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Faster oxygen atom transfer catalysis with a tungsten dioxo complex thanwith its molybdenum analog†

T. Arumuganathan, Ramasamy Mayilmurugan,‡ Manuel Volpe and Nadia C. Mosch-Zanetti*

Received 14th February 2011, Accepted 17th May 2011DOI: 10.1039/c1dt10248f

The synthesis and characterization of a series of molybdenum ([MoO2Cl(Ln)]; L1 (1), L2 (3)) andtungsten ([WO2Cl(Ln)]; L1 (2), L2 (4)) dioxo complexes (L1 = 1-methyl-4-(2-hydroxybenzyl)-1,4-diazepane and L2 = 1-methyl-4-(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane) of tridentateaminomonophenolate ligands HL1 and HL2 are reported. The ligands were obtained by reductiveamination of 1-methyl-1,4-diazepane with the corresponding aldehyde. Complexes 3 and 4 wereobtained by the reaction of [MO2Cl2(dme)n] (M = Mo, n = 0; W, n = 1) with the corresponding ligand inpresence of a base, whereas for the preparation of 1 and 2 the ligands were deprotonated by KH priorto the addition to the metal. They were characterized by NMR and IR spectroscopy, by cyclicvoltammetry, mass spectrometry, elemental analysis and by single-crystal X-ray diffraction analysis.Solid-state structures of the molybdenum and tungsten cis-dioxo complexes reveal hexa-coordinatemetal centers surrounded by two oxo groups, a chloride ligand and by the tridentate monophenolateligand which coordinates meridionally through its [ONN] donor set. In the series of compounds 1–4,complexes 3 and 4 have been used as catalysts for the oxygen atom transfer reaction between dimethylsulfoxide (DMSO) and trimethyl phosphine (PMe3). Surprisingly, faster oxygen atom transfer (OAT)reactivity has been observed for the tungsten complex [WO2Cl(L2)] (4) in comparison to itsmolybdenum analog [MoO2Cl(L2)] (3) at room temperature. The kinetic results are discussed andcompared in terms of their reactivity.

Introduction

Molybdenum and tungsten complexes in high oxidation statesare of considerable interest not only because of their occurrencein the active site of oxygen atom transferring enzymes but alsobecause of their use in industrial catalysis. A large number ofimportant transformations both in vivo and in vitro are catalyzedby molybdenum complexes, for example, hydrodesulfurization,olefin epoxidation, alcohol oxidation, olefin metathesis, andoxidation of aldehydes, xanthine and other purines etc.1 Molyb-denum enzymes are ubiquitous and can be classified into threedifferent families: (a) xanthine oxidase (XO) (b) dimethyl sulfoxidereductase (DMSOR) and (c) sulfite oxidase (SO), based on theirstructural motifs. The crystal structures of several molybdenumand tungsten containing enzymes demonstrate that the active site

Institut fur Chemie, Bereich Anorganische Chemie, Karl-Franzens-Universitat Graz, Schubertstrasse 1, 8010, Graz, Austria. E-mail:nadia.moesch@uni-graz.at† Electronic supplementary information (ESI) available. CCDC refer-ence numbers 810207 (4), 810208 (1) and 810209 (3). For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c1dt10248f‡ Current address: School of Chemistry/Department of Physical Chem-istry, Madurai Kamaraj University, Madurai-625021, India. E-mail:ramamayil@gmail.com

containing the mononuclear metal center is coordinated by oneor two oxygen atoms as well as one or two additional sulfurcontaining pyranopterin ligands.2 DMSO reductase is the mostdiverse among the existing three families of molybdenum enzymeswhere the metal center is frequently coordinated by the serinealkoxide oxygen, cysteine thiolate sulfur or a carboxylate oxygenfrom aspartate. These enzymes use a range of alkylaryl and dialkylsulfoxides as substrates for the oxidation.2c In addition, interest hasemerged to evaluate the properties of a Mo-enzyme with those ofits W-substituted analog. For example, W can replace Mo in sulfiteoxidase at the same site but with prominent differences in theirproperties.3 Furthermore, the replacement of Mo by W in DMSOreductase shows that W-DMSOR is very effective in catalyzing thereduction of DMSO (ca. 17 times faster than Mo-DMSOR), butis inactive in the reverse catalytic reactions, the oxidation of DMSto DMSO.4 Most often, substitution of W in place of Mo showsthe loss or retention5 and very rarely enhancement in reactivity.4

Hence, mimicking model complexes of this category are mostnoteworthy in understanding the structure and mechanism of theseenzymes. Early on, Mo and W based model complexes mimickingthese metalloenzymes have been synthesized with a variety ofligands, e.g. with dithiolene ligands,6 non-dithiolene ligands suchas tris(pyrazolyl)borate,7 b-ketiminates,8 3,5-tert-butylpyrazolate,9

and bisphenolate ligands.10 Molybdenum complexes with

7850 | Dalton Trans., 2011, 40, 7850–7857 This journal is © The Royal Society of Chemistry 2011

