Dash, A. C.

Mohapatra, S. K.

English

NOPR

Indian Journal of ChemistryVol. ISA, October 1977, pp, 871-874Kinetics & Mechanism of Hydrolysis of cis-(Bromo) bistethylene-diamine) (z-aminothiazole )cobalt(III) CationA. C. DASH*Department of Chemistry, Ut.kal University, Bhubaneswar 751004andS. K. MOHAPATRADepartment of Chemistry. Regional College of Education, Bhubaneswar 751006Received 3 February 1977; accepted 12 April 1977The kinetics of aquation of cis-(bromo)(2-aminothiazole)bis(ethylenediamine)cobalt(III)cation have been Investigated at 30-50° in HCIO. and acetate buffer media of 1= 0·3 mol drrr+,The rate of aquation obeys the rate law -d In [Complex]T/dt= k, + k2 KNH/[H+]+ k'3 [MeCO-.]where k, and k2 are the rate constant for cis-[CoBr(NH,tz) (en),]"+and its amido conjugate baserespectively; KNH is the dissociation constant of the amine proton (believed to be that of co-ordinated NH,tz) and k'3 is a constant characterfztng the acetate ion catalysed path. The valuesof k"k2KNHand k'3 at 45° are (2·95± 0·04) x 10-<see-I, (5·18± 0·15) x 10-9 mol drn= sec" and(4·75:.'::0·25) x 10-<drn" mol! see"! respectively. Solvent assistance is invoked in the kI path.The activation parameters indicate that the mechanism is essentially dissociative for all thethree paths. From the activation entropy data, the transition states in the kI and ki paths arebelieved to be tetragonal in nature. 'I'r igonal bipyramidal transition state appears to be morelikely in the k2 path.EXTENSIVE iIlve~tigations h<l:ve .been car.riedout on the kinetics of substitution reactionsof octahedral cobalt(III) complexes of thetype cis-[CoXL(en)2J2+ where en represents ethy-lenediamine, L is a monodentate non-replaceableamine ligand and X stands for C1 or Brl: "Ye havebeen interested in the study of the kinetics andmechanism of substitution reactions of the complexesof the type mentioned above with L as a weaklybasic amine ligand. Our aim is to (i) examine theacid ionization of the NH group of the coordinatedamine, and (ii) assess the factors governing the ratesand mechanism of substitution reactions of suchcomplexes as well as their conjugate bases. Wereport in this paper the synthesis and kinetics ofhydrolysis of cis-(bromo) bis(ethylenediamine)(NH2tz)cobalt(III) ion (NH2tz = 2-aminothiazole). Theresults obtained have been compared with similardata for certain other weakly basic amine complexesof the type cis-[CoBr(amine)(en)2J2+.Materials and MethodsThe bromo-bis(ethylenediamine) (2-aminothiazole)-cobalt(III) bromide was prepared by the methodof Bailar and Clapp". The complex was recrystal-lized at 0° as the perchlorate salt from aqueoussolution acidified with perchloric acid using saturatedsodium perchlorate solution as the precipitant. Thecomplex was washed successi vely with ice· cold water,ethanol, diethvl ether and then stored over fusedcalcium chloride in a desiccator painted black{Found: Co, 10·26; Br, 14·45 [CoBr(C3N2SH4) (en)2]-(C104)2 requires Co, 10·55; Br, 14·32%}. The corres-ponding chloro complex could not be preparedwith high degree of purity by this method.2-Aminothiazole (Koch Light) was used withoutfurther purification. Sodium perchlorate used forionic strength adj ustment was Riedel's reagentgrade sample. All other chemicals were Analargrade. Copper(II) and mercury(II) perchloratesolutions were prepared by dissolving the metal(II)oxides in excess of perchloric acid. Copper(II) wasestimated iodometrically and mercury(II) by EDTAtitration. Back titration procedure