Polyhedron Vol[ 06\ No[ 19\ pp[ 2446Ð2453\ 0887Þ 0887 Elsevier Science Ltd\ Pergamon All rights reserved[ Printed in Great Britain9166Ð4276:87 ,08[99¦9[99PII] S9166Ð4276"87#99038Ð0

Ruthenium substituted Keggin typepolyoxomolybdates] synthesis\ characterization anduse as bifunctional catalysts for the epoxidation

of alkenes by molecular oxygen

Ronny Neumann� and Mazal Dahan

Casali Institute of Applied Chemistry\ Graduate School of Applied Science\ The Hebrew University ofJerusalem\ Jerusalem\ Israel\ 80893

"Received 13 February 0887^ accepted 03 April 0887#

Abstract*The ruthenium substituted polyoxomolybdate of the Keggin structure\ Q3PRuIII"H1O#Mo00O28

"Q�n!Bu3N#\ has been synthesized and characterized[ The IR spectra show that this compound is isostructuralwith the known manganese and cobalt analogs[ The cyclic voltammogram showed similar redox potentialsand the UVÐvis spectra showed similar energies for the dÐd transitions compared to the correspondingtungstate\ Q3PRuIII"H1O#W00O28[ The catalytic activity of the molybdate versus tungstate in reactions withmolecular oxygen was\ however\ signi_cantly di}erent[ IR and 20P NMR evidence indicated that treatment ofQ3PRuIII"H1O#Mo00O28 with oxygen showed no structural changes whereas\ for Q3PRuIII"H1O#W00O28\ a clearchange was observed[ This _nding probably explains the lack of catalytic activity for the latter in the co!oxidation of cumene and 0!octene to cumyl alcohol and 0!octene oxide[ For the molybdenum compound\ thisreaction took place by a kinetic balance of ruthenium metal!catalyzed autooxidation of cumene to cumenehydroperoxide and the molybdenum catalyzed oxygen transfer from cumene hydroperoxide to 0!octene toyield the products[ High catalyst loading led to reaction inhibition whereas low loading and excess cumene ledto increased autooxidation[ Þ 0887 Elsevier Science Ltd[ All rights reserved

Keywords] polyoxomolybdates^ bifunctional catalysts^ alkenes^ molecular oxygen[

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INTRODUCTION

Transition metal substituted polyoxometalates have\over the last decade\ attracted considerable interest asoxidatively stable catalysts for oxidation of hydro!carbons ð0Ł[ Transition metal substituted polyox!ometalates can be de_ned as having a transition metaloctahedrally or tetragonally coordinated by alacunary polyoxometalate[ The lacunary polyoxo!metalate may also be viewed as integrating {hard|donor oxide ions with adjacent d9 addenda atoms"MoVI and:or WVI# providing acceptor orbitals[ Theinteraction of a substituted transition metal with theaddenda should be a function of the reduction poten!tial of the heteropolyoxometalate\ especially for asecond!row transition metal such as ruthenium whichhas extended d!orbitals[ Despite the known di}erence

�Author to whom correspondence should be addressed[

2446

in the redox properties of polyoxotungstates versuspolyoxomolybdates\ the transition metal polyox!omolybdates and their catalytic properties have beenonly scarcely investigated[ A typical transition metalsubstituted Keggin structure with the transition metalin a polyhedral representation is presented in Fig[ 0[Numerable oxidation reactions for these structuresand some others have been reported for polyox!otungstates containing chromium ð1Ł\ manganese ð2Ł\iron ð3Ł\ cobalt ð4Ł\ nickel ð5Ł\ niobium ð6Ł\ rhodium ð7Łand ruthenium ð8Ł active centers\ using a variety ofoxidants such as iodosobenzene\ t!butylhydro!peroxide\ periodate\ amine!oxides\ hydrogen peroxideand oxygen[ One current interest of our researchgroup is the utilization of oxygen in catalytic trans!formations involving ruthenium substituted polyoxo!metalates ð8Ł[ In this context\ we describe in this paperthe synthesis and characterization of ruthenium sub!stituted polyoxomolybdates of the Keggin structureand their use as bifunctional catalysts for the aerobic

