Salophen and salen oxo vanadium complexes as catalysts of sulfides oxidation with H 2O 2: Mechanistic insights

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Catalysis

Today

jo u r n al h om epa ge :w w w . e l s e v i e r . c o m / l oc a t e / c a t t o d

Salophen

and

salen

oxo

vanadium

complexes

as

catalysts

of

sulfides

oxidation

with

H

2

O

2

:

Mechanistic

insights

A.

Coletti

_,

_P.

_Galloni

_,

_A.

_Sartorel

_,

_V.

_Conte

_,

_B.

_Floris

a_{DipartimentodiScienzeeTecnologieChimiche,UniversitàdiRomaTorVergata,viadellaRicercaScientificasnc,00133Roma,Italy} b_{DipartimentodiScienzeChimiche,UniversitàdiPadova,viaMarzolo1,35131Padova,Italy}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received8November2011

Receivedinrevisedform5March2012 Accepted8March2012

Available online 26 April 2012 Keywords: Catalytcoxidation Vanadiumcomplexes Substituenteffect 51_VNMR DFTcalculation

a

b

s

t

r

a

c

t

TheapplicationofV(V)catalystsinoxidationofsulfideswithperoxidesoffersanefficientprocedure,that iscompatiblewithdifferentfunctionalgroups,andleadstogoodyieldsandselectivities.However,the understandingofthefactorsaffectingthereactivityofdifferentcatalystsisstillfartobecomplete.An experimentalandtheoreticalstudyonaseriesofV(V)complexescontainingvariouslysubstitutedsalen andsalophenligandsisreportedwiththeaimtocorrelatetheactivityofthecatalystswiththeelectronic characterofthevanadiumcenter.Theresultsobtainedindicatethatstericfactorsplayamajorrolein determiningtheoutcomeofthereaction,oftenovercomingtheelectroniceffects.Theoreticalresults suggesttheinterventioninthecatalyticcycleofanhydroperoxovanadiumspecies.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Oxidationreactions,andinparticulartheircatalyticversions, oftenrepresentkey processes totransformbulk chemicalsinto valuablematerials.Millionsoftonsofsuchcompoundsareyearly producedworldwide,andfindapplicationsinallareasofchemical industries[1].

On the basis of economic and environmental reasons [2,3], oxidationprocesseswithH2O2 aresustainableprocesses:firstof

all H2O2 is, besides dioxygen, themost appealing oxidant and

theone withthe highest “atomefficiency” (47% of active oxy-gen);inaddition,aqueoushydrogenperoxideisreadilyaccessible, safetouseanduponreductiononlywaterisformed[4–7]. Cat-alytic systems based on different metals have been employed in conjunction with hydrogen peroxide for oxidation reactions

[5].Vanadiumcatalyzedoxidationswithhydrogenperoxidehave attractedinterestofseveralresearchgroupsbecauseofthe inter-estingpropertiesofthis metalinterms ofselectivity,reactivity andstereoselectivity.Inthisperspective,thesereactionsarealso interesting because of the relationship with the reactivity of vanadium-dependent haloperoxidases(VHPO) enzymes,species abletoactivatehydrogenperoxidefortheoxidationof halides, thusproducing halogenatedcompounds,and forstereoselective sulfoxidationsofappropriatesubstrates[8–10].

∗ Correspondingauthor.Tel.:+390672594014;fax:+390672594328. E-mailaddress:valeria.conte@uniroma2.it(V.Conte).

Peroxovanadium complexes, formedupon interactionof the metalionwithH2O2,dependingonthenatureoftheligands

coor-dinatedtothemetalandontheexperimentalconditions,canact eitheraselectrophilicoxygentransferreagentsorasradical oxi-dants [11]. Typical electrophilic processes are the oxidation of sulfidesand tertiaryaminesand theepoxidationofallylic alco-hol or simple alkenes [11–13]. The oxidation of alcohols and the hydroxylation of aliphatic and aromatic hydrocarbons are examples of homolyticreactivity [5,12,14]. Severalreview arti-cles have appeared recently on vanadium-catalyzed oxidations

[5,15,16].

Additionally, the oxidation of sulfur containing compounds remains an hot research topic both from a synthetic point of view [11,17–19] i.e. thepreparation of chiral sulfoxides and of sulfones, aswellas consideringmodernprocesses for desulfur-ization of fuels [20]. Theremoval of sulfur from dieseloil and gasolineisbecomingmoreandmoreimportant,notonlyto pro-tect car exhaust catalysts, but also because of the more strict environment-protecting regulations. Up-to-date, hydrodesulfur-ization is themethod of choice toremove thiols, sulfides, and disulfides, but withthis process it is not possibleto efficiently removearomaticsulfur-containingcompounds,suchas dibenzoth-iopheneand4,6-dimethyldibenzothiophene.Toachieveultra-deep desulfurizationoffuels,oxidationreactionappearstobea promis-ingprocedure[21].

Thisbackgroundpromptedustoexplorethecatalyticactivity of selectedvanadium complexes in oxidationreactionsstarting fromamodelsulfide,Ph S CH3,selectivelytothecorresponding

0920-5861/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

Scheme1. Vanadium(V)salenandsalophencomplexes.

sulfoxide. The structure of the catalysts used in this study is reportedinScheme1,theyarealloxovanadium(V)species, lig-atedwithSchiffbases derivedfromtheappropriatesubstituted salicylaldehydes and o-phenylendiamine or 1,2-ethanediamine respectively.

Salophenandsalenderivativesareconsidered“privileged lig-ands” because their easy synthesis by condensation between aldehydesand amines, theyalso coordinates a wide variety of metals[22].Inaddition,stereogeniccentersorotherelementsof chiralitycanbealsointroduced.Moreoverafterappropriate mod-ificationheterogeneouscatalysts[23]canbeprepared.

Thevariationofthecoordinationspherewasdesignedinorder tohighlight correlation between the electronic distribution on themetalcenterwiththereactivityinoxidationreactions, sim-ilarapproach hasbeenrecentlyusedwithvanadiumcontaining stronglyrelatedsalenandsalancomplexes[24].

The catalytic activity of some of the species here studied wasalreadyrecentlyanalyzedinourlabs,intheepoxidationof cyclooctene[25].Quitesurprisingly,electronwithdrawinggroups intheperipheryofthesalophenligandsreducedthecatalytic activ-ityofthevanadiumcomplextowarddoublebonds.However,this aspectwasattributedtofasterdecompositionoftheperoxide.On theotherhand,withoxovanadiumsalophencomplexpossessing fourt-butylgroupsinthebackbone,theobservedreactivitytoward cyclooctenewasalmostzero.Inthislattercasetheprevalenceof thestericeffectovertheelectroniconeisevident,i.e.the bulki-nessoftheligandheavilyhampertheapproachofthesubstrate totheperoxocomplex.Accordingly,inthatwork[25]noclearcut correlationbetweentheobservedreactivityandthecoordination environmentofthemetalcenterwasenvisaged.

