Twenty-two base adducts of uranyl o-hydroxyarylcarbonyl complexes [UO2L2B] (L = o-hydroxyarylcarbonyl compound, B = H2O, pyridine, pyridine N-oxide) have been prepared. Pure field substituent parameters are used to derive a quantitative order of the electronic effects of the chelate ring substituents. Values of v(U=O) correlate well with these values. Evidence is cited suggesting almost complete non-aromaticity of the chelate ring, and the transmission of the electronic effects of the chelate ring substituent through the uranium atom in the pyridine and pyridine N-oxide base adducts

Haigh, J M

Haigh, John M

South East Academic Libraries System (SEALS)

Journal of MolecularStructure,23 (1974)281-288~ ElsevierScientificPublishingCompany,Amsterdam- Printedin The NetherlandsLIGAND SUBSTITUTION EFFECTS IN URANYL o-HYDROXYARYL-CARBONYL COMPLEXESJ. M. HAIGHDepartmentof PharmaceuticalSciences,RhodesUniversity,Grahamstown(SouthAfrica)(Received21December1973)ABSTRACTTwenty-twobase adductsof uranyl o-hydroxyarylcarbonylcomplexes[U02L2B] (L =o-hydroxyarylcarbonylcompound,B = H2O, pyridine,pyri-dineN-oxide)havebeenprepared.Purefieldsubstituentparametersareusedtoderivea quantitativeorderof theelectroniceffectsof thechelateringsubstituents.Valuesof v(U=O)correlatewellwiththesevalues.Evidenceis citedsuggestingalmostcompletenon-aromaticityof thechelatering,andthetransmissionof theelectroniceffectsof thechelateringsubstituentthroughtheuraniumatomin thepyridineandpyridineN-oxidebaseadducts.INTRODUCTIONIt hasrecentlybeenshown[1] thatpurefieldandresonancesubstituentparameterscanbeusedtoderiveaquantitativeorderof theelectroniceffects(Eo")of theligandsubstituentsin a seriesof uranylp-ketoenolates.Althoughthereissomeevidence[2]for electrondelocalizationi thechelateringin p-ketoenolates,it hasbeensuggested[3,4] thatthemetalion providesa barrierto completefreedomof movementof then-electrons.In o-hydroxyarylcarbonylchelatecom-plexesit hasbeenshown[5]thatthereis acertainamountof electronlocalizationin thechelatering,thusdecreasingthepossibilityof resonanceplayinga partinthestabilizationof thesecomplexes.Herewediscusstheapplicationof fieldandresonancesubstituentparametersin uranylo-hydroxyarylcarbonylcomplexes.Ligandsusedin this studyweresalicylaldehyde(SAL), ethyl salicylate(ESAL), o-hydroxyacetophenone(HAP), o-hydroxypropiophenone(HPP), methylsalicylate(MSAL) andphenylsalicylate(PSAL). Basesusedin this studywereH2O,pyridine(py)andpyridineN-oxide(pyNO).282Theelectroniceffectsof thechelateringsubstituentandtheadductedbaseon characteristicgroupfrequenciesof the complexeswill be discussed.ThesemoleculesareparticularlysuitedtoanIRinvestigationsincetheyareallisostoiochio-metricandtheU~Oasymmetricstretchingmode,v(U=O),canbeunambiguouslyassigned.EXPERIMENTALThemajorityof complexesdescribedherearenewlyreported.Aquo adductsr--The o-hydroxyary1carbonylcompound(0.0052mole)in methanol(10m!)is addedto uranylnitratehexahydrate(1 g, 0.0026mole)in methanol(40ml).The mixtureis vigorouslystirredand25%aqueousNaOH is addeddropwise.The complexprecipitates.The amorphousorangesolidis thenfiltered,washedwithwaterandairdried.Pyridine adductsUranyl nitratehexahydrate(lg , 0.0026mole)is dissolvedin methanol(40 ml) anda mixtureof the o-hydroxyary1carbonylcompound(0.0052mole)and pyridine(0.0026mole)is added.The mixtureis allowedto standat roomtemperature.After 24 h the red crystallineproductis filtered,washedwithmethanolandair dried.An alternativemethodinvolvesrecrystallizationof theaquoadductfromexcessof pyridine.Thecomplexesthusformedarefiltered,washedwithmethanolandair dried.Thelattermethodis to bepreferredwhereyieldsareof importance.,....PyridineN-oxideadductsA procedureanalogousto thatusedfor thepyridineadductsyieldsthepure,redcrystallineproducts.The additionof a littlemethanolis necessarytodissolvethepyridineN-oxide.InfraredspectraInfraredspectraweredeterminedonNujol