Chakravarty, B.

Sil, A. K.

English

NOPR

Indian "Pumal of ChemistryVol. 15..1,June 1977,pp. 502-505MechanisticStudieson the Reactionof cis-Diaquobisbiguanide-cobalt(III) Ion & Phenylbiguanidein AqueousAcidicSolution: Formationof Mixed Chelateof Cobalt(III)B. CHAKRA VARTY & A. K. SILDepartmentof Chemistry, Kalyani University, Kalyani, West Bengal 741235Received 30 August 1976; accepted 29 November 1976Theformationofmixedchelatecomplex,bis-biguanidemonophenylbiguanidecoba1t(III)fromdiaquobisbi~uanidecoba1t(III)and its deprotonatedform hydroxoaquobisbi~uanidecobalt(III)withdifferentprotonatedformsof phenylbi~uanidehasbeenstudiedspectrophotometricallyinthepH range1·5to6·0. Onlyoneformoftheligand,themonoprotonatedphenylbi~uanide,takespartin theformationreaction.Therateconstants,kcorrandk~orrfor diaquo-andhydroxoaquo-formsrespectivelyofthecomplexcation,arederivedfromkobsandfractionsofcomplexandligand(nucleophile)species.Secondorderrateconstants,k1andk2, havebeenderivedfromtheequa-tionskcorr=kl[PhBi~H]totalandk~orr=k2[PhBi~H]total.Thehydroxoaquo-formofthecomplexis 4-5timesmorereactivethanthediaquo-formandthereactionis entropy-controlledas theenthalpiesofthereactionswithbothformsarealmostequal. Ion-pairformationiscompletelyab-sentandmultiplehydro~enbondin~seemstoberesponsiblefortheassociationsofthereactants.Si~nificantbondformationtakesplacebeforebondbreaka~eis completein thetransitionstate.R 'ACTIONS involving replacementof aquoligandsin cobalt(III) complexesby variousanions have been reported by differentauth rsl-9,17.Suchreactionsusuallyproceedthroughion-p ir formation. With neutral species (e.g.oxali acid)formationof ion-pairis untenableandsomesort of hydrogenbondingis responsibleforthe eaction1.In the presentinvestigation,be-causeof like chargeson the reactingspecies,ion-pair ormationis doubtfuland it is interestingtostud whichfactorsareresponsiblefor the process.The resentstudywasmadEovera wide rangeofpH ( ,5 to 6·5)to distinguishbetweenthe reactionsof v rious complexand ligandspecies.BelowpHca1· aquationand abovepH ca8 basehydrolysisof te mixedchelatecomplexbecomeappreciableand tudiesin theserangeswereleft out in thisinve. igation.Mat rials and Methodscis Diaquobisbiguanidecobalt(III)perchloratewasprepred as follows: Hydroxoaquobisbiguanide-cobat(III) sulphatewas preparedfollowingRayand hosh10.It crystallizedoutwith two and halfmoleules of water (Found: Co, 13·64;N, 31·21.Req ired: Co, 13-50;N, 32·03%).T perchloratesalt was preparedin solution,whe ever requiredby the metathesisof the com-plex sulphatewith bariumperchloratein solution.Belo pH 4·0thecomplexion (pK 6·0)existsalmostenti'ly in the diaquo-form.Bi -biguanidemonophenylbiguanidecobalt(III)per-chlo te waspreparedin solutionby themetathesisof c mplexsulphatewith bariumperchlorate.Thecomlex sulphatewas preparedfollowing Duttaand arkarI1 (Found:Co, 7,94;N, 34·81.Required:Co, 8,01; N, 36·20%).502Phenylbiguanidehydrochloridewas preparedasdescribsd by Smolka and Friedrich12• It wastwice recrystallizedbefore use.All other reagentswereeitherof reagentgradeor elsewerepurifiedby suitablemethods. Conduc-tivity waterwas used throughoutfor makingthesolutions.Determinationof thepK of the comPlexequilibria- It wasdeterminedgraphicallyfrom thepH titra-tionof a0·025M hydroxoaquocomplex(thiscomplexion is very little solublein water and almost asaturatedsolutionwas used)with O·lM perchloricacid. The pH at the half-neutralizationpoint isthe requiredpK value.Absorbance measurements- A Beckman DU-2spectrophotometerwasusedfor absorbancemeasure-ments. The absorptionspectraof the diaquobis-biguanidecomplexion, in its protonatedor depro-tonatedforms,are identical. It exhibitedabsorp-tion maximaat 490nm (EM 125)and the productcomplexion at 480nm (EM 178). The spectraofa 1X 10-3M solutionof the complexion in the pre-senceof an excessof thechelatingagentat differentpH, in the range1·5to 6,5,changeswith time atvarious rates and ultimately becomesidenticalwith the spectraof bis-biguanidemonophenylbigua-nide complexion. (Actuallya smalldifferencewasalways observed,Efio being 175.) Kinetic runswerecarriedout in stopperedsilica cellsplacedina thermostated chamberwhere the temperaturewasmaintainedto within +0.10 of thedesiredvalue.Ionic strengths(IL) of the;olutionsweremaintainedby adding sodium perchlorate.The rates of formationof the mixed chelatecomplexwereconvenientlyfollowedin self-bufferedsolutions(pH adjustedwith sodiumhydroxideorperchloricacid) containingexcessof the reagent--_.