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Schiff-base ligands, and their oxo transfer properties with phos-phines, were first investigated by Topich and Lyon.11 The iso-lation of mononuclear Mo and W complexes in their higheroxidation states are sometimes tricky because the coordinationsphere around the metal center is frequently completed throughdimerization. The reaction between MoVIO2 and MoIVO leads tothe formation of dimeric MoV

2O3 species which are not biologicallysignificant. EXAFS studies give evidence for the presence of cis-dioxo MoO2

2+ groups in the oxidized forms of xanthine oxidase12

and sulfite oxidase13 and a single terminal oxo ligand in thereduced forms of xanthine dehydrogenase and reduced sulfiteoxidase.14 Consequently, it is of biologically great interest toisolate mononuclear molybdenum and tungsten complexes in theirhighest oxidation state +6. Mo and W complexes with tetradentate[ONNO] or [SNNS] functionality10b,c,15 and tridentate ligandswith [ONO] and [ONS] functionality have been well examined.16

However, tridentate complexes with [ONN] donors are not muchexplored. Only a few related complexes are described in theliterature.17 We report here the synthesis and characterization ofa series of novel dioxo MoVI and WVI complexes with tridentatemonophenolate ligands coordinated by their [ONN] donors andtheir oxo transfer reaction towards trimethyl phosphine in pres-ence of DMSO at room temperature. The tridentate [ONN] ligandsdescribed here are designed based on our previous investigationof a similar tetradentate [ONNO] bisphenolate system.10b Due tothe monophenolate nature of the [ONN] ligand, one Cl atomremains in the coordination sphere of the metal, potentiallyallowing the substitution by other monoanionic ligands such ase.g. OH- ion, which could result in the formation of mimickingmodel complexes for molybdenum containing hydroxylases. Thesystem described here was found to be too sensitive for this kindof substitution reactions. However, interesting OAT reactivity ofthe unsubstituted complexes was found, as the tungsten complex[WO2Cl(L2)] (4) was revealed to be a significantly more efficientOAT catalyst than its molybdenum counterpart [MoO2Cl(L2)] (3).Syntheses of the complexes and kinetic data of the OAT catalysisare reported.

Results and discussion

Syntheses and spectroscopic characterization of complexes

The appropriate ligands HL1 and HL2 were readily prepared byreductive amination of 1-methyl-1,4-diazepane with the corre-sponding aldehyde using sodium cyanotrihydridoborate as reduc-ing agent with slight modifications to a procedure for previouslyreported related bisphenols.18 The ligand HL1 was obtained asa deep orange viscous liquid and HL2 as a colorless solid.The molybdenum(VI) and tungsten(VI) complexes of the type[MO2Cl(Ln)] (M = Mo, W, n = 1 (1, 2) and n = 2 (3, 4)) weresynthesized by the treatment of [MoO2Cl2] and [WO2Cl2(dme)],respectively, with the ligand. Whereas complexes 1 and 2 have beensynthesized by starting from the potassium salt of its ligand (KL1)precursor, complexes 3 and 4 were obtained by using HL2 andtriethylamine as base (Scheme 1). Attempts to prepare Mo and Wcomplexes (3 and 4) starting from their respective potassium saltfailed to achieve the target complexes but rather unidentified red–brown oils were obtained. The immediate formation of a yellowor orange color after addition of the [MoO2Cl2] or [WO2Cl2(dme)]solution to the ligand solution indicates the formation of thecorresponding molybdenum or tungsten complexes in solution.Complexes 1–4 were successfully obtained after purification fromthe reaction mixture. Handling of compounds 1 and 2 was difficult,as they were prone to decomposition, which explains the relativelylow yields obtained. They also decomposed in DMSO solutionwhich prevented the investigation of OAT reactivity (vide infra).

In general, compounds 3 and 4 employing the tert-butylsubstituted ligand L2 were more stable. This allowed the isolationof pure compound 3 in good yield. Also the respective tungstencompound 4 was in principle formed in high yield. However,purification was somewhat hampered due to similar solubilityproperties of 4 and formed triethylammonium chloride in organicsolvents. Removal of triethylammonium chloride was partiallyachieved by the addition of diethyl ether to an acetonitrile mixture.The initially precipitated triethylammonium chloride was removedby filtration through a pad of Celite. After evaporation, the

Scheme 1 Syntheses of ligands and formation of complexes.

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obtained orange solid material was dissolved in acetonitrile andagain ether was added and the same process was repeated forthree to four times to obtain reasonable purity. All the complexesdecompose readily in the presence of wet solvents and moist airbut can be stored in a glovebox for extended times.