Moreover,itistobekeptinmindthatinvanadiumcatalyzed oxidationswithH2O2twocompetitivereactionsoccur:the

oxida-tionofthesubstrateandthevanadiumcatalyzeddecomposition ofhydrogenperoxide[26],forthatreasonahigheractivityofa vanadiumcomplexmaysimplyresultinafasterdecompositionof H2O2.

Inthispaperwepresentanexperimentalandtheoreticalstudy, concerningtheeffectsofstericandelectronicmodificationofthe ligandsonthecatalyticactivityofsalophenandsalenoxo vana-dium(V)complexesintheoxidationofPhSMewithH2O2.Thiseasy

tooxidizemodelsubstratehasbeenchosenwiththeaimtoobtain fastandcleanoxidationprocess,goodprerequisiteforanalyzing aclassofcatalystswhichhavealreadyshownambiguouskinetic behaviorasfunctionofthesubstituentspresentinthesalophenor salenscaffolds[25].

2. Procedures

2.1. Experimental

Caution:Theuncontrolledheatingoflargeamountsofperoxides mustbeavoided.Careshouldbeexercisedinordertoavoidpossible explosion.

2.1.1. Instruments

1_{HNMRspectrawererecordedona BrukerAvance300MHz}

spectrometer,withCDCl3,CD3CNorCD3COCD3 assolvents.51V

NMRspectrawererecordedonaBrukerAvance400MHz spectrom-eter,inCD3CN.HPLCanalyseswerecarriedoutwithaShimadzu

LC-10 ADvp instrument withUV–vis SPDM10Avp detectorand a Kromasil 100 C18 (250mm×4.6mm, 5␮m) column with VV

complexesoraMetachemMetaphor50ODS-2(250mm×4.6mm, 5␮m)foroxidationreactions.UV–visiblespectrawererecordedon aSHIMADZU2450spectrometerequippedwiththeUVProbe2.34 program.

2.1.2. Materials

HPLC-purity grade MeCN was used. Methanol and dichloromethane commercial grade purity were purchased and used as such. Spectroscopic-grade solvents were used for UV–visspectra.CommerciallyavailableaqueoussolutionofH2O2

wereusedafteriodometrictitration(10.4±0.2MH2O2inwater).

2.1.3. Synthesisofligands

TheSchiffbases usedforthesynthesis ofthecatalystswere prepared by thewell known reaction between salicylaldehyde anddiamine.Slightexperimentalvariationswereintroducedwith respecttoliteraturemethods[27]andtheresultingprocedurewas successfullyappliedtoanumberofdifferentlysubstituted aldehy-des.

General procedure: Two equivalents of the appropriate sali-cylaldehyde weredissolved inthe minimumvolume of boiling methanol(generally,20ml)andaddeddropwisewithone equiva-lentofdiamine(either1,2-diaminoethaneor1,2-benzenediamine) in5mlmethanol.Thesolutionwasrefluxeduntilallthealdehyde disappeared(TLCanalysis)andthencooledtoroomtemperature, thus causing precipitation of theSchiff base,as a yellowsolid. Thefilteredsolidwaswashedwithasmallamountofmethanol, thenwithdiethyletheranddried.ThefollowingSchiffbaseswere prepared:

Salophen, [1,2-bis-(salicylideneamino)benzene]: yield 95.3%; 5,5-Cl2salophen, [1,2-bis-(5-Cl-salicylideneamino)benzene]:

yield >99%; 5,5-(t-Bu)2salophen

[1,2-bis-(5-t-Bu-salicylidene-amino)benzene]: yield 75%; 3,3-(OMe)2salophen,

[1,2-bis-(3-OMe-salicylideneamino)-benzene]: yield 77%; 5,5-(OMe)2

salophen, [1,2-bis-(5-OMe-salicylideneamino)-benzene]: yield 79%; 3,3,5,5-Cl4salophen [1,2-bis-(3,5-Cl2

salicylideneamino)-benzene]: yield 95%;3,3,5,5-(t-Bu)4salophen

[1,2-bis-(3,5-di-t-Bu-salicylideneamino)benzene]:yield75%.

Salen, [1,2-bis-(salicylideneamino)ethane]: yield 88%; 5,5 -Cl2salen, [1,2-bis-(5-Cl-salicylideneamino)ethane]: yield 67%;

5,5-(t-Bu)2salen, [1,2-bis-(5-t-Bu-salicylideneamino)ethane]:

yield 91%; 3,3-(OMe)2salen,

[1,2-bis-(3-methoxy-salicy-lideneamino)ethane]: yield 92%; 5,5-(OMe)2salen,

[1,2-bis-(5-OMe-salicylideneamino)ethane]: yield 92%;3,3,5,5-Cl4salen,

[1,2-bis-(3,5-Cl2-salicylideneamino)ethane]: yield 70%; 3,3,5,5

-(t-Bu)4salen, [1,2-bis-(3,5-(t-Bu)2-salicylideneamino)ethane]:

yield72%.

Allthecompoundsgave1_{HNMRandUV–visspectraconsistent}

withthestructureandwithliteraturedata[25,28–30].

2.1.4. SynthesisofVIV_complexes

Thesynthesisofthesecomplexeswasaccomplishedbya proce-dureslightlydifferentformthatreportedintheliterature[31,32]. Anumberofthevanadiumderivativesusedintheliteraturewere testedasprecursors,i.e.VO(acetylacetonate)2[31],vanadylsulfate

di-hydrate[29,33],andV(acetylacetonate)3[34].Thebestresults

wereobtainedwithVIII_(acac)

3,intermsofreproducibilityofthe

reactionandsolubilityofcomplexes.

General procedure: The Schiff base was dissolved in 100ml of boiling methanol, or suspended when scarcely soluble. The equimolaramountof V(acac)3 wascompletely dissolvedin the

minimumvolumeofMeOHwiththehelpofsonicationandadded dropwisetothesolution(ortothesuspension),causing immedi-atecolorchangefromyellowtogreen.Afteranovernightstirring inanopenvesselatroomtemperature,thereactionwasstopped andtheprecipitatedsolidwascollected,washedwithdiethylether, anddried.NotraceofSchiffbasewaspresent.Eventuallyunreacted V(acac)3waswashedoffwithwarmacetone.

ThefollowingVIV_{Ocomplexeswereprepared,theirpuritywas}

checkedwithTLCandHPLCanalyses.

