CHEMICAL SPECIATION OF TERNARY COMPLEXES OF LASPARTIC ACID AND ETHYLENEDIAMINE WITH Co(II), Ni(II) AND Cu(II) IN LOW DIELECTRIC MEDIA
*R.SwaroopaRani,G.NageswaraRao1DepartmentofChemistry,ANITSEngg.College,SangivalasaVisakhapatnam,India.(*CorrespondingAuthor)
2DepartmentofInorganic&AnalyticalChemistry,SchoolofChemistry,AndhraUniversity,Visakhapatnam,India.
ABSTRACT
Chemical speciation of ternary complexes of Co(II), Ni(II) and Cu(II) ions with L-aspartic acid and ethylenediamine was studied pH metrically in 0.0–60.0% v/v Dioxan-water mixtures maintaining an ionic strength of 0.16 mol L-1 at 303.0±0.1 K.Alkalimetric titrations were carried out in different relative concentrations (M:L:X=1:2.5:2.5,1:2.5:5.0,1:5.0:2.5)ofmetal(M)toasparticacid(L)toethylenediamine(X).Stabilityconstantsofternarycomplexeswerecalculatedandvarious modelswererefinedwithMINIQUAD75.Thebestfitchemicalmodelswereselectedbasedonstatisticalparametersandresidualanalysis.Thespeciesdetectedwere MLX,ML2XandMLXHforCo(II),Ni(II)andCu(II)inDioxan-watermixtures.ThestabilitiesofthecomplexesfollowedtheIrving-Williamsorder,i.e.,Co(II)< Ni(II)<<Cu(II).
KEYWORDS:Chemicalspeciation,ternarycomplexes,asparticacid,ethylenediamine,Dioxan,MINIQUAD75.
INTRODUCTION:
Chemicalspeciationoftransitionmetalsisimportanttostudytheirmobility,distribution, bioavailability, toxicity and setting environmental standards Bioavailabilityofametaliondependsonwhetheritisinfreestateorcomplexed withvariousconstituentsduringbiologicalreactions.Controllingfactorssuchas pH,temperatureandionicstrengthchangethecomplexationbehaviourofmetals andbindingsites.Thuscomplexationcansignifythebioavailabilityofthemetal ions in various biosystems. Most metabolic reactions are catalyzed by metalcontainingenzymes,theactivityofwhichisduetometal-enzyme-substratecomplexes.The active site of the enzymes has a lower polarity than biofluids.The specificity and selectivity of enzyme-substrate reactions can be achieved by manipulating the equivalent solution dielectric constants (ESDC) at the active site[1].Acidityandbasicityofamoleculeisgovernedbyitsstructureandsolvent effects [2, 3]. L-aspartic acid (Asp), a non-essential amino acid, plays an importantroleinmaintainingthesolubilityandioniccharacterofproteins[4].It actsasatridentateligandandalsoasaneurotransmitter[5].
Ethylenediamine(en)isusedasmonodentate,bidentateorabridgingligand[6]. It is used in the manufacture of EDTA, carbamate fungicides, surfactants and dyes. It is also useful in manufacturing accelerator or curing agent in epoxy industry Itisinvolvedinthesynthesisofseven-memberedringcomponentswith β-ketoestersresultingsecondaryaminesandβ-enaminoesters[7].Theenplays animportantroleinthesynthesisofSchiffbases[8].Theprotonationconstants ofethylenediaminewerereportedearlierbytheoreticalcalculations[9,10].
Cobalt is essential for the production of the red blood cells, and its salts are widelyusedinindustrialmaterials,paintproducts,fertilizers,feedsanddisinfectants [11-13]. Special cobalt-chromium-molybdenum alloys are used for prosthetic parts such as hip and knee replacements [14]. Nickel is found in urease, which accounts for 6% of the soluble cellular proteins [15] and catalyses the hydrolysisofureatoyieldammoniaandcarbamate.Coppercontainingenzymes and proteins constitute an important class of biologically active compounds, whosebiologicalfunctionsincludeelectrontransfer,dioxygentransport,oxidation,reductionanddisproportionation[16].