Complexes 1–4 are not very soluble in common organic solvents,which only allowed characterization by 1H NMR spectroscopy,and failed to produce 13C NMR spectra. Coordination of thephenolate ligand is evident in the 1H NMR spectra of 1–4 as thetwo asymmetric protons of the methylene group give rise of twodistinct doublets (4.45–4.68 ppm, J = 11.83–12.26 Hz; 2.61–3.59ppm, J = 11.71–12.21 Hz), whereas in the free ligand, the samemethylene group protons (symmetric) exist as a singlet around3.41–3.75 ppm.

The IR spectra of molybdenum and tungsten complexes 1–4show two strong bands at 894 (1), 902 (2), 891 (3), 899 cm-1 (4)and 925 (1), 943 (2), 920 (3), 941 cm-1 (4) which are characteristic ofthe asymmetric and symmetric n(M O) stretches respectively inthe C2v cis-MO2

2+ moiety (where M = Mo or W), thus confirmingthe formation of mononuclear molybdenum(VI) or tungsten(VI)complexes. These stretching frequencies are closely related toreported similar dioxo compounds.16f ,19

Molecular structures

Solid-state structures of compounds [MO2Cl(Ln)] (M = Mo, n =1 (1) and n = 2 (3) and (M = W, n = 2 (4)) were determinedby single-crystal X-ray diffraction analysis. The crystal structuresdemonstrate that all complexes exist as monomers. The complexstructures are formed by chelation of the deprotonated monophe-nolate ligand, having an [ONN] donor set which consists of aphenoxy oxygen atom and 1-methyl-1,4-diazepane nitrogen atoms.All the compounds show identical connectivity and crystallized inmonoclinic, P21/c (for 1 and 3) and C2/c (for 4) space groups.The asymmetric units for complexes 1, 3 and 4 contain only onecomplex molecule in the unit cell and ORTEP representationsare presented in Fig. 1, 2 and 3, respectively. The relevantcrystallographic data are provided in Table 1. Due the identicalconnectivity, the bond lengths and bond angles are similar to eachother. Selected bond lengths and bond angles of compounds 1, 3and 4 are summarized in Table 2. All three complexes exhibit adistorted octahedral geometry. The ligands are coordinated to the

Fig. 1 Molecular structure and atom numbering scheme for complex 1.Thermal ellipsoids have been drawn at 50% probability level.

Fig. 2 Molecular structure and atom numbering scheme for complex 3.Thermal ellipsoids have been drawn at 50% probability level (disorderedtert-butyl group has been omitted for clarity).

Fig. 3 Molecular structure and atom numbering scheme for complex 4.Thermal ellipsoids have been drawn at 50% probability level (disorderedtert-butyl group has been omitted for clarity).

cis-MO22+ core (where M = Mo or W) through O3, N1 and N2

atoms, where the O3 atom is from monophenolate and N1 andN2 atoms are from the diazepane backbone. The fourth and fifthcoordination site of the metal centers are occupied by two cis oxogroups and the sixth coordination of the distorted octahedron iscompleted by a chloride ion. The coordination site of oxo groupO1 is located trans to amine nitrogen atom N1 and the otheramine nitrogen atom N2 is located trans to phenolate oxygen atomO3. The M–N(1) bond trans to the oxo group (Mo(1)–N(1) bonddistance is 2.3772(12) A for complex 1, 2.3881(13) A for complex3 and W(1)–N(1) bond distance is 2.360(2) A for complex 4) islonger than the M–N(2) bond, which is trans to the phenolateoxygen atom ((Mo(1)–N(2) bond distance is 2.2588(13) A for1, 2.2794(12) A for complex 3 and W(1)–N(2) bond distance is2.265(2) A for complex 4). This is due to the strong trans effectexerted by an oxo group in comparison to the phenolate oxygen.The same tendency we found in related bisphenolate complexes.10b

The bond angles and bond lengths of the cis-dioxo groups aresimilar to reported six-coordinated dioxomolybdenum(VI) anddioxotungsten(VI) complexes.10e,20

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Table 1 Crystal data and structure refinement for [MoO2Cl(L1)] (1), [MoO2Cl(L2)] (3) and [WO2Cl(L2)] (4)

1 3 4·0.5THF

Empirical formula C13H19N2O3ClMo C21H35N2O3ClMo C21H35N2O3ClW·0.5THFMr 382.69 494.90 618.86T/K 100(2) 100(2) 100(2)l/A 0.71073 0.71073 0.71073Crystal system Monoclinic Monoclinic MonoclinicSpace group P21/c P21/c C2/ca/A 11.1092(7) 18.7474 31.6788 (12)b/A 10.7576(7) 9.8561(3) 13.2452(6)c/A 12.9658(8) 12.5930(5) 12.4882(6)b 111.837(2)◦ 91.276(1)◦ 108.774(3)V/A3 1438.34(16) 2326.31(14) 4961.2(4)Z 4 4 8Dc/g cm-3 1.767 1.413 1.657m/mm-1 1.106 0.701 4.792F(000) 776 1032 2480Crystal size/mm 0.44 ¥ 0.28 ¥ 0.15 0.35 ¥ 0.25 ¥ 0.14 0.70 ¥ 0.33 ¥ 0.172q range/◦ 2.54–30.03 2.17–36.30 1.68–30.0Refl. collected/unique 32266/4150 13023/8528 114459/7233Rint 0.0349 0.0212 0.0346Goodness-of-fit on F 2 1.080 1.053 1.064Final R indices [I > 2s(I)] R1 = 0.0218 R1 = 0.0288 R1 = 0.0228