----.-._---_._-----_._------_._---------------~-~-----------..,CHAKRAVARTY & SIL: KINETICS OF FORMATION OF MIXED CHELATE OF Co(II1)(phenylbiguanide) by measuring the change inabsorbanceof the solutions with time at 480 nm.The pseudofirst-order rate constantswereevaluatedgraphically by the usual method of plottinglog (At,-A)/ta(At,-Ata) vs (t2-tI), At" At, and Atocbeing absorbanceat time tv t2 and after attainmentof equilibrium.Resultsand DiscussionThe pK value for the reaction (1) was determinedat various temperatures(28-35°) and 11. (0'025-0'52\11)and found to be 5·93±0·07. It was almost indepen-dent of temperature and ionic strength. BelowpH 4, the complex ion exists almost entirely as thediaquo species (I) and betweenpH 4 and 6 boththe protonated (I) and the deprotonated(II) formsexist simultaneously.The ioni,,';ationconstants of the diprotonatedand the monoprotonated ligands (phenylbiguanide)were determined according to Ray et aP3 at 32°as pKa,=1O'7 and PKa, =2·2. Our determinationofpKa, at 28° and 22° givesthe values as 2·25 and 2·30respectively.In the experimentalpH range, only two forms ofthe substituting reagent, the monoprotonatedandthe diprotonated species were present, with com-plete absenceof any deprotonatedform.Considering all the forms of the complex andligand species which exist between pH 1·5 andpH 6·5, the overall reaction may be representedasshown in Scheme 1.The pH rangesin which diprotonated ligand anddeprotonated complex exist are far apart, therebyruling out the possibility of their interreaction.Ion-pair formation is also eliminated because oflike chargeson each reacting species. It was laterverified by the plots of kobsvs total [ligand] underdifferentconditions,all of which werelinear indicat-ing no ion-pair formation.kobsvalues were derived from the initial slopesof the conventional first-order plots to avoid thecomplications of any pH drift. The plots of rateconstant (kobs)versus [nucleophile]at a given pH,(4 and temperatureWerelinear with zero intercept.Consequently there is no reagent independentpath or at least it should be extremelyslow. More-over, it also eliminates the possibility of any ion-pair formation, where a non-linear relationshipbetween the rate and [nucleophile] exists. Ratelaw which holds good up to pH ",4,whereboth thek,Co(BigHh(HPJl++PhBigH;+->-Co(BigH)2(PhBigH)'+slowKa,J~ ... (2)k,Co(BigH)2(H20)~++ PhBigH~~Co(BigH)2(PhBigH)3+K J~ ... (3)k,Co(BigH)2(OH)H202++PhBigH~->-Co(BigH)2(PhBigH)~... (4)(BigH= biguanide,PhBigH= phenylbiguanide)Scheme1protonated forms of the ligand are present, may bederived asRate= kl[Co(BigHh(H20):+][PhBigH~]+k.[Co(BigH)2(H20)~+][PhBigH~+]Hence kobs= kl[PhBigH~]+ k.[PhBigH~+] ...(5)Table 1 records the observed rates at constant (L.and [nucleophile] up to pH 3·82 at two differenttemperatures. In this pH range the complexexists entirely as the diaquo species. If 'j'be thefraction of the ligand which exists in the mono-protonated form at any specifiedpH, then kObs/f--kcorrdenotesthe corrected first-order rate constantfor the speciesinvolved in reaction (3). The relativeconstancyof kcorrindicatesthat only monoprotonat-ed species is reactive enough to be observableunder this condition. The diprotonated form ispresent as a non-reactive species due to strongelectrostaticrepulsiveforcesbetweenthe like chargeson the reacting species. Eq. (5), after replacingthe non-reactingterm and kobsby kcorr,simplifiestokcorr=kl [nuc1eophile]total ... (6)ki being the second-order rate constant