The oxidative decarboxylation of polyaminocarboxylic acidm—VI1

Talanta, 1972, Vol. 19, pp. 1097 10 1104. PecmmonReu.

PrimedinN-Ireland

THE OXIDATIVE DECARBOXYLATION OF POLYAMINOCARBOXYLIC ACIDS-VI* REACTION OF DIETHYLENETRIAMINEPENTA-ACETIC ACID WITH CERIUM(IV) IN SULPHURIC ACID MEDIA Slrw~ B. HANNA,RITA K. HI?%=, WILLJAMR. CARROLLand WILLIAMH. WEBB Chemistry Department, Colkgc-eutsts~~~Urdvemity , . . .

of Missouri-Rolla,

(Received 22 October 1971. Accepted20 January W72) Fhnmwrp-The oxidation of dktby~ta-acetic

acid (DTPA) by Cc(IV) in sulphuric acid was im‘estigatad speumphotometrWlybythe&opped-flowtechnique. Therateofmactionisinfluenced by the acidity, but can be expressed by a simplified rate law -

%$?! = k'[Ce(iV)J[DTPA]

At [H&30,] below 0*75iU the reaction p”ctdr stepwise mb= by formation of a 1:l C&V)-DTPA compkx with osasura formation and decay. At hi acid stnxlgths. the formation of an intern&iateisnotevident. 4Y rate is max&al in - @7SM sulphuric acid. The overall stokhiometry varks with time.

of ceric salts as oxidants for metal ions, anions and organic compounds in a variety of media, but interest in their use for oxidation of aminocarboxylic acids is more recent,14 perhaps because the primary function of these compounds is as chelating rather than reducing agents. We have undertaken a comprehensive study of the oxidation of polyaminocarboxylic acids with cerium(IV) in acid media. The rate and mechanism of the oxidation of EDTA and a comparative rate study of DCTA, DTPA, EDTA and NTA oxidations have been conducted in these laboratories.4-6 Spectrophotometric evidence that complex formation occurs at a measurable rate in the oxidation of DTPA6 prompted us to further study of that reaction. This paper deals with a stoichiometric study of the complex formation and the reduction of cerium(IV) by DTPA and with a rate study of the two processes in sulphuric acid media. MANY STUD= have been made

EXPERIMBNTAL Reqents Acid media were prepa by dilution from a standard&d sulphuric a&i solution; a standardized solution of (NHJ&e(SO,),~2H,O in O-O?86Nsulphuric acid was diluted to desired concentrations of Ce(IV). Aqueous DTPA solutions d known concentration were prepamd by direct weighing. Solutions of (NH,),Fe@O~,~6H,O were also pmpared by direct weighing of the salt, and a standard procedure7was used to prepare the ferroin indicator. All chemicals were of the highest quality and used without further purification. Procehre Stoichiometty of the ce(IV)-DTPA reaction. The number of moles of Ce(IV) consumed per mok of DPTA was determined at several acidities and as a function of time both by spe&ophotometric and titrimetric studies. In spectml studies, the consumption of Ce(IV) with time was followed at * Part V-!ke

Reference 16. 1097

1098

SA~QRB. HANNAet al.

the absorbance of the solution with time was rccodod. Ceriumm solutiom in O-1M. 0.6M. 0.7M. 1*3M, 15M and 2.OMsulphuric acid were used to study the dependence of the stoichiom& on the acidity of the media. To follow the stokhiometry titrfmetricaly. known volumes of standardi& cerlc solution were mixed thoroughly with a 5xed volume (5 ml) of DTPA at room temperature for specik periods of time.Thereactiaaewlsrsq~with~~and~i~voLnmarof~100Mf~ammonium suiphate, and the excess of iron(D) was back-titrated with aric sulphate, with ferroin as indicator. Sroichkme#y of compkx fomuth.For investigation of the stoichiomotry of the Ce(IV)-DTPA complex a Beckman DK-2A spectrophotomeW, a Gary-14 spectrophotometer, and a Sargent automatic mcordin titrim&er were used. Standard procedures8were followed for the continuous variations and moei -ratio dewminatiom. For the met&d of conthmous wwiatiw, the total concentration of the mixed reactants was 2.0 x IO-CM.For molar-ratio studies, the concentration of cerium(IV) was lIxed at 2-O x 104M in O-1M sul huric acid and the concentration of DTPA was variedfrom1~Ox1O+Zto69xlO4M. The 9 travioIetspa#raofthereactantsinl-cmsilica Theabsorbanceofthecompkxformedwas cellswererecordedovertherange230+Onmat30°. readat264nm.e Fortllspsars,ths~odlooataiwd~phuric~dd~spmecorrcentrationasthatusedhlpsw+wwgthecericso1utions. en~dtratlanswarealSouwdto Following the pnxcdwe outlined by W’ oso’ Mu&moin~1Msulphuricaddwere dCWlIliOCtlWSt0 &.hionsof2*Oxl titrated with 2.0 x-O=% lo-*M PTA, or an acid&d solution of 29 x WaM DTPA was titrated with