SalophenVIV_{O, yield 73%, UV–vis in MeCN [}

max, nm (ε,

M−1cm−1)] 242 (40,000), 314(22,000) and 396(18,000); 5,5 -Cl2salophenVIVO, yield 78%, UV–vis in MeCN [max, nm (ε,

M−1cm−1)]305(12,000),409(10,700);5,5-(t-Bu)2salophenVIVO,

yield82%,UV–visinMeCN[max,nm(ε,M−1cm−1)]246(40,900),

318(22,700),409(15,400);3,3-(OMe)2salophenVIVO,yield79%,

UV–vis in MeCN [max, nm (ε, M−1cm−1)] 221 (42,000), 301

(24,000),313(22,000)and335(13,000);5,5-(OMe)2salophenVIVO,

yield87%,UV–visinMeCN[max,nm(ε,M−1cm−1)]216(70,000),

243(41,000),289(30,000),300(32,000),337(26,000)and 434 (9000); 3,3,5,5-Cl4SalophenVIVO: yield 78%, UV–vis in MeCN

[max, nm (ε, M−1cm−1)] 315 (9800), 412 (10,000); 3,3,5,5

-(t-Bu)4salophenVIVO,yield 87%, UV–vis in MeCN [max, nm (ε,

M−1cm−1)]250(43,300),327(25,900),416(16,100). SalenVIV_{O,yield89%,UV–visinMeCN[}

max,nm(ε,M−1cm−1)]

242 (39,000), 277 (18,000) and 362 (7900); 5,5-Cl2salenVIVO,

yield 67% UV–vis in MeCN [max, nm] 248 (49,000), 280 sh,

370 (7400); 5,5-(t-Bu)2salenVIVO, yield 67%, UV–vis in MeCN

[_max,nm(ε,M−1cm−1)]246(56,000),278(27,000),370(9200);

3,3-(OMe)2salenVIVO,yield93%, UV–visin MeCN[max,nm(ε,

M−1cm−1)] 224 (16,000), 296 (14,000) and 381 (2800); 5,5 -(OMe)2salenVIVO, yield 86%, UV–vis in MeCN [max, nm (ε,

M−1cm−1)]251(20,000),286sh(8800)and392(9000);3,3,5,5 -Cl4salenVIVO,yield70%,UV–visinMeCN:max=370nm(doesnot

dissolvecompletely);3,3,5,5-(t-Bu)4salenVIVO,yield72%,UV–vis

in CH2Cl2 [max,nm (ε, M−1cm−1)]252(41,000), 288(27,000)

and386(3600).UV–visspectraareconsistentwithliteraturedata

[35,36].

2.1.5. SynthesisofVV_complexes

Thesynthesisofthesecomplexeswasaccomplishedbya proce-dureslightlydifferentformthatreportedintheliterature[34].For allcompounds1_{HNMRspectraareinagreementwiththestructure}

(inCD3CN,aromaticprotonsresonate intherange 7.1–7.8ppm,

CH Nprotons intherange8.2–8.9ppm,and CH2 protons of

salencomplexesarearound4.4ppm).Bycomparisonwith authen-ticsamples,TLCandHPLCanalysesexcludedthepresenceofVIV

species.

Generalprocedure:200mgofVIV_{Ocomplexweredissolvedin}

30ml CH2Cl2 under stirring. Dioxygen wasbubbled 5min into

thesolutionkeptat0◦CandO2 atmospherewasensuredwitha

latexreservoir.1.2Equivalentstrifluoromethanesulfonicacidwere rapidlyadded,causingdarkeningofthesolutionandprecipitation ofasolid.Thereactionmixturewasallowedtoreachroom tem-peratureandstirreduntildisappearanceofVIV_{species(5÷20h).}

FinepowderofVV_{complexwasisolatedaftercentrifugationofthe}

reactionmixture(6000r.p.m.)anddecantationofthesupernatant solution.

ThefollowingVV_{Ocomplexeswereprepared.}

[salophenVV_O]CF

3SO3,yield95%UV–visin MeCN[max,nm

(ε,M−1cm−1)]242(40,000),304(26,000)and393(10,000);[5,5 -Cl2salophenVVO]CF3SO3,yield98%,UV–visinMeCN[max,nm(ε,

M−1cm−1)]303(21,300),408(11,200);[5,5-(t-Bu)2salophenVVO]

CF3SO3,yield35%,UV–visinMeCN[max,nm(ε,M−1cm−1)]245

(42,700), 320(24,300),407(14,400); [3,3-(OMe)2salophenVVO]

CF3SO3,yield75%,UV–visinMeCN[max,nm(ε,M−1cm−1)]220

(64,000),251(37,000),302(45,000),310(42,000),340(24,000)and 437(7200);[5,5-(OMe)2salophenVVO]CF3SO3,yield82%,UV–vis

in MeCN [max,nm (ε, M−1cm−1)]215 (68,000), 242(41,000),

292(46000),300(45000),341(29000)and435(4900);[3,3,5,5 -Cl4SalophenVVO]CF3SO3,yield35%,[max,nm(ε,M−1cm−1)]313

(19200),410(13,700);[3,3,5,5-(t-Bu)4salophenVVO]CF3SO3,yield

87%,[max,nm(ε,M−1cm−1)]244(30,700),326(35,300),416sh

(9300).

[salenVV_O]CF

3SO3,yield89%,UV–visinMeCN[max,nm(ε,

M−1cm−1)] 230 (30,000), 296 (14,000) and 347 (7400); [5,5 -Cl2salenVVO] CF3SO3, yield67%, UV–vis in MeCN[max,nm (ε,

M−1cm−1)]230(30,000),248(24,000),and 285(17,000); [5,5 -(t-Bu)2salenVVO] CF3SO3, yield 5%,UV–vis in MeCN [max, nm

(ε, M−1cm−1)] 230 (33,400), 250 (27,500) 290 (19,000) and 348(7500);[3,3-(OMe)2salenVVO]CF3SO3,yield58%,UV–visin

MeCN[max,nm(ε, M−1cm−1)]281(12,000),308(10,000)and

359 (4200); [5,5-(OMe)2salenVVO] CF3SO3, yield 89%, UV–vis

in MeCN [max, nm (ε, M−1cm−1)] 288 (12,000) and 391 sh

(3000);[3,3,5,5-Cl4salenVVO]CF3SO3,yield66%,UV–visinMeCN

[max, nm (ε, M−1cm−1)] 293 sh (1300) and 345 sh (710);

3,3,5,5-(t-Bu)4salenVVO,yield80%.UV–vis inCH2Cl2 [max,nm

(ε,M−1cm−1)]263(8000),307(5600)and367sh(2900). Cyclicvoltammetry(CV)characterizationreportedelsewhere

[37]confirmedtheelectroniceffectofthesubstituentsonthe vana-diumcentersinthesesalenVIV_{OandsalophenV}IV_{Ocomplexes.The}

higherredoxpotentialsofthesalophencomplexeswithrespectto thesalenonesindicatethatthearomaticringconferstovanadium astrongerLewisacidity.