R[ Neumann and M[ Dahan2447

Fig[ 0[ Representation of the Keggin transition metal substituted polyoxometalate[

epoxidation of alkenes using alkanes as the source ofelectron donors ð09Ł[ In the past\ there have been sev!eral reports on the combined use of twometals for such reactions in the intramolecular"cyclohexene# ð00Ł and intermolecular mode ð01Ł[ Thescheme for epoxidation is summarized in Scheme 0[First\ there is an autooxidation of alkanes\ for exam!ple\ cumene presumably catalyzed at the rutheniumsite to intermediate hydroperoxides[ Instead of theusual metal catalyzed decomposition of the hyd!roperoxide\ there occurs heterolytic oxygen transfer"epoxidation# from the intermediate hydroperoxidecatalyzed at a molybdenum site to the alkene yieldingepoxide and alcohol as co!product[

RESULTS AND DISCUSSION

Synthesis and characterization

As has been pointed out in the past\ the relativeinstability of Keggin type lacunary polyoxo!

molybdates prevents the preparation of such tran!sition metal substituted polyoxomolybdates in waterð02Ł[ Instead however\ they may be prepared in non!aqueous solvents such as acetonitrile using quaternaryammonium cations\ as has been described forQ3HPMMo00O28 where M�CoII\ MnII and CuII andQ�"n!Bu3N# ð02Ł[ We have now adapted the non!aqueous procedure for the preparation of Q3PRuIII

"L#Mo00O28 for L�H1O and "CH2#1SO[ The synthesisand puri_cation of the lacunary species\Q3H2PMo00O28\ was carried out as published ð02Ł[The lacunary species was then reacted both with anequivalent amount of solid RuCl2×H1O or RuII

"DMSO#3Cl1 to yield Q3PRuIII"H1O#Mo00O28\ 0\ andQ3PRuIII"DMSO#Mo00O28 ð03Ł\ 1\ respectively\ whichwere puri_ed by recrystallization[ The labile ligand onthe ruthenium atom in 0 is rather arbitrarily given asbeing water although it could well be acetonitrile or amixture of both "see analysis of IR spectrum below#[The IR spectra of the two ruthenium substitutedpolyoxomolybdates are shown in Fig[ 1\ along withthe spectrum of Q3PRuIII"H1O#W00O28\ 2 ð04Ł\ for com!

Ruthenium substituted Keggin type polyoxomolybdates 2448

Fig[ 1[ IR spectra of ruthenium substituted polyoxometalates[ Top] Q3PRuIII"H1O#W00O28 0979\ 0925 cm−0 PÐO^ 850 cm−0

WÐOt^ 770 cm−0 WÐOcÐW^ 791 cm−0 WÐOeÐW[ Middle] Q3PRuIII"H1O#Mo00O28 0950\ 0927 cm−0 PÐO^ 841\ 825 cm−0

MoÐOt^ 767 cm−0 MoÐOcÐMo^ 702 cm−0 MoÐOeÐMo[ Bottom] Q3PRuIII"DMSO#Mo00O28 0987\ 0910 cm−0 PÐO^ 822 cm−0

MoÐOt^ 773 cm−0 MoÐOcÐMo^ 795 cm−0 MoÐOeÐMo^ 626 cm−0[

parison[ The spectra "not shown# measured forQ3HPMMo00O28 "M�CoII and MnII#02 were prac!tically identical to that of Q3PRuIII"H1O#Mo00O28[ TheIR peaks attributable to stretching vibrations at cor!ner shared bridges\ MÐOcÐM\ and edge sharedbridges\ MÐOeÐM\ were similar for all threecompounds[ The peaks attributable to PÐO vibrationswere all split due to the reduction in symmetry fromTd in the parent Keggin structure to a Cs symmetryfound in the transition metal substituted analog ð05Ł[The large split observed in Q3PRuIII"L#Mo00O28 withDMSO as ligand indicates a large displacement ofthe ruthenium cation from the heteroatom with thestrongly coordinating DMSO[ For the less coor!dinating water or acetonitrile ligands\ a much smallersplit is observed[ Thus\ there is a smaller displacementand a more compact structure is postulated ð05Ł[ Forcompounds 1 and 2\ there were single peaks attribu!table to the terminal MÐOt moiety[ However\ forQ3PM"H1O#Mo00O28 "M�Co\ Mn and Ru# two peaksat 841 and 825 cm−0 were observed[ We have no cer!

tain explanation for this phenomenon but hypothesizethat it may be connected to the labile ligand at theruthenium atom with compounds with di}erentligands yielding di}erent peaks ð06Ł[

