Relevance of Protons in Heterolytic Activation of H2O2 over Nb(V): Insights from Model Studies on Nb-Substituted Polyoxometalates

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Article

Relevance of Protons in Heterolytic Activation of H2O2 over Nb(V).

Insights from Model Studies on Nb-substituted Polyoxometalates

Nataliya V. Maksimchuk, Gennadii M. Maksimov, Vasilii Yu. Evtushok, Irina D. Ivanchikova, Yuriy A. Chesalov,

Raisa Maksimovskaya, Oxana A. Kholdeeva, Albert Solé-Daura, Josep M. Poblet, and Jorge J. Carbó

ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02761 • Publication Date (Web): 10 Sep 2018

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Relevance of Protons in Heterolytic Activation of H₂O₂over Nb(V).

Insights from Model Studies on Nbꢀsubstituted Polyoxometalates

Nataliya V. Maksimchuk,^{†, ‡}Gennadii M. Maksimov,^†Vasilii Yu. Evtushok,^{†, ‡}Irina D. Ivanchikova,^†

Yuriy A. Chesalov,^{†, ‡}Raisa I. Maksimovskaya,^†Oxana A. Kholdeeva,^{†, ‡,}* Albert SoléꢀDaura,^§Josep

M. Poblet,^§and Jorge J. Carbó^§,*

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^†Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russia

^‡Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russia

^§Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, 43005 Tarragona, Spain

ABSTRACT:

Nbꢀmonosubstituted

Lindqvist-type

polyoxometalates

(POM),

(Bu₄N)₄[(NbW₅O₁₈)₂O]

(1)

and

(Bu₄N)₃[Nb(O)W₅O₁₈] (2), catalyze epoxidation of alkenes with hydrogen peroxide and mimic the catalytic performance of heteroꢀ

geneous Nbꢀsilicate catalysts. Dimer 1 is more active than monomer 2, but the catalytic activity of the latter increases in the presꢀ

ence of acid. Kinetic and spectroscopic studies suggest a mechanism that involves generation of monomer (Bu₄N)₂[Nb(OH)W₅O₁₈]

(3), interaction of 3 with H₂O₂leading to a protonated peroxo niobium species, (Bu₄N)₂[HNb(O₂)W₅O₁₈] (4), followed by oxygen

transfer to a C=C bond in alkene. The previously unknown peroxo complex 4 has been isolated and characterized by elemental

analysis, UVꢀvis, FTꢀIR, Raman, ⁹³Nb, ¹⁷O and ¹⁸³W NMR spectroscopy, cyclic voltammetry, and potentiometric titration. The

physicochemical techniques support a monomeric Lindqvist structure of 4 bearing one peroxo ligand attached to Nb(V) in a η²ꢀ

coordination mode. While the unprotonated peroxo complex (Bu₄N)₃[Nb(O₂)W₅O₁₈] (5) is inert toward alkenes under stoichioꢀ

metric conditions, 4 readily reacts with cyclohexene to afford epoxide and 1,2ꢀtransꢀcyclohexane diol, which proves the key role of

protons for heterolytic activation of H₂O₂over Nb(V). The IR, Raman, UVꢀvis, and ¹⁷O NMR spectroscopic studies along with

DFT calculations showed that the activating proton in 4 is predominantly located at a Nb–O–W bridging oxygen. However, DFT

calculations revealed that the protonated peroxo species ‘HNb(O₂)’ is present in equilibrium with a hydroperoxo species ‘Nb(η²

–

OOH)’, which has a lower activation barrier for the oxygen transfer to cyclohexene and is, therefore, the main epoxidizing species.

The calculations indicate that protonation is crucial to generate the active species and to increase POM electrophilicity.

KEYWORDS: epoxidation, hydrogen peroxide, niobium, peroxo complex, polyoxometalate, Lindqvist structure, DFT

W(VI),^11,14ꢀ17Re(VII),^18ꢀ20Ti(IV),^21ꢀ23Mn(II),^12,24ꢀ26and

INTRODUCTION

Fe(II/III)^12,20,27,28, have been reported for epoxidation of a wide

range of alkenes. Among heterogeneous catalysts, the miꢀ

croporous titanium–silicalite TS–1 developed by the ENI

group at the beginning of the 1980s remains a unique material

that enables efficient transformations of linear olefins into

epoxides using dilute H₂O₂.^29,30The development of mesopoꢀ

rous Tiꢀsilicates in the 2000s was targeted at oxidative transꢀ

formations of large organic molecules; however, the hydroꢀ

philic nature of these materials favors adsorption and unproꢀ

ductive decomposition of hydrogen peroxide, which deterioꢀ

rates selectivity of epoxidation, with the exception of some

specific cases.^31,32

On the way to sustainable production of valuable chemicals,

a challenging goal is the development of economic and ecoꢀ

logically sound oxidation processes which employ green,

cheap and readily available oxidizing agents.^1ꢀ4Along with

molecular oxygen, hydrogen peroxide is the most environꢀ

mentally friendly oxidant since it is atomꢀefficient, easy to

handle and safe (<60 wt% in water and <20% wt% in organic

solvent) and produces water as the sole byproduct.^5ꢀ7Likewise

dioxygen, H₂O₂itself is inert toward most organic substrates

and its use in selective oxidation processes requires employꢀ

ment of catalysts. Redox transition metals readily perform

homolytic activation of H₂O₂, leading to the formation of

radical species and Fentonꢀtype chemistry.^8,9However, in this

case, unproductive decomposition of H₂O₂with evolution of

molecular oxygen competes with the target oxygenation reacꢀ

tion, leading to very low oxidant utilization efficiency. Moreꢀ

over, the formation of radicals (HO·, HO₂·) is detrimental for

selectivity of oxidation processes.

In recent years, niobiumꢀcontaining materials have attracted

growing interest as selective oxidation catalysts. In particular,

mesoporous Nbꢀsilicates turned out to be active and recyclable

catalysts for alkene epoxidation using dilute H₂O₂.^33ꢀ42Several

research groups noticed that Nb,Siꢀcatalysts reveal superior

selectivity in H₂O₂ꢀbased epoxidation relative to their Ti counꢀ

terparts.^37,38,41,42The reasons for this are not completely clear.

While high levels of understanding mechanisms of alkene

epoxidation have been reached for W, Re and Ti,^30,43ꢀ45as well

as biomimetic Fe and Mn catalysts,^{24ꢀ27,43,46,47}epoxidation

catalyzed by Nb is still poorly understood. A few attempts at

rationalizing the catalytic behavior of Nb(V) have been reꢀ

ported.^42,48ꢀ51Nevertheless, the nature of the active epoxidizing

The selective catalytic epoxidation of alkenes with hydrogen

peroxide is of both academic and industrial interest.^10ꢀ13To

accomplish oxygen atom transfer from H₂O₂to alkenes and to

avoid the formation of intermediate radical species, heterolytic

activation of the oxidant is required. Several highly effective

homogeneous catalysts, including specific complexes of

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species still remains under debate. Our recent kinetic and alcohol) with H₂O₂in the presence of 2 equiv. of protons

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spectroscopic studies on mesoporous Nbꢀsilicates implicated a

protonated, presumably hydroperoxo niobium species Nb–

OOH as the active form responsible for epoxidation of elecꢀ

tronꢀrich C=C double bonds in alkenes.^42,52On the other hand,

Bregante et al. proposed that a superoxo Nb species is responꢀ

sible for alkene epoxidation over Nbꢀsubstituted Betaꢀ

zeolite.^50,51More recently, they discarded superoxo Nb and

suggested that, in sharp contrast to Ti singleꢀsite catalysts for

which hydroperoxo species TiOOH has become a widely

accepted concept,^30,53,54simply Nb(η²–O₂) surface intermediꢀ

ates epoxidize alkenes on Nbꢀbased catalysts.⁵⁵To shed more

light on the nature of active Nb peroxo species, detailed experꢀ

imental and theoretical model studies using soluble molecular

compounds are needed.

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Anionic metalꢀoxygen clusters or polyoxometalates (POMs),

owing to their metal oxide like structure, thermodynamic

stability to oxidation and hydrolysis as well as tunable acidity

and redox properties, have found applications in a wide range

of research areas, covering catalysis, molecular magnetism,

electronics and medicine.^56–66The evident structural analogy

between POMs and metal oxide surfaces makes possible conꢀ

sidering POMs as discrete, soluble fragments of extended

metal oxide lattices, which can be characterized and investiꢀ

gated at the atomic/molecular level.^67–72Since lacunary polyꢀ

anion frameworks can function as totally inorganic, hydrolytiꢀ

cally and oxidationꢀresistant multidentate ligands toward a

heterometal M, transitionꢀmetal monoꢀsubstituted POMs (Mꢀ

POMs) meet the criteria of an important class of singleꢀsite

catalysts⁷³and can serve as tractable molecular models for

studying mechanisms of oxidation catalysis.^74–80Earlier, some

of us demonstrated that Tiꢀsubstituted POMs mimic well the

catalytic performance of mesoporous titaniumꢀsilicate cataꢀ

lysts in a range of selective oxidations with H₂O₂.^76–78Alkene

epoxidation mediated by two different POMs, the Tiꢀ

monosubstituted Keggin type POM [PTi(OH)W₁₁O₃₉]^4–and

the sandwich type POM [Ti₂(OH)₂As₂W₁₉O₆₇(H₂O)]^8–, conꢀ

taining wellꢀdefined 6ꢀ and 5ꢀcoordinated titanium atoms,

respectively, have been studied in detail using experimental

and computational techniques.^45,78,81The higher selectivity of

the sandwich anion was attributed to the larger energy cost of

homolytic O–O bond breaking in the hydroperoxo intermediꢀ

ate ‘TiOOH’.⁴⁵It was also demonstrated that electrophilicity

and reactivity of the peroxo titanium derivative of the Keggin

TiꢀPOM can be increased through polyanion protonation.^82,83

Figure 1. 3D representation for several monosubstituted Nbꢀ

tungtates of the Lindqvist structure.