for thecomplex formation with the diaquo species. k1values at various temperaturesare given in Table 2.kl values have also been derived in another way.Dependenceof rate constant, kobson the fractionof nucleophile in monoprotonatedform in the pHrange 1 to 3,8is shown in Fig. 1 at various tempera-tures. The second-orderrate constants have beenderivedby extrapolating theplots to l1l! nuc1eophileTABLE1- RATEOFFORMATIONOFCo(BigH)2(PhBigH)3+BYTHE REACTIONOF Co(BigHMHPH+ AND PhBigH~ATVARIOUSACIDITIESANDTEMPERATURES{fl=0'5;[complex]=0·001M;[PhBigH]=0·020M}pH 102kobsf102kc rrmin-1min-1TEMP.28°1·50,36·15NO2·201-530887'980473 09,64TEMP.32°2 023 6721TABLE2- SECOND-ORDERRATECONSTANTSFORTHEFORMATIONREACTIONS(fl=0'5M)Temp.1 2k110lk2°Csec-1sec-1221'385,602785-93(2'50)282· 08325353 2·7503INDtAN J. CIIEM'f VOL. 15A,JUNE 19774·0•.....ICE 3·0~ui.0o(I/" 2·0Q1·002 06 '·0+FRACTION OF (PhBigH2)0·2 0·6 1-0FRACTION OF COMPLEXIN HYDROXOAQUO FORMFig. 1 Dependenceof kobson the fractionof the nucleophilein the monoprotonatedformFig. 2 - Dependenceof rate observed for hydroxoaquospecieson thefractionof thecomplexin thedeprotonatedformconce tration, and dividing the rate thus ob-taine by the fraction of PhBigH which is in themono rotonated form at the particular pH. Thevalue thus obtained are also included in Table 2.The econd-order rate constants, kI arrived byeithe method are similar. At pH 4 and above,both forms of the complex cation, I and II, areprese t together. kobsvalues above pH 4 are madeof tw terms due to two reacting complex species.Their contributions being proportional to theirfracti ns present at the specified pH. If 11 and12are the fractions of diaquo- (I) and hydroxoaquo-(II) pecies at any time, then kobs=keorrxll-1-k~orr ,where k~orris the correctedfirst-order rate forthe r ction of the species(II). 11>12weredetermin-ed fr ill the pK value of reaction (1). 12h~orrvalueswere btained by subtracting l1keorrfrom kobsandare r corded in Table 3. A plot of rate due tohydrxoaquo species versus 12 (Fig. 2) gives l<~orrat 12 1. The second-order rate constant, k2, forthe r action of hydroxoaquo complex is given byEq. ( )..,(7)k2 vajlues at various temperatures are also gIVenin Table 2. The hydroxoaquo complex is muchmore (4-5) times reactive than the diaquo complex,owing to the well-known labilizing influence ofOH- adjacent to H20.In reactions (3) and (4), no intermediate withmonodentate phenylbiguanide and with one aquoor hydroxyl ligand could be detected on an ion-exchange column or by spectral method. Thisintermediate species must be sufficiently reactiveto ring closureto avoid its detectionby any means.Similar fast ring closing steps have been observedin the anation of Coen2(H20W(en=ethylenediamine)and oxalic acid or oxalate ion in acidic aqueoussolutionl. This may be correlated with the factthat monodentatebiguanide or substituted bigua-nide complexesareunknown. Any attemptat theirpreparation, lea(1sto ring closure. In reaction (4),H20 is replacedbefore OH- is based on the reacti-vity of thesetwo ligandson coba1t(III) pentamminesand also on the fact that while few complexesof the type !Co(BigH)2(OH)XJn+ are knownl3,[Co(BigH)2(H20)X](n+1J+complex ion would havebeen so unstable that not a single one of the latertype could be isolated.The activation parameters corresponding tosecond-orderreaction rates, kI and k2, were evaluat-ed by the usual Eyring plot. The !1Ht and !1Stvalues for these and few other similar reactionsareTABLE3- RATEOFFORMATIONOFCo(BigH)2(PhBigH)3+BY THEREACTIONOFCo(BigH)2(OH)(H20)"+AND(PhBigH)iATVARIOUSACIDITIESA.NDTEMPERATURES{Complex]=1xl0-3M; [PhBigH]2xl0-"M; [L=0'5M}. ,TernpH10"kobs1110"/ keorrI"o"12kc rrCmin-1rnin-1mi323·823,50,99·92·01,58324 2068 00 875,56·7295713 8' 606744-19'78 653641' 3288214'8 1376, 56 3- 4504--~----..... --IReferences1. BROWN, P. M. & HARRIS, G. M., Inorg. Chern.,7 (1968),1872.2. ELDIK, R. V. & HARRIS, G. M., Inorg.Chern.,14(1975),10.3. SWADDLE, T. W. & GUASTALLA, G., Inorg. Chern.,8(1969),1604.4. CHEFFEE, E., DASGUPTA, T. P. & HARRIS, G. M., J. Arn.chern.Soc.,95 (1973),4167..5. JOUBERT, P. R. & VAN ELDIK, R., Inorg. chirn..Acta,14 (1975),259; J. inorg.nucl. Chern.,37 (1975),1817.6. HAGUE, D. N. & HALPERN, ]., Inorg.Chern.,6 (1967),2059.7. SAMUS, N. M. & LUKYANETS, T. S., Z. neorg. Khim.20(4) (1975),1010.8. BANERJEA, D. & SENGUPTA, S., Z. anorg. Chern.,397(1973),215.9. BANERJEA, D. & RoY,]., Z. anorg.Chem.,399(1973),215.10. RAY, P. & GHOSH, S. P., J. Indian chern.Soc.,19 (1942),1-11. DUTTA, R. L. & SARKAR, S., J. Indian chern.Soc., 44(1967),842.12. SMOLKA, T. & FRIODRICH W., Mh. Chern.,9 (1888),227.13. RAY, P., Chern.Rev.,61 (1961),313.14. WELLMAN, K. M., HARRIS, D. L. & MURPHY, P. J., Chern.Comrnun.,(1967),568.15. KRUSE, W. & TAUBE, H., J. Am. chern.Soc.,83 (1961),1280.16. SEN, D., J. chem.Soc., (A) (1969),1304.17. HAIM, A. & WILMARTH, W. K., Inorg. Chern.,1 (1962),573;HAIM, A. & TAUBE, G., Inorg. Chern.,2 (1963)1199.CHAKRAVARTY & SIL: KINETICS OF FORMATION OF MIXED CHELATE OF Co(IIl)The approachof the monoprotonatedligandto thecomplexion takesplacethroughthenon-protonatedend. Large tendencyof ring formation,removesthe attachedproton from the ligand along withaquo ligand as HaO+.Becauseof the protonationon both ends, theapproachof the diprotonatedligand towardsthemetal atom througheither end is restricteddueto electrostaticeffects.The lossof unsaturationofthe diprotonatedform14alsolosesits quasi-aromaticring formationtendency.This is the reasonforno reactionwith this form of ligand.The.associationof the reactantsmay greatlybefacilitatedby the multiplehydrogenbondingpos-sibilities of the aquo ligandof the complexion.Hydrogenbonding tendencyof H20 over OH-facilitatesthe approachof the ligandthroughthisend,with its removal. Becauseof its basicnature,phenylbiguanideforms strongerbonds than H20.Oncetheattachmentat oneendis formed,the ringclosuretakesplaceimmediatelywith simultaneousremovalof HaO+or H20.TABLE 4 - ACTIVATION PARAMETERS OF VARIOUSSUBSTITUTION REACTIONS OF COBALT(III)CHELATE COMPLEXESComplexEnteringb.Htb.stRef.ligandkcal/e.n.moleCo en2(H2O)\+H2O28·9+15,115Co en2(H2OWC204,4-81'HC204 Co(DMGh(N02)Br-2'5-6(H2O) Co(DMGhI(H20) Br-17'8-14·06Co(BigHh( 20 PhBigH~3·20,8Thi workCo BigHh(OHWphBi R~07-doH2O)2+ (H20)~Cl-, Br, 1-6·197includedin Table 4. AHt and ASt valuesfor k1and k2 areidenticalwithinexperimentallimitations,indicatingsimilarmechanismsfor the replacementof aquo ligandsin eachform of the complexion.It is interestingto notethat AHt and ASt valuesdecreaseas the 7t-backbondingtendencyof thebisbidentatechelateon the aquocomplexincreasesas en <DMG7 <BigH (DMG=dimethylglyoxime).BigH is an unsaturatedligand14,and the extrastability of the biguanidechelatering is due to'It-bondingphenomenon,intrOducinga quasi-aromaticcharacterto the chelatering16. The rate of sub-stitutionreactionis alsoin the sameorderas theirbasicitywould warrant,i.e. en (l·O)<DMG (10·0)<BigH (103). This suggeststhat bis-biguanidecomplexof Co(III) has someclass (b) character.It is likely that the 'It-bondingof the bischelateligandremoveselectroncloudaroundcobaltatomfrom t2g orbitals, makingtheseorbitals increasedacceptorof the incomingligand. This resultsina fasterreactionrate for biguanidecomplexes.Inthis case,significantbond formationtakes placebeforeany bondbreakagein the transitionstate.After the removalof the first aquo ligand,thereplacementof secondH20 or OH- ligand takesplaceimmediatelywith ring closure,due to highringformationtendencyoftheunsaturatedbiguanideligandformingsix-memberedquasi-aromaticring16•505

Mechanistic Studies on the Reaction of cis-Diaquobisbiguanide-cobalt(III) Ion & Phenylbiguanide in Aqueous Acidic Solution: Formation of Mixed Chelate of Cobalt(III)

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Mechanistic Studies on the Reaction of cis-Diaquobisbiguanide-cobalt(III) Ion & Phenylbiguanide in Aqueous Acidic Solution: Formation of Mixed Chelate of Cobalt(III)

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