wR2 = 0.0574 wR2 = 0.0640 wR2 = 0.0461Final R indices (all data) R1 = 0.0253 R1 = 0.0460 R1 = 0.0288

wR2 = 0.0599 wR2 = 0.0726 wR2 = 0.0483Drmax/min/e A-3 1.067/-0.811 0.775/-0.764 1.223/-1.330

Table 2 Metal-centered bond distances (A) and bond angles (◦) for 1, 3 and 4 (M = Mo for 1, 3, M = W for 4)

[MoO2Cl(L1)] (1) [MoO2Cl(L2)] (3) [WO2Cl(L2)] (4)

M(1)–O(1) 1.7026(11) 1.7032(11) 1.7290(17)M(1)–O(3) 1.9314(11) 1.9229(10) 1.9112(17)M(1)–N(2) 2.2588(13) 2.2794(12) 2.265(2)M(1)–O(2) 1.7078(12) 1.7032(13) 1.7260(17)M(1)–N(1) 2.3772(12) 2.3881(13) 2.360(2)M(1)–Cl(1) 2.5458(4) 2.5310(5) 2.5250(6)

O(1)–M(1)–O(2) 103.39(6) 103.00(6) 101.45(8)O(2)–M(1)–O(3) 94.99(5) 96.58(5) 96.76(8)O(2)–M(1)–N(2) 88.83(5) 88.36(5) 87.72(8)O(1)–M(1)–N(1) 161.20(5) 162.07(5) 162.86(8)O(3)–M(1)–N(1) 85.45(4) 85.42(4) 85.64(7)O(1)–M(1)–Cl(1) 86.65(4) 87.00(5) 87.28(6)O(3)–M(1)–Cl(1) 84.77(3) 85.67(4) 85.98(6)N(1)–M(1)–Cl(1) 82.22(3) 81.88(4) 81.70(5)O(1)–M(1)–O(3) 108.64(5) 107.80(5) 106.73(8)O(1)–M(1)–N(2) 96.16(5) 97.53(5) 98.12(8)O(3)–M(1)–N(2) 153.19(5) 152.26(5) 153.24(8)O(2)–M(1)–N(1) 87.21(5) 86.98(5) 88.48(8)N(2)–M(1)–N(1) 68.20(5) 67.54(4) 68.06(7)O(2)–M(1)–Cl(1) 169.41(4) 168.43(4) 169.59(6)N(2)–M(1)–Cl(1) 86.73(3) 84.52(4) 85.41(5)

The chloride ligand is bonded to the metal center with a bonddistance of 2.5458(4) A in 1, 2.5310(5) A in compound 3 and2.5250(6) A in compound 4. This is considerably longer than forsimilar reported compounds,17f ,21 where Mo–Cl bond lengths inthe range of 2.384(3)–2.417(16) A are found. In the publishedcompounds the phenol oxygen atom is located trans to chlorineatom whereas in compounds 1, 3 and 4 the oxo group O2 ispositioned trans to chlorine atom. As discussed earlier, the transeffect of an oxo group is stronger than that of the phenol oxygenand hence the M–Cl bond lengths are significantly elongatedin compounds 1, 3 and 4. Due to this elongation the chloroligand becomes more labile and hence a decreased overall stability

is observed. This explains the moderate yields of monomericcomplexes 1–4 obtained.

Electrochemical studies

The cathodic reduction potential of compounds 3 and 4 wereobtained by cyclic voltammetry. All cyclic voltammetry mea-surements were performed in dried acetonitrile with NBu4PF6

(TBAP) used as supporting electrolyte. The reduction observed forcis-dioxo molybdenum(VI) complex 3 and cis-dioxo tungsten(VI)complex 4 are uniformly irreversible in this study at various scanrates such as 20, 50, 100, 200 and 500 mV s-1. The electrochemical

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Table 3 Electrochemical data for MoVI/MoV and WVI/WV reductions inCH3CN at room temperature; scan rate = 100 mV s-1