In the present work, we first explored the catalytic perforꢀ

mance of Nbꢀmonosubstituted Lindqvist tungstates, i.e. ꢀꢀoxo

dimer (Bu₄N)₄[(NbW₅O₁₈)₂O] ((NbW₅)₂O, 1) and monomer

(Bu₄N)₃[Nb(O)W₅O₁₈] (Nb(O)W₅, 2) in alkene epoxidation

with hydrogen peroxide. Hydrolysis of dimer 1 was anticipatꢀ

ed to produce

a

monomer, (Bu₄N)₂[Nb(OH)W₅O₁₈]

(Nb(OH)W₅, 3) with terminal Nb–OH bond. Figure 1 shows

the 3D molecular structures of anions 1, 2 and 3. Given that

both Nb=O and Nb–OH surface groups are present in Nbꢀ

silicates and zeolites^89–91and both may potentially lead to an

active peroxo niobium species,^{42,49,52,92,93}it was interesting to

compare their reactivity towards H₂O₂using wellꢀdefined

molecular compounds. To elucidate the role of protons in the

activation of the peroxo niobium group, we first synthesized a

protonated

niobium

peroxo

complex,

(Bu₄N)₂[HNb(O₂)W₅O₁₈] (HNb(O₂)W₅, 4), and fully characꢀ

terized it by elemental analysis, potentiometric titration, cyclic

voltammetry, UVꢀvis, FTꢀIR, Raman, and ¹⁸³W, ⁹³Nb, and ¹⁷

O

NMR spectroscopy. The structure and reactivity of 4 have

been investigated by experimental and computational techꢀ

niques in comparison with the unprotonated peroxo complex

(Bu₄N)₃[Nb(O₂)W₅O₁₈] (Nb(O₂)W₅, 5).

EXPERIMENTAL

Materials. Acetonitrile (Panreac, HPLC grade) was dried

and stored over activated 3Å molecular sieves. Cyclohexene

and cyclooctene were purchased from SigmaꢀAldrich and

purified prior to use by passing through a column filled with

neutral alumina to remove traces of possible oxidation prodꢀ

ucts. All the other compounds were the best available reagent

grade and used without further purification. A solution of 77%

Н₂О₂was obtained by concentration of a commercial 30%

aqueous solution in vacuum. The concentration of hydrogen

peroxide was determined iodometrically prior to use. Semiꢀ

quantitative Quantofix peroxide test sticks were used for estiꢀ

mation of the amount of H₂O₂at the end of catalytic reactions.

Tetraꢀnꢀbutylammonium hydroxide (TBAOH) (0.39 M in

water) was titrated with HCl (0.1 M) prior to use. All the other

compounds were the best available reagent grade and were

used without further purification.

Although a large number of Nbꢀcontaining POMs have been

reported in the literature,⁸⁴in particular in relation to their

antitumor and antiviral activity,^84,85potential of NbꢀPOMs in

catalysis is much less explored compared to TiꢀPOMs. Among

them, very few works were devoted to oxidation catalysis.^86–88

Nbꢀtrisubstituted POMs, (Bu₄N)₅H₂[(NbO₂)₃SiW₉O₃₇]⁸⁶and

(Bu₄N)₄H₂[(NbO₂)₃PW₉O₃₇],⁸⁷could accomplish epoxidation

of unsaturated allylic alcohols to the corresponding diols with

aqueous H₂O₂, but they were inert toward unfunctionalized

alkenes.⁸⁶On the basis of kinetic and NMR data, the authors

concluded that the NbꢀPOMs function primarily as catalyst

precursors that produce active W, Nb or polyoxoanion fragꢀ

ments.^86,87More recently, Mizuno and coworkers reported

synthesis and catalytic properties of a diꢀNbꢀsubstituted siliꢀ

codecatungstate, (Bu₄N)₅[γ-HSiW₁₀O₃₈Nb₂(η²-O₂)₂].⁸⁸This

POM was able to catalyze oxidation of some organic subꢀ

strates (cyclooctene, thioanisole, 1ꢀphenylethanol, and allyl

Synthesis and Characterization of POMs

Na₇HNb₆O₁₉·xH₂O was prepared according to the known

procedure.⁹⁴Nb₂O₅(25.5 g) was fused with 31 g of NaOH

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(1:8 molar ratio) in a nickel crucible at 600 C for 5 h. The

reaction product was dissolved in water (1.5 L) under heating

to give 1.8 g of product in 70% yield. Anal. calcd (%) for

C₆₄H₁₄₄N₄Nb₂W₁₀O₃₇: C, 21.44; H, 4.05; N, 1.56; O, 16.51;

Nb, 5.18; W, 51.26. Found: C, 21.64; H, 4.07; N, 1.69; O,

16.37; Nb, 5.13; W, 50.2. IR (KBr, 1000ꢀ400 cm^–1): 994 (w,

sh), 974 (s), 877 (m), 835 (w, sh), 812 (s), 733 (w, sh), 720

(m, sh), 695 (s, NbONb), 588 (m), 547 (m), 447 (s). ⁹³Nb

NMR (ppm, in CH₃CN): –950 (broad).

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(90 C). The resulting solution was slowly evaporated to 200

mL at ambient temperature and the precipitate was washed

with ethanol and recrystallized from water. The white product

was washed with ethanol and airꢀdried overnight. The recrysꢀ

tallization yielded an analytical sample with 90% yield. FTIR

(KBr, 1200–400 cm^–1): 860, 772, 677, 534.

(Bu₄N)₂[HNb(O₂)W₅O₁₈] (HNb(O₂)W₅, 4). Protonated

peroxo complex 4 was synthesized by the addition of 30%

H₂O₂(0.2 mL, 2.3 mmol) to a suspension of (NbW₅)₂O (2.8 g,

0.7 mmol) in 20 mL CH₃CN. The yellow solution was stored

for 30 min at room temperature and diluted with 120 mL of

ether. The resulting yellowish solid (2.7 g, ca. 95% yield) was

isolated by decantation and dried in air at room temperature.

Anal. calcd (%) for C₃₂H₇₃N₂NbW₅O₂₀: C, 21.14; H, 4.05; N,

1.54; O, 17.60; Nb, 5.11; W, 50.57. Found: C, 21.22; H, 3.51;

N, 1.68; O, 17.37; Nb, 4.96; W, 48.4. IR (KBr, 1000–400 cm^–

¹): 984 (w, sh), 973 (vs), 883 (w, TBA), 870 (w), 848 (m), 816

(m), 770 (m), 739 (w, TBA), 621 (w), 595 (m), 567 (w), 541

(w), 455 (m). ⁹³Nb NMR (ppm, in CH₃CN): –1015. ¹⁷O NMR

(ppm, in CH3CN): 754 (W=O), 742 (NbO₂), 414 (NbOW),

409 (WOW), 402 (WOW), and –63 (µO).

(Bu₄N)₃[Nb(O₂)W₅O₁₈] (Nb(O₂)W₅, 5). The synthesis of

peroxo

complex

5

was

adapted

from

ref.⁹⁵

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Na₇HNb₆O₁₉·15H₂_oO (3.55 g, 2.7 mmol) was dissolved in 140

mL of water (20 C) and 5 mL of 30% aqueous H₂O₂. The

resulting turbid solution was stirred overnight, then heated to

50 ^oC followed by the addition of 20 g (60.6 mmol) of

Na₂WO₄·2H₂O in 50 mL H₂O and 15% HNO₃(slowly) to

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adjust the pH at 2.3. After 1 h of stirring at 100 C, a turbid

yellow solution was centrifuged to remove precipitate of unreꢀ

acted peroxoniobate. Then 12 g (37 mmol) of TBABr was

added to the clear solution to afford a yellow precipitate which

was collected by suction filtration, washed with 100 mL of

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water, dried at 100 C, and purified by reprecipitation with

water from CH₃CN. The resulting yellow solid (13.3 g) was

dissolved in 100 mL of dichloroethane, centrifugated to reꢀ

move insoluble impurities, evaporated in air, and then repreꢀ

cipitated with the aqueous solution of H₂O₂(2 mL of 30%

H₂O₂in 160 mL H₂O) from CH₃CN (80 mL). The resulting

yellowish solid (11.2 g, yield 44% on W) was dried in air at

(Bu₄N)₂[W₆O₁₉] (W₆O₁₉) was prepared and characterized as

described elsewhere.⁹⁷

(Bu₄N)₄PW₁₁NbO₄₀(PW₁₁Nb). H₃PW₁₂O₄₀·6H₂O (9 g, 3

mmol) was dissolved in 20 mL of water. Then 0.9 mL of

0.325 М H₃PO₄, 50 mL of hot aqueous solution of

Na₇HNb₆O₁₉·15H₂O (0.702 g, 0.5 mmol), and 1 mL of 30%

Н₂О₂(11 mmol) were subsequently added and the reaction

mixture was stirred upon heating during 1 h. The resulting

yellow solution of the peroxo species (Bu₄N)₄PW₁₁Nb(O₂)O₂₉

(³¹P NMR: –13.7 ppm) was evaporated until dryness and then

dried in air at 140 ^oC for 2 h. The white residue was dissolved

in water (20 mL), an insoluble precipitate was separated by

centrifugation, and TBA salt was precipitated by the addition

of TBABr (4.5 g). The resulting white solid was isolated by

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30 C overnight and at 90 C for 1 h. The presence of one

peroxo group per molecule of 5 was confirmed by titration

with triphenylphosphine (PPh₃) followed by monitoring with

³¹P NMR.⁸⁷Anal. calcd (%) for C₄₈H₁₀₈N₃NbW₅O₂₀: C, 27.99;

H, 5.29; N, 2.04; O, 15.54; Nb, 4.51; W, 44.63. Found: C,

27.71; H, 5.40; N, 1.96; O, 14.34; Nb, 4.16; W, 44.60. IR

(KBr 1000–400 cm⁻¹): 980 (w, sh), 960 (s,W–Ot), 885 (w,

TBA), 857 (sh), 806 (vs), 735 (sh, TBA), 639 (m), 602 (m),

586 (w), 567 (w), 546 (w), 442 (w), 428 (s). ¹⁸³W NMR (ppm,

in CH₃CN): 62 (4W_eq), 101 (1W_ax). ⁹³Nb NMR (ppm, in

CH₃CN): –1045. ¹⁷O NMR (ppm, in CH₃CN): 734 (W=O),

725 (NbO₂), 460 (NbOW), 396 (WOW), 390 (WOW), –72

(µО).

o

filtration, washed with water, dried at 100 C and then recrysꢀ

tallized from CH₃CN. Large colorless crystals were dried in

air at room temperature and then at 90 ^oC. Yield ca. 50%. The

number of TBA cations (4) was determined by ignition. IR

(KBr, 1100–400 cm^–1): 1082, 1070, 965, 890, 805, 600, 510.