mullsbetweencaesiumbromideplateson a BeckmanIR-12 spectrophotometercalibratedagainstwatervapourandcarbondioxide.For optimumreproducibilityfrequencieswerereadfromthewavenumberdrum,not the chartpaper.Duplicatespectra,determinedwithordinateandabscissascaleexpansionandagainwitha freshsampleof thecom-pound,yieldeda maximumdiscrepancyof 0.5cm-1in v(U~O).UraniumanalysesUraniumanalyseswereperformedashasbeenpreviouslydescribed[6]byignitionto 750-850°c for severalhoursandweighingtheresidueasU 308,-.. RESULTSAND DISCUSSIONRaman,IR andtheoreticalevidence[7-9]unanimouslysupportstheassign-mentof theintensebandnear900cm-1 in thespectraof a11uranylcompoundstotheasymmetricstretchingfrequencyof theuranylgroup.It hasbeenshown[1]thatin uranylf3-ketoenolatestwobandsoccurringat around528and403cm-1areprobablymetalsensitivemodes.In uranylo-hydroxyary1carbonylcomplexesthecomplexityof theIR spectrais muchgreaterthanthatof thecorrespondingf3-ketoenolates,andfor thisreasonno attemptis madehereto interprethefarIR spectra ndassignmetalsensitivemodes.A verystrongabsorptioncenteredat around270cm-1 hasrecentlybeenassigned[10]as v(U-O) betweenthe uraniumatomand the oxygenatomofpyridineN-oxidein a seriesof compoundsUOzLz . pyNO. This bandis presentina11thecomplexesdiscussedin thiswork,andisalsopresentinuranylf3-ketoenol-atecomplexes[11].Thisbandhaspreviouslybeenassigned[7],basedon theoret-ical treatments,to the bendingmodeof the uranylgroup,c5(O=U~O).Ourfindingsareconsistentwiththelatterassignment.In metalacetylacetonatestwo bandsareapparentin the 1500-1600""""cm-1 range.Earlierassignmentsof v(C~O)andv(C=C)by normalco-ordinateanalyses[12,13]havebeenreversedby a furtheranalysis[14],and by 180-labe11ingof thecarbonyloxygen[15].Thusthehigherbandis assignedto v(C~O)andthelowerto v(C=C).Theseassignmentsareupheldin a recentpaper[16]concerningfirst-rowtransitionmetal(III)f3-ketoenolateswhereit wasshownthatv(C~O)exhibitsadependenceontheCFSE thatisgenera11ysimilartothatexhibitedby v(M-O) andtheinverseof thatshownby v(C~C)on thebasisof thehigherfrequencybandbeingassignedto v(C=O).In thespectraof theuranylo-hydroxy-ary1carbonylcomplexestudiedhere,two bandsarealsoobservedbetween1500and1650em-1. On thebasisof theassignmentsof thesetwobandsin spectraofthef3-ketoenolatesdiscussedabove,thebandof higherfrequencyis assignedtov(C=O)andthelowerfrequencybandto v(C=C).Thatv(C~O)occursathigherfrequencyin theo-hydroxyary1carbonylcomplexesthanin thef3-ketoenolatesisevidencefor a greaterdegreeof electronlocalizationabouttheC=Obondin the284TABLE IANALYTICAL AND IR DATA FOR URANYL O-HYDROXYARYLCARBONYL BASE ADDUCT COMPLEXES [U02(LhB]formercomplexes.Evidentlyfusionof the benzenering with the chelateringleadsto considerablelocalizationof electronsin thechelatering systemwhichwouldappearto existessentiallyin theformrepresentedin formulaI..5=o,,~/o-oU-O/II\'o~0 \ RB(I)Supportfor theunsymmetricalstructureof thechelateringis providedbytheX-raycrystallographicexamination[17,18]ofbis(salicylaldehydato)c pper(II) ~and diaquobis(salicylaldehydato)nickel(II) in whichit wasdeterminedthattheC=Obond(in formulaI) is considerablyshorterthantheC-O bond.Any attempto correlategroupfrequencieswiththeelectroniceffectsof asubstituentrequiressomequantitativeindexof themodeof transmissionof theelectroniceffect.Hammettsubstituentparameters[19]cannotbe usedfor 0-hydroxyarylcarbonylcomplexesasthiswouldassumearomaticityof thechelatering.It hasbeenindicatedabovethatsincetheoxygenatomsof thechelateringarenon-equivalentandtheC~Obondshavedifferentlengths,it is unlikelythatthechelateringshouldbearomatic.Wethereforeusetherecentlyderived[20]purefield(~) andresonance(~) parameterswhichareindependentof thesiteof sub-stitution.Theelectroniceffect«(J)of theo-hydroxyarylcarbonylchelateringsub-stituent(R in formulaI) is givenbyeqn.