2.1.6. OxidationofphenylmethylsulfidewithH2O2,catalyzedby

VV_complexes

Thecatalyticactivityofallthecomplexeswastestedinaprobe reaction,i.e. oxidationof PhSMewithH2O2 in MeCN.The

reactantspecies,at25◦Cinathermostatedvessel.Productanalysis wasobtainedbyHPLC,toavoidthermaldecompositionof prod-ucts,addingaknownamountoftolueneasinternalstandardand usingtheresponsefactorsobtainedbycalibrationstraightlines. StocksolutionsofsulfideandtolueneinMeCNwereused.The lat-terwasperiodicallycheckedforthepresenceofoxidationproduct thatwerealwaysabsent.SolutionsofH2O2andthecatalystwere

preparedimmediatelybeforeuse.

0.5ml of PhSMe 0.151M in MeCN and 0.15ml of catalyst 2.2×10−3Minthesamesolventwereaddedto5mlMeCN.0.5mlof theresultingsolutionweretakenanddilutedto5mlina volumet-ricflaskcontaining2mlofPhMe1.2×10−3MinMeCN,tomeasure initialsulfideconcentration.Thereactionstartedafteradditionof 0.18mlfreshlypreparedH2O20.145MinMeCN(obtaineddiluting

to5ml70␮l10.4±0.2MH2O2inwater).Samplesweretakenat

selectedreactiontimes(2,5,15,and40min)andanalyzedatthe HPLC.

2.2. Calculations

PseudopotentialLACVPwasusedfortheenergyoptimization withoutsymmetry constraints.Gaussian 03 program wasused for geometryoptimizations withthe continuumsolvent model

[38].GeometryoptimizationswereperformedwithDensity Func-tionalTheory(DFT)calculationsusingtheB3LYPfunctionalwith the6-31G*basis set[39,40]. Geometrieswereoptimizedatthe B3LYP/LANL2DZlevel,includingsolventeffectswiththeIEF-PCM method(MeCN).

Alltheinvacuumquantum-mechanicalcalculationswere car-riedoutbyusingSpartan’08[41].

3. Resultsanddiscussion

3.1. Experimental

3.1.1. Reactivity

ThecomplexespreparedandusedinthisworkcontainedSchiff baseligandspossessingvarioussubstituentsintheperipheryofthe aromaticscaffold,seeScheme1.

All salophen and salen molecules have been prepared following literature procedures [27] starting from the appro-priately substitutedsalicylaldehydes and o-phenylendiamine or 1,2-ethanediaminerespectively. The various ligands have been subsequentlyusedtosynthesizeallVIV_andVV_{complexesinvery}

goodyields(seeSection2),followingslightlymodifiedliterature procedures[27,31,32,34]andthespeciesobtainedhavebeen iden-tifiedbycomparisonoftheirpropertieswithliteraturedata.

Forcomparisonpurposes,theVIV_{species,obtainedinthefirst}

stepofthesynthesisofvanadatesderivatives,werealsotestedas catalystsintheoxidationreactions,andthecollecteddatawere comparabletothoseobserved(anddiscussedbelow)withVV

com-plexes.However,thepossible incursionof radicalreactions, an expectedeventwhenVIV_{derivativesareusedascatalystsin}

reac-tionswithhydrogenperoxide[11],promptedustoexaminethe reactivityinoxidationreactionwithhydrogenperoxideonlywith thecatalystsintheirhighestoxidationstate.Insuchaway, mini-mizationofthevanadiumcatalyzeddecompositionofH2O2should

occur.

Thevarioussubstituentsintroducedinthescaffoldofthe com-plexes were chosen with the scope to modify the electronic character of the VV _{ion. Furthermore, a consistent}

compari-son between the salophen and salen structure should help in dividetheelectronic fromthe structural effects, beingthe for-mer species forced intoa simil-planar configuration,while the latter can experience a fluctional behavior [22]. Accordingly,

broadersignalsweredetectedin1_{HNMRspectrawithsalen}

com-plexes:i.e.signalfor CH Nat9.23ppm showsa ␯½=7Hzfor

[3,3,5,5-(t-Bu)4salophenVVO]CF3SO3 whileforthe

correspond-ing[3,3,5,5-(t-Bu)4salenVVO]CF3SO3derivativethesameCH N

signalat8.86ppmshowsa␯½=22Hz.

ThemodelreactionhasbeentheoxidationwithH2O2ofPhSMe

tothecorrespondingsulfoxide,inMeCNatroomtemperature,the ratioH2O2: Catwasca.50:1.Atwo-foldexcessofthesubstrate

withrespecttotheoxidantwasusedinordertoensure selectiv-ity towardthe sulfoxideand alsoto keeproughly constant the concentration ofthe substrate, atleast at thebeginning of the reaction.

The sulfide consumption and the sulfoxide formation were determinedwithquantitativeHPLCanalysisofthereaction mix-ture,seeSection2fordetails.Sulfideconsumptionandsulfoxide yield, at selected reaction time, were taken as parameters to compare the reactivity of thevarious catalysts, thetwo values (see Tables) are equal withinthe experimental error ±5%. The data concerning the results with salophen complexes are col-lected in Table 1 whilst those related to salen species are in

Table2.

TheTables alsocontainthe␴[42] whichis the arithmeti-calsumofthevaluesforthesubstituentsintroducedintothe salophenandsalenscaffolds.Thesevaluesaregiventosketchout theelectroniceffectdictatedbytheirpresence.Forthecomplexes used,itcanbenotedthat,spanningfroma␴ofabout+1,dueto thepresenceoffourchlorineatoms,toavalueofabout−0.8,when fourt-butylgroupsarepresent,alargevariationoftheelectronic characterofthemetalion,i.e.itsLewisacidity,shouldbeobtained. Lookingatthedatacollectedinthetwo tablessomegeneral commentscanbedone:thecatalyticeffectofthesalophenand salenoxovanadium(V)complexesisquitegood;thereactiontimes areshort,mostoftheoxidationsarecompletein15minatroom temperature.Thislastobservationindicatesthatthevanadium cat-alyzeddecompositionofH2O2 isnotcompetingwiththeoxygen

transferreaction.