³¹P NMR (ppm, in CH₃CN): –13.2. ¹⁸³W NMR (ppm, in

CH₃CN): –67, –88, –93, –99, –106.

(Bu₄N)₃[Nb(O)W₅O₁₈] (Nb(O)W₅, 2). Monomeric Nbꢀ

substituted tungstate 2 was synthesized by reduction of

Nb(O₂)W₅with PPh₃. To a solution of 5 (3.8 g, 1.8 mmol) in

20 mL CH₃CN, PPh₃(0.56 g, 2.1 mmol) dissolved in 10 mL

of CH₃CN was added, and the reaction mixture was stored for

1 h. A white solid was precipitated by a twoꢀfold excess of

water, isolated by filtration, washed with ethanol and dried in

air at 30 ^oC. Yield 83%. Anal. calcd (%) for

C₄₈H₁₀₈N₃NbW₅O₁₉: C, 28.21; H, 5.33; N, 2.06; O, 14.88; Nb,

4.55; W, 44.98. Found: C, 28.06; H, 6.06; N, 2.12; O, 14.98;

Nb, 4.34; W, 45.0. IR (KBr 1000–400 cm^–1): 977(w, sh), 957

(s, W–Ot), 915 (m, Nb–Ot), 883 (w, TBA), 804 (vs), 735 (sh,

TBA), 589 (m), 572 (m). ¹⁸³W NMR (ppm, in CH₃CN): 71

(4W_eq), 27 (1W_ax). ⁹³Nb NMR (ppm, in CH₃CN): –899. ¹⁷O

NMR (ppm, in CH₃CN): 732 (W=O), 730 (sh, W=O), 455

(NbOW), 392 (WOW), 390 (sh, WOW), –68 (µO).

Interaction with H₂O₂. Interaction of NbꢀPOMs with 30%

H₂O₂was performed by the addition of H₂O₂(0.1–50 equiv.,

taken as 30% solution in water) to a 0.01 M (⁹³Nb NMR moniꢀ

toring) or 0.0005 M (UVꢀvis monitoring) solution of POM in

dry CH₃CN at 20 ^oC.

Catalytic reactions. Catalytic oxidations were performed

under vigorous stirring (500 rpm) in thermostated glass vesꢀ

sels. Typical reaction conditions were as follows: alkene 0.2

mmol, H₂O₂0.2 mmol, POM 0.004 mmol (0.002 mmol for

o

(NbW₅)₂O), CH₃CN 1 mL, 50 C. Reactions were started by

the addition of 30% H₂O₂to the reaction mixture containing

acetonitrile solvent, alkene, POM and, in some cases, 1 equiv.

(0.004 mmol) of HClO₄. Samples of the reaction mixture (0.5

µL) were withdrawn periodically during the reaction course

by a syringe. The oxidation products were identified by gas

chromatography−mass spectrometry (GS–MS). The product

yields and substrate conversions were quantified by gas chroꢀ

matography (GC) using an internal standard, biphenyl. For

GC analysis, the method described by Shul’pin⁹⁸was used,

(Bu₄N)₄[(NbW₅O₁₈)₂O] ((NbW₅)₂O), 1). The synthesis of

dimer 1 was adapted from ref.⁹⁶Acetylchloride (0.11 mL, 1.5

mmol) was added to a solution of Nb(O)W₅(3.0 g, 1.5 mmol)

in 20 mL CH₃CN. The reaction mixture was stirred for 15 min

o

at room temperature (20 C), and the resulting white precipiꢀ

tate was isolated by filtration and washed with 50 mL of ether

ACS Paragon Plus Environment

ACS Catalysis

Page 4 of 16

natural ¹⁷O abundance (0.037 %). Chemical shifts, δ, were

which involves treatment of the reaction mixture with PPh₃in

referenced to NbCl₅, H₂O, and Na₂WO₄for ⁹³Nb, ¹⁷O, and

¹⁸³W NMR spectra, respectively. For convenience, secondary

external standards were used: 0.4 M H₄SiW₁₂O₄₀for ¹⁸³W

NMR (−103.6 ppm) and 0.05 M H₅SiW₁₁NbO₄₀(–975 ppm)

for ⁹³Nb NMR. Infrared spectra were recorded as 0.5–2.0 wt

% samples in KBr pellets on an Agilent Cary 600 FTIR specꢀ

trometer. Electronic absorption spectra were run on a Cary–50

spectrophotometer using a 0.2 cm quartz cells. FT–Raman

spectra (3600–100 cm^–1, 300 scans, resolution 4 cm^–1, 180°

geometry) were recorded using a RFS 100/S spectrometer

(Brüker). Excitation (1064 nm) was provided by a Nd–YAG

laser (100 mW power output). Cyclic voltammetric measureꢀ

ments were performed under Ar using a Pꢀ8nano voltammetric

1

2

3

4

5

6

7

8

order to reduce unreacted H₂O₂and possible organic peroxides

formed and comparison of chromatograms of the reaction

mixture before and after the reduction of peroxides with PPh₃.

Each experiment was reproduced at least 2 times.

Kinetic experiments. Kinetic experiments were performed

in temperatureꢀcontrolled glass vessels under vigorous stirring

(600 rpm). Reactions were initiated by the addition of 30 or

77% H₂O₂to a CH₃CN solution containing CyOct substrate,

catalyst (NbW₅)₂O, and internal standard for GC (biphenyl).

The total volume of the reaction mixture was 1 mL. The reacꢀ

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

o

tion temperature was kept at 50 C. Samples (0.5 µL) of the

reaction mixture were withdrawn periodically during the

reaction with a syringe and analyzed by GC. Each experiment

was reproduced at least 2–3 times. To rule out the possibility

of evaporative losses of the substrate, blank experiments

without catalyst and oxidant were carried out at the reaction

temperature using the internal standard.

analyzer (Elins Electrochemical Instruments),

a threeꢀ

electrode cell, a Pt working electrode, a Pt auxiliary electrode,

and an Ag/AgCl reference electrode (BAS). TBAClO₄(0.1 M)

was used as a supporting electrolyte. The analysis of W and

Nb content was done by ICP–OES using a Perkin Elmer Opꢀ

tima–430 DV instrument. The concentrations of C, H, N and

O in the samples were determined with a CHNSO analyzer

Vario EL Cube (Elementar Analysensysteme GmbH).

Reaction order in catalyst. The catalyst (NbW₅)₂O concenꢀ

tration was varied in the range 0.0006–0.004 M. Concentraꢀ

tions of other reactants were held constant: [CyOct] = 0.1 M,

[H₂O₂] = 0.1 M.

Computational details. The DFT analysis of the reaction

pathway was carried out with Gaussian09 rev. C01 software.⁹⁹

Geometry optimization of reagents, intermediates, transition

states and products was made using B3LYP density functionꢀ

al.^100–102LANL2DZ pseudopotential¹⁰³was used for W and Nb

atoms and 6–31g(d,p) basis set^104–106was used for other atoms.

The geometry optimization was full and without any symꢀ

metry constrains, and solvent effects of acetonitrile were

included using the IEFꢀPCM implicit solvation model as imꢀ

plemented in Gaussian09. This level of theory has been

proved to be accurate and reliable enough to study the reactivꢀ

ity concerning POMs and their transition metalꢀsubstituted

analogues, always showing a high degree of consistency with

experimental outcomes and kinetic studies.^107–109Timeꢀ

dependent DFT (TD–DFT) calculations were performed with

the longꢀrange corrected CAM–B3LYP functional¹¹⁰as imꢀ

plemented in Gaussian 09 using the same basis set. In order to

compute protonation energies (ꢁG_H+), we took the experiꢀ

mental standard free energy of a proton in aqueous solution

272.2 kcal·mol^–1.¹¹¹NMR calculations were performed with

ADF2107 package using a TZVP Slater type allꢀelectron basis

set and the OPBE functional¹¹²with spin–orbit (SO) correcꢀ

tions.¹¹³

Reaction order in substrate. The initial substrate concentraꢀ

tion was varied between 0.02 and 0.3 M while maintaining

constant concentrations of H₂O₂(0.1 M) and catalyst (0.002

M).

Reaction order in H₂O₂. The initial oxidant concentration

was varied in the range of 0.1–0.9 M. The concentration of

water in these experiments was kept constant by the addition

of corresponding amounts of H₂O. To reduce the amount of

water in the reaction mixture, 77% H₂O₂was employed. The

concentrations of other reactants were as follows: [CyOct] =

0.1 M, [(NbW₅)₂O] = 0.002 M.

Reaction order in H₂O. The initial concentration of water

was varied from 0.08 to 1.8 M. Other parameters were held

constant: [CyOct] = 0.1 M, [H₂O₂] = 0.1 M, and [(NbW₅)₂O]

= 0.001 M.

Determination of activation energy. Temperature dependꢀ

ence of the reaction rate was studied in the range of 30–80 ^oC

in CH₃CN using the following reaction conditions: [CyOct] =

0.1 M, [H₂O₂] = 0.1 M, and [(NbW₅)₂O] = 0.002 M.

Stoichiometric interaction with CyH. Stoichiometric reacꢀ

tions between CyH and peroxo complexes Nb(O₂)W₅and

HNb(O₂)W₅were performed under Ar in thermostated glass

vessels equipped with a magnetic stirrer at [POM] = 0.016 M,

[CyH] = 0.08 M, and [HClO₄] = 0.016 M (if any) in dry

CH₃CN (1 mL) at 50 °C. The reaction course was monitored

by UVꢀvis and GC. The oxidation products were identified by

GC–MS and quantified by GC using internal standard.

RESULTS AND DISCUSSION

Catalytic performance of NbꢀPOMs in alkene oxidation.

The catalytic properties of the wellꢀknown Lindqvist type

dimer (NbW₅)₂O and monomer Nb(O)W₅were first assessed

in the oxidation of сyclohexene (CyH) with hydrogen peroxꢀ

ide using equimolar amount of the oxidant (Table 1). CyH

possesses highly reactive H atoms in the allylic position,

which can be easily abstracted by radical species. Hence, the

formation of allylic oxidation products, viz. cyclohexenyl

hydroperoxide (HP), 2ꢀcyclohexeneꢀ1ꢀol (enol) and 2ꢀ

cyclohexeneꢀ1ꢀone (enone), is a clear indication of a homolytꢀ

ic oxidation mechanism. Contrary, the selective formation of

epoxide along with the ring opening product transꢀ

cyclohexaneꢀ1,2ꢀdiol (diol) and its overoxidation product 2ꢀ

hydroxycyclohexanone (ketol) points to heterolytic oxidation

mechanism.