(1)(J =f~ +r~ (I)Complex R in (1) Calculated(%) Found(%) v(U-O) v(CO)C H N U C H N U (cm-1) (cm-l)U02(HPPh . H2O Et 36.9 3.4 - 40.6 36.2 3.2 - 40.4 925 1611U02(HPPh . py Et 42.7 3.6 2.2 36.8 41.9 3.7 2.1 36.6 899 1607U02(HPPh . pyNO Et 41.6 3.5 2.1 35.9 41.3 3.6 2.2 36.2 890 1612U02(HAPh' H2O Me 34.4 2.9 - 42.6 34.7 2.7 - 42.2 925 1613U02(HAPh. py Me 40.7 3.1 2.3 38.4 40.7 2.7 2.4 38.5 900 1609U02(HAPh' pyNO Me 39.7 3.0 2.2 37.5 39.3 2.7 2.4 37.6 891 1614U02(SALh . H2O H 31.7 2.3 - 44.9 31.4 2.1 - 44.3 925 1627U02(SALh . py H 38.6 2.6 2.4 40.3 39.2 2.6 2.4 40.0 901 1614U02(SALh . pyNO H 37.6 2.5 2.3 39.2 37.2 2.6 2.2 39.5 895 1627U02(ESALh' H2O OEt 35.0 3.3 - 38.5 35.3 3.2 - 38.2 925 1639U02(ESALh . py OEt 40.7 3.4 2.1 35.0 40.6 3.4 2.1 34.9 907 1631U02(ESAL)2' pyNO OEt 39.7 3.3 2.0 34.2 39.7 3.1 2.0 34.9 900 1639U02(MSALh . py OMe 38.7 2.9 2.2 36.5 38.2 2.8 2.2 36.5 908 1630U02(pSALh . py OPh 48.0 3.0 1.8 30.7 48.4 2.8 1.6 30.4 915 1652where f and r ate the relativeweightingsof fF and [JJl respectively.It hasbeenshown[1]that,exceptfor phenylsubstituents,useoff = 0.75and r = 0.25yieldsvaluesof (J whichdecreasemonotonicallywith decreaseinv(U~O)in a seriesof uranylp-ketoenolates.Phenylsubstitutedp-ketoenolatesrequireanincreasedcontributionfrom[JJlto accountfor thecontributionof thevariousresonanceforms possible.In the caseof the o-hydroxyarylcarbonylcomplexes,theweightingof [JJlmustbeconsiderablydecreasedsince,theoretically,resonanceis highlyunlikelyin thechelateringbecauseof electronlocalization.Henceanytransmissionof electroniceffectsbythesubstituentmustbeconsideredTABLE 2----SUBSTITUENT DATA FOR URANYL O-HYDROXYARYLCARBONYL COMPLEXES1620/. ESAL/ 0 ESAL0 MSALjW /e HAP 0 SALeHPP I1610~/ 0 HAP0 HPP/ESAL x1""'E'"""" *.1630u:> SALxHAP xHPP x1600890 900 910 920vU=O!em-') ;;.-Fig. 1.Relationshipbetweenv(C~O)andv(U~O)for complexes[U02(LhBI. (e), B = pyNO,(0), B = py; (x), B = H2O.Ligand R ff fJ1l f r a v(UO) py adduct(cm- 1)HPP C2Hs -0.07 -0.11 0.75 0.25 -0.08 899HAP CH, -0.05 -0.14 0.75 0.25 -0.07 900SAL H 0.00 0.00 - - 0.00 901ESAL OC2Hs +0.36 -0.44 0.75 0.25 +0.16 907MSAL OCH, +0.41 -0.50 0.75 0.25 +0.19 908PSAL OC6Hs +0.75 -0.74 0.75 0.25 +0.54 915/0 PSAL1650286to takeplaceviaanalmostpurefieldmechanism.Substituentdataaresummarisedin Table2.As canbe seenfrom Fig. 1,v(U~O)andv(C=O)aresimultaneouslyin-creasedor decreasedby substitutionin the chelatering for the pyridineandpyridineN-oxidebaseadducts.In thecaseof the aquo adductshowever,sub-stitutionin thechelateringcausesa shiftof v(C=O),but v(U~O)remainscon-stantthroughoutheseries.For anyparticularo-hydroxyarylcarbonylcomplex,substitutionof pyridineN-oxidefor pyridinecausesa shiftof v(U~O)to lowerfrequencies,implying,onthebasisofanearliercommunication[21],thatpylidineN-oxidehasa higherligandfieldstrengththanpyridine,assumingU ~ pyridineand U ~ pyridineN-oxiden-bondingto be of a similarmagnitude.The more~polarnatureof theoxygenatomintheamineoxidescomparedtootheroxo-donorssuchas waterand the alcoholsmakesthe bondingby its lone pair electronsstronger.In thecaseof theaquoadductsit is of interestonotethatwhilstthereis asubstantialshiftofv(C=O)onchangingR (formulaI), v(U-O) remainsconstant.A possible xplanationis thatsinceH2OcannotenterintostrongU ~ H2On-bonding,theelectroniceffectof thering substituentmaywellbe confinedtothechelatering.In thecaseof pyridineand pyridineN-oxidehowever,the aro-maticheterocyclicring couldactasanelectronsink,thereforethepossibilityof0.80.70.60.50.4 ::)0.3 ~0.2 ~0.100 00-0.1/I/'0/°0///// ~I IOS'/°0.40.3'C02~~:;u0.1~.]b000 /-0.1~900-L90.5I . I910 915vU=O(em-I) ~Fig. 2. Relationshipbetweenv(U-O) and(J (calculated)andg; values.287transmissionof electroniceffectsthroughtheuraniumatombecomesapparent.If thisis so,pyridineandpyridineN-oxideadductswouldbeexpectedto showchangesin v(U~O)with changeof R (formulaI), whereasthe aquaadductswouldnot.Figure2 showsa plot of thecalculated(J values(assumingf = 0.75andr = 0.25)againstv(U~O)for thepyridineadductsof