However (see Fig.1),nosimple correlationcan bedetected betweenthe␴andthesulfoxideyieldsinbothsalophenandsalen series;additionally,nosimplecomparisoncanbemadeinequally substitutedcouplesofsalophenandsalenderivatives;thisbehavior beingindeedexpectedonthebasisofourpreviousstudies[25].

Therefore,adeeperinsightoftheresultsisnecessaryinorderto highlight,foreachcatalyst,ifelectronicorstericeffectsareplaying themajorroleindeterminingthereactivity.

Interestingly,thecatalyticactivityofsubstitutedsalophen com-plexes,measuredassulfoxideyieldataspecificreactiontime(left graph),roughlyfollowsthetrendofthe␴valueswiththe excep-tionofthe3,3-(OMe)2derivative,thisdeviationfromtheexpected

resultwillbediscussedbelow.Theobservedtendencyapparently indicatesthatthereactivitysomewhatfollowstheLewisacidity characterofvanadium,␴+0.9for3,3,5,5-Cl4-salophen.

On theother hand,with substitutedsalen derivatives(right graph)nosimplecorrelationcanbeestablishedbetweenthe sul-foxideyieldsand the␴values;noteworthy,alsointhis series the3,3-(OMe)2derivativeshowsthehighestreactivity.Inaddition,

withinthiscatalystsserieshighreactivityisobservedwith5,5 sub-stitutedligandsindependentlyfromtheirinductiveeffect,whereas for3,3,5,5-tetrasubstitutedspeciesthereactivityappearstotrack theelectrondensityaroundvanadium.

Atthispointofthediscussion,abetterunderstandingofthe resultsobtainedinthereactivity studiescanbelikely obtained lookingalsoattheclassicalreactionmechanism[3–6,11]proposed formetalcatalyzedsulfidesoxidationwithperoxides.

Theacceptedreactionmechanismforvanadiumcatalyzed oxi-dationprocesseswithhydrogenperoxiderequirestheformation ofaperoxidevanadiumcomplexuponcoordinationofH2O2tothe

Table1

V-salophen(0.00056mmol)catalyzedoxidationofphenylmethylsulfide(0.059mmol)withH2O2(0.026mmol)inCH3CN(5.33ml)atroomtemperature.

S S O H2O2 cat. MeCN, 25°C 0.059 mmol 0.026 mmol 5.6 x 10-4 mmol

+

Catalyst Substituent t,min %Sulfideconsumed %Sulfoxideformeda _␴b

N N O V _O O Cl Cl Cl CF3SO3 _Cl 2 93 94 3,3_,5,5_{-Cl4 5 93 96 0.908} 15 94 94 N N O V O O Cl Cl CF3SO3 2 87 92 5,5_{-Cl2 5 91 94 0.454} 15 94 92 N N O O V O CF3SO3 2 54 56 H 5 90 98 0.000 15 95 100 N N O V _O O CF3SO3 2 12 1.2 5,5-(t-Bu)2 5 14 4.5 −0.394 15 54 53 N N O _O O _O V O CF3SO3 2 54 48 3,3_{-(OMe)2 5 89 88 −0.536} 15 97 96 O N N O V _O O O CF3SO3 2 27 9 5,5_{-(OMe)2 5 40 26 −0.536} 15 70 62

Table1(Continued)

Catalyst Substituent t,min %Sulfideconsumed %Sulfoxideformeda _␴b

N N O V _O O CF3SO3 2 18 0 3,3_,5,5_{-(t-Bu)4 5 24 1.5 −0.788} 15 20 6

a_{Sulfideconsumptionandsulfoxideformationcorrespondswithintheexperimentalerror.} b_{␴isthearithmeticalsumofallthevalues[42].}

Scheme2.ProposedcatalyticcycleforsulfidesoxidationwithH2O2.Lstandsfor

salenorsalophenderivatives.

metalcenter,suchintermediateisthenthebonafideoxidantof thesubstrate.Forthesakeofsimplicity,weanticipateherethat thisactivespecieslikelyisanhydroperoxoone.Thecoordination oftheoxidanttotheLewisacidmetalcenteractivatestheoxygen towardthenucleophilicattackofanelectronrichsubstrate,such asthephenylmethylsulfoxide.Inthetransitionstateofthe oxi-dationstep,coordinationofthesecondperoxidicoxygenatomto vanadiumhelpstheoxygentransfertothesubstrate,subsequent eliminationofthesulfoxideandawatermoleculeregeneratesthe catalystprecursor(Scheme2).

With salophen and salenspecies formation of theperoxidic intermediatecanbestrongly influencedbythestructure ofthe precursor.Inordertoconsiderthisstructuralaspect,energy mini-mizationoftheperoxidicspecies,formedfromsalophenandsalen VV_{O precursors hasbeen carried out and the outcome for the}

unsubstitutedhydroperoxooxovanadiumcomplexes(the plausi-bleactiveoxidantspecies)isshowninFig.2.

Forexample,salophenligandsforcetheprecursorandthe pos-sibleperoxo complexesina planarconformation,i.e. theligand residesontheplaneofasimil-tetrahedricalshape(seeFig.2a).On theotherhandwithsalenligands,togetherwiththeplanar confor-mation(Fig.2b)alsoabentonecanbeadopted,inwhichoneofthe nitrogenoftheligandistranstotheoxogroup(seeFig.2c).Itis interestingtonotethatfromtheoreticalcalculation(seefollowing paragraphs),thislatterconformationappearstobefavorite.Thegas phaseoptimizedsalenVV_{(O)(OOH)andsalophenV}V_(O)(OOH)

struc-turesshowthatthehydroperoxogrouphasadifferentapproachto themetaldependingontheconformationoftheligand.In partic-ular,whentheligandhasaplanarconformation,likeinthecase ofsalophenVV_{(O)(OOH),theperoxogroupalmostadoptthe}

side-onconfigurationaround themetal,thus facilitatingthe oxygen

transfertothesubstrate.Onthecontrary,whentheligandhasabent conformation,likeinthecaseofsalenVV_{(O)(OOH)(seeFig.2c)the}

approachofthesecondoxygentothemetalishamperedasshown bythelongerdistanceinthecalculatedstructure,thuspreventing thestabilizationofthetransitionstate.

Summarizing, theoretical calculations suggest that salophen ligandsforcethehydroperoxovanadiumcomplexinaplanar con-formationthat allowsthestabilizationofthetransitionstateof theoxidationstep,resultinginahigherreactivitywithrespectto salencomplexes,wherethemorestablebentconformationcanbe adopted.

