 0.116 0.222 × 0.162 × 0.087 0.142 × 0.094 × 0.018 0.235 × 0.152 × 0.066 0.110 × 0.080 × 0.058

θ range for data collection

[deg]

3.98 to 27.86

3.92 to 26.89

3.41 to 25.04

3.34 to 25.24

3.53 to 26.19

index ranges

−11 ≤ h ≤ 14

−16 ≤ k ≤ 16

−17 ≤ l ≤ 17

16 143

−14 ≤ h ≤ 15

−15 ≤ k ≤ 16

−16 ≤ l ≤ 16

16 097

−9 ≤ h ≤ 10

−15 ≤ k ≤ 18

−12 ≤ l ≤ 17

8588

−9 ≤ h ≤ 10

−18 ≤ k ≤ 18

−17 ≤ l ≤ 17

10 142

−11 ≤ h ≤ 10

−18 ≤ k ≤ 16

−18 ≤ l ≤ 12

11 309

reﬂections collected

independent reﬂections

7571 (R_int= 0.026)

7758 (R_int= 0.030)

99.7

3618 (R_int= 0.068)

99.8

3850 (R_int= 0.032)

99.8

4123 (R_int= 0.033)

99.8

completeness to 2θ = 50° [%] 99.8

max and min transmission

data/restraints/parameters

goodness-of-ﬁt on F²

0.667 and 1.000

0.643 and 1.000

7758/0/615

1.066

0.740 and 1.000

3618/0/273

1.011

0.602 and 1.000

3850/0/272

1.053

0.671 and 1.000

4123/0/272

1.075

7571/0/543

1.079

ﬁnal R indices [I > 2σ(I)]

R1 = 0.0597

wR2 = 0.1628

R1 = 0.0708

wR2 = 0.2115

R1 = 0.0771

wR2 = 0.1835

R1 = 0.0501

wR2 = 0.1279

R1 = 0.0468

wR2 = 0.1205

R indices (all data)

R1 = 0.0799

wR2 = 0.1746

R1 = 0.0932

wR2 = 0.2299

R1 = 0.1621

wR2 = 0.2209

R1 = 0.0747

wR2 = 0.1395

R1 = 0.0693

wR2 = 0.1311

largest diﬀ peak and hole

0.92 and −0.61

1.26 and −0.84

1.32 and −0.44

2.44 and −1.57

2.13 and −1.21

[e Å⁻³

]

CCDC no.

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 H₂O₂and ROOH can be easily reduced with PPh₃to

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, 5-

NO₂-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.¹²Here we study the catalytic

potential of complexes [VO(OMe)(N^∩O)₂] (1−6) in the

oxidation of alkanes with H₂O₂and compare their activity with

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.

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.¹³Since 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%. High-

resolution mass spectrometry (HRMS) electrospray ionization (ESI):

calcd for C₁₈H₁₀N₂O₃Cl₂V 422.9508, found 422.9505. IR (KBr, cm⁻¹):

B

DOI: 10.1021/acs.inorgchem.7b02684

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

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),

atoms were placed in calculated positions and reﬁned with riding

constraints: d(C−H) = 0.93 Å, U_iso(H) = 1.2 U_eq(C) (for aromatic)

and d(C−H) = 0.96 Å, U_iso(H) = 1.5 U_eq(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.

1

460(m). H 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, λ_max, nm (ε, dm³·mol⁻¹·cm⁻¹)): 245

(50 100), 327 (5800), 398 (4800), 497 (3200).

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 CDCl₃or

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). Full-

scan 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⁻¹with 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

× 10⁻⁵M, 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 H₂O₂with 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,¹⁵the 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,¹⁵allows us to estimate the real concentration of an

alkyl hydroperoxide, ketone (aldehyde), and alcohol present in the

reaction solution. Addition of solid PPh₃to 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 PPh₃were 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 cyclo-

hexanone 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.