using standardZn(II) solution and Erio T indicator was adoptedfor Hg(I1) estimation". The free acid contents ofthe metal perchlorate soluti ons were estimated bya combined procedure in volvir.g cation exchangeand acidimetry. The alkali was standardized agair.stpotassium hydrogen phthalate. All ion-exchangeexperiments were performed with Dowex 50-WX-8resin in acid form. All solutions were prepared infreshly distilled water. .Infrared spectra (nujol) were recorded with aPer ki n-Elmer 337spectroph otometer. The electronicspectra were obtained with a Beckman DU2spectrophotometer using 1 em quartz cells. Allnumerical calculations were performed on an IBM1130 computer.Kinetic measurements - Rate measurements weremade at 35-50° in perchlorate medium of 1=0·3mol drrr '. The concentration of the complex was(2-3) X 10-3 mol dm=. The reaction mixtures ofappropriate composition (see Tables I and 2) wereprepared in 50 ml measuring flasks and thermo-st~ted to ± 0·1°. The reaction mixture (5 ml) waswithdrawn at convenient time intervals; the aquoproduct and the unreacted bromo complex wereremoved by Dowex 50.:WlX.8resin in acid form andthen the li berated bromide estimated potentio-871INDIAN J. CHEM .• VOL. 15A. OCTOBER 1977metrically using silver nitrate (0·01 mol drrr"). Thepseudo-first-order rate constants (kobs) were calcu-lated from the gradients of the least squares bestline plots of In (V'" - Vel against time where Vt andV", stand for the titre values at time t and infinity(i.e. for complete release of bromide) respectively.kobS was weighted inversely as its variance whilecalculating its average from replicate runs.Results[CoBr(NH2tz)(en)2p+ has the absorption maximumat 525 nm with molar absorption coefficient of 93·0dm" mol-l crrr! in 0·1 mol drrr? HCI04 medium whichcompares satisfactorily with the position and inten-sity of lA[g~IT[g transition of several cis-halo-(amine)-bis(ethylenediamine)cobalt(III) complexes+,Thesecondligandfield band due to+zl , ~IT2gtransi-tion is. however. masked by the charge transferspectrum of the bromo complex at 300-400 nm. TheUV spectra [Amax. nm (€\ drn" moltcrrr '}: 250 (15530)in 0·1 mol drrr" HClO. medium] indicate thecoordinated 2-aminothiazole5. Recently House ei al.shave shown that the chloro(amine)-bis(ethylene-diamine)cobalt(III) complexes obtained fromBailer and Clapp's preparations have the cis-configuration. Considering the method of prepara-tion and the visible spectral data it is reasonableto assign cis-configuration to [CoBr(NH2tz)(en)2]2+.Singh and Shukla' from the consideration of thelowering of the asymmetric vNH of NH2tz from3400 to 3310 crrr ' concluded that z"aminothiazolecoordinates to Sn(IV) through the primary aminogroup. The IR spectrum of [CoBr(NH2 ~z)(en)~]2+exhibits bands at 3360. 3275 and 3240cm-1 indicatinglowering of asymmetric '1NH of 2-aminothiazole oncoordination to cobalt(III). Bands at 1600. 1560and 1525 with shoulders at 1555. 1480 and 1350crrr+ are observed. Apparently the asymmetricvNH2 is lowered and the stretching frequency ofthe thiazole ring is not raised up as NH2tz coordi-nates to cobalt(III). It is. therefore. most likelythat the NH2tz is bound to cobalt(III) by theprimary amine functions.We attempted to study the protonation of the ringnitrogen of the coordinated NH2tz spectrophot o-metrically at 25° and 1= 0·3 mol dnr". The UV spec-trum of cis-[CoBr(NH2tz)(en)zJ2+ (240-260 nm) wasfound to be virtually acid independent in the rangeof [H+] = 0·001-0·3 mol drrr" suggesting