Returningtotheexperimentaldata,thehigherreactivityofthe salophenderivatives,togetherwiththeobservedrelationshipof thesulfoxideyieldswiththeLewisacidcharacterofthevanadium center,appearstoberelatedtotheplanararrangementofsuch specieswhichinturnisdictatedbythestructureoftheligands. Ontheotherhand,withsalenderivatives,whichcanadoptseveral differentconformationsbecauseofthemobilityoftheligand,the sterichindranceofthesubstituentscombines,inanunpredictable way,withtheelectroniccharacterofthecatalyst.

The 3,3 dimethoxy substituted salophen and salen oxo-vanadium complexes deserve a specific discussion. In fact, the catalytic activity of [3,3-(OMe)2salophenVVO]CF3SO3 and [3,3

-(OMe)2salenVVO]CF3SO3ismuchhigherthanthatofalltheother

derivativesinthecorrespondingseries(seeFig.1).Thisbehavior maysuggestaneasierformationofthemetalperoxidespecies.In thatrespect,asindicatedinFig.3,thepresenceof3,3-(OMe)2

sub-stituents maystabilizetheapproach ofa H2O2 moleculetothe

metalcenter,throughtheinterventionofanhydrogenbond.

3.1.2. 51_VNMRstudies

Itiswellknownthatthenumberandthenatureofthe perox-ometalspeciesstronglydependsonthereactionconditions,suchas thepHvalue,theligandandthesolvent.Sinceitisnotalways pos-sibletoobtainperoxovanadiumcomplexesinthesolidstate,and oftentheirsolutionstructureisdifferent,theirstructureisoften investigatedbymeansofseveraltechniques.Forexample,51_VNMR

andelectrosprayionizationmassspectrometry(ESI-MS)or theo-reticalcalculationshavebeenrecentlyusedinordertounderstand theirbehaviorincatalyticcycles[43].

Themostcommonstructuresof monoperoxovanadium com-plexes are the side-on cyclic peroxo species, in which both the oxygenatoms are coordinated to vanadium atom, and the hydroperoxospecies,inwhichonlyoneoxygeniscovalentlylinked tothemetalcenter. Inthisworkwehavetried toobtain infor-mation on the formation in solution of theperoxo derivatives fromsalophenand/orsalenV(V)species,inthepresenceof dif-ferentexcessesofhydrogenperoxide.51_{VNMRspectrahavebeen}

acquiredforsomeofthevanadium(V)precursorsinacetonitrile and,inallcases,singlepeaksappearedaround−600ppm.Tonote,

Table2

V-salen(0.00056mmol)catalyzedoxidationofphenylmethylsulfide(0.059mmol)withH2O2(0.026mmol)inCH3CN(5.33ml)atroomtemperature.

S S O H2O2 cat. MeCN, 25°C 0.059 mmol 0.026 mmol 5.6 x 10-4 mmol

+

Catalyst Substituents t,min %Sulfideconsumed %Sulfoxideformeda _␴b

N N O V O O Cl Cl Cl CF3SO3 Cl 2 12 9 3,3_,5,5_{-Cl4 5 18 12 0.908} 15 37 35 N N O V O O Cl Cl CF₃SO₃ 2 76 76 5,5_{-Cl2 5 98 99 0.454} 15 96 98 N N O V O O CF3SO3 2 8 1 H 5 10 4 0.000 15 18 15 N N O V _O O CF3SO3 2 92 94 5,5_{-(t-Bu)2 5 88 98 −0.394} 15 92 96 N N O _O O _O V O CF3SO3 2 >99 >99 3,3-(OMe)2 5 >99 >99 −0.536 15 >99 >99 O N N O V _O O O CF₃SO₃ 2 34 34 5,5-(OMe)2 5 62 64 −0.536 15 – 93 N N O V _O O CF3SO3 2 92 94 3,3_,5,5_{-(t-Bu)4 5 88 98 −0.788} 15 92 96

a_{Sulfideconsumptionandsulfoxideformationcorrespondswithintheexperimentalerror.} b _{␴isthearithmeticalsumofallthevalues[42].}

Fig.1.PhSOMeyieldsvs.␴ oftheligands.Left:V-salophencatalyzedoxidationofPhSMewithH2O2inCH3CNatroomtemperatureat2min(experimentalconditions

reportedinTable1).Right:V-salencatalyzedoxidationofPhSMewithH2O2inCH3CNatroomtemperatureat15min(experimentalconditionsreportedinTable2).

Fig.2. Gasphaseoptimizedstructures ofLVV_{(O)(OOH)complexes(L=salenorsalophen).(a)PlanarconformationofsalenV}V_{(O)(OOH).(b)Planar conformationof}

salophenVV_{(O)(OOH).(c)BentconformationofsalenV}V_(O)(OOH).

thechemicalshiftsforsalenandsalophenspeciesareverysimilar, despiteofthepresenceofdifferentsubstituentsonthearomatic scaffold.

UponadditionofincreasingamountsofH2O2toasolutionof

[salophenVV_O]CF

3SO3or[salenVVO]CF3SO3,thesignalofthe

pre-cursordecreasesand,forbothcomplexes,nosignalsappearhaving referencetovanadiumfreeligandspecies;ingeneral,noother51_V

signalswerepresent in NMR spectradifferentfromthat ofthe

Fig.3.PlausibleformationofanhydrogenbondbetweenanapproachingH2O2

moleculeandthemethoxyoxygensin[3,3_{-(OMe)2salophenV}V_O]CF

3SO3complex.

catalyst precursor, over a wide range of chemical shift (as an example,seeFig.4for[salenVV_O]CF

3SO3).Peroxovanadiumspecies

usuallygivesignalsathighfields[44];inthiscase,theabsenceof anysignalmaybeduetothelowsolubilityofperoxometalspecies atroomtemperature.Infact,theformationofayellowprecipitate wasobserveduponadditionofalargeexcessofH2O2toasolution

ofsalenVO+_{inacetonitrile.}

Fig.4. 51_{VNMRspectraof[SalenV}V_O]CF

3SO3inacetonitrile(1mM).(a)Initial

solu-tion(b)afteradditionof2equivalentsofH2O2totheprevioussolution;(c)after

Table3

51_{VNMRchemicalshiftsfordifferent(V)complexesinCH}

3CN. Catalyst ␦51_VNMR(ppm) [salenVV_O]CF 3SO3 −597 [5,5_-(Cl)2salenVV_O]CF 3SO3 −596

[5,5_{-(t-Bu)2salenV}V_O]CF

3SO3 −584

[salophenVV_O]CF

3SO3 −604

[3,3_,5,5_{-(t-Bu)4salophenV}V_O]CF

3SO3 −595

TheIRspectrumoftheyellowprecipitateshowednosignals intherangeof918–954cm−1weretheO Ostretchingisusually presentforside-onmonoperoxovanadiumcomplexes[12],thus supportingthehypothesisthatsalenandsalophenoxovanadium complexesformanend-onhydroperoxidicspeciesinthepresence ofH2O2(seeSection3below).