Theaimofthepresentstudyistounderstandtheroleofmetalionsatactivesite cavitiesinbioactivemoleculeslikeenzymesandproteinsandtoknowtheeffect of dielectric constant of the medium on the chemical speciation of the ternary complexesofCo(II),Ni(II)andCu(II)withAspanden.Sincethedielectricconstant at the active site cavities is very small compared to that in biofluids, low dielectric constant is mimicked by using a water soluble organic solvent like Dioxan(Dox)whichisanon-polarsolventcapableofactingashydrogenbond acceptorwithrandomstructure.Protonationconstants[17,18]andbinarystability constants ofAsp [19] and en [20] with Co(II), Ni(II) and Cu(II) have been reported earlier Hence, chemical speciation of their ternary complexes is reportedinthepresentcommunication.
Experimental:
Aqueous solutions (0.1 mol L-1) of Co(II), Ni(II) and Cu(II) chlorides (GR Grade,E-Merck,Germany)werepreparedbydissolvingthemintripledistilled water 0.05molL-1solutionsofEthylenediamine(AR,Qualigen,India)andLAsparticacid(AR,Qualigen,India)werealsoprepared.Toincreasethesolubilityoftheligandsandmetalsalts,0.05molL-1hydrochloricacidwasmaintained
inthesolutions.1,4-Dioxan(Finar,India)wasusedasreceived.Thestrengthof alkaliwasdeterminedusingtheGranplotmethod[21,22].Errorsintheconcentrationsofligands,metalionsandalkaliweresubjectedtoanalysisofvariance (ANOVA)[23].
Titrationswerecarriedoutinthemediumcontainingvaryingconcentrationsof Dox maintaining an ionic strength of 0.16 mol L-1 with sodium chloride at 303.0±0.1K.ThemeasurementswererecordedwithanELICO(ModelLI-120) pHmeterof0.01readabilityinconjunctionwithaglassandcalomelelectrode. ThepHmeterwascalibratedwith0.05molL-1potassiumhydrogenphthalatein acidicregionand0.01molL-1boraxsolutioninbasicregion.Theglasselectrode was equilibrated in a well stirred Dox-water mixtures containing inert electrolyte. The effect of variations in asymmetry potential, liquid junction potential, activity coefficient, sodium ion error and dissolved carbon dioxide on the response of glass electrode were accounted for in the form of correction factor (logF)[24]whichwascomputedfromtheexperimentalandsimulatedacid-base titrationdatacalculatedbySCPHDprogram[25].Italsoaccountsforthesolvent effectonpH.Titrationofstrongacidwithalkaliwascarriedoutatregularintervals to check whether complete equilibration was achieved.The calomel electrodewasrefilledwithDioxan-watermixturesofequivalentcompositionasthat ofthetitrand.Ineachofthetitrations,thetitrandconsistedof1mmolofhydrochloricacidinatotalvolumeof50mL.Titrationswerecarriedoutinthepresence of different relative concentrations of the metal (M) toAsp (L) to en (X) (M:L:X=1:2.5:2.5,1:2.5:5.0,1:5.0:2.5)with0.4molL-1NaOH(Table1).The best-fitchemicalmodelforeachsysteminvestigatedwasarrivedatusinganonlinear least squares analysis program MINIQUAD75 [26]. Results and discussion.
Modelingofchemicalspeciation:
Apreliminaryinvestigationofalkalimetrictitrationsofmixturescontainingdifferent mole ratios ofAsp and en in the presence of hydrochloric acid and inert electrolyte indicates that no condensed species were formed. The protonation constantsandthestabilityconstantsofthebinarymetalcomplexesoftheseligands were fixed in refining ternary complexes and in testing various chemical modelsusingMINIQUAD75.Thebestfitmodelwaschosenbasedonthestatisticalparameterslikeχ2,R-factor,skewnessandkurtosisgiveninTable2.Theternary complex species detected are MLX, ML2X and MLXH for Co(II), Ni(II) andCu(II).