Instrumentation and Methods. GC analyses were perꢀ

formed using a gas chromatograph Tsvetꢀ500 equipped with a

flame ionization detector and a quartz capillary column (30

m×0.25 mm) filled with Agilent DB–5MS. GC–MS analyses

were carried out using an Agilent 7000B system with a triple–

quadrupole mass–selective detector Agilent 7000 and a GC

Agilent 7890B apparatus (quartz capillary column

30m×0.25mm/НР–5ms). ⁹³Nb, ¹⁷O, and ¹⁸³W NMR spectra

were recorded at 97.94, 54.24, and 16.67 MHz, respectively,

on an Avanceꢀ400 Brüker spectrometer using a high resoluꢀ

tion multinuclear probe head with 10 mm o.d. (3 mL solution

volume) sample tubes. ¹⁷O NMR spectra were measured at

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ACS Catalysis

Table 1. CyH oxidation with H₂O₂in the presence of NbꢀPOMs^a

1

2

3

Entry

POM

Time,

h

CyH

H₂O₂

Product selectivity,^c%

conv.,%

eff.,^b%

epoxide

diol

ketol

HP

enol

3

enone

4

5

6

7

8

9

1

2

3

4

5

6

7

8

Nb(O)W₅

5

55

2

64

53

traces

8

25

ꢀ

14

ꢀ

2

3

5

n.d.^d

98

n.d.^dtraces

traces

W₆O₁₉

PW₁₁Nb

5

20

74

76

74

5

20

43

51

40

traces

44

5

50

4

10

7

5

Nb(O)W₅+ 1 eq. H⁺

(NbW₅)₂O ^e

Nb(O)W₅+ 0.1 eq. H⁺

No catalyst

2

3

37

33

27

ꢀ

1

1.5

3

97

3

1

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

92

20

21

25

4

1

5

n.d.^d

33

ꢀ

33

25

11

1 eq. H⁺

5

8

ꢀ

traces

^aReaction conditions: 0.2 M CyH, 0.2 M H₂O₂(30%), 0.004 M POM, 0.004 M HClO₄(if any), 50 ^oC, 1 mL CH₃CN. ^bH₂O₂utilization

efficiency = total yield of products based on the oxidant consumed. Based on substrate consumed. Not determined. ^eConcentration of

c

d

dimer (NbW₅)₂O was 0.002 M.

In the presence of Nb(O)W₅, CyH conversion attained 55%

after 5 h at 50 °C. Total selectivity toward heterolytic oxidaꢀ

tion products (epoxide, diol, and ketol) and efficiency of hyꢀ

drogen peroxide utilization were 92% and 64%, respectively

(Table 1, entry 1). Epoxide prevailed among the oxidation

products. Importantly, the Nbꢀfree tungstate (Bu₄N)₂[W₆O₁₉]

was almost inactive in this reaction, indicating that the substiꢀ

tution of W(VI) for Nb(V) is indispensable for the observed

activity (Table 1, entry 2). It is noteworthy that the Nbꢀ

substituted Keggin phosphotungstate (Bu₄N)₄[PW₁₁NbO₄₀]

revealed very poor activity and selectivity, producing HP as

the main oxidation product and thereby indicating predominaꢀ

tion of a homolytic oxidation pathway (Table 1, entry 3),

which was also confirmed by reduction of the reaction rate in

the presence of the conventional radical scavenger ionol. The

reaction in the presence of Nb(O)W₅was greatly accelerated

with the addition of 1 equiv. of acid (Figure S1). Moreover,

H₂O₂utilization efficiency increased reaching 98% (Table 1,

entry 4). A similar result was observed with dimer (NbW₅)₂O

(Table 1, entry 5; Figure S1). In contrast to the reaction with

Nb(O)W₅, the epoxide ringꢀopening product, diol, predomiꢀ

nated among the oxidation products and the yield of the

overoxidation product, ketol, also increased. Even minor (0.1

equiv.) additives of acid produced a significant effect on CyH

oxidation rate and selectivity in the presence of Nb(O)W₅

(Table 1, entry 6). If 1 equiv. of acid was used as the sole

catalyst, the selectivity toward heterolytic oxidation products

(epoxide and diol) increased in the blank experiment either

(Table 1, compare entries 7 and 8), but the reaction rate was

insignificant relative to the NbꢀPOMꢀcatalyzed reaction.

which is, most likely, a manifestation of a partial protonation

of Nb(O)W₅(see Figure S4).

The catalytic properties of Nb(O)W₅and (NbW₅)₂O were

also tested in epoxidation of various alkenes with aqueous

H₂O₂(Table S1). For cyclic alkenes, such as cyclooctene

(CyOct) and caryophillene, selectivity toward corresponding

epoxides reached >99%. In the oxidation of methyl oleate and

cisꢀstilbene, only cisꢀepoxides were formed, indicating a conꢀ

certed mechanism of oxygen atom transfer that involves no

radical or ionic intermediate capable of rotation around C–C

bond. Note that stereospecificity in epoxidation of cisꢀalkenes

is typical of mesoposous niobiumꢀsilicates.^42,114,115Styrene and

stilbene produced, along with epoxides, a significant amount

of benzaldehyde, the product of the oxidative C=C bond

cleavage, which is also characteristic of heterogeneous Nbꢀ

catalysts.^40,42Therefore, the catalytic performance of the Lindꢀ

qvist Nbꢀsubstituted tungstates is very similar to that of mesoꢀ

porous niobiumꢀsilicates, which justifies use of the NbꢀPOMs

as soluble molecular models for studying activation of H₂O₂

over Nb(V) sites.

Since unproductive decomposition of hydrogen peroxide is

the main side reaction that competes for H₂O₂with the target

selective oxidation, activity of (NbW₅)₂O and Nb(O)W₅was

also evaluated in decomposition of H₂O₂in the absence of any

organic substrate. While monomer Nb(O)W₅revealed rather

low activity, the addition of acid strongly enhanced the H₂O₂

degradation rate (Figure S5). Dimer (NbW₅)₂O exhibited

activity similar to that of monomer Nb(O)W₅in the presence

of 1 equiv. of acid. Therefore, protons accelerate both the

target (epoxidation) and side (H₂O₂degradation) reactions.

However, given that a significant improvement of the oxidant

utilization efficiency was observed in the presence of acid

(Table 1, compare entries 1 and 4), we conclude that the effect

of acid on epoxidation is more pronounced.

Importantly, the stability of the Lindqvist structure under the

turnover conditions of CyH oxidation was confirmed. The IR

spectra of the catalysts recovered by precipitation with ether

revealed all the main vibrations of the Lindqvist tungstate and

were very close to those of peroxo complex Nb(O₂)W₅(Figure

S2, compare spectra E and F with C). This is indicative that

peroxo species participates in the reaction mechanism. The

⁹³Nb NMR spectrum of the reaction mixture after 24 h of the

CyH oxidation revealed a sole resonance with δ –900 ppm,

which is very close to that of Nb(O)W₅(Figure S3, A and B),

as one could expect after complete consumption of the oxiꢀ

dant. In turn, the ⁹³Nb NMR spectrum of the reaction mixture

after the CyH oxidation in the presence of 1 equiv. of HClO₄

showed a resonance slightly shifted upfield (Figure S3, C),

Kinetic Studies. To shed more light on the epoxidation

mechanism over NbꢀPOM, the kinetics of CyOct oxidation

with H₂O₂in the presence of dimer (NbW₅)₂O was studied.

CyOct was chosen as a model substrate because it is less volaꢀ

tile than CyH and its epoxide is more stable toward ring openꢀ

ing and overoxidation (compare data in Table 1 and Table S1).

Typical kinetic curves of CyOct consumption and epoxide

product accumulation showed no induction period, autocatalyꢀ

sis or inhibition behavior. The absence of effect of light, oxyꢀ

gen or conventional chain radical scavengers, e.g., ionol, on

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ACS Catalysis

Page 6 of 16

the reaction rate discarded radical chain oxidation mechanism,

which is consistent with the minor amount of allylic oxidation

products formed in the oxidation of CyH (vide supra).

ening of the ⁹³Nb NMR signal (Figure S9, spectrum E), which

reflects the process of condensation of Nb(O)W₅in the presꢀ

ence of acid. The formation of HNb(O₂)W₅in the presence of

H₂O₂is discussed in more detail in the next section.

1

2

3

4

5

6

7

8

The rate of CyOct oxidation in the presence of (NbW₅)₂O

exhibited a typical Arrhenius dependence (Figure 2), which

implies that there was no change in the rateꢀlimiting step over

the evaluated temperature range. The value of the apparent

activation energy (11.7 kcal mol^–1) turned out close to that

previously found for CyOct epoxidation over mesoporous Nbꢀ

catalysts (11–12 kcal mol^–1).⁴²It is also noteworthy that the E_a

for epoxidation is significantly lower than the E_aestablished

for decomposition of H₂O₂in the absence of organic substrate:

11.7 vs 16.3 kcal mol^–1(Figure 2). The same trend was recentꢀ

ly observed for mesoporous niobiumꢀsilicates.⁴²

2-

[(NbW₅O₁₈)₂O]^4-+ H₂O

2[Nb(OH)W₅O₁₈]

(1)

(3)

[Nb(OH)W₅O₁₈

]

^2-+ H₂O₂

[HNb(O₂)W₅O₁₈]

^2-+ H₂O

(2)

(3)

(4)

(3)

9

[HNb(O₂)W₅O₁₈

]

^2-+ alkene

[Nb(OH)W₅O₁₈]

^2-+ epoxide

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

(4)

(3)

The first reaction order in (NbW₅)₂O found for 30% H₂O₂

implies that, at high concentration of water, dissociation of the

dimer is not involved in the rateꢀlimiting step of the oxidation

process otherwise one would anticipate ca. 0.5 reaction order

in (NbW₅)₂O, as it was observed earlier for thioether oxidation

catalyzed by a Tiꢀmonosubstituted ꢁꢀhydroxo Keggin dimer.¹¹⁶

Indeed, the reaction order in catalyst became close to 0.5 when

the amount of water in the system was reduced (see Figure

S7b). In turn, at low concentration of water, the overall reacꢀ

tion rate is proportional to H₂O concentration since water

accelerates of the formation of Nb(OH)W₅via Eq. 1. Howevꢀ

er, when water concentration becomes significant and all the

NbꢀPOM is present in the form of monomer, the oxidation rate

tends to decrease, most likely, because equilibrium (Eq. 2)

shifts left, thereby decreasing the concentration of the active

peroxo Nb species. The linear dependence of the reaction rate

on the concentration of H₂O₂at high concentration of water

and saturation dependence at low water concentration are also

consistent with the proposed mechanism. Note that stages

similar to those described by Eq. 2 and Eq. 3 were recently

suggested for epoxidation of electronꢀrich alkenes over mesoꢀ

porous niobiumꢀsilicates on the basis of product, kinetic and

spectroscopic data.⁴²

ꢀ5

E_a(H₂O₂) = 16.3 ± 1.5 kcal/mol

ꢀ6

ꢀ7

ꢀ8

ꢀ9

E_a(CyO) = 11.7 ± 0.8 kcal/mol

0.0028

0.0030

0.0032

1/T, K^ꢀ1

Figure 2. Arrhenius plots for (NbW₅)₂Oꢀcatalyzed CyOct epoxiꢀ

dation and H₂O₂decomposition in CH₃CN. Reaction conditions:

CyOct 0.1 M, H₂O₂0.1 M, and (NbW₅)₂O 0.002 M (for CyOct

epoxidation) and Н₂О₂0.2 М, (NbW₅)₂O 0.004 M (for H₂O₂

decomposition).