Complex Epc/V Reduction

3 -1.395 MoVI → MoV

4 -1.517 WVI → WV

data are summarized in Table 3 and their corresponding cyclicvoltammograms are given in Fig. S1 (ESI†). Sterically less de-manding complexes 1 and 2 were excluded from electrochemistrystudies because of their rapid decomposition in solution. TheMoVI/MoV cathodic reduction potential of 3 (Epc, -1.395 V) ismore positive than that of 4 (Epc, -1.517 V). This result is inagreement with common observation that the molybdenum centercan be more easily reduced than its tungsten analog. Furthermore,no reduction of MV/MIV is observed. This is consistent toour earlier finding with related molybdenum dioxo bisphenolatecomplexes.10b In general, electron releasing groups such as methylor tert-butyl on phenolate rings decreases the reduction tendencyof the molybdenum center. Whereas p-methyl substitution showsMoVI/MoV as well as MoV/MoIV redox potentials, ortho andp-methyl or ortho and p-tert-butyl substitution shows only theMoVI/MoV redox potential.10b

Catalytic oxygen atom transfer

Oxygen atom transfer activity of the molybdenum complex 3and tungsten complex 4 was investigated. Attempts to investigatecatalytic OAT activity of complexes 1 and 2 failed because of theirdecomposition in DMSO. In a typical experiment, 0.025 mmol ofcomplexes 3 or 4 and 10 equivalents of PMe3 were dissolved in 1.0mL of deoxygenated dimethyl sulfoxide (DMSO-d6). The samplewas subjected to 31P NMR spectroscopy at room temperature afterits immediate preparation and the progress of the oxo transferreaction was monitored. The OPMe3 product can be identifiedeasily by its characteristic peak at 36.60 ppm. The concentrationof PMe3 (at -62.06 ppm) decreases gradually with time of reaction.Substrate conversion employing catalyst [MoO2Cl(L2)] (3) wasfound to be complete after 16 h whereas with [WO2Cl(L2)] (4)reaction was complete after 30 min at room temperature (eqn(1)).10b,f

PMe + OSMe OPMe SMe3 2MO2 (L2 )Cl

M = Mo or W 3 2[ ]⎯ →⎯⎯⎯⎯ + (1)

The pseudo-first order rate constants kobs of the OAT reactionbetween PMe3 and DMSO catalyzed by 10 mol% of 3 and 4was calculated as 8.0 ¥ 10-5 s-1 for 3 and 2 ¥ 10-3 s-1 for 4,respectively. The corresponding 31P NMR spectroscopy kineticplots are provided in Fig. S2 (ESI†). Proton NMR spectraof the catalytic reaction solutions after complete conversionof the phosphine show complexes 3 and 4 to remain fairlystable in solution along with some amount of free ligand dueto decomposition. No evidence for the formation of dimericMo(V) species by the often occurring dimerization of Mo(VI)dioxo and Mo(IV) monooxo complexes can be found.16f ,22 Dimeric[Mo2O3(L)2] could, in principle, be diamagnetic or paramagneticdepending on the orientation of the M O moieties vs. each other.We do not observe additional resonances in the spectrum fora diamagnetic compound, nor a broadening of the resonances,

pointing to paramagnetic species in solution. Fully decomposedsamples, indicated by a color change from yellow to colorless,were inactive in the catalytic OAT, as even after 24 h no OPMe3

was formed as evidenced by 31P NMR spectroscopy. This clearlyindicates that OAT is catalyzed by compounds 3 and 4. The rateof the molybdenum catalyzed oxo transfer is significantly fasterthan related previously reported systems where usually highertemperatures are required in order to observe any transfer.

The dramatic increase of OAT reactivity by substitutingmolybdenum by tungsten is surprising and is opposite to ourearlier observations where the pyrazolate complex [MoO2(t-Bu2pz)2] shows faster OAT than its W-analog [WO2(t-Bu2pz)2] atelevated temperatures.9 Generally, OAT reactions in molybdenumcomplexes occur via the formation of two transition states: (i)nucleophilic attack of phosphine at the oxygen atom on theMo O p* orbital to give the phosphine oxide intermediate and(ii) substitution at the octahedral Mo(IV) intermediate, whichleads to product release. A similar pathway with tungsten wouldrequire the formation of a W(IV) intermediate. This seems lesslikely as no reaction occurs between complex 4 and PMe3 in theabsence of DMSO-d6, even at elevated temperatures. In contrast,complex 3 is reduced by trimethyl phosphine in deuteratedbenzene, as indicated by an immediate color change to green.This behavior has previously been observed where the reductionof molybdenum dioxo complexes with phosphine led to complexesof the type [MoO(L)2(PMe3)].9 Here, we were not able to isolatea reduced species due to its sensitive nature but we have nodoubt that reduction occurred, as we observe OPMe3 by 31PNMR spectroscopy. Consistent to this, CV measurements showthe tungsten complex is more difficult to reduce. This suggestsdifferent mechanistic pathways within the two metal systems.Interestingly, biological systems show a similar behavior. Mostmolybdoenzymes, such as sulfite oxidase5a,c or nitrate reductase,5d–j