theo-hydroxyarylcarbonylcomplexes.Whilstit is apparenthatthereis a generaltrend,i.e.,electronwith-drawingsubstituentscausean increasein thebondorderof theU~O bond,itwouldappearthatuseoff = 1.00andr = 0.00(i.e.,pure:# values,Fig. 2) givesbetteragreementin termsof relativeelectroniceffect.It is interestingto notethatplotsof thisnatureutilizingtheHammett(J functionor puref!ltvaluesshowno-- trend.In thelightof thisargumentit wouldappearthattheelectroniceffectsofthe substituentR (formulaI) are transmittedby a purefield effect,electronlocalizationreducingthearomaticityof thechelatering to a minimum.On thesamebasis,it hasbeenargued[1]that,in uranylfi-ketoenolates,thechelateringdoescomprisea certaindegreeof aromaticity.In an attempto duplicatetheresultsfoundfor theuranylfi-ketoenolatesystem[1],thesynthesisof a seriesof substitutedpyridineadductsof theuranylsalicylaldehydecomplexwas attempted.The compoundswerepreparedin ananalogousmannerto thepyridineadduct.It wasfoundthatovertherangefromstronglyelectronwithdrawing(4-CNCsH4N) to stronglyelectronreleasing(4-NHzCsH4N) substituents,v(U~O)and v(C~O)shiftedover2 and 4 cm-lrespectively(Table3).Substitutionof thepyridineringthereforehasa negligibleeffectonthestrengthof U -* pyridinen-bondingin thisseriesofcomplexeswherethechelateringis non-aromatic,indicatingessentiallyno transmissionof theseeffectsthroughtheuraniumatom.* Pyridineswith substituentsuchas-CN and-NHz areboundto uraniumthroughthehetero-cyclicnitrogen,sincethecharacteristicgroupfrequenciesof thesubstituentin thecomplexesarecloseto thoseof thefreepyridines.TABLE 3--..R AND SUBSTITUENTDATA FOR COMPLEXES[UOz(SAL),(R-CsH4N)I*R Hammett (j v(UO) v(CO)4-CN +0.63 902 16124-COC6Hs +0.46 901 16124-COzCzHs +0.40 902 16134-C6Hs +0.01 902 1612H 0.00 901 16144-CzHs -0.15 902 16144-CH3 -0.17 901 16164-NHz -0.60 902 16144-N(CH3), -0.66 900 1616288ACKNOWLEDGMENTSThis workwassupportedby Universityof CapeTown,RhodesUniversityandC.S.I.R. researchgrants.WethanktheUniversityof CapeTownfor theuseof theIR spectrophotometer.REFERENCES1 J. M. Haigh andD. A. Thornton, J. Mol. Struct.,8 (1971)351.2 J. P. Collmann,Reactionsof Co-ordinatingLigands,AmericanChemicalSociety,Washington,1963,p. 78.3 D. W. Barnam,J. Inorg.Nucl. Chern.,22 (1961)183.4 J. P. FacklerandF. A. Cotton, Inorg. Chern.,2 (1963)102.5 D. P. GraddonandG. M. Mockler,Austr.J. Chern.,20 (1967)21.6 A. E. Comyns,B. M. GatehouseandE. Wait, J. Chern.Soc., (1958)4655.7 G. K. T. Conn andC. K. Wu, Trans.FaradaySoc.,34 (1938)1483.8 H. D. Bist, J. Mol. Spectrosc.,16(1968)542.9 K. Nakamoto,Y. Morimoto andA. E. Martell, J. Arner.Chern.Soc.,83 (1961)4533.10 M. S. SubramanianandV. K. Manchanda,J. Inorg.Nucl. Chern.,33 (1971)3001.11 J. M. Haigh,Ph.D. Thesis,Universityof CapeTown, 1970.12 K. NakamotoandA. E. Martell, J. Chern.Phys.,32 (1960)588.13 M. Mikami, J. NakagawaandT. Shimanouchi,Spectrochirn.Acta, Ser.A, 23 (1967)1037.14 G. T. BehnkeandK. Nakamoto,Inorg. Chern.,6 (1967)433.15 S. Pinchas,B. L. Silverand1.Laulicht, J. Chern.Phys.,46 (1967)1506.16 R. D. HancockandD. A. Thornton,J. Mol. Struct.,4 (1969)361.17 A. J. McKinnon, T. N. WatersandD. Hall, J. Chern.Soc., (1964)3290.18 J. M. Stewart,E. C. LingafelterandJ. D. Breazeale,Acta Crystallogr.,14(1961)888.19 H. H. Jaffe, Chern.Rev.,53 (1953)191.20 C. G. SwainandE. C. Lupton, J. Arner.Chern.Soc.,90 (1968)4328.21 S. P. McGlynn, J. K. SmithandW. C. Neely,J. Chern.Phys.,35(1961)105./'

Ligand substitution effects in uranyl ο-hydroxyarylcarbonyl complexes

Rhodes Repository (SEALS)