The CyOct epoxidation reaction showed a firstꢀorder deꢀ

pendence on alkene substrate (Figure S6a). The reaction order

in oxidant and catalyst depended on the concentration of Н₂О.

With 30% H₂O₂([H₂O] was adjusted to 1.4 M in all experiꢀ

ments), the reaction was first order in oxidant (Figure S6b).

While using 77% H₂O₂([H₂O] = 0.35 M), the reaction was

slower and its rate exhibited a saturation behavior, indicating

the formation of an active peroxo niobium intermediate (Figꢀ

ure S7a). In turn, the reaction was first order in catalyst with

30% H₂O₂(Figure S6c) while the order changed to the fracꢀ

tional value (close to 0.5) when concentrated H₂O₂was emꢀ

ployed (Figure S7b). Finally, the rate of CyOct oxidation

revealed a complicated, bellꢀshaped dependence on the conꢀ

centration of water (Figure S8).

Interaction of dimer

1 and monomer 2 with H₂O₂. To

confirm and understand the formation of the active peroxo

niobium species, we studied the interaction of dimer

(NbW₅)₂O and monomer Nb(O)W₅with hydrogen peroxide by

using UVꢀvis and ⁹³Nb and ¹⁸³W NMR spectroscopy. Scheme

1 summarizes possible equilibriums between various NbꢀPOM

species in the presence of aqueous hydrogen peroxide.

The observed kinetic trends allowed us to suggest a reaction

mechanism that involves hydrolysis of (NbW₅)₂O to form

monomeric species Nb(OH)W₅(Eq. 1), interaction of

Nb(OH)W₅with H₂O₂generating a peroxo species HNb(O₂)W₅

(Eq. 2), followed by oxygen transfer from HNb(O₂)W₅to

alkene, producing epoxide (Eq. 3).

Interconversions between (NbW₅)₂O, Nb(O)W₅and

Nb(OH)W₅in CH₃CN solution can be followed by ⁹³Nb NMR

spectroscopy. The addition of water to a solution of (NbW₅)₂O

causes the narrowing of the ⁹³NMR resonance indicating the

formation of Nb(OH)W₅(Figure S9, spectrum B). After the

addition of 1 equiv of acid to Nb(O)W₅, an upfield shift is

initially observed (Figure S9, spectrum D) followed by broadꢀ

Scheme 1. Possible NbꢀPOM species and interconversion in

the presence of aqueous H₂O₂.

First, after the addition of H₂O₂to Nb(O)W₅, the solution

slowly turned pale yellow and a shoulder at 309 nm appeared

ACS Paragon Plus Environment

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ACS Catalysis

similar high field shift was recently observed for peroxonioꢀ

in the UVꢀvis spectrum (Figure 3a). Note that yellow or light

1

2

3

4

5

6

7

8

yellow color is characteristic of several Nb(V) peroxo comꢀ

plexes for which bidentate (η²) coordination of the peroxo

ligand has been proved by Xꢀray analysis.^87,117,118Hence, the

shoulder at 309 nm can be assigned to peroxo ligandꢀtoꢀmetal

charge transfer.

bate (Bu₄N)[NbO(O₂)(OH)₂] as compared to the spectrum of

(Bu₄N)[NbO(OH)₂R].¹²¹The intensity of this resonance inꢀ

creased with increasing concentration of H₂O₂and, after the

addition of 2 equiv., the initial signal of Nb(O)W₅(δ –898

ppm) disappeared completely (Figure 4, spectrum D). Evidentꢀ

ly, the resonance at –1048 ppm belongs to peroxo complex

Nb(O₂)W₅formed via reaction of Nb(O)W₅with H₂O₂. The

correctness of this process was confirmed by experiments with

additives of water. Indeed, the resonance of initial Nb(O)W₅(δ

–898 ppm) appeared if 10 vol% of H₂O was added to the

equilibrated mixture of Nb(O)W₅and H₂O₂(Figure 4a, specꢀ

trum E). The approximately fiveꢀfold broadening of the signal

of Nb(O)W₅indicates that the ⁹³Nb quadrupolar relaxation rate

in it is five times faster,¹²⁰which is due to the increase of the

Nb coordination number from six to seven and consequently

to the increase of the electric field gradients, produced by

more distorted oxygen environment. The Lindquist structure

of the peroxo NbꢀPOM (Nb(O₂)W₅) proved to be stable toꢀ

ward excess of hydrogen peroxide. No signs of the structure

degradation were detected by ⁹³Nb NMR even after exposure

to a 50ꢀfold excess of H₂O₂during one week. DFT calculaꢀ

tions on Nb(O)W₅and Nb(O₂)W₅also showed an upfield shift

of the ⁹³Nb NMR signal when the oxo ligand in the Lindqvist

structure was replaced by peroxo group with η²coordination,

although the computed upfield shift of 106 ppm was someꢀ

what lower than the observed one of 150 ppm.

(a)

Nb(O)W₅

1.2

+5 eq 30% H₂O₂:

5 min

9

15 min

30 min

1 h

1.5h

2h

0.9

0.6

0.3

0.0

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

2.5h

250

300

350

400

λ, nm

(b)

Nb(O)W₅

+1eq H⁺+

1.2

5 eq 30% H₂O₂:

0.9

0.6

0.3

0.0

5 min

15 min

30 min

1 h

1.5h

2h

(a)

ꢀ898

ꢀ1015

(b)

2.5h

A

250

300

350

400

λ, nm

B

C

B

ꢀ1048

Figure 3. UVꢀvis spectra of Nb(O)W₅(0.0005 M) after addition

of (a) 5 equiv. of H₂O₂and (b) 1 equiv. of HClO₄and 5 equiv. of

H₂O₂. CH₃CN, 20^oC.

ꢀ1015

ꢀ1048

⁹³Nb NMR spectroscopy is very useful to characterize the

Nb(V) compounds both in solids¹¹⁹and solution. ⁹³Nb has the

high NMR sensitivity, high spin (9/2) and the large electric

quadrupolar moment which, coupling to the local electric

field gradients in typical for Nb(V) distorted environments,

accelerates the nuclear magnetic relaxation, significantly

broadening the signal. In solution, the signal widths increase

also with the complex dimension and concentration. The

Lindqvistꢀtype Nbꢀsubstituted tungstates are appropriate for

D

C

ꢀ1015

D

E

ꢀ1000

(ppm)

ꢀ800

ꢀ1200

ꢀ800

ꢀ1200

ꢀ1000

(ppm)

Figure 4. (a) ⁹³Nb NMR spectra of Nb(O)W₅(A); Nb(O)W₅in

the presence of 0.5, 1 and 2 equiv. of H₂O₂(B, C and D, respecꢀ

tively) and spectrum D after addition of 10 vol% H₂O (E). (b)

⁹³Nb NMR spectra of Nb(O)W₅in the presence of 1 equiv. of

H₂O₂and 1 equiv. of HClO₄(A), (NbW₅)₂O in the presence of 1

equiv. of H₂O₂(B), Nb(O₂)W₅(C), Nb(O₂)W₅in the presence of 1

equiv. of HClO₄(D), and HNb(O₂)W₅(E). NbꢀPOM 0.01 M,

CH₃CN, 20 ^oC.

studying by ⁹³Nb NMR.^96,The width of the ⁹³Nb NMR

120

signal of anion Nb(O)W₅in its 0.008M CH₃CN solution (δ –

898 ppm) is about 1000 Hz, and it is the sharpest signal among

those of the derivative anions studied in this work. Niobium in

the composition of this anion has a distorted C_4vꢀlike octaheꢀ

dral oxygen environment with one terminal unshared oxygen

atom. A significant signal broadening observed for dimer

In the presence of 1 equiv. of acid, the shoulder in the UV

spectrum of H₂O₂ꢀtreated Nb(O)W₅displayed a substantial red

shift (Figure 3b). The TDꢀDFT simulated spectra for Nbꢀ

peroxo species Nb(O₂)W₅shows one peak centered around

305 nm, as shown in Figure S10. The absorption arises from

the HOMO to LUMO excitation, which has a π*_O–O→d(M)

nature. The protonation of Nb(O₂)W₅at the bridging Nb–O–W

(NbW₅)₂O (Figure S9, A) can be explained by almost twoꢀfold

increase of the anion dimensions which results in a larger

correlation time of molecular rotation,⁹⁶although the symꢀ

metry of the ⁹³Nb environment in the dimer is similar to that in

monomers Nb(O)W₅and Nb(OH)W₅.