are inactive or less active upon substitution of Mo by W. Incontrast, by replacement of Mo by W in DMSO reductase, a 17times faster reduction of DMSO to DMS but no reverse catalyticoxidation of DMS to DMSO was observed.4a The native Moenzyme catalyzes both the reduction of DMSO to DMS and theback reaction. The authors explain this by the different redoxproperties of the MVI/MV (+26 mV for Mo and -194 mV for W)and MV/MIV (+200 mV for Mo and -134 mV for W) couples. Themolybdenum data are closer to the DMSO/DMS couple (+160mV) allowing back and forth reactivity. Thus, whether activity isobserved upon substitution of Mo by W strongly depends on thesubstrate, explaining why other enzymes are usually inactive. Webelieve that the superior ligand properties of DMSO compared toe.g. nitrate or sulfite plays a crucial role. Thus, we suggest in the heredescribed catalytic oxidations of phosphine follows two differentpathways depending on the metal. In case of molybdenum, webelieve reduction of the metal occurs forming a reduced Mo(IV)intermediate which is subsequently reoxidized by DMSO, closingthe catalytic cycle.10b

In the case of tungsten, however, reduction of the metal isharder so that a concerted transfer of the oxygen atom could occurwithout the formation of a W(IV) intermediate (Scheme 2). DMSOcould possibly coordinate prior to the nucleophilic attack of PMe3

on the oxo group, as tungsten tends to form seven-coordinatecomplexes. For this reason, we carried out a 1H NMR experimentin which tungsten complex 4 was dissolved in CDCl3 in presence of

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Scheme 2 Proposed intermediate in complex 4 during OAT.

a small amount of dried DMSO. In addition to the resonances ofthe complex, a singlet at 2.59 ppm assignable to the methyl protonsof DMSO was observed. However, a control experiment withoutcomplex (CDCl3 in presence of dried DMSO with identicalconcentration) showed the identical resonance at 2.59 ppm,evidencing against the formation of a seven-coordinate DMSOadduct. Accordingly, in contrast to the molybdenum catalyzedreaction, one six-coordinate transition state is conceivable, leadingpossibly to the observed fast catalysis. Theoretical calculationssupporting the hypothesis are in progress.

Conclusions

Tridentate amino monophenolate ligands were developed for theformation of molybdenum and tungsten dioxo complexes whichwere prepared by the reaction of [MoO2Cl2] and [WO2Cl2(dme)]with the respective ligands. NMR spectral study and single-crystal structure analyses allowed the conclusion that tridentatemonophenolate ligands have coordinated to the cis-dioxo MO2

2+

core (where M = Mo or W), through the [ONN] donor setand the final coordination is satisfied by a chloride ligand. Thecatalytic activity of [MoO2Cl(L2)] (3) and [WO2Cl(L2)] (4) towardsoxygen atom transfer to PMe3 as substrate, with DMSO as oxygendonor, has been investigated. Unusually, the tungsten complex[WO2Cl(L2)] (4) shows faster OAT reactivity than its molybdenumanalog [MoO2Cl(L2)] (3) at room temperature. This is interestingas in a W-substituted DMSO reductase, faster conversion ofDMSO to DMS than with the native molybdenum based enzymewas observed,4a rendering the here reported complexes suitablebiomimetic models. The reason for this reversed reactivity is as yetunclear, but support for these findings by computational methods,is currently underway in our laboratory.

Experimental

General aspects

All syntheses were performed under an atmosphere of dryargon using a glovebox or Schlenk techniques. Sodium cyanotri-hydridoborate, 1-methyl-1,4-diazepane, 2-hydroxybenzaldehyde,3,5-ditertiarybutyl-2-hydroxybenzaldehyde and [MoO2Cl2] wereused as received without any further purification. Triethylaminewas dried with KOH and distilled with CaH2 prior to use.DMSO was dried by stirring overnight with CaH2, subsequentdistillation and storing over molecular sieves (3 A). Startingprecursor [WO2Cl2(dme)] was prepared by following the publishedliterature procedures.23 All deuterated solvents for NMR studieswere purchased from Deutero GmbH and dried over molecularsieves. All the solvents were dried by a Pure Solv MD-4-ENsolvent purification system from Innovative Technology, inc. and

flushed with argon prior to use. Celite and aluminum oxide werepurchased from commercial sources and dried in vacuo at 300 ◦C.Trimethylphosphine was obtained commercially, stored in glovebox, and used as received without any further purification.

Physical measurements

All the NMR spectra were recorded on a Bruker Avance 300 MHzspectrometer. Chemical shift values are given in parts per million(ppm). Spectra were obtained at 25 ◦C unless otherwise noted.Cyclic voltammetry (CV) was performed in a glove box usinga three-electrode cell configuration. A platinum disc, platinumwire and Ag(s)/Ag+ were used as working, auxiliary and referenceelectrodes, respectively, and NBu4PF6 (TBAP) was used as thesupporting electrolyte. The cathodic potential values were mea-sured under identical conditions for various scan rates. The instru-ments utilized included a GAMRY Potentiostat/Galvanostat andGAMRY framework software to carry out the experiments and toacquire the data. IR spectra were recorded with a Bruker alpha FT-IR spectrometer. Electron impact mass spectra were measured on aAgilent 5973 MSD-Direct Probe. Elemental analyses were carriedout using a Heraeus Vario Elementar automatic analyzer at theInstitute of Inorganic Chemistry, Graz University of Technology,Austria.