Q3PRuIII"DMSO#Mo00O28 proved to be cata!lytically inactive due to the non!lability of the DMSOligand ð04Ł[ Thus\ further characterization was limitedto the catalytically active Q3PRuIII"H1O#Mo00O28[ The20P NMR spectrum in acetonitrile showed a singlevery broad peak at −094 ppm "Dn0:1�0199 Hz# whichindicated a pure paramagnetic compound[ The UVÐvis spectrum ðFig[ 2"a#Ł shows the expected chargetransfer peak at l�139 nm\ e�6×093 M−0 cm−0 withrelatively intense dÐd transitions at 309 and 465 nm"e�09999 and 2999 M−0 cm−0 respectively#[ These dÐd absorptions are slightly shifted "09Ð14 nm# relativeto the reported spectrum of the tungsten analog ð04Ł[The cyclic voltammetry measurement in CH2CN "Fig[3# showed two reversible waves assigned by literatureprecedence of other ruthenium subtituted poly!oxometalates ð04Ł to Ru"III#:Ru"II# at −9[00 V and

R[ Neumann and M[ Dahan2459

Fig[ 2[ UVÐvis spectrum of Q3PRuIII"H1O#Mo00O28 in ace!tonitrile[ "a# *** without treatment[ "b# ! ! ! after bubbling

O1 for 01 h[

Ru"IV#:Ru"III# at 9[97 V[ These reduction potentialsare only marginally lower compared to those reportedfor the tungsten compound at neutral pH[ NoRu"V#:Ru"IV# transition was observed due to solventinterference[ In the past ð04Ł it had been concludedthat the low spin con_guration of the ruthenium atomallows partial p!delocalization into vacant pp�orbitals of oxygen atoms bridging ruthenium and the

Fig[ 3[ Cyclic voltammogram of Q3PRuIII"H1O#Mo00O28[

addenda atoms[ Inasmuch as it is well known thatpolyoxomolybdates\ e[`[\ PMo01O39

2−\ are signi_cantlystronger oxidants than their analogous polyoxo!tungstates e[`[\ PW01O39

2− and show signi_cant batho!chromic shifts ð07Ł\ a more signi_cant e}ect on boththe ruthenium redox chemistry and electronic spec!trum was expected ð04Ł\ however\ only slight di}er!ences were observed[ It would appear that there islittle p!delocalization between the ruthenium cationand the neighboring addenda atoms\ which has littlee}ect on the properties at the ruthenium center[Despite these observations\ the Q3PRuIII"H1O#Mo00O28 compound showed di}erent "a# reactivity tomolecular oxygen and "b# catalytic activity and sel!ectivity compared to the Q3PRuIII"H1O#W00O28

analog\ as will be shown below[

Reaction of Q3PRuIII"H1O#M00O28 with molecular oxy!`en

Since we were interested in the catalytic co!oxi!dation of two substrates with molecular oxygen aspresented in Scheme 0\ the reaction of Q3PRuIII"H1O#M00O28 with molecular oxygen was examined byFTIR\ UVÐvis and 20P NMR[ Reaction of the mol!ybdenum compound\ 9[90 M Q3PRuIII"H1O#Mo00O28

in acetonitrile with 0 atm molecular oxygen for 7 h at59>C yielded only the disappearance of the peak at825 cm−0 in the IR spectrum "Fig[ 4# and no formation

Ruthenium substituted Keggin type polyoxomolybdates 2450

Fig[ 4[ IR spectrum of Q3PRuIIIM00O28 before and after treat!ment with O1[ Upper box] Q3PRuIIIW00O28 "top] before treat!ment with O1\ bottom] after treatment with O1#[ Lower box]Q3PRuIIIMo00O28 "top] before treatment with O1\ bottom]

after treatment with O1#[

of a new absorption peak[ An identical experimentreplacing dioxygen with dinitrogen yielded the samechange in the IR spectrum[ A further UVÐvis experi!ment "9[0 mM in acetonitrile\ 0 atm O1\ 7 h\ 59>C#with both the molybdenum and tungsten compoundsshowed the disappearance of the peaks in the visibleregion ðFig[ 2"b#Ł[ Our explanation of these results isthat the bubbling of dioxygen or dinitrogen througha solution of Q3PRuIII"H1O#Mo00O28 in an apolar sol!vent only leads to the removal of the labile ligand ð08Ł[This removal of the labile ligand manifests itself in theabsence of peaks in the visible spectrum as has beenreported also for the ZnWRu"ZnW8O23#101− com!pound ð8Ł[ For the Q3PRuIII"H1O#Mo00O28 compound\