Upon addition of H₂O₂to Nb(O)W₅, a new broad signal (δ –

1048 ppm) appeared in the ⁹³Nb NMR spectrum (Figure 4a). A

ACS Paragon Plus Environment

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Page 8 of 16

the presence of 1 equiv. of acid (Figure S11), corroborating

the presence of proton in HNb(O₂)W₅.

oxygen to give HNb(O₂)W₅shifts the peak for π*_O–O→d(M)

excitation to 315 nm, reproducing nicely the experimental red

shift. The analysis of molecular orbitals revealed that upon

protonation at the POM framework the dꢀtype orbitals of W

suffer a larger stabilization than the π*_O–O–type orbital of

peroxo moiety reducing the energy gap. Moreover, the addiꢀ

tion of protons greatly accelerated generation of the peroxo

species, which completed within few minutes (compare Figꢀ

ures 3a and 3b). Dimer (NbW₅)₂O reacted with H₂O₂very fast

and the UVꢀvis spectrum of (NbW₅)₂O after interaction with

H₂O₂was similar to the spectrum of Nb(O₂)W₅in the presence

of acid (Figure S11). A pronounced bathochromic shift was

earlier observed in the UV–vis spectra of the H₂O₂–Nb₂O₅–

H₂SO₄system as the concentration of sulfuric acid was inꢀ

creased.¹²²It is noteworthy that a blue shift of the DR UVꢀvis

band, which was preliminary assigned to a ‘NbOOH’ species

(307 nm), was detected upon addition of base to an H₂O₂ꢀ

treated mesoporous Nbꢀsilicate.⁴²Hence, we may conclude

that the UVꢀvis absorption associated with the peroxo Nb

species generated in mesoporous Nbꢀsilicate and homogeneꢀ

ous NbꢀPOM reveal a similar response to acid/base additives.

1

2

3

4

5

6

7

8

The purity of HNb(O₂)W₅was verified by using ⁹³Nb NMR

spectroscopy. The only peak detected in the ⁹³Nb NMR specꢀ

trum was located at δ –1015 ppm (Figure 4b, spectrum E). The

same chemical shift was observed for the peroxo species genꢀ

erated in situ upon addition of 1 equiv. of H⁺to Nb(O₂)W₅

(Figure 4b, spectrum D). ⁹³Nb NMR monitoring of the stepꢀ

wise addition of HClO₄to Nb(O₂)W₅(Figure S15a) and

Bu₄NOH to HNb(O₂)W₅(Figure S15b) clearly indicated that

species HNb(O₂)W₅and Nb(O₂)W₅are in fast exchange on the

⁹³Nb NMR time scale, which implies that both peroxo comꢀ

plexes have a similar structure and differ only in their protonaꢀ

tion state. As one can judge from Figure S15, the interconverꢀ

sion between HNb(O₂)W₅and Nb(O₂)W₅is reversible and the

Lindqvist structure of the NbW₅peroxo complexes is stable

toward, at least, 3 equiv. of acid and 1.5 equiv. of base.

9

10

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12

13

14

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17

18

19

20

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53

54

55

56

57

58

59

60

The structure of HNb(O₂)W₅, in particular a possible site of

the proton localization, was probed by vibrational spectroscopꢀ

ic techniques. No IR bands were observed for solid

HNb(O₂)W₅in the characteristic 1800–1600 cm^–1region (Figꢀ

ure S16), indicating that no H₃O⁺was present and that the

proton is directly attached to the POM surface.^123,124The FTꢀ

IR spectra of HNb(O₂)W₅and Nb(O₂)W₅confirm the retention

of the Lindqvist structure (see Figure S2). The absence of the

vibration stretches of Nb=O and Nb–O–Nb at 915 and 695

cm^–1present in the parent compounds Nb(O)W₅and

(NbW₅)₂O, respectively, clearly indicates changes at the termiꢀ

nal position of the Nb atom. The FTꢀIR spectrum of Nb(O₂)W₅

revealed new bands at 857, 638 and 602 cm^–1. On the basis of

the literature data reported for various peroxo niobium comꢀ

plexes,^{86,95,125–131}the band at 857 cm^–1can be assigned to the

stretching fundamental of the O–O bond while the couple at

638 and 602 cm^–1is associated with symmetric and asymmetꢀ

ric Nbꢀperoxide stretches. The IR spectrum of HNb(O₂)W₅

revealed some important distinctive features relative to that of

Nb(O₂)W₅. First, the frequencies of ν(W=O) and ν(O–O) are

shifted to higher energy: 973 vs 960 cm^–1and 870 vs 857 cm^–1,

respectively, which reflects a decrease in the corresponding

bond lengths due to the contraction of the structure via reducꢀ

tion of the negative charge. This indicates that protonation of

the peroxo derivative occurs at Nb–O–W bridging oxygen

rather than at the peroxo group, because, in the latter case, one

would expect a lowerꢀenergy shift due to lengthening of the

O–O bond. A higher energy shift of ν(W=O) was observed

earlier upon protonation of the diꢀNbꢀsubstituted Lindqvist

tungstate [Nb₂W₄O₁₉]^4–.¹³²

The protonation of the NbꢀPOM peroxo species was also

manifested in the ⁹³Nb NMR spectra (Figure 4b). In the presꢀ

ence of 1 equiv. of acid, the resonance of H₂O₂ꢀtreated

Nb(O)W₅shifted downfield relative to the protonꢀfree system

(δ –1015 versus –1048 ppm). A resonance with the same

chemical shift (δ –1015 ppm) was observed in the ⁹³Nb NMR

spectrum of dimer (NbW₅)₂O upon interaction with hydrogen

peroxide (Figure 4b, spectrum B). In turn, the broad ⁹³Nb

NMR resonance of peroxo complex Nb(O₂)W₅(δ –1048 ppm)

narrowed significantly in the presence of protons and moved

to –1015 ppm (Figure 4b, spectrum D). Thus, the observed

changes in the UVꢀvis and ⁹³Nb NMR spectra allowed us to

suggest that the same protonated niobium peroxo species

could be generated via interaction of H₂O₂with either

(NbW₅)₂O or Nb(O)W₅in the presence of H⁺(see Scheme 1).

Importantly, the ¹⁸³W NMR spectrum of (NbW₅)₂O in the

presence of 3 equiv. of H₂O₂(Figure S12, spectrum D)

showed two resonances with the intensity ratio of 4:1 typical

of monosubstituted Lindqvist tungstates.

Characterization and reactivity of Nbꢀperoxo species.

While our attempts to isolate a protonated peroxo niobium

complex from a solution of Nb(O)W₅in the presence of acid

and an excess of H₂O₂failed, we managed to synthesize it

starting from dimer (NbW₅)₂O and H₂O₂(Scheme 1).

The presence of one peroxo group per Nb atom was conꢀ

firmed by titration with PPh₃monitored by ³¹P NMR accordꢀ

ing to the procedure described in the literature.⁸⁷The eleꢀ

mental analysis data agreed with the formulation of

(Bu₄N)₂[HNb(O₂)W₅O₁₈]. Although attempts to grow Xꢀray

quality crystals of HNb(O₂)W₅failed, this compound could be

comprehensively characterized using various analytic and

spectroscopic techniques. Potentiometric titration of

HNb(O₂)W₅in CH₃CN with aqueous Bu₄NOH showed a sharp

breakpoint at 0.73 equiv. of OH^–(Figure S13), indicating that

the exact [H⁺]/[Nb] ratio is a little less than 1. Cyclic voltamꢀ

metry indicated a significant increase in the oxidation potential

of HNb(O₂)W₅relative to Nb(O₂)W₅(Figure S14). The UVꢀ

vis spectrum of HNb(O₂)W₅differed from that of Nb(O₂)W₅,

but it practically coincided with the spectrum of Nb(O₂)W₅in

The assignment of the IR bands in the peroxo Nb species

and the hypothesis about protonation of the Nb–O–W bridging

oxygen were further supported by Raman spectroscopy. In

comparison with the spectra of peroxoꢀfree compounds

(NbW₅)₂O and Nb(O)W₅, new bands appeared in the Raman

spectrа of the peroxo derivatives at 860–863 and 603–623 cm^–

1

(Figure S17) and can be assigned to ν(O–O) and ν(NbO₂)

vibrations, respectively.^{118,125,126,133–135}Similarly to the IR

spectra, a higherꢀenergy shift of the stretching vibrations of

terminal W=O groups was clearly observed in the Raman

spectrum of solid HNb(O₂)W₅as compared to the spectra of

Nb(O₂)W₅and Nb(O)W₅. The Raman spectrum of

HNb(O₂)W₅generated in situ from Nb(O₂)W₅and 1 equiv. of

H⁺in CH₃CN solution (Figure 5) showed shearing of ν(W=O)

and ν(NbO₂) to higher energies (ca. 13 and 7 cm^–1, respectiveꢀ

ACS Paragon Plus Environment

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ACS Catalysis

The protonation site in HNb(O₂)W₅was also confirmed by

ly). Thus, both IR and Raman spectroscopy provided evidence

in favor of the sideꢀon (η²) bonding mode of the peroxo ligand

in HNb(O₂)W₅and Nb(O₂)W₅, with proton located at Nb–O–

W oxygen site in the former. In agreement with this interpretaꢀ

tion, the DFTꢀsimulated Raman spectra of Nb(O₂)W₅and

HNb(O₂)W₅(Figure S18) revealed a shift to higher energies

for ν(W=O) and ν(NbO₂) bands of complex HNb(O₂)W₅by 14

and 5 cm^–1, respectively, which is close to what was observed

experimentally. Moreover, DFT calculations (vide infra)

showed that in HNb(O₂)W₅protonation at Nb–O–W is therꢀ

modynamically favored relative to protonation at the W–O–W

bridging oxygen and at the peroxo moiety by 3.0 and 8.7

kcal·mol^–1, respectively.

¹⁷O NMR spectroscopy. As shown in Figure 6 and Table S2,

the ¹⁷O NMR spectrum of HNb(O₂)W₅revealed a pronounced

upfield shift of the Nb–O–W signal (δ 414 ppm) relative to its

position in the ¹⁷O NMR spectra of Nb(O)W₅(δ 456 ppm)^97,120

and Nb(O₂)W₅(460 ppm). This shift in combination with

downfield shifts for the rest of signals is, according to the

literature,^132,136an unambiguous prove for the protonation of

Nb–O–W bond in solution.¹³⁷Hence, IR, Raman, and ¹⁷O

NMR spectroscopic techniques along with DFT calculations

(see below) all support the conclusion that in HNb(O₂)W₅the

proton is preferably located at the Nb–O–W bridging oxygen

site rather than at the peroxo ligand.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

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41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

Finally, we investigated the stoichiometric interaction of

Nb(O₂)W₅and HNb(O₂)W₅with CyH as model alkene subꢀ

strate. The results on the substrate conversion and product

selectivity are shown in Table 2. Peroxo complex Nb(O₂)W₅

was found to be completely inert toward CyH. However, it

became reactive and produced heterolytic oxidation products,

epoxide and diol, if 1 equiv. of protons was added. In turn,

protonated peroxo complex HNb(O₂)W₅was also able to

oxidize CyH under stoichiometric conditions, leading to epoxꢀ

ide and diol. The amount of the oxygenated products correꢀ

sponded to the amount of active oxygen (i.e., peroxo oxygen

activated by proton) in HNb(O₂)W₅. These experiments unꢀ

ambiguously prove that protons play a crucial role in the actiꢀ

vation of the peroxo niobium moiety and that namely protoꢀ

nated Nb peroxo species is responsible for heterolytic oxidaꢀ

tion of C=C bond in alkene.