OAT reactions were carried out by reacting freshly preparedcomplexes 3 or 4, respectively, and PMe3 in a 1 : 10 ratio in 1 mL ofdeoxygenated dimethyl sulfoxide (DMSO-d6). Decrease of PMe3

was monitored by 31P NMR spectroscopy at room temperature byrecording a spectrum every 30 min for complex 3 and every 2 minfor complex 4. The amount of PMe3 (in % vs. the total amount ofphosphorous in the sample) was determined by integration of theresonances of PMe3 and POMe3.

Synthesis of ligands

Synthesis 1-methyl-4-(2-hydroxybenzyl)-1,4-diazepane, HL1.To a solution of 2-hydroxybenzaldehyde (1.22 g, 10 mmol) in100 mL of methanol were added 1-methyl-1,4-diazepane (1.14g, 10 mmol) and a small amount of acetic acid. Sodium cyan-otrihydridoborate (0.94 g, 15 mmol) in methanol (5 mL) wasadded dropwise to the resulting solution with stirring. After thesolution was stirred for 3 days at 25 ◦C, it was acidified by addingconc. HCl and then evaporated almost to dryness under reducedpressure. The residue was dissolved in saturated aqueous solution(50 mL) of Na2CO3 and extracted with CHCl3 (3 ¥ 50 mL). Thecombined extracts were dried over anhydrous MgSO4 and filtered.On slow evaporation of the filtrate a deep orange viscous liquidwas obtained. The product was further purified by flash columnchromatography using isocratic elution of hexane and followed bymethanol. Yield: 1.18 g (54%). 1H NMR (360 MHz, C6D6): d 7.16(2H, aryl), 6.84 (2H, aryl), 3.41 (s, 2H, ArCH2N), 2.38 (m, 8H,CH2N), 2.08 (s, 3H, CH3), 1.49 (m, 2H, CH2N).

Preparation of KL1. A THF solution of 0.670 g (3.04 mmol) ofHL1 was added dropwise to the suspension of 0.244 g (6.08 mmol)of KH in THF. The reaction mixture was allowed to stir overnightand then filtered through 2.5 cm pad of Celite. The resulting slightyellowish colored solution was evaporated to dry to obtain puresolid material which was used as such in subsequent reactions(yield 0.579 g, 75%).

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 7850–7857 | 7855

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Synthesis of 1-methyl-4-(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane, HL2. This ligand was prepared in an analogousmanner to that described above for HL1, except 3,5-di-tert-butyl-2-hydroxybenzaldehyde (2.34 g, 10.0 mmol) was used instead of2-hydroxybenzaldehyde. Yield: 1.93 g (58%). 1H NMR (360 MHz,CDCl3): d 7.21 (1H, aryl), 6.81 (1H, aryl), 3.75 (s, 2H, ArCH2N),2.72 (m, 8H, CH2N), 2.38 (s, 3H, CH3), 1.89 (m, 2H, CH2N), 1.42(s, 9H, C(CH3)3), 1.28 (s, 9H, C(CH3)3).

General complex synthesis procedure employing ligand L1. Asolution of 0.25 mmol of [MoO2Cl2] or [WO2Cl2(dme)] in 2 mLof THF was added slowly to the solution of 0.25 mmol of KL1

in 5 mL of THF. The reaction mixture was allowed to stir foranother 5 h and then evaporated to dryness. The compounds 1and 2 were washed with dry diethyl ether to remove unreactedligand (KL1 is soluble in ether) and followed by extracted intodichloromethane. The suspensions were filtered through a pad ofCelite. After evaporation the pure yellow compounds were isolatedand used for further studies.

[MoO2Cl(L1)] (1). Yellow solid, yield: 0.030 g (31%). 1H NMR(360 MHz, CD3CN): d 7.33 (2H, aryl), 6.97 (1H, aryl), 6.76 (1H,aryl), 4.49 (d, 1H, ArCH2N), 3.78 (m, 4H, CH2N), 3.51 (d, 1H,ArCH2N), 3.47 (m, 1H, CH2N), 2.96 (m, 6H, CH2N, CH3), 2.68(m, 2H, CH2N). Anal. Calc. for C13H19MoN2O3Cl·0.2C5H12: C42.34, H 5.43, N, 7.05. Found: C 43.42, H 5.40, N 7.26%.