there was also a change in the IR i[e[\ the dis!appearance of the peak at 825 cm−0[ As stated above\we have postulated that the split peaks at 841 and825 cm−1 were due to the presence of a labile ligandat the ruthenium atom[ Removal of the ligand leadsdue a single peak indicative of only MoÐOt stretchingvibrations^ importantly the Keggin structure isretained[ Another indication of the absence of a sig!ni_cant structural change was obtained by 20P NMR[The original broad peak at −094 ppm was unchangedafter treatment with oxygen[

By contrast\ in the case of the tungsten compound\Q3PRuIII"H1O#W00O28\ after treatment with oxygen\there appeared in the IR spectrum a new distinct peakat 0908 cm−0 whilst the other peaks remainedunchanged "Fig[ 4#[ However\ in this case\ applicationof dinitrogen instead of dioxygen yielded only theoriginal spectrum[ The change in the IR spectrumof the tungsten analog\ Q3PRuIII"H1O#W00O28\ wouldseem to indicate some structural change in the poly!oxometalate[ There was also a clear change in the 20PNMR spectrum[ The original compound has a verybroad peak at −69 ppm ð04Ł whereas\ after treatment\a new\ narrower peak at 157 ppm "Dn0:1 � 019 Hz# isformed[ In addition\ to the change in the IR and 20PNMR spectra\ heating of Q3PRuIII"H1O#W00O28 wasalso coupled with the formation of a black precipitatewhich was insoluble in a wide range of polar andapolar solvents[ We have no certain explanation ofwhich change has occurred\ but the formation of adimeric m!oxo bridged species followed by pre!cipitation of RuO1 is a very distinct possibility ð19Ł[Unfortunately\ our hypothesis concerning the identityof the new ruthenium tungstate could not be veri_eddue to our inability to obtain single crystals ofsu.cient quality[ This speculation is also supportedby the catalytic inertness of the tungsten compoundversus the active molybdate "see below# ð09Ł[ Anotherpuzzling point to explain concerns the reasoningbehind the di}erent reactivity of the molybdate andtungstate with molecular oxygen[ The IR spectra ofthe original compounds "see above# indicate that theruthenium is more tightly bound in the molybdateversus tungstate\ as borne out by the smaller split inthe vibrations atttributed to the PÐO stretch[ In theone case where a dimeric compound with a RuIVÐOÐRuIV bridge has been isolated ð19Ł\ the bridge is placedover rather than in the lacunary position[ It is possiblebut of course not certain that the more labile structurevis a vis the ruthenium position in the tungsten versusmolybdenum compound may lead to a more facileformation of the bridged compound and rutheniumremoval from the lacunary site of the tungstate[

Catalytic oxidation

In our previous communication\ we reported thatin a reaction according to Scheme 0 "0 M cumene\ 0 M0!octene\ 0 mM Q3PRuIII"H1O#M00O28 "M�Mo\ W#

R[ Neumann and M[ Dahan2451

in CH2CN 0 atm O1\ 79>C\ 13 h#\ the tungstate showedessentially no reactivity\ whereas the molybdate\½099 turnovers\ gave ½0]0 ratio of 0!octene oxide:cumyl alcohol and only traces of acetophenone ð09Ł[For the tungstate compound\ the formation of an inertcompound is postulated as mentioned and discussedabove[ For the molybdenum compound\ there was aclear balance in the rate of formation of the inter!mediate cumene hydroperoxide and the oxygen trans!fer reaction from the intermediate to the alkene[ Aninduction period of three to four hours was typicallyobserved for these reactions[ The ratio of cumene:0!octene "Fig[ 5# was a critical parameter for deter!mining the activity and selectivity in the Q3PRuIII