Averylowstandarddeviation(SD)intheoverallstabilityconstants(logβ)indicates the precision of these parameters. The small values of Ucorr (sum of squares of deviations in the concentrations of the metal, the ligands and the hydrogenionatallexperimentalpointscorrectedfordegreesoffreedom)indicatethatthemodelsrepresenttheexperimentaldata.Smallvaluesofmean,standarddeviationandmeandeviationforthesystemscorroboratethattheresiduals arearoundazeromeanwithlittledispersion.Foranidealnormaldistribution,the values of kurtosis and skewness should be three and zero, respectively The kurtosis values in the present study indicate that most of the residuals are very nearertoleptokurticandafewformmesokurticpatterns.Thevaluesofskewness recordedinTable2arebetween-2.34and3.07.Thesedataevincethattheresidualsformapartofnormaldistributionhence,least–squaresmethodcanbeapplied tothepresentdata.Thesufficiencyofthemodelisfurtherevidentfromthelow crystallographicR-valuesrecorded.
Effectofdielectricconstantonstabilityofternarycomplexes:
Dox is a protophilic dipolar protic solvent and acts as a structure former [27]. Hence, it removes water from the coordination sphere of metal ions, making themmorereactivetowardstheligands.Asaresult,thestabilityofthecomplexes isexpectedtoincrease.Atthesametime,itisacoordinatingsolventanditcompeteswiththeligandsforcoordinatingthemetals.Thisdecreasesthestabilityof the complexes. Hence, the stability of the complexes is expected to either increaseordecrease.Thevariationofoverallstabilityconstantswithco-solvent contentdependsuponelectrostaticandnon-electrostaticfactors.Born'sclassical treatmentholdsgoodinaccountingfortheelectrostaticcontributiontothefree energychange[28].Accordingtothistreatment,theenergyofelectrostaticinteraction is related to dielectricconstant. Hence, the log β values should vary linearlyasafunctionofreciprocalofthedielectricconstant(1/D)ofthemedium. Thelinearvariationobservedinthepresentstudy(Figure1)indicatesthatelectrostaticforcesaredominatingtheequilibriumprocessunderthepresentexperimentalconditions.Thisisduetoprotophilicdipolarproticandstructureforming natureofthesolvent.ThestabilitiesofthecomplexesfollowtheIrving-Williams order,i.e.,Co(II)<Ni(II)<<Cu(II).
Quantificationofchangeinstabilityofternaryspecies: Thechangeinthestabilityoftheternarycomplexesascomparedtotheirbinary analogueswasquantifiedbasedonthedisproportionationconstant(logX)given [29,30] byEquation1 whichcorrespondstotheequilibriumML +Mx 2MLX. 2 2 Undertheequilibriumconditionsonecanexpecttheformationof50%ternary complexesand25%eachofthebinarycomplexesstatisticallyandthevalueof log X shall be 0.6.Avalue greater than this accounts for the extra stability of MLX.
The distribution of metal ions in various complexes species (chemical speciation)withpHisrepresentedasdistributiondiagrams.SometypicaldistributiondiagramsaregiveninFigure2.Figure2ArepresentstheformationofCoAsp-encomplexes.InthepHrange4.0-10.0,theprotonatedligandsinteractwith metal ion (Equilibria 1 and 3) to form MLXH and MLX. And also MLX is formedbythedissociationofMLXH(Equilibrium2).SimilarlyMLXisformed 2 eitherfrominteractionofML,LHandXH (Equilibrium4)orfromMLX(Equi- 2 librium5),becausetheconcentrationsofbothMLXandLHaredecreasingwith increasingconcentrationofMLXinthepHrange8.0-10.0.Figure2Bshowsfor- 2 mation of Ni-Asp-en complexes. The concentrations of MLX and MLXH increaseinthepHrange4.0-10.0.Figure2CshowstheformationofCu(II)complexesinthepHrange5.0-11.0.