[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),

1

664(w), 626(m), 491(s), 410(w). H 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, λ_max, 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),

1

597(m), 516(m), 497(s), 478(m). H 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, λ_max, 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

C₁₈H₈N₂O₃Cl₂VI₂674.7449, 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, λ_max, 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

C₁₈H₈N₂O₃VI₄858.6147, 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,

λ_max, 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 four-

circle 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 CrysAlis^Prosoftware.^14aThe

structures were solved by the direct methods using SHELXS and

reﬁned by full-matrix least-squares on F²using SHELXL-2014.^14bAll

the non-hydrogen atoms were reﬁned anisotropically, and hydrogen

C

DOI: 10.1021/acs.inorgchem.7b02684

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. General Scheme of Syntheses of 1−6

Catalytic Reactions under ¹⁸O₂Atmosphere. The reactions

were typically performed under N₂atmosphere in thermostated

Schlenk tube under vigorous stirring. The reagents were introduced in

the same order as for a normal catalytic reaction. After addition of

H₂O₂the Schlenk tube was closed with the septum, and the mixture

was immediately frozen with liquid nitrogen; the gas atmosphere was

pumped and ﬁlled with N₂a 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 ¹⁸O₂through 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⁻¹ﬂow 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 hydro-

peroxide.

The total energies corrected for solvent eﬀects E_swere 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 version¹⁹with

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 (S_s) was calculated according to the procedure

described by Wertz^20aand Cooper and Ziegler^20busing eqs 1−4:

ΔS₁= R ln(V^s_m,liq/V_m,gas

)

(1)

(2)

(3)

ΔS₂= R ln(V°_m/V^s

)

m,liq

α = [S°^,s− (S°^,s+ ΔS₁)]/[S°^,s+ ΔS₁]

liq

gas

S_s= S_g+ ΔS_sol= S_g+ [ΔS₁+ α(S_g+ ΔS₁) + ΔS₂]

= S_g+ [(−12.21 cal/mol·K) − 0.23(S_g− 12.21 cal/mol·K)

+ 5.87 cal/mol·K]

(4)

where S_gis the gas-phase entropy of solute, ΔS_solis the solvation

s

entropy, S_liq^o,s, S_gas^o,s, and V_m,liqare 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), V_m,gasis the

o

molar volume of the ideal gas at 25 °C (24 450 mL/mol), and V_mis

the molar volume of the solution that corresponds to the standard

conditions (1000 mL/mol). The enthalpies and Gibbs free energies in

solution (H_sand G_s, respectively) were estimated using the expressions

5 and 6.

H_s= E_s(6‐311+G**) + H_g(6‐31+G*) − E_g(6‐311+G**)

(5)

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 functional^16awith the help of the Gaussian

09 program package.^16bNo 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]¹⁷for the

vanadium atoms and the 6-31+G* basis set for other atoms. Single-

point 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.

G_s= H_s− TS_s

(6)

where E_sand E_gare the total energies in solution and the gas phase,

and H_gis 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.

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

RESULTS AND DISCUSSION

■

Syntheses and Structures of the Complexes. The

vanadium(V) complexes were obtained by reaction of [V^IVO-

(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

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

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-NO₂-quin)₂]·1/2CHCl₃(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 −OCH₃donor 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⁻¹is assigned to V−OCH₃fragment.

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

for the C−O stretching of the quinolin-8-olate occur at

∼1290−1360 cm⁻¹.

1

The H NMR spectra of 1−5 (Figure S1) reveal two sets of

¹H 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

1·1/2CHCl₃and 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 P1, 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

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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 six-

coordinate 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 least-

squares 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 mono-

nuclear complexes of vanadium(V).^13,22,24,25The 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.

Figure 2. Oxidation of cyclohexane (0.46 M) with H₂O₂(50%, 2 M)

catalyzed by complex 5 (2 × 10⁻⁴M) 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 W₀of the cyclohexane

oxidation on temperature of the process (Figure 3) allowed us

estimating the eﬃcient activation energy E_a= 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 H₂O₂under 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:

Catalyzed Oxidation of Alkanes with H₂O₂to 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 PPh₃to the corresponding alcohols. For

the oxidation of cyclohexane, the chromatograms obtained after

the reduction with PPh₃are 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.¹⁵The 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.