WhensalenVV_{OBrwasusedasprecursor,}51_{VNMRspectrain}

thepresenceofincreasingamountofhydrogenperoxideshowed disappearanceofthesignalat−595ppm,alreadyaftertheaddition oftenequivalentsofH2O2.Inthisconditionsnoformationofthe

yellowprecipitatewasobserved.Thissuggeststhattheformation oftheperoxovanadium speciesismorefavorablewhentheless tightlycoordinatedBr−isthecounterioninplaceofCF3SO3−.In

agreementwithsuchobservation,whensalenVV_{OBrwastestedas}

catalyst,afasterdecompositionofhydrogenperoxideisobserved

[5,26].

3.1.3. Theoreticalstudies

The unclear results obtained with the NMR spectroscopy promptedustoapplyDFTtheoreticalcalculations(seeSection2.1

fordetails)inordertoshedmorelightonthestructureofthe reac-tiveperoxocomplexesformedinsolution,whensalophenandsalen vanadium(V)precursorsareused.

Therefore,calculationonrelevantspecies wereperformedat B3LYP/lanl2dzlevel, includingsolvent effectswiththeIEF-PCM method (acetonitrile). These include vanadium hydroperoxides and2_{peroxides,indifferentprotonationforms,andinthe}

pres-enceofaresidualoxoorhydroxoligand(Scheme3).Frequency calculationswereperformedinordertoconfirmthatthe equilib-riumstructureswereactualminimumenergyspecies(Table3).

Optimized distances between the vanadium center and the coordinated oxygen of the peroxide moiety fall in the range 1.82–2.05 ˚A,whileoxygen–oxygendistancesoftheperoxidegroup arefoundbetween1.46–1.51 ˚A;bothareingoodagreementwith literaturevalues[45,46].Ingeneral,adecreaseoftheV O(O) dis-tance is observed increasing thepositive charge of thespecies

[45,46].Whentheperoxideisprotonated(vanadium hydroperox-ides),theprotonatedoxygenisnotlocatedatbindingdistancefrom thevanadiumcenter,confirmingamonodentatemodeof coordina-tionoftheO Ogrouptothemetal;differently,whentheperoxide moietyisnotprotonatedbothoxygenatomsarecoordinatedto vanadium,ina2_mode[47].

AsillustratedinFig.2,onlyplanarconformationswere consid-eredforsalophencomplexes,whilebentandplanarconformations wereexploredfor all thesalenVV _{species. To note,as reported}

in Table4,some ofthe salenVV _{derivatives aremore stable in}

thebent conformation,and inparticular theV(O)(OOH)peroxo complex,which isthereasonable activespecies responsiblefor oxygentransfer(videinfra).

Theacceptedmechanism,alreadyproposedintheliterature,for theformationofperoxidicspeciesresultingfromthereactionof vanadiumspecieswithhydrogenperoxide[43,45,46]hasbeenused asthebasetoanalyzetheenergeticpathleadingtotheformation oftheperoxospeciespossiblyresponsibleforoxygentransfer,see

Scheme4.

Table4

EnergeticallypreferredconformationsforsalenVV_{complexesasderivedbyDFT}

calculations.

salenVV_{complex Bent Planar G}a_(kcalmol−1₎

[salenVV_(OH)(OOH)]+ _{× 2.3}

[salenVV_{O(OOH)] × 6.7}

[salenVV_{(OO)(OH)] × 1.0}

[salenVV_O(OO)]− _{× 14.6}

a_{Freeenergydifferencebetweenmostandlessstableconformationsforeach}

species.

Insuchschemetheoriginalstateofthecatalystwas consid-eredtobeaLVV_O+_{species(Lstandsforsalenorsalophenligand)}

originatedfromdissociationofLVV_OCF

3SO3insolution;addition

ofH2O2toLVVO+toformLVV(OH)(OOH)+specieswerefoundto

beendothermicprocesses,withG=+18.1and+21.4kcalmol−1 forL=salenandsalophen,respectively.Forthisreason,and con-sideringalsothesterichindranceoftheligandsaroundthemetal, onlytheformationofmonoperoxospecieswastakenintoaccount. Moreover,thefactthatadditionofH2O2areendoergonicprocesses,

suggestthatonlysmallamountsofvanadiumperoxocomplexes forminsolution;thisresultisconsistentwith51_VNMRspectra,

wherenosignalsdifferentfromtheoneoftheprecursorhavebeen detected.

Subsequentcalculationswereaimedatestablishingthe reactiv-ityoftheLVV_(OH)(OOH)+_{species.Acrucialobservationisthatsuch}

intermediatesbehaveasstrongacids,byreleasingaprotonfrom thehydroxogrouptoformLVV_{(O)(OOH)derivatives}

(deprotona-tionstepinScheme4).ThisresultedbycalculationofthepKaofthe species,bycomparisonwithareferenceacid/basecoupleaccording toWangetal.[48].ThisprocedureavoidsdirectcalculationonH3O+

species,thataresubjecttorelevantsolventeffects,difficultto eval-uateemployingacontinuumapproximation.Inthisparticularcase, thepyridinium/pyridinecouple(pKa=5.25)waschosenasthe ref-erence,inordertomaintainthesametotalchargeoftheacid/base coupleunderinvestigation,LVV_(OH)(OOH)+_/LVV_(O)(OOH).

Calculated pKa for theLVV_(OH)(OOH)+_/LVV_{(O)(OOH) couples}

were found −6.9 and −4.6 for salen and salophen derivatives, respectively, thusconfirminghighacidbehaviorofthehydroxo groupcoordinatedtothevanadiumcenter.

The LVV_{(O)(OOH) species were confirmed to be the most}

probable ones,since possibleisomers were observed at higher energy. Indeed, LVV_{(OH)(OO) species are higher in energy by}

12.5 and 4.1kcalmol−1 for salen and salophen, respectively, while a H-LVV_{(O)(OO) derivative (where the proton is located}

attheoxygenatomoftheLligand)is2.2–8.7kcalmol−1 higher when L=salophen (the same structure with L=salen failed to converge).Protonationoftheperoxidespecies,withno involve-ment of the oxo group, was recently demonstrated by XANES spectroscopy for a [HheidaV(O)(OO)]− complex (heida N-(2-hydroxyethyl)iminodiaceticacid)tobethekeysteptoactivatethe speciesforoxygentransfercatalysis[49].