Journal of Molecular Structure, 23 (1974) 281-288~ Elsevier Scientific Publishing Company, Amsterdam - Printed in The NetherlandsLIGAND SUBSTITUTION EFFECTS IN URANYL o-HYDROXYARYL-CARBONYL COMPLEXESJ. M. HAIGHDepartment of Pharmaceutical Sciences, Rhodes University, Grahamstown (South Africa)(Received 21 December 1973)ABSTRACTTwenty-two base adducts of uranyl o-hydroxyarylcarbonyl complexes[U02L2B] (L = o-hydroxyarylcarbonyl compound, B = H2O, pyridine, pyri-dine N-oxide) have been prepared. Pure field substituent parameters are used toderive a quantitative order of the electronic effects of the chelate ring substituents.Values of v(U=O) correlate well with these values. Evidence is cited suggestingalmost complete non-aromaticity of the chelate ring, and the transmission of theelectronic effects of the chelate ring substituent through the uranium atom in thepyridine and pyridine N-oxide base adducts.INTRODUCTIONIt has recently been shown [1] that pure field and resonance substituentparameters can be used to derive a quantitative order of the electronic effects (Eo")of the ligand substituents in a series of uranyl p-ketoenolates. Although there issome evidence [2] for electron delocalization in the chelate ring in p-ketoenolates,it has been suggested [3, 4] that the metal ion provides a barrier to completefreedom of movement of the n-electrons. In o-hydroxyarylcarbonyl chelate com-plexes it has been shown [5] that there is a certain amount of electron localizationin the chelate ring, thus decreasing the possibility of resonance playing a part inthe stabilization of these complexes. Here we discuss the application of field andresonance substituent parameters in uranyl o-hydroxyarylcarbonyl complexes.Ligands used in this study were salicylaldehyde (SAL), ethyl salicylate(ESAL), o-hydroxyacetophenone (HAP), o-hydroxypropiophenone (HPP), methylsalicylate (MSAL) and phenyl salicylate (PSAL). Bases used in this study wereH2O, pyridine (py) and pyridine N-oxide (pyNO).282The electronic effects of the chelate ring substituent and the adducted baseon characteristic group frequencies of the complexes will be discussed. Thesemolecules are particularly suited to an IRinvestigation since they are all isostoiochio-metric and the U~O asymmetric stretching mode, v(U=O), can be unambiguouslyassigned.EXPERIMENTALThe majority of complexes described here are newly reported.Aquo adductsr--The o-hydroxyary1carbonyl compound (0.0052 mole) in methanol (10 m!)is added to uranyl nitrate hexahydrate (1 g, 0.0026 mole) in methanol (40 ml).The mixture is vigorously stirred and 25 % aqueous NaOH is added dropwise.The complex precipitates. The amorphous orange solid is then filtered, washedwith water and air dried.Pyridine adductsUranyl nitrate hexahydrate (lg , 0.0026 mole) is dissolved in methanol(40 ml) and a mixture of the o-hydroxyary1carbonyl compound (0.0052 mole)and pyridine (0.0026 mole) is added. The mixture is allowed to stand at roomtemperature. After 24 h the red crystalline product is filtered, washed withmethanol and air dried.An alternative method involves recrystallization of the aquo adduct fromexcess of pyridine. The complexes thus formed are filtered, washed with methanoland air dried.The latter method is to be preferred where yields are of importance.,....Pyridine N-oxide adductsA procedure analogous to that used for the pyridine adducts yields the pure,red crystalline products. The addition of a little methanol is necessary to dissolvethe pyridine N-oxide.Infrared spectraInfrared spectra were determined on Nujol mulls between caesium bromideplates on a Beckman IR-12 spectrophotometer calibrated against water vapourand carbon dioxide. For optimum reproducibility frequencies were read from thewavenumber drum, not the chart paper. Duplicate spectra, determined withordinate and abscissa scale expansion and again with a fresh sample of the com-pound, yielded a maximum discrepancy of 0.5 cm-1 in v(U~O).Uranium analysesUranium analyses were performed as has been previously described [6] byignition to 750-850 °c for several hours and weighing the residue as U 308,-.. RESULTS AND DISCUSSIONRaman, IR and theoretical evidence [7-9] unanimously supports the assign-ment of the intense band near 900 cm-1 in the spectra of a11uranyl compounds