ν

(W=O)

D

C

ν

(NbO₂)

B

A

1200

1000

800

600

400

200

Raman shift, cm^ꢀ1

Table 2. Interaction of HNb(O₂)W₅and Nb(O₂)W₅with CyH^a

Figure 5. Raman spectra of CH₃CN (A) and CH₃CN solutions of

Nb(O)W₅(B), Nb(O₂)W₅(C), and Nb(O₂)W₅in the presence of 1

equiv. of HClO₄(D). POM 0.1 M, CH₃CN, 20 ^oC.

Entry

POM

CyH

Product yield,^b%

conv. %

epoxide

diol

0

WOW

1

2

3

a

Nb(O₂)W₅

Nb(O₂)W₅+ 1 eq. H⁺

HNb(O₂)W₅

0

19.5

16

13.5

10

6

W=O

6

ꢁ₆ꢀO

NbOW

Reaction conditions: 0.016 M POM, 0.016 M HClO₄(entry

2), 0.08 M CyH, CH₃CN, 50 ^oC, 2 h. ^bBased on substrate.

A

The stoichiometric reaction can be easily followed by UVꢀ

vis. The decrease in the characteristic absorption region due to

the ligandꢀtoꢀmetal charge transfer (310–350 nm) in the UVꢀ

vis spectra of HNb(O₂)W₅(Figure 7) or HNb(O₂)W₅generated

in situ from Nb(O₂)W₅and H⁺(Figure S19) is much slower

than in the presence of CyH. This implies that thermal decomꢀ

position of HNb(O₂)W₅is rather slow and that the observed

decay of the absorption is due to the reaction with CyH. ⁹³Nb

NMR monitoring of the reaction between HNb(O₂)W₅and

CyH revealed transformation of the sharp resonance at δ –

1015 ppm into the broad resonance at ca. –950 ppm attributed

to (NbW₅)₂O,⁹⁶which appeared at the end of the reaction as a

result of dimerization of Nb(OH)W₅. Importantly, interaction

of HNb(O₂)W₅with cisꢀstilbene produced exclusively cisꢀ

epoxide along with the C–C cleavage product benzaldehyde,

corroborating a concerted oxygen transfer mechanism.

B

**

*

C

(ppm)

0

600

200

400

800

Figure 6. ¹⁷O NMR spectra of Nb(O)W₅(A), Nb(O₂)W₅(B), and

HNb(O₂)W₅(C). 0.1 M POM, CH₃CN, 20 C. The spectrum of

HNb(O₂)W₅was accumulated in the presence of a 10ꢀfold excess

of 30% H₂O₂. Peaks corresponding to H₂O₂and H₂O are marked

with asterisks * and **, respectively.

o

ACS Paragon Plus Environment

ACS Catalysis

Page 10 of 16

above, the bridging W–O–Nb is the thermodynamically preꢀ

ferred protonation site.

(a)1.2

0.9

HNb(O₂)W₅

heating:

1

2

3

4

5

6

7

8

We have analyzed the heterolytic oxygen transfer to CyH

from both sideꢀon Nbꢀhydroperoxo (4b) and ꢀperoxo (4) speꢀ

cies. Figure 9 collects the geometries of the reactants and the

transition states, as well as, the corresponding free energy

barriers. The less stable hydroperoxo anion 4b shows a lower

free energy barrier than the peroxo anion 4 (17.6 vs 27.1

kcal·mol^–1). In Nbꢀhydroperoxo species 4b, the alkene attack

occurs preferably at the proximal, nonꢀprotonated αꢀoxygen

because the σ*O–O orbital is polarized toward Oα, favoring

the donation from the nucleophilic double bond.⁴⁵In this case,

the transition state for βꢀoxygen transfer (see Figure S20) is

2.3 kcal·mol^–1higher than the corresponding transition state

for αꢀoxygen transfer TS_4b–P. Nevertheless, the βꢀoxygen

transfer could be favored in complexes with metal ions in

sterically hindered or strongly embedded in nonꢀflexible enviꢀ

ronment that make the Oβ more accessible to the substrate.¹³⁹

15 min

30 min

1 h

1.5 h

2 h

0.6

3 h

0.3

time

9

10

11

12

13

14

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60

0.0

250

300

350

400

λ

, nm

1.2

0.9

0.6

0.3

0.0

(b)

HNb(O₂)W₅

+5 eq CyH:

5 min

15 min

30 min

1 h

1.5 h

2 h

3 h

time

250

300

350

400

λ

, nm

Figure 7. UVꢀvis spectra of HNb(O₂)W₅(a) in time and (b) in

time in the presence of 5 equiv. of CyH. Reaction conditions:

0.0005 M HNb(O₂)W₅, 0.0025 M CyH, CH₃CN, 50 ^oC.

DFT study of reaction mechanism. To understand the reacꢀ

tion mechanism at atomistic level and identify the active speꢀ

cies of the oxygen transfer process, we have performed DFT

calculations on Nbꢀsubstituted Lindqvist anions using CyH as

substrate. Our previous study on the effect of the metal nature

on the oxygen transfer mechanism showed that when we move

from Ti to Nb (down in the periodic table), both peroxo and

hydroperoxo energy barriers decrease, but the latter does it

more appreciably.⁴⁵However, in order to obtain a complete

picture of the reaction, one should also evaluate the effect of

the larger radius of Nb(V) ion on the stabilization of 7ꢀ

coordinated species such as η²ꢀperoxo and η²ꢀhydroperoxo

ones.¹³⁸Figure 8 depicts the calculated free energy profile for

the epoxidation of CyH with H₂O₂catalyzed by

[Nb(OH)W₅O₁₈]^2–(3) anion. In the first step, the interaction of

H₂O₂with the Nbꢀhydroxo moiety of 3 results in the hydropꢀ

eroxo species 4a, in which the OOH group is η¹ꢀcoordinated,

and the release of a water molecule. The H₂O₂activation proꢀ

ceeds through transition state TS_3–4a(see Figure S20), and the

computed freeꢀenergy barrier is moderate, 15.3 kcal mol^–1.

The generated η¹ꢀOOH species (4a) can evolve to the more

stable η²ꢀOOH species 4b because the size of the secondꢀrow

Nb(V) ion allows to accommodate 7ꢀfold coordination. The

η²ꢀhydroperoxo species 4b is lower in energy (–3.5 kcal·mol^–

¹) than the corresponding η¹ꢀhydroperoxo 4a, but higher in

energy (+8.7 kcal·mol^–1) than the corresponding η²ꢀperoxo

complex 4 in agreement with experimental data. Nbꢀperoxo

complex 4 can be formed from 4b via hydrogen transfer from

the hydroperoxo moiety to a bridging W–O–Nb oxygen overꢀ

coming a moderate free energy barrier of 15.1 kcal·mol^–1(see

transition state structure TS_4b–4in Figure S20). As commented

Figure 8. Calculated Gibbs free energy profile (kcal·mol^–1) for

the epoxidation of cyclohexene (CyH) with H₂O₂catalyzed by

Nb(OH)W₅(3).

Another important feature of the potential freeꢀenergy proꢀ

file (Figure 8) is the relative rates between the hydrogen and

the oxygen transfer from 4b to form the peroxo species 4

(TS_4b–4) and the epoxide (TS_4b–P), respectively. The hydrogen

transfer is computed to be faster, its free energy barrier being

2.5 kcal·mol^–1lower in energy. Thus, it is reasonable to asꢀ

sume that a large proportion of the reacting species, reaching

the Nbꢀhydroperoxo 4b intermediate, evolve to the Nb–peroxo

complex 4, which becomes the restingꢀstate of the catalytic

process. From Nbꢀperoxo species 4, the reaction can proceed

directly to oxygen transfer to the alkene through TS_4–Pstrucꢀ

ture, overcoming a computed freeꢀenergy barrier of 27.1

kcal·mol^–1, or it can come back to the less stable but more

reactive Nbꢀhydroperoxo species 4b, overcoming an overall

freeꢀenergy barrier (4 ꢀ TS_4b–P) of 26.3 kcal·mol^–1. In sumꢀ

mary, our calculations indicate that most of the reaction proꢀ

ceeds via heterolytic oxygen transfer from Nbꢀη²ꢀhydroperoxo

species 4b, involving a previous formation of protonated Nbꢀ

peroxo species 4, which acts as the restingꢀstate of the catalyst.

However, other two energetically accessible paths can conꢀ

tribute to the formation of epoxides: (i) the direct oxygen

transfer from 4b without the formation of peroxo species 4,

and (ii) the heterolytic oxygen transfer from the Nbꢀperoxo

ACS Paragon Plus Environment

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ACS Catalysis

species 4. According to the computed values, at 50º C, the

contribution of the two latter paths is approximately 2 and

21%, respectively.

Table 3. Calculated protonation freeꢀenergies (ꢁG_H+) and freeꢀ

energy barriers for oxygen transfer (ꢁG^‡_Otrans) from Lindqvist

(Nb(O₂)W₅5 and HNb(O₂)W₅4) and Keggin (5k and 4k)

anions in kcal·mol^ꢀ1.

1

2

3

4

5

6

7

8

9

ꢁG_H+

–

ꢁG^‡

Otrans

Catalyst

Active species

q/M

0.50

0.33

0.25

31.6

5

–

29.6

26.3^a

23.8^a

5k

5 + 1H⁺

5k + 1H⁺

5k

4

–3.7

+2.7

4k

10

11

12

13

14

15

16

17

18

19

20

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22

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41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

^aFreeꢀenergy barriers through hydroperoxo path.