"H1O#Mo00O28 catalyzed reaction[ As the amount ofcumene was increased\ the turnover frequency tocumene hydroperoxide decomposition products "ace!tophenone and cumyl alcohol in excess of the amountof 0!octene oxide formed# increased signi_cantly\whereas the amount of 0!octene oxide formed\ "½099turnovers# remained more or less constant[ This showsthat increased amounts of cumene had no positivee}ect on the rate of epoxide formation in the oxygentransfer reaction but\ rather simply\ was {wasted| inthe formation of decomposition products[ The e}ectof Q3PRuIII"H1O#Mo00O28:substrate ratio on the con!versions and selectivities is presented in Fig[ 6 "ratiocumene:0!octene�0#[ Here\ one may observe thathigh catalyst loading inhibited the reaction[ AtQ3PRuIII"H1O#Mo00O28:substrate ratios up to 0:0499\there was an excellent selectivity to 0!octene oxide[ Asthe solution became more dilute in catalyst and theQ3PRuIII"H1O#Mo00O28:substrate ratios decreased\there was a loss of activity in parallel with loss ofselectivity for formation of the epoxide[

The results of the catalytic reaction are most easilyexplained via a combination of the classical ð10Ł lowvalent metal "RuIII# catalyzed autooxidation mech!anism together with the heterolytic oxygen transfermechanism at the high valent metal "MoVI#\ see

Fig[ 5[ Catalytic activity of Q3PRuIII"H1O#Mo00O28 as a func!tion of the amount of cumene and 0!octene[ Reaction con!ditions] 9[4Ð1[9 mmol cumene\ 0 mmol 0!octene\ 0 mmolQ3PRuIII"H1O#Mo00O28\ 0 mL acetonitrile\ 0 atm O1\ 79>C\

13 h[

Fig[ 6[ The e}ect of the amount of Q3PRuIII"H1O#Mo00O28 onactivity and selectivity[ Reaction conditions] 0 mmol cumene\0 mmol 0!octene\ 9[22Ð1 mmol Q3PRuIII"H1O#Mo00O28\ 0 mL

acetonitrile\ 0 atm O1\ 79>C\ 13 h[

Scheme 1 ð11Ł[ First\ the catalyst "Ru# initiates a chainreaction for the formation of cumene hydroperoxide[The intermediate alkyl hydroperoxo or peroxo speciesreact with the molybdenum based polyoxometalateligand to form an active MoÐOÐO!alkyl species whichtransfers oxygen to the alkene to form epoxide andalcohol[ When the catalyst loading is high the inter!mediate cumene peroxide is shunted by the catalystfrom the propagation "autooxidation# reaction\ lead!ing to low turnover numbers and apparent reactioninhibition[ Such phenomena have been observed in thepast for transition metal substituted polyoxometalatecatalyzed reactions ð12Ł[ When the amount of cumeneis too high or the amount of catalyst is too low\ thepropagation of the cumene peroxide and hydro!peroxide\ independent of the catalyst\ becomes themajor reaction pathway[ This then leads to excessautoxidation products\ especially acetophenone viadecomposition of the cumyl oxo radical ð13Ł[

CONCLUSION

A ruthenium substituted polyoxomolybdate\Q3PRuIII"H1O#Mo00O28\ of the Keggin structure hasbeen prepared and characterized[ The compound isan e}ective bifunctional catalyst for the co!oxidationof cumene and 0!octene\ in contrast to the analogoustungsten compound which undergoes\ in the presenceof molecular oxygen\ an apparent structural changeto an inert form[

EXPERIMENTAL PART

Materials and polyoxometalate synthesis

Organic substrates of highest available purity"Aldrich and Fluka# were puri_ed prior to use bypassing the compound over a neutral alumina column[The known polyoxometalates\ n−"C3H8#3NŁ3PRuIII

"H1O#W00O2804 and ðn!"C3H8#3NŁ3HPM"H1O#Mo00O28\

Ruthenium substituted Keggin type polyoxomolybdates 2452

M � Mn "II#\ and Co"II# were prepared by the pub!lished literature procedures[ Similarly\ ðn!"C3H8#3NŁ3PRuIII"H1O#Mo00O28 was prepared by dissolving9[29 mmol "799 mg# ðn−"C3H8#3NŁ3H2PMo00O28 ð02Łin 04 mL acetonitrile to form a light green solutionfollowed by addition of 9[20 mmol "54 mg# solidRuCl2×H1O[ After stirring for two hours under argonat room temperature\ a few drops of water were added\air was passed through the solution for one hour andthe solvent was evaporated[ It may be noted that theinsertion reaction failed when an aqueous solution ofRuCl2×H1O was used[ The dark green rutheniumsubstituted polyoxomolybdate\ ðn!"C3H8#3NŁ3PRuIII