K&t& or&r of qmpk ormtim. The kit&cc of complex formationwere date&n& by mwuringthaabwrbuwat comxmrations of the l 64nmasafunctionofvarlationsintheinitial reactantaandoftheacldity. RESULTS

AND

Stoichfommy of the Cc(IV)-DTPA reacti&,

DISCUSSION

spectral studies

In our earlier work with EDTA’ we observed that four molas of cerium(IV) were immediately consumed per mole of EDTA. It was expected that one mole of DTPA would inu&ately reduce five moles of cerium(IV), but experiment showed that this reaction was not instantaneous, some ccrium remaining unruiuccd, but being reduced slowly. This was not too surprising, hawever, since we had observed progressive reduction of ccrium@V) by EDTA,’ and Raol reported a large consumption of ccrium(IV) by DTPA after several hours, which confirmed our earlier obscrvations.” Rao also reported that at least one product, glycine, is not fitrther oxidized, but t&t other intcrm4iat.e~ are oxidized to formic acid and carbon dioxide. Jones and LambcrP reported that the oxidation of polyaminocarboxylic acids with ferricyanide initially produces glyoxals and amines that are oxid.imd to glycinc, formic acid and formaldehyde. We have previously obscn&O that carbon dioxide and formaldehyde am produ& in the reactions. Preliminary work with electrophoresis~ and paper chromatography indicati that N,N’- and N,N-cthylencdiaminediacetic acids, glycine, ethylcncdiamine and 2-0x0-l-piperazineacetic acid are among the products of the ceric oxidation of EDTA.

Decarboxylation of polyaminocarboxylic acids

1099

When additional cerium(IV) was injected into solutions in which the first quantity of cerium(IV) added had already been consumed, the absorbance at 316 MI reappeared, but decayed slowly as the absorbance at 254 run and 239 run [due to cerium(III)~*6 simultaneously increased. In 0.6M sulphuric acid, five equivalents of cerium(JV) were consumed per mole of DTPA in 5.25 min. When the initial concentration of cerium(IV) was ten times that of the DTPA, 75 % of it was consumed in I hr and all of it within 24 hr. After 40 hr, no further change in the absorbance peaks of the reaction mixture could be detected. At higher acidities, the consumption of cerium(IV) was slower, in line with the earlier result@ which showed that the rate of reduction of cerium(IV) was decreased at sulphuric acid concentrations >0*75M. However, the reaction occurred in two stages, one mole of ceric per mole of DTPA being consumed immediately and the rest slowly. E$ct ofacidity. Varying the acidity of the ceric solution did not alter the initial stoichiometry. However, the spectra of equal concentrations of the reactants mixed in 0.1M sulphuric acid did not indicate the initial reduction of cerium(IV); a strong absorbance appeared almost instantaneously at 264 nm, characteristic of the relatively stable complex expected at low acidities.6 After several hours, the only absorbance in those solutions was at 254 and 239 mn, characteristic of cerium(III). Spectra of equimolar mixtures in O-65,0-75 and l+OMsulphuric acid showed no absorbance at 316 nm, presumably because of rapid reduction of cerium(IV). However, the absorbance at 316 run reappeared when more ceric solution was introduced and then decayed with time. Titrimetric studies. Aliquots of O-1M DTPA were mixed with known volumes of @0933M cerium(IV) in 1*7Msulphuric acid and the reaction immediately quenched by injection of excess of iron@). Back-titration of the iron(u) showed that at room temperature only one mole of Ce(IV) was consumed per mole of DTPA (Table I). Table I also shows results for reactions quenched in the same manner after various times. The quantity of cerium(IV) consumed incmases with time, but only slowly after about the first 5 min. In view of the many oxidizable centres in the DTPA mokcuk (I), the variable stoichiometry is not unusual. It is surprising, however, that whereas EDTA (II) TABU I.-Co

NWMPTION OF Ce(ll’) BY DTPA IN 125M 3’. AS A FUNCTION OF TRAM

Concentration, M* K’TPAI Fxwl 0462-t 0.076 O-076 O-082 O-082 O-082 O-082 0.082 O-082

0*033t

o-020 O-020 O-014 0.014 0.014 o-014 o-014 0.014

Tiirm,min 0

2 : 1: :: 30

&SO,

AT

&o:DTPA 0.966f

3.486 3486 5102 5.122 5254 5.216 5.274 5682

*Nominal initialconantrationin the mixtureof Cc(W) and DTPA. t Average of seven values.