tothe asymmetric stretching frequency of the uranyl group. It has been shown [1]that in uranyl f3-ketoenolates two bands occurring at around 528 and 403 cm-1are probably metal sensitive modes. In uranyl o-hydroxyary1carbonyl complexesthe complexity of the IR spectra is much greater than that of the correspondingf3-ketoenolates, and for this reason no attempt is made here to interpret the farIR spectra and assign metal sensitive modes.A very strong absorption centered at around 270 cm-1 has recently beenassigned [10] as v(U-O) between the uranium atom and the oxygen atom ofpyridine N-oxide in a series of compounds UOzLz . pyNO. This band is presentin a11the complexes discussed in this work, and is also present in uranyl f3-ketoenol-ate complexes [11]. This band has previously been assigned [7], based on theoret-ical treatments, to the bending mode of the uranyl group, c5(O=U~O). Ourfindings are consistent with the latter assignment.In metal acetylacetonates two bands are apparent in the 1500-1600""""cm-1 range. Earlier assignments of v(C~O) and v(C=C) by normal co-ordinateanalyses [12, 13] have been reversed by a further analysis [14], and by 180-labe11ingof the carbonyl oxygen [15]. Thus the higher band is assigned to v(C~O)and the lower to v(C=C). These assignments are upheld in a recent paper [16]concerning first-row transition metal(III) f3-ketoenolates where it was shown thatv(C~O) exhibits a dependence on the CFSE that is genera11ysimilar to that exhibitedby v(M-O) and the inverse of that shown by v(C~C) on the basis of the higherfrequency band being assigned to v(C=O). In the spectra of the uranyl o-hydroxy-ary1carbonyl complexes studied here, two bands are also observed between1500and 1650 em-1. On the basis of the assignments of these two bands in spectra ofthe f3-ketoenolates discussed above, the band of higher frequency is assigned tov(C=O) and the lower frequency band to v(C=C). That v(C~O) occurs at higherfrequency in the o-hydroxyary1carbonyl complexes than in the f3-ketoenolates isevidence for a greater degree of electron localization about the C=O bond in the284TABLE IANALYTICAL AND IR DATA FOR URANYL O-HYDROXYARYLCARBONYL BASE ADDUCT COMPLEXES [U02(LhB]former complexes. Evidently fusion of the benzene ring with the chelate ringleads to considerable localization of electrons in the chelate ring system whichwould appear to exist essentially in the form represented in formula I..5=o,,~/o-oU-O/II\'o~0 \ RB(I)Support for the unsymmetrical structure of the chelate ring is provided bythe X-ray crystallographic examination [17, 18] of bis( salicylaldehydato )copper(II) ~and diaquobis(salicylaldehydato )nickel(II) in which it was determined that theC=O bond (in formula I) is considerably shorter than the C-O bond.Any attempt to correlate group frequencies with the electronic effects of asubstituent requires some quantitative index of the mode of transmission of theelectronic effect. Hammett substituent parameters [19] cannot be used for 0-hydroxyarylcarbonyl complexes as this would assume aromaticity of the chelatering. It has been indicated above that since the oxygen atoms of the chelate ringare non-equivalent and the C~O bonds have different lengths, it is unlikely thatthe chelate ring should be aromatic. We therefore use the recently derived [20] purefield (~) and resonance (~) parameters which are independent of the site of sub-stitution. The electronic effect «(J)of the o-hydroxyarylcarbonyl chelate ring sub-stituent (R in formula I) is given by eqn. (1)(J =f~ +r~ (I)Complex R in (1) Calculated (%) Found (%) v(U-O) v(CO)C H N U C H N U (cm-1) (cm-l)U02(HPPh . H2O Et 36.9 3.4 - 40.6 36.2 3.2 - 40.4 925 1611U02(HPPh . py Et 42.7 3.6 2.2 36.8 41.9 3.7 2.1 36.6 899 1607U02(HPPh . pyNO Et 41.6 3.5 2.1 35.9 41.3 3.6 2.2 36.2 890 1612U02(HAPh' H2O Me 34.4 2.9 - 42.6 34.7 2.7 - 42.2 925 1613U02(HAPh. py Me 40.7 3.1 2.3 