"H1O#Mo00O28 was recrystallized from 2 mL acetoni!trile\ yield 67)[ Carbon\ hydrogen\ nitrogen andphosphorous were determined by microanalysis\ruthenium as RuO1 and molybdenum as the 7!hy!droxy quinolate were determined gravimetrically afterdecomposition with 0 M NaOH[ Analysis] found "cal!culated# C 17[01 "16[35#^ H 4[46 "4[15#^ N 1[24 "1[99#^P 0[99 "0[00#^ Ru 2[2 "2[50#^ Mo 25[82 "26[69#[ 20PNMR −094 ppm "Dn0:1 � 0499 Hz#[ IR 0950\ 0927\841\ 825\ 767\ 702 cm−0

In an analogous procedure\ ðn!"C3H8#3NŁ3PRuIII

"DMSO#Mo00O28 was prepared by reacting 9[29 mmol"799 mg# ðn!"C3H8#3NŁ3H2PMo00O28 in 04 mL with9[20 mmol "029 mg# solid RuII"DMSO#3Cl1 ð14Ł[ Afterremoval of the solvent overnight under vacuum\ theproduct was recrystallized from 2 mL acetonitrile\yield 37)[ Analysis] found "calculated# C 17[47"16[61#^ H 4[54 "4[18#^ N 1[44 "0[85#^ P 0[99 "0[97#^ Ru2[2 "2[42#^ Mo 27[34 "25[80#[ IR 0987\ 0910\ 822\ 773\795\ 626 cm−0[

Instruments and polyoxometalate characterization

UVÐvis spectra were measured using a diode arrayspectrometer "HP 7341# using 0 cm quartz cuvettes[

Solutions\ 19 mM\ were prepared by dissolvingQ3PRuIII"L#Mo00O28 "Q�n!"C3H8#3N\ L�H1O\DMSO# in acetonitrile[ IR spectra were measured ona Nicolet 409 M FTIR instrument[ Spectra were mea!sured neat by placing a few drops of the polyox!ometalate dissolved in acetonitrile on a KBr plate[The acetonitrile was evaporated o} and the spectrawere measured[ 20P NMR spectra were measuredusing a Bruker DRX!399 instrument at 050[865 MHzwith 74) D2PO3 as external standard[ Approximately09 mM solutions were prepared in 79) CH2CN\ 19)CDCl2[ A cyclic voltammogram of 4 mM Q3PRuIII

"H19#Mo00O28 was measured using a BAS!0 instru!ment in a 9[0 M solution of n!"C3H8#3NBF3 in ace!tonitrile using a glassy!carbon working electrode\ anAg:AgNO2 reference electrode and a platinum auxili!ary electrode^ scan rate 299 mV:sec[ Potentials werereferenced to the ferrocene:ferrocenium couple"−9[11 V#[

Epoxidation of alkenes

The epoxidation of alkenes were carried out in½02 mL pressure tubes "Ace Glass#[ In a typical pro!cedure\ 0[9 mmol 0!octene\ 0[9 mmol cumene and0[9 mmol ðn!"C3H8#3NŁ3PRuIII"H1O#Mo00O28\ dissolvedin 0 mL acetonitrile\ were placed in the glass tubewith a te~on stirrer[ Dioxygen\ ×88) purity\ wasintroduced to the reaction at 79>C and 0 atm dioxygenafter a series of three pumpÐthaw operations[ Thetubes were immersed in a temperature controlled oilbath equipped with a magnetic stirrer[ Samples ofthe reaction mixture were taken at the desired timeintervals and directly injected into a gas chromato!graph "HP 4789\ FID detector\ He carrier gas# _ttedwith a capillary methyl silicone bonded phase column"RTX 0\ length 04 m\ ID 9[21 mm\ coating thickness9[14 mm#[

R[ Neumann and M[ Dahan2453

Acknowled`ements*This research was supported by theIsrael Science Foundation founded by the Israel Academyof Science and Humanities[ Professor Michael T[ Pope ofGeorgetown University is kindly thanked for his sample ofQ3PRuIII"H1O#W00O28[

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11[ The reduced RuÐPOM species is representedwhere the electron is localized at the rutheniumatom[ This is not necessarily so\ although thedegree of delocalization onto the molybdenumaddenda is unknown[

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