[WO2Cl(L1)] (2). Yellow solid, yield: 0.035 g (30%). 1H NMR(360 MHz, CD3CN): d 7.33 (2H, aryl), 6.98 (1H, aryl), 6.84 (1H,aryl), 4.51 (d, 1H, ArCH2N), 4.04 (m, 1H, CH2N), 3.85 (m, 3H,CH2N), 3.58 (d, 1H, ArCH2-N), 3.35 (m, 2H, CH2N), 3.19 (m,2H, CH2N), 3.09 (s, 3H, CH3), 2.80 (m, 1H, CH2N), 2.36 (m, 1H,CH2N). Anal. Calc. for C13H19WN2O3Cl: C 33.18, H 4.08, N, 5.95.Found: C 32.03, H 3.90, N 5.62%.

General complex synthesis procedure employing ligand L2. Tothe mixture of 0.25 mmol of HL2 and 0.25 mmol of triethylaminein 5 mL of THF, 0.25 mmol of [MoO2Cl2] or [WO2Cl2(dme)] in 2mL of THF was added slowly. The reaction mixture was allowedto stir for another 5 h and evaporated to dryness. Compound3 was washed with dry pentane to remove unreacted ligand andtriethylamine, followed by extraction into toluene and then filteredthrough Celite. After evaporation, pure orange compound 3 wasisolated in good yields and used for further studies. Compound 4was purified partially by precipitating triethylamine hydrochloridewith ether followed by filtration through a pad of Celite.

[MoO2Cl(L2)] (3). Orange solid, yield: 0.075 g (61%). 1H NMR(360 MHz, C6D6): d 7.55 (1H, aryl), 6.93 (1H, aryl), 4.59 (d, 1H,ArCH2N), 3.95 (m, 1H, CH2N), 3.56 (m, 2H, CH2N), 2.94 (m,1H, CH2N), 2.62 (d, 1H, ArCH2N), 2.46 (s, 3H, CH3), 2.23 (m,1H, CH2N), 2.10 (m, 2H, CH2N), 1.69 (m, 3H, CH2N), 1.53 (s,9H, C(CH3)3), 1.35 (s, 9H, C(CH3)3). MS (EI): m/z 461 (M+ - Cl,5%). Anal. Calc. for C21H35MoN2O3Cl·C7H8: C 52.41, H 7.19, N,5.46. Found: C 53.57, H 7.05, N 6.05%.

[WO2Cl(L2)] (4). Orange solid, yield: 0.066 g (45%). 1H NMR(360 MHz, CDCl3): d 7.37 (1H, aryl), 6.98 (1H, aryl), 4.64 (d, 1H,ArCH2N), 4.30 (m, 1H, CH2N), 4.04 (m, 3H, CH2N), 3.68 (m,1H, CH2N), 3.37 (d, 1H, ArCH2N), 3.19 (s, 3H, CH3), 2.71 (m,2H, CH2N), 2.26 (m, 1H, CH2N), 1.76 (m, 2H, CH2N), 1.38 (s,9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3). MS (EI): m/z 582 (M+ 5%).Anal. Calc. for C21H35WN2O3Cl: C 43.28, H 6.07, N, 4.81. found:C 42.80, H 5.69, N 5.09%.

X-Ray crystallography

Single-crystals of 1, 3 and 4 of suitable size were selected fromthe mother-liquor and immersed in paraffin oil, then mountedon the tip of a glass fiber. The X-ray quality yellowish orangesingle crystals were grown by vapor diffusion of pentane into aTHF–dichloromethane mixture for complex 1, from a THF andpentane mixture for 3 and vapour diffusion of diethyl ether intoTHF for complex 4. Data for the crystals were collected usingMo-Ka radiation (0.71073 A) on a Bruker AXS-SMART APEX2 diffractometer equipped with a CCD area detector at 100 K. Thedata for the three compounds were reduced, corrected for Lorentzand polarization effects and for absorption using SAINT24 andSADABS25 programs, respectively. The structures were solved bydirect methods (SHELXS-97)26 and refined by full-matrix least-squares method.27 One of the t-Bu groups from complexes 3 (C18,C19, C20 and C21) and 4 (C19, C20 and C21) was found tobe disordered and were modeled as split in two positions, whoseoccupancies were refined to 0.73/0.27 and 0.58/0.42, respectively.Low theta completeness in complex 3 is due to a hardware crashduring the dataset collection, followed by decay of the crystal.Complex 3 features a disordered t-Bu group which was modeledwith the help of SAME and ISOR restraints. In compound 4, adisordered lattice solvent molecule was removed from the datasetby PLATON SQUEEZE and was calculated for 0.5 moleculeof THF per complex. If not noted otherwise, all non-hydrogenatoms were refined anisotropically; hydrogen atoms were locatedin calculated positions to correspond to standard bond lengthsand angles.

Acknowledgements

Financial support by the Austrian Science Foundation FWF(P19309-N19) is gratefully acknowledged.

References

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