!&am B. HANNAet al.

1100

consumes 4 equivalents of cerium(IV) per mole, almost instantaneously, under identical conditions the Ce(IV)-DTPA system displays only a 1 :l reaction ratio in the initial stages. This may be because the DTPA molecule must undergo certain conformational adjustments before it can effectively co-ordinate cerium(IV) and is expected to do so more slowly than EDTA. This is supported by the fact that DCTA (III), which is endowed with the rigid and most favourable geometry for complexation, is oxidixed about 100 times faster than EDTA or DTPA by cerium(IV).6

CH,-CH,

X ‘F<

Ix

& -0

N

X

-0

X

X

N

X = -CH,-CO,H

\X

'X

Stoichiometry of complex formation, spectral studies The &up initial deaease in the absorbance of cerium(IV) at 316 nm when mixed with DTPA lad us to suspect the rapid formation of a Ce(IV)-DTPA complex and spectral studies showed that there was in fact a new strong absorbance, presnmably of a complex, at 264 run.* The metal:ligand ratio in the complex was deWmined by the continuous-variations and molar-ratio methods,’ and found to be 1: 1. Potentiometric studies. Flaschlca* showed that the stoichiometry of Fe(III)EDTA complexes could be determined by continuously measuring the potential of the solution as complexation occur&. For an n-electron redox system where both species (Ox and Red) form 1: 1 complexes with Y, with stability constants de&red by K, = [OxY]/[Ox]M and KW = [RedYl/[Red]M, the potential E is given by substitution into the Nernst quation: E=P+---

0959 n

IW &ted

log [Red] K,,

if the stability constants are high enough for dissociation of the complex to be negligible. The potential is recorded as the ratio of [M”f]/[M(-“)+I changes during complexation. Because cerium(III) complexes much less strongly than cerium(IV),ls the standard potential of the complexed couple is much less than that of the uncomplexed one. In this study, the amount of cerium(III) present in the ceric solutions was extremely small. As DTPA was added the potential changed slowly because cerium(IV) remained in large excess over cerium(III), but dropped sharply when all the cerium(IV) was complexed. To be certain that the results were not intluenced by the solution of the reactant in excess being titrated, the order of addition of reactants

Decarboxylation of polyaminocarboxylic acids

1101

Both tests showed, within lo%, that complete complexation occurs at a 1:l molar ratio. Earlier work with EDTA showed the existence of a 1:l complex with cerium(IV).*

~8s reversed.

Rates of oxidation of DlIPA As in our earlier work on the Ce(IV)-EDTA reaction: and as Mishra and Gu~ta,~

and Brubaker and Smcia@ also observed in metal-ion oxidations with cerium(IV), the in&encc of the acidity of the media on the Ce(IV)-DTBA reaction rate is not linear, but a maximum is observed in the plot of log kg us. log w&Q] at about 0~75il4sulphuric acid (Fig. 1). The position of the maximum is chamcteristic for both the substrate and the medium. For DCTA, EDTA and NTA oxidations with cerium(IV) in sulphuric acid media, the maxima occur at b05, @65 and 04OM respe&vely.s For perchloric aeid media, however, the maxima occur at 3aOiUfor DCTA, 3.OM for DTPA, 3&U for EDTA and 24iU for NTA.” The observations can be explained qualitatively in terms of variation in the concentrations of the active species in the reaction media. In sulphuric acid media, Ce(SOs+ and Ce(SO,), are believed to be the most abundant species;17’3sthe fraction of the reactive species Cti present is probably very small. Also, in the high acidity range l-10M sulphuric acid, there are large concentrations of H!QWsr’which would complex further with the Ce(SO3, species: Ce(SO& + HSO,- ir Ce(SO&,*- + H+ 6040-

20-

7

IOQ

6-

-;5 6" AL? 4-

I to.2

FIQ.l.-Variations

2

4 I I,,,, 0.4 0.6 Ob I.0

I 2.0

I

J

4.0

in the rate constants for C@V)-DTPA reaction with I&SO, amcmtration.

1102

SAMIRB. HANNA et al.

The increase in the concentration of the heavily complexed Ce(SO&% and HCe(SOA~pecies~*u*~as the acidity is increased, would explain the decrease in the rate observed at [H,SO,) > 0*75M. In addition, extensive protonation of DTPA (98.3% of DTPA exists