H₂O₂+ catalyst 1 → X (rate W)

i

X + RH → ROOH (constant k_RH

)

X + MeCN → products (constant k_MeCN

)

Assuming quasi-stationary concentration of species X, the

analysis of this scheme leads to the equation

W

d[ROOH]

i

W =

=

0

Under optimal conditions ([1]₀= 2 × 10⁻⁴M; [cyclo-

hexane]₀= 0.46 M; [H₂O₂]₀= 2 M; 50 °C, 1 h) the yield was

k

_MeCN[MeCN]

dt

1 +

k

_RH[RH]

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equation allowed us to measure values W_iand k_MeCN[MeCN]/

k

_RH: W_i= 1.3 × 10⁻⁵M s⁻¹and k_MeCN[MeCN]/k_RH= 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 × 10⁻⁴M and [H₂O₂] = 2 M, the ratio of

oxidation rates at [RH] = 0.12 and 0.3 M is 0.73. At [1] = 5 ×

10⁻⁴M and the same other conditions, the rate ratio is 0.8. At

concentrations [1] = 2 × 10⁻⁴M 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 k_MeCN[MeCN]/k_RH= 0.10 M is in agreement with

the assumption that the oxidizing species X is hydroxyl radical

(Table S7).²⁶Experiments 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, CCl₄or 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.

Figure 3. Oxidation of cyclohexane (0.46 M) with H₂O₂(50%, 3 M)

catalyzed by complex 1 (2 × 10⁻⁴M) at diﬀerent temperatures.

Concentrations of cyclohexanol and cyclohexanone were measured

after reduction with PPh₃.

Catalytic Reactions under ¹⁸O₂Atmosphere. Chromato-

grams recorded in the course of oxidation of cyclohexane (0.46

M) with H₂O₂(2 M), catalyzed by complex 1 (2 × 10⁻⁴M),

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,27aApart from cyclo-

hexanol, cyclohexanone, and cyclohexyl hydroperoxide plus an

Figure 4. Oxidation of cyclohexane (RH) with H₂O₂(50%, 2 M)

catalyzed by complex 1 (2 × 10⁻⁴M) 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 PPh₃to

reaction sample (15 min reaction time).

G

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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.^15bHence, the incorporation of

the labeled oxygen from ¹⁸O₂into 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 ¹⁸O₂molecule is captured by the Cy· radical.

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 PPh₃to the reaction samples. In contrast,

cyclohexanone revealed notable amounts of labeled oxygen

only before the PPh₃addition (Figure 7). The negligible levels

18

of Cy O after addition of PPh₃can 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 chromato-

grams 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 mass-

spectral library as acetone peroxide. However, the presence of

signals at 91 m/z (absent in the reference spectrum of acetone

peroxide)²⁸suggests 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 works^{1a,g,h,2c,e,4b,29}including our

previous publications.⁸⁻¹⁰In 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.

Figure 6. Incorporation of ¹⁸O from ¹⁸O₂into cyclohexyl hydro-

●

peroxide (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 H₂¹⁶O₂with formation of ¹⁶O₂(in this way

changing the ¹⁸O₂atmosphere to the mixed ¹⁸O₂/¹⁶O₂one)

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

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).^30aThis 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 H₂O₂toward the homolytic O−O bond

cleavage. The ﬁrst step of such an activation is the coordination

Figure 7. Incorporation of ¹⁸O from ¹⁸O₂into cyclohexanol and

cyclohexanone measured before and after addition of PPh₃.

H

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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.

a

Scheme 2. Most Favorable Mechanism I of the HO· Generation Involving One H₂O₂Molecule

a

Relative ΔG_svalues are indicated (in kcal/mol).

of H₂O₂to a metal center. Since the coordination sphere of V is

saturated in complex 1, the ligation of H₂O₂requires 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 H₂O₂to give 8 (ΔG_s= 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.

stabilized by addition of one electron to give the OH⁻anion. In

other words, the activation of H₂O₂toward 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:

(p+1)+

[M^mL_n]^p++ H₂O₂→ [M^(m+1)L_n]

+ 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

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

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energy being reduced by 24.1−40.2 kcal/mol in comparison

with free H₂O₂.^30b−dAnalysis 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

Mechanism I Involving Two H₂O₂Molecules. In the

previous works,^30b−dwe demonstrated that two H₂O₂

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 H₂O₂is a

precursor of the redox-active OOH⁻ligand. Hence, the

possibility of the involvement of the second H₂O₂molecule

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 Fenton-

like mechanism is generally accepted.^10b,31It 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 H₂O₂molecule by the

reduced catalyst, which recovers its initial oxidation state at this

second step:

Scheme 3. HO−OH Bond Cleavage and Intramolecular

a

Electron Transfer in Complexes

[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 H₂O₂activation 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

(p−1)+

[M^mL_n]^p++ H₂O₂→ [M^(m−1)L_n]

+ HOO· + H⁺

(p−1)+

[M^(m−1)L_n]

+ H₂O₂→ [M^mL_n]^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,32aand Re(VII).^10d,32bThe possibility of its realization

for catalyst 1 was investigated in this work.