therebythat the complex does not undergo pr.otona~ion todetectable extent in the range of [H+] Investigated.Aquation - Both the ligand field bands are ob-served for the [Co(NH2tz)(en)z(OHz)]3+ generatedby the spontaneous aquation of the bromo complexin 0·1 mol drrr" HCI04 medium at 60° [Ama.. nm (€max,dm" mol-l crrr-}: 486 (77·0); 338 (148)). Ourattempt to generate the aquo complex by Hg(II)-catalysed aquation of the bromo complex and checkits spectral data quoted above was unsuccessful asa pale vellow precipitate gradually developed onadding 'Hg(CI04)2 to the solution of the bromocomplex. The observed spectral ~hi~t due to theaquation of the bromo complex lS 111 accordancewith the spectrochemical order of OH2 and Br9.Table 1 presents the rate data at .45°. for theaquation of cis-[CoBr(NH2tz)(en)2J2+ studied 111HCl04872and acetate buffer media of 1= 0·3 mol drrr". Itis evident that the pseudo-first-order rate constant(kobs) virtually le\'els off to a constant value at [H+]= 0·001 mol dm". The plots of kobs against [H+]-lat constant [MeC02] and kob,. against [MeC02J atconstan t [H+J are linear with positive gradients.The inverse acid dependence of kobs is reconciledwith the formation of a reactive conjugate base(C.B.) presumably [CoBr(XHtz)(en)2J+. Catalysis ofaquation of the bromo complex by acetate ion maybe attributed to the formation of a reactive ion-pair.{[CoBr(NH2tz)(en)2] MeC02)} +. The various steps in-volved in the aquation mav be delineated as shownin Scheme 1.K:-;H[Complexj+ ?=+ H,OI "t k,Product[Cornplex-H]+ + H+c.B.-'- H.Ot k.ProductK,[Complex Pr + ;\'IeCO' .= {[Complex] ~IeC02}++ HoO.j. k3ProductScheme 1Consistent with this mechanism. the rate law ofaquation takes the form (1)-d In[ComplexJT/dt =k _ kl +k2K:-lH/[H+]...!...k3K,lMeCO;;!obs - 1 K /"H ~·I 0 ... (1)+ NH L +J+Ks['veC :i]where kl' k'2 k3 stand for the rate constant forcis-[CoBr(NH2tz)(en)2J2+. its conjugate base and itsacetate ion pair respectively; K:-;H is the N-Hdissociation constant of the complex and K, is theformation constant of its acetate ion-pair. Eq.(1) reduces to Eq. (2) if KNH![H+J+K.[MeCO;J~land k3Ks is taken to be k;kobs= kI +k2KNH/[H+]+k~[MeC02] ... (2)Eq. (2) is in accord with the observed dependenceof kobs on [H+J-l and [MeCO;J as mentioned earlier.An alternative interpretation may also be givento the conjugate base path as shown in Scheme 2K[Complexjs+ + OH- ~ [Complex - HJ+ + HoOCBt k.ProductScheme 2where K stands for the equilibrium constant of forma-tion of the conjugate base as stated above. But thisis kinetically equivalent to the proton dissociationscheme mentioned earlier as it can be shown that KNH= KKw where Kill is the ionic product of water.Apparently the mode of representation of the conju-gate base equilibrium is ambiguous. This is however,of little importance in the present context due to thekinetic equivalence of both the schemes and as weare concerned with the rate of aquation and equili-brium constant of formation of the conjugate base.The rate data in Table 1 were fitted to Eq.(2) by means of a computer programme whichDASH & MOHAPATRA: KINETICS OF AQUATION OF Co(III) COMPLEXTABLE 1 - RATE DATA FOR AgUATIONOF cis-[CoBr(NH.tz) (en),]'+ IN PERCHLORICACID AND ACEoTATEBUFFER :'IIEDIAOF I = 0·3 mol dm-3 AT 45 C[HClO,]mol drrr=10'koDs'*see"!10'kobs. *see"![:'IIeCOii]mol dm-3105[H+]mol dm-30·0010·0020·0050·010·020·050·100·200·300·010·080·050·100·100·150·010·020·050·100·150·250·100·250·150·250·103'32 .L 0·244-13 ± 0·064·21 .