Finally, we analyzed computationally the reactivity of Nbꢀ

substituted Keggin anions [PW₁₁O₃₉Nb(O₂)]^4–(5k) and

[HPW₁₁O₃₉Nb(O₂)]^3–(4k), which also display a poor perforꢀ

mance in alkene epoxidation (Table 1). Two main trends are

observed in Table 3, which collects the computed protonation

energies (ꢁG_H+) and the freeꢀenergy barriers for oxygen transꢀ

fer ( G_Otrans) through peroxo (5/5k → TS_5–P/TS_5k–P) and hyꢀ

ꢁ

droperoxo (4/4k → TS_4b–P/TS_4bk–P) paths. First, ongoing from

Lindqvist to Keggin anion the charge density decreases as

reflected in the total charge per metal ratio (q/M).^{107,140–142}

Consequently, the POM becomes better electrophile favoring

the oxygen transfer to the nucleophilic alkene by ~2 kcal·mol^–

1 83

.

Second, the less basic Keggin anion is more difficult to

protonate resulting in an endergonic process according to our

calculations (+2.7 kcal·mol^–1for 5k + 1H⁺→ 4k). The comꢀ

puted atomic charge from electrostatic potential at the bridging

NbꢀOꢀW oxygen for 5k (ꢀ.74 au) is less negative than that of

the corresponding oxygen for Lindqvist species 5 (ꢀ.86 au).

Thus, although Keggin structures are more reactive as oxygen

transfer agents than Lindqvist ones, they are more difficult to

be activated via protonation. Overall, these opposite trends

might result in a loose of reactivity for Keggin structures. In

summary, for the alkene epoxidation by Nbꢀsubstituted POMs

the art consists of tuning the charge density of the anion. The

POM should have low electron density in order to favor the

electrophilic oxygen transfer to alkenes, but basic enough to

be protonated and enable the hydroperoxo path.

Figure 9. Combined ballsꢀandꢀsticks and polyhedral representaꢀ

tion of some of the most relevant intermediates (4b and 4) and

transition states (TS_4b–Pand TS_4–P) in reaction mechanism. Main

distances are shown in Å. Free energy barriers for the electroꢀ

philic Oꢀtransfer step occurring from 4b and 4 are shown in kcal

mol^–1.

To further validate our proposed mechanism, we analyzed

the zeroꢀpoint corrected energy profile (Figure S21) for CyOct

epoxidation by dimer (NbW₅)₂O and compared it with the

experimental E_a(11.7 ± 0.8 kcal·mol^–1) in Figure 2. In the

absence of entropic effects, the computed overall energy barriꢀ

er for direct oxygen transfer from hydroperoxo species 4b is 8

kcal·mol^–1(too low) while those involving the formation of

peroxo species 4 are about 16 kcal·mol^–1(too high). However,

assuming that several paths contribute to the reaction, the

weighted average of the computed energy barriers should be

close to experimental E_avalue.

CONCLUSIONS

Nbꢀsubstituted tungstates of the Lindqvist structure are highꢀ

ly active and selective catalysts for H₂O₂ꢀbased oxidation of

alkenes via heterolytic pathway. In this work, we demonstratꢀ

ed that the Lindqvist NbꢀPOMs mimic well the catalytic perꢀ

formance of heterogeneous Nbꢀsilicate catalysts and can be

successfully employed as soluble molecular models for studyꢀ

ing mechanism of H₂O₂activation on Nb(V). Product, spectroꢀ

scopic and computational studies revealed that protons play a

crucial role in the catalytic activity of niobium sites. While a

suitable source of protons is required to accelerate interaction

of the terminal Nb=O bond in monomer Nb(O)W₅with H₂O₂,

ꢁꢀoxo dimer (NbW₅)₂O is highly active without any additives.

Interaction of (NbW₅)₂O (alternatively Nb(O)W₅+ H⁺) with

H₂O₂affords protonated Nb peroxo derivative HNb(O₂)W₅

which, in contrast to protonꢀfree peroxo complex Nb(O₂)W₅,

is able to epoxidize alkenes under stoichiometric conditions.

Therefore, protons are important not only for the generation of

peroxo niobium species but also for its reactivity. Spectroꢀ

scopic (UVꢀvis, IR, Raman, and ¹⁷O NMR) and computational

techniques strongly support a monomeric Lindqvist structure

Next, to understand the role of protons on the reactivity, we

evaluated feasibility of the epoxidation process using the nonꢀ

protonated peroxo species [Nb(O₂)W₅O₁₈]^3–(5). The free

energy barrier for the oxygen transfer from 5 to CyH was

computed to be 31.6 kcal mol^–1, which is larger than that for

the epoxidation by protonated peroxo species 4, 26.3

kcal·mol^–1(from 4 to TS_4b–P). This is consistent with lack of

reactivity observed over species 5. Moreover, the freeꢀenergy

barrier for H₂O₂activation by the nonꢀprotonated species 2 is

~4 kcal·mol^–1higher than that from the protonated species 3

(see Figure 8) in full agreement with the accelerated generaꢀ

tion of peroxo species observed upon addition of protons

(Figure 3). Thus, protonation has two important consequences

in the catalytic performance of Nbꢀsubstituted anions: (i) it

allows forming the more reactive Nbꢀhydroperoxo species,

and (ii) it reduces the overall negative charge of the POM

favoring the electrophilic oxygen transfer to the alkene as

observed also for Tiꢀsubstituted POMs.⁸³

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of HNb(O₂)W₅with one peroxo ligand attached to Nb(V) in a

Page 12 of 16

η²ꢀcoordination mode and proton located at a Nb–O–W

bridging oxygen. DFT calculations also revealed that η²ꢀ

hydroperoxo niobium species, which is present in equilibrium

with the protonated peroxo species HNb(O₂)W₅in CH₃CN

solution, is the main epoxidizing species since it drives the

reaction by a less energyꢀdemanding reaction path. The protoꢀ

nation of the NbꢀPOMs plays a crucial role in activating the

oxidant by allowing the formation of the more reactive hyꢀ

droperoxo intermediate and increasing its electrophilicity that

favors the oxygen transfer to nucleophiles. Future catalyst

designs should fineꢀtune its electrostatic properties reducing

electron density but allowing protonation in order to enable

the hydroperoxo path.

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AUTHOR INFORMATION

(9) Masarwa, A.; RachmilovichꢀCalis, S.; Meyerstein, N.; Meyerꢀ

stein, D. Oxidation of Organic Substrates in Aerated Aqueous Soluꢀ

tions by the Fenton Reagent. Coord. Chem. Rev. 2005, 249, 1937–

1943.

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kenes with Hydrogen Peroxide. Chem. Rev. 2003, 103, 2457–2474.

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opments on the Epoxidation of Alkenes Using Hydrogen Peroxide as

an Oxidant. Green Chem. 2003, 5, 1–7.

(12) Bryliakov, K. P.; Talsi, E. P. Chemo– and Stereoselective C–

H Oxidations and Epoxidations/cis–Dihydroxylations with H₂O₂,

Catalyzed by Non–Heme Iron and Manganese Complexes. Coord.

Chem. Rev. 2012, 256, 1418–1434.

(13) Hauser, S. A.; Cokoja, M.; Kühn, F. E. Epoxidation of Olefins

with Homogeneous Catalysts – Quo Vadis? Catal. Sci. Technol. 2013,

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Corresponding Author

* Eꢀmails: khold@catalysis.ru; j.carbo@urv.cat

Author Contributions

The manuscript was written through contributions of all authors. /

All authors have given approval to the final version of the manuꢀ

script.

Notes

The authors declare no competing financial interests.

ASSOCIATED CONTENT

Supporting Information. Data on product yields for various

alkenes,¹⁸³W and ⁹³Nb NMR, IR, Raman, and UVꢀvis spectra, ¹⁷

O

NMR data, potentiometric titration, cyclic voltammograms, kinetꢀ

ic data on CyOct epoxidation, structures of transition state, energy

profile.

(14) Venturello, C.; D’Aloisio, R. Quaternary Ammonium

Tetrakis(diperoxotungsto)phosphates(3–) as a New Class of Catalysts

for Efficient Alkene Epoxidation with Hydrogen Peroxide. J. Org.

Chem. 1988, 53, 1553–1557.

(15) Bregeault, J.ꢀM. Transition–Metal Complexes for Liquid–

Phase Catalytic Oxidation: Some Aspects of Industrial Reactions and

of Emerging Technologies. Dalton Trans. 2003, 3289–3302.

(16) Mizuno, N.; Yamaguchi, K.; Kamata, K. Epoxidation of Oleꢀ

fins with Hydrogen Peroxide Catalyzed by Polyoxometalates. Coord.

Chem. Rev. 2005, 249, 1944–1956.

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and Dimeric Oxido–Peroxido Tungsten(VI) Complexes in Catalytic

and Stoichiometric Epoxidation. Coord. Chem. Rev. 2014, 268, 83–

100.

(18) Herrmann, W. A.; Fischer, R. W.; Scherer, W.; Rauch, M. U.

Methyltrioxorhenium(VII) as Catalyst for Epoxidations: Structure of

the Active Species and Mechanism of Catalysis. Angew. Chem. Int.

Ed. 1993, 32, 1157–1160.

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Reactions Catalyzed by Rhenium, Molybdenum, and Iron Complexes.

Chem. Rec. 2016, 16, 349ꢀ364.

ACKNOWLEDGMENT

We thank Dr. I. Y. Skobelev for preliminary DFT calculations,

Dr. M. V. Shashkov for the product identification by GC–MS, Dr.

S. S. Arzumanov and Dr. D. F. Khabibulin for their help in NMR

measurements. The authors are grateful to Dr. R. J. Errington

(Newcastle University/UK) for providing the sample of

(Bu₄N)₂[W₆O₁₉].

This work was carried out within the framework of the budget

project АААА-А17-117041710080-4 for the Boreskov Institute of

Catalysis and was partially supported by the Russian Foundation

for Basic Research (grant N 16-03-00827). We also thank the

Spanish Ministry of Science (CTQ2017-87269-P and UNRV15-

EE–3935), the Generalitat de Catalunya (2017SGR629) and the

URV for generous support. J.M.P. also thanks the ICREA foundaꢀ

tion for an ICREA ACADEMIA award.

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Relevance of Protons in Heterolytic Activation of H2O2 over Nb(V): Insights from Model Studies on Nb-Substituted Polyoxometalates

DOI: 10.1021/acscatal.8b02761

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