This pathway starts with the coordination of H₂O₂to

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 H₂O₂molecule, stepwise H-transfer from H₂O₂to

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.

Figure 9. HO−OH bond cleavage and intramolecular electron transfer

in complex 8 and spin density distribution in [9]L·.

Analysis of the calculated energies shows that most of the H-

transfer 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).

reducing species in this case is one of the 5-chloroquinolin-8-

olate 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 significant delocalization of spin density in [9]L· are the

factors that determine tremendous activation of H₂O₂in 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).

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 H₂O₂but also demonstrate a type of

the H₂O₂activation that is diﬀerent from the common Fenton

or Fenton-like mechanisms. Such an activation is associated

The oxidized quinolin-8-olate ligand can be reduced by

another H₂O₂molecule to give the HOO· radical and 8-

hydroxyquinoline (ΔG_s= 7.4 kcal/mol), which protonates and

substitutes the OH⁻ligand in 9 restoring the initial catalyst and

completing the catalytic cycle (Scheme 2).

J

DOI: 10.1021/acs.inorgchem.7b02684

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

a

Scheme 4. Conventional Fenton-like Mechanism

a

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

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 [V^IVO(acac)₂] with the nitro or halogen substituted

8-hydroxyquinoline and then characterized by Fourier trans-

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 catalysts^30b−dis 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-butylhy-

droperoxide.^6mMoreover, 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

1

form infrared (FT-IR), H 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

C₆D₁₂and under ¹⁸O₂atmosphere, 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 decom-

posed to the corresponding alcohol and ketone.

ASSOCIATED CONTENT

■

S

* Supporting Information

It is usually accepted that the generation of the HO· radical

from H₂O₂catalyzed 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-chloroquinolin-

8-olate moiety. These results suggest that, namely, the redox-

active 5-Cl-quin ligand in the catalyst molecule but not the

metal center plays the crucial role in the generation of HO·

radicals from H₂O₂and, therefore, in the oxidation of alkanes.

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.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 chromato-

grams, 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 crystal-

lographic data for this paper. These data can be obtained free of

charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-

ing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

K

DOI: 10.1021/acs.inorgchem.7b02684

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: bmachura@poczta.onet.pl. (B.M.)

*E-mail: max@mail.ist.utl.pt. (M.L.K.)

*E-mail: shulpin@chph.ras.ru. (G.B.S.)

■

ORCID

Anna Chrobok: 0000-0001-7176-7100

Maxim L. Kuznetsov: 0000-0001-5729-6189

Dmytro S. Nesterov: 0000-0002-1095-6888

(4) (a) Sakurai, H.; Tsuchiya, K.; Nukatsuka, M.; Kawada, J.;

Ishikawa, S.; Yoshida, H.; Komatsu, M. Insulin-mimetic action of

vanadyl complexes. J. Clin. Biochem. Nutr. 1990, 8, 193−200.

(b) Crans, D. C.; Mahroof-Tahir, M.; Keramidas, A. D. Vanadium

chemistry and biochemistry of relevance for use of vanadium

compounds as antidiabetic agents. Mol. Cell. Biochem. 1995, 153,

17−24. (c) Thompson, K. H.; Orvig, C. Coordination chemistry of

vanadium in metallopharmaceutical candidate compounds. Coord.

Chem. Rev. 2001, 219−221, 1033−1053. (d) Sakurai, H.; Kojima, M.;

Yoshikawa, Y.; Kawabe, K.; Yasui, H. Antidiabetic vanadium(IV) and

zinc(II) complexes. Coord. Chem. Rev. 2002, 226, 187−198.

(e) Evangelou, A. Vanadium in cancer treatment. Crit. Rev. Oncol.

Hematol. 2002, 42, 249−265. (f) Sakurai, H.; Yoshikawa, Y.; Yasui, H.

Current state for the development of metallopharmaceutics and anti-

diabetic metal complexes. Chem. Soc. Rev. 2008, 37, 2383−2392.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

This work has been partially supported by the Russian

Foundation for Basic Research (Grant No. 16-03-00254) and

■

the Fundaca̧ o para a Ciencia 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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High Catalytic Activity of Vanadium Complexes in Alkane Oxidations with Hydrogen Peroxide: An Effect of 8-Hydroxyquinoline Derivatives as Noninnocent Ligands

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