-:-0·084·50 ± 0·154·58 ± 0·095'31 .i. 0·235·17 ::;::0·115·24 :+: 0·215·22 ± 0·185·32+- 0·255-49 ± 0·255·86 ± 0·106·71 + 0·228'31 ::;::0·088·57 =::: 0·2312·9 ± 0·219·3 ± 0·63·20'- 0·103·09 -;: 0·112·81 ::;::0·122'79:::;:: 0·092·92 :::;::0·043·01 ::;::0·102·82::;:: 0·072·79;:: 0·072·81 ± 0·0830·17·526·026·024·514·013·013·013·013'013·013·011·501'201'000'6020·301•Average of at least duplicate runs.floated the values of kl' k2KNH' k; and minimizedn~ W,(kcaJi-kabsi)2. The reciprocal of the variancei=lof kabs was taken to be the weighting factor. Thevalues of kl' k2KNH' and k~ used as inputs wereobtained from the graphical plots mentioned earlier.The least squares best values of these parametersat different temperatures are given in Table 2.The effect of Cu(II) on the rate of aquation of thecomplex was examined at 35° and 1= 0·3 mol drrr".With [H+h = 0·01 and [Cu2+] = 0'02, 0·04, 0·06mol drrr", we obtained kabs= (0·86± 0·01) X 10-4see"! which agree satisfactorily with the value ofkabsat [Cu2+]= 0, [H+h = 0·01 mol dm" (see Table 2).Thus evidence for the association of Cu2+ withthe thiazole ring leading to metal ion catalysedaquation of [CoBr(NH2tz)(en)2J2+ is lacking.TABLE 2 - PARAMETERSOF Eq, (2)Temp. 10' kl 10' k2KNH 10' k'3°C Sec-1 mol dm-a see'? dma mol-1 sec-135 0·89 .-:-0·01 0·74 ± 0·02 H± 0·140 1·68 ± 0·02 2·1 ± 0·1 2·8 ± 0·245 2'95 ± 0·04 5·2 ± 0·2 4·8 ± 0·350 4'79 ± 0·10DiscussionTable 3 presents the kinetic data for the aquationof several cis-bromo- (amine) -bis(ethy lenediamine)-cobalt(III) complexes derived from weakly basicamines10•ll. The kinetic patterns of such complexesale alike. Differences ill their substitutional labilities,which are controlled by both enthalpy and entropyof activation, are evident. The acid independentrate constants at 50° (see Table 3) indicate that the2-aminothiazole complex is 9 and 15 times morereactive than its imidazole and aniline counterpartsrespectively. This reactivity difference cannot berationalized in terms of the steric influence of thevicinal amine ligand on the departing bromide.~St values being either low positive or negative thetransition state of the k, path of aquation of aniline,imidazole and 2-aminothiazole complexes appearsto be essentially dissociative in nature involving atetragonal pyramidal configurat ion12 • The disso-ciative activation for the 2-aminothiazole complexis also supported by the observed acetate ioncatalysis; strikingly the temperature dependenceof ka gave ~Ht = 95·7 ± 7·4 kJ mol-l and ~St =-8·1 ±23JK-I mol-l which are close to the valuesof the analogous parameters for the kj path. Thekinetic parameters of spontaneous aquation of thecomplexes listed in Table 3 remaining essentiallythe same, the observed reactivity order might beindicative of the preferential solvation of the2-aminothiazole complex due to the cooperativehydrogen bonding effect (structure-I) which is notavailable to its aniline and imidazole counterparts.We attempted to study the base hydrolysis ofcis-[CoBr(NH2tz)(en)2J2+ at 25° in diethanolaminebuffer of pH 8,6-9·2. But the reaction was toofast to be studied. This precluded independentassessment of k2 and KNH. The values of KXH notbeing known it is not possible to accuratelypredict the reactivities of the conjugate bases ofNH2tz and aniline complexes. Disregarding theminor effect of ionic strength, it is, however,important to note that the value of (k2KNH)c,H,NH,/(k2KNHJtzNH2is 26 at 50° (Table 3). The acid disso-TABLE 3 - RATE AND ACTIVATIONPARAMETERSOF THE AgUATIONOF cis-[CoBrL(en)2jl~L