38.4 40.7 2.7 2.4 38.5 900 1609U02(HAPh' pyNO Me 39.7 3.0 2.2 37.5 39.3 2.7 2.4 37.6 891 1614U02(SALh . H2O H 31.7 2.3 - 44.9 31.4 2.1 - 44.3 925 1627U02(SALh . py H 38.6 2.6 2.4 40.3 39.2 2.6 2.4 40.0 901 1614U02(SALh . pyNO H 37.6 2.5 2.3 39.2 37.2 2.6 2.2 39.5 895 1627U02(ESALh' H2O OEt 35.0 3.3 - 38.5 35.3 3.2 - 38.2 925 1639U02(ESALh . py OEt 40.7 3.4 2.1 35.0 40.6 3.4 2.1 34.9 907 1631U02(ESAL)2' pyNO OEt 39.7 3.3 2.0 34.2 39.7 3.1 2.0 34.9 900 1639U02(MSALh . py OMe 38.7 2.9 2.2 36.5 38.2 2.8 2.2 36.5 908 1630U02(pSALh . py OPh 48.0 3.0 1.8 30.7 48.4 2.8 1.6 30.4 915 1652where f and r ate the relative weightings of fF and [JJl respectively.It has been shown [1] that, except for phenyl substituents, use off = 0.75and r = 0.25 yields values of (J which decrease monotonically with decrease inv(U~O) in a series of uranyl p-ketoenolates. Phenyl substituted p-ketoenolatesrequire an increased contribution from [JJlto account for the contribution of thevarious resonance forms possible. In the case of the o-hydroxyarylcarbonylcomplexes, the weighting of [JJlmust be considerably decreased since, theoretically,resonance is highly unlikely in the chelate ring because of electron localization.Hence any transmission of electronic effects by the substituent must be consideredTABLE 2----SUBSTITUENT DATA FOR URANYL O-HYDROXYARYLCARBONYL COMPLEXES1620/. ESAL/ 0 ESAL0 MSALjW /e HAP 0 SALeHPP I1610~/ 0 HAP0 HPP/ESAL x1""'E'"""" *.1630u:> SALxHAP xHPP x1600890 900 910 920vU=O !em-') ;;.-Fig. 1. Relationship between v(C~O) and v(U~O) for complexes [U02(LhBI. (e), B = pyNO,(0), B = py; (x), B = H2O.Ligand R ff fJ1l f r a v(UO) py adduct(cm - 1)HPP C2Hs -0.07 -0.11 0.75 0.25 -0.08 899HAP CH, -0.05 -0.14 0.75 0.25 -0.07 900SAL H 0.00 0.00 - - 0.00 901ESAL OC2Hs +0.36 -0.44 0.75 0.25 +0.16 907MSAL OCH, +0.41 -0.50 0.75 0.25 +0.19 908PSAL OC6Hs +0.75 -0.74 0.75 0.25 +0.54 915/0 PSAL1650286to take place via an almost pure field mechanism. Substituent data are summarisedin Table 2.As can be seen from Fig. 1, v(U~O) and v(C=O) are simultaneously in-creased or decreased by substitution in the chelate ring for the pyridine andpyridine N-oxide base adducts. In the case of the aquo adducts however, sub-stitution in the chelate ring causes a shift of v(C=O), but v(U~O) remains con-stant throughout the series. For any particular o-hydroxyarylcarbonyl complex,substitution of pyridine N-oxide for pyridine causes a shift of v(U~O) to lowerfrequencies, implying, on the basis of an earlier communication [21], that pylidineN-oxide has a higher ligand field strength than pyridine, assuming U ~ pyridineand U ~ pyridine N-oxide n-bonding to be of a similar magnitude. The more ~polar nature of the oxygen atom in the amine oxides compared to other oxo-donorssuch as water and the alcohols makes the bonding by its lone pair electronsstronger.In the case of the aquo adducts it is of interest to note that whilst there is asubstantial shift ofv(C=O) on changing R (formula I), v(U-O) remains constant.A possible explanation is that since H2O cannot enter into strong U ~ H2On-bonding, the electronic effect of the ring substituent may well be confined tothe chelate ring. In the case of pyridine and pyridine N-oxide however, the aro-matic heterocyclic ring could act as an electron sink, therefore the possibility of0.80.70.60.50.4 ::)0.3 ~0.2 ~0.100 00-0.1/I/'0/°0///// ~I IOS'/°0.40.3'C02~ ~:;u0.1~.]b000 /-0.1~900-L90.5I . I910 915vU=O(em-I) ~Fig. 2. Relationship between v(U-O) and (J (calculated) and g; values.287transmission of electronic effects through the uranium atom becomes apparent.If this is so, pyridine and pyridine N-oxide adducts would be expected to showchanges in v(U~O) with change of R (formula I), whereas the aqua adductswould not.Figure 2 shows a plot of the calculated (J values (assuming f = 0.75 andr = 0.25) against v(U~O) for the pyridine adducts of the