Analternativepathway(notrepresentedinScheme4),where water eliminationfromtheLVV_(OH)(OOH)+ _leadstothe

forma-tion of a LVV_(OO)+ _{species was found to be endothermic by}

5.4kcalmol−1 inthecaseofL=salen,confirmingthatvanadium complexeswithoutoxoorhydroxomoietiesarenotstable[12]:

Therefore, calculations suggest that a vanadium oxo-hydroperoxo species, LVV_{(O)(OOH), is the most plausible}

intermediate formed upon H2O2 addition to vanadium Salen

Scheme3.RepresentationofrelevantspeciesconsideredinDFTcalculations,withoptimizedinteratomicdistancesindicatedinblackandredforsalenandsalophenspecies, respectively.Thesalenorsalophenligandisrepresentedas“L”forclarityreasons.

Scheme4. EnergiesfortheformationpathwaysofvariousvanadiumbasedperoxocomplexeswithL=salen(blackvalues)orsalophen(redvalues).Theenergyofthe reactionsinvolvingH+_{ionswerenotdirectlycalculated,butdeterminedindirectlyfromthecalculationofthepKaofthecompetentspecies.(Forinterpretationofthe}

referencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

potential,seconddeprotonationofLVV_{(O)(OOH)togiveananionic}

LVV_(O)(OO)−_{specieswasalsoconsidered;inthesecasescalculated}

pKaare4.1and7.8forL=salenandsalophen,respectively(inthis caseaceticacid/acetatehasbeenusedasthereferenceacid/base couple, in order to preserve the charges of the species in the equilibriumprocess).Itisinterestingtonoticethatthecalculated pKacomparereasonablywiththevaluesof5.4–5.9observedfor thevanadium-dependenthaloperoxidasesenzymes[50,51]. Nev-ertheless,deprotonatedperoxidessuchasLVV_(O)(OO)−_areknown

tobepoorlyactiveinoxygentransfer[45,46,49],therefore,even iftheycouldactuallybeformedinsolutionaccordingtothepKa valuesdiscussedabove,theyarenotthereasonablecatalytically activespeciesinthepresentsystem.

Therefore,thepresenceoftheLVV_{(O)(OOH)peroxocomplexes}

insolutionisthemostreliablescenario.Thisisinagreementwith resultsfoundintheliteratureforsimilarvanadiumcomplexeswith modelsalanligands[52],wheretheoxohydroperoxovanadium derivativesappearstobethemostfavorableactivespeciesinalkene epoxidationprocesses.Insuchpaper,thesalanligandis coordi-natedtoVatominabentconformation,inwhichtwoOandone Noccupytheequatorialpositions,whiletheremainingOisinthe axialone;thisconformationisconsistentwiththebent arrange-mentoftheligandaroundthevanadiumatomthatwefoundfor salenVOperoxocomplexes.

In the caseof salenV(O)(OOH) peroxocomplex, the energies forthebentand planarconformationsofallthe5,5-substituted derivativeswerecalculatedin thegasphasein orderto under-stand whether the presence of substituents could affect their conformations.Thecomparisonbetweenthetwoconformations, representedby theirenergy differences,is reported in Table5. Interestingly,inallcasesthemostfavorablestructureisthebent one. For the unsubstituted salen complex, energies in acetoni-trileasthemodelsolventwereevaluated,obtainingcomparable results.

Afinalargumentconcernsthepossibleoxygentransferactivity oftheseLV(O)(OOH)peroxides,namelyiftheycanbetheactual speciesresponsibleforsulfideoxidation.

Asdiscussedabove,theoptimizedgeometriesofLV(O)(OOH) peroxidesstronglysuggestamonodentatebindingofthe perox-idetothevanadiumcenter,anditoccurswiththenonprotonated peroxidicoxygen,whiletheprotonatedoxygenisnotlocatedat bindingdistancefromthevanadium(V O(H)=2.962 ˚Aand2.520 ˚A for L=salenand salophen,respectively).Thisdiffersfromother vanadiumhydroperoxospeciesreportedintheliterature,where thecalculatedV O(H)distanceismuchshorter[45,46]andcanbe relatedtothehighersterichindranceofsalenandsalophenligands. Asa result,two differentscenario canbeenvisaged:(a)the LV(O)(OOH) are the actual species for oxygen transfer; indeed the O O ␴* orbitals, usually recognized as the one giving the mostimportantcontributionintheelectrophilicoxidative activ-ity of metalperoxides [53–55],are foundat reasonableenergy levelsforbothspecies(Fig.5);(b)theLV(O)(OOH)speciesarein equilibriumwithotherforms,wherealsothesecondprotonated oxygeniscoordinatedtovanadium,andtheselatterspeciesare responsibleforoxygentransfer:

Table5

Differencesofenergiesforplanarandbentconformationsofdifferentlysubstituted salenV(O)(OOH)species.

salenV(O)(OOH)complex E(bent-planar)(kcalmol−1₎ [5,5_{-(t-Bu)2salenV}V_{O(OOH)] −7.71}

[5,5_{-(MeO)2salenV}V_{O(OOH)] −8.79}

[salenVV_{O(OOH)] −7.46(−6.7inCH3CN)}

Fig.5.RepresentationoftheO O␴*orbitalforSalenV(O)(OOH)(LUMO+6)and SalophenV(O)(OOH)(LUMO+10)species.

Equilibriaofthistypehavealreadybeenconsideredforother V(V)species,wherethe2_{coordinationofthehydroperoxidewas}

necessarytoobserveoxygentransferactivityandthefreeenergy ofthereactionwasinfluencedbythepresenceofLewisbasesas coligands[56].Equilibriaofthistypearecurrentlyunderevaluation forsalenandsalophenderivatives.However,beingthedistance oftheprotonatedoxygenshorterinthesalophencomplexes,they maylikely establishmoreeasilysuchequilibrium.On thisbasis thehigher activityof salophencomplexes withrespecttosalen onescouldbeexplained,furthersupporttothisisgivenbytheir observedhigherLewisacidityasmeasuredwithCVexperiments

[37].

Fullmechanisticstudy,includingevaluationoftransitionstates, isinprogressandwillbereportedinaseparatecommunication.

4. Conclusions

Theexperimentalandtheoreticalresultsobtained,eventhough donotallowtodrawadecisivepictureofthemechanism,letto emphasizethatstericfactorsplayamajorroleindeterminingthe outcomeofthereaction,oftenovercomingtheelectroniceffects. Theoreticalresultssuggesttheinterventioninthecatalyticcycleof anhydroperoxovanadiumspecies.

Acknowledgements

Research carried out in theframework of COST D40 Action “InnovativeCatalysis–NewProcessesandSelectivities”. Experi-mentalworkofS.UmmarinoandG.DiCarmineisacknowledged.

AppendixA. Supplementarydata

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.cattod.2012.03.032.

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