I 10'k1 (50°) t>Ht t>St 109k2KNH t>Ht t>St Ref.mol dm-a see'< kJ mol"! JK-1mol-1 (50°) kJ mol"! JK-l mol-1mol dm-asec-1Aniline 0·06 0·31 110 +7 420 113 -12 (10)Imidazole 0·3 0·55 94·5 -34 0'33 102 -113 (11)LO·Ol =,=0·3 ±1 ±O·Ol ±3 -1-82-Aminothiazole 0·3 4·79 92·1 -23,7 15·0- 157 88 (This work)±0'10 ±3'0 ±9'8 +4 ±12a, extrapolated value.873INDIAN J. CHEM., VOL. ISA, OCTOBER 1977ciation constant of benzimidazole (bzimH) in cis-[CoBr(bzimH)(en)2J2+ has been found to be ,.......,10times higher than that of imidazole (imH) in cis-[CoBr-(imH)(en)2J2+ under comparable conditiont--!". Onthis basis and considering the conjugative inter-action of the lone pair of the amido nitrogen withthe tertiary nitrogen of the thiazole ring, the amineproton of the coordinated NH2tz is expected to bemore acidic than that of coordinated aniline. Thiswould then mean that the reactivity of cis-[CoBr-(NHC6Hs)(en)2J+ is ,.......,26times higher than that ofcis-[CoBr(NHtz) (en)2J+. Strikingly the conjugatebase, cis-[CoBr(bzim) (en)2J+, has been found to beless reactive than the cis-[CoBr(im)(en)2J+(ref. 11,13).These results point to the fact that the depletion ofelectron density from the amido nitrogen bondedto cobalt(III) reduces its labilizing action on theCo-X bond.A near constancy in the values of 6.H and 6.5 forthe dissociation of N-H proton from coordinatedaniline, imidazole, benzimidazole and 2-aminothia-zole is most likely. For the cis-[CoX(bzimH)(en)2J2+we obtained 6.H (KNH) = 41 k] mol! and 6.5 (KNH)= -23 JK-l mol? (ref. 13). Thermodynamic datafor other cis-[CoX(amine)(enhJ2+ (X = Cl or Br)complexes are not available. Therefore, with 6.H(KNH) and 6.5 (KNH) data for the benzimidazolecomplex as the guide we are led to believe from theobserved high positive values of the overall activation874parameter data (6.5t in particular) for the NH2tZcomplex (acid dependent path) that the conjugatebase undergoes considerable rearrangement andpasses through the trigonal bipyramidal transitionstate4•12,14 for which there may arise considerablegain in entropy.References1. BASOLO, F. & PEARSON, R. G., Mechanisms of inorganicreactions (Wiley, New York), 1967.2. BAILAR, J. C. & CLAPP, L. B., J. Am. chem, Soc., 67 (1945),171.3. VOGEL, A. 1., A textbook of quantitative inorganic analysis(Longmans, Green, London), 1964, 436.4. FENEMOR, D. & HOUSE, D. A., J. inorg. nucl. Chem., 38(1976), 1559.5. RAo, C. N. R., Ultraviolet and visible spectroscopy (Butter-worths, London), 1967, 77.6. KINDRED, 1. J. & HOUSE, D. A., Inorg. nucl. chem. Leu.,10 (1974), 25.7. SINGH, P. P. & SHUKLA, U. P., Aust. J. Chem., 27 (1974),1827.8. RAo, C. N. R.. Chemical applications of infrared spectro-scopy (Academic Press, New York), 1963, 109.9. JORGENSEN, C. K., Absorption spectra and chemical bond-ing in complexes (Pergamon Press, Oxford), 1962, 109.10. NANDA, R. N., Ph.D. thesis, Utkal University, Cuttack,1970.11. DASH, A. C. & MOHAPATRA, S. K., J. chem. Soc. Dalton,(1977), 246.12. TOBE, M. L., Inorg. Chem., 7 (1968), 1260.13. DASH. A. C. & MOHAPATRA, S. K., J. chem. Soc. Dalton,(1977), 1207.14. TOBE, M. L., Accounts chem. Res., 3 (1970), 377.

Kinetics & Mechanism of Hydrolysis of cis-(Bromo) bis (ethylenediamine) (2-aminothiazole)cobalt(III) Cation

http://nopr.niscair.res.in/bitstream/123456789/53284/1/IJCA%2015A%2810%29%20871-874.pdf

Kinetics & Mechanism of Hydrolysis of cis-(Bromo) bis (ethylenediamine) (2-aminothiazole)cobalt(III) Cation

Abstract

Similar works

Full text

Available Versions

NOPR