Structuresofcomplexes:
Theliterature[37,38]suggeststhatCo(II),Ni(II)andCu(II)complexesshallbe octahedral.AminonitrogenatomcanassociatewithhydrogenionsinphysiologicalpHranges.Hence,thereisoftensignificantcompetitionbetweenhydrogen andmetalionforthisdonorsite.Thissituationresultsinthesimultaneousexistence of a number of equilibria producing an array of protonated complexes, whicharedetectedinthepresentstudy Aspactsasbidentateortridentateligand dependingontheexperimentalconditionsandenactsasbidentateligand.Thus basedontheaboveequilibriatheplausiblestructuresofthecomplexesarepresentedinFigure3.
CONCLUSIONS:
The following conclusions have been drawn from the modeling studies of the speciationofternarycomplexesofCo(II),Ni(II)andCu(II)withAspandenin Dox-watermixtures.
Anotherapproachtoquantifytheextrastabilityofternarycomplexeswasbased onthedifferenceinstability(ΔlogK)forthereactionsMLwithXandM(aq) withLandX[31-33],whereListheprimaryligand(Asp)andXisthesecondary ligand (en). It is compared with that calculated purely on statistical grounds as giveninEquation2.TheequationsforthecalculationofΔlogKandlogXare giveninTable3.
The electrostatic theory of binary complex formation and statistical arguments suggest the availability of additional coordination positions of the hydrated metalionforthefirstligandthanforthesecond.Hence,theusualorderofstabil-
M ML ityK >K applies.ThissuggeststhatΔlogKshouldbenegative,although ML ML2 several exceptions have been found [34]. The statistical values of Δlog K for tridentateLandbidentateXare,-15.56to3.67inDox-watermixtures(Table3). Negative values of Δlog K can be understood as the secondary ligand forms a morestablecomplexwithhydratedmetalionthanwithL.WhenevertheexperimentalvaluesofΔlogKexceedthestatisticalvalues,itcanbeinferredthatthe ternarycomplexisformedasaresultofinteractionofMLwithXorMXwithL.
ThelogXandΔlogKvaluescouldnotbecalculatedforsomesystemsduetothe absenceofrelevantbinaryspecies.ThelogXvaluesrangefrom1.39to10.50, some of which are found to be higher than those expected on statistical bases (0.6). These higher values account for the extra stability of the ternary complexes.Thereasonfortheextrastabilityofthesecomplexesmaybeduetointeractionsoutsidethecoordinationspheresuchastheformationofhydrogenbonds betweenthecoordinatedligands,chargeneutralization,chelateeffectandstacking interactions [35, 36]. The extra stability of ternary complexes makes them moreamenablefoemetaltransport.Thelessstablebinarycomplexesmakethe metalsbioavailable.
Distributiondiagrams:
+ - 2- 2+ +
Basedontheactiveformsoftheligands(Lh ,LH,LH,L andXH ,XH and 3 2 2 X)andtheexistenceofthebinarycomplexspecies(ML,ML,MLH MLH and 2 2 2 2 3 MLH and MX, MX and MX), the plausible forms of species are assumed to 2 4 2 3 formthroughthefollowingequilibria.
1. TheternarymetalcomplexspeciesdetectedareMLX,ML2XandMLXH, forCo(II),Ni(II)andCu(II),whereL=AspandX=en.
2. ThevaluesofΔlogKandlogXindicatethattheternaryspecieshaveextra stabilitycomparedtotheirbinaryspecies,maybeduetotheinteractionsoutside the coordination sphere, such as the formation of hydrogen bonds between the coordinated ligands, charge neutralization, chelate effect, stacking interactions and the electrostatic interactions between noncoordinatedchargegroupsoftheligands.
3. The linear trend in the stabilities of ternary complexes with decreasing dielectricconstantsisduetothedominanceofelectrostaticforcesovernonelectrostaticforces.
4. Thestudyalsogivesaninsightintothemetalavailability/metaltransportin biofluids.Theternarycomplexesaremoreamenablefor“metaltransport” becauseoftheirextrastability Acknowledgements:
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