o-hydroxyarylcarbonylcomplexes. Whilst it is apparent that there is a general trend, i.e., electron with-drawing substituents cause an increase in the bond order of the U~O bond, itwould appear that use off = 1.00 and r = 0.00 (i.e., pure :# values, Fig. 2) givesbetter agreement in terms of relative electronic effect. It is interesting to note thatplots of this nature utilizing the Hammett (Jfunction or pure f!lt values show no-- trend.In the light of this argument it would appear that the electronic effects ofthe substituent R (formula I) are transmitted by a pure field effect, electronlocalization reducing the aromaticity of the chelate ring to a minimum. On thesame basis, it has been argued [1] that, in uranyl fi-ketoenolates, the chelate ringdoes comprise a certain degree of aromaticity.In an attempt to duplicate the results found for the uranyl fi-ketoenolatesystem [1], the synthesis of a series of substituted pyridine adducts of the uranylsalicylaldehyde complex was attempted. The compounds were prepared in ananalogous manner to the pyridine adduct. It was found that over the range fromstrongly electron withdrawing (4-CNCsH4N) to strongly electron releasing(4-NHzCsH4N) substituents, v(U~O) and v(C~O) shifted over 2 and 4 cm-lrespectively (Table 3). Substitution of the pyridine ring therefore has a negligibleeffect on the strength of U -* pyridine n-bonding in this series of complexes wherethe chelate ring is non-aromatic, indicating essentially no transmission of theseeffects through the uranium atom.* Pyridines with substituents such as -CN and -NHz are bound to uranium through the hetero-cyclic nitrogen, since the characteristic group frequencies of the substituent in the complexes areclose to those of the free pyridines.TABLE 3--..R AND SUBSTITUENTDATA FOR COMPLEXES[UOz(SAL),(R-CsH4N)I*R Hammett (j v(UO) v(CO)4-CN +0.63 902 16124-COC6Hs +0.46 901 16124-COzCzHs +0.40 902 16134-C6Hs +0.01 902 1612H 0.00 901 16144-CzHs -0.15 902 16144-CH3 -0.17 901 16164-NHz -0.60 902 16144-N(CH3), -0.66 900 1616288ACKNOWLEDGMENTSThis work was supported by University of Cape Town, Rhodes Universityand C.S.I.R. research grants. We thank the University of Cape Town for the useof the IR spectrophotometer.REFERENCES1 J. M. Haigh and D. A. Thornton, J. Mol. Struct., 8 (1971) 351.2 J. P. Collmann, Reactions of Co-ordinating Ligands, American Chemical Society, Washington,1963, p. 78.3 D. W. Barnam, J. Inorg. Nucl. Chern., 22 (1961) 183.4 J. P. Fackler and F. A. Cotton, Inorg. Chern., 2 (1963) 102.5 D. P. Graddon and G. M. Mockler, Austr. J. Chern., 20 (1967) 21.6 A. E. Comyns, B. M. Gatehouse and E. Wait, J. Chern. Soc., (1958) 4655.7 G. K. T. Conn and C. K. Wu, Trans. Faraday Soc., 34 (1938) 1483.8 H. D. Bist, J. Mol. Spectrosc., 16 (1968) 542.9 K. Nakamoto, Y. Morimoto and A. E. Martell, J. Arner. Chern. Soc., 83 (1961) 4533.10 M. S. Subramanian and V. K. Manchanda, J. Inorg. Nucl. Chern., 33 (1971) 3001.11 J. M. Haigh, Ph.D. Thesis, University of Cape Town, 1970.12 K. Nakamoto and A. E. Martell, J. Chern. Phys., 32 (1960) 588.13 M. Mikami, J. Nakagawa and T. Shimanouchi, Spectrochirn. Acta, Ser. A, 23 (1967) 1037.14 G. T. Behnke and K. Nakamoto, Inorg. Chern., 6 (1967) 433.15 S. Pinchas, B. L. Silver and 1. Laulicht, J. Chern. Phys., 46 (1967) 1506.16 R. D. Hancock and D. A. Thornton, J. Mol. Struct., 4 (1969) 361.17 A. J. McKinnon, T. N. Waters and D. Hall, J. Chern. Soc., (1964) 3290.18 J. M. Stewart, E. C. Lingafelter and J. D. Breazeale, Acta Crystallogr., 14 (1961) 888.19 H. H. Jaffe, Chern. Rev., 53 (1953) 191.20 C. G. Swain and E. C. Lupton, J. Arner. Chern. Soc., 90 (1968) 4328.21 S. P. McGlynn, J. K. Smith and W. C. Neely, J. Chern. Phys., 35 (1961) 105./'

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Ligand substitution effects in uranyl ο-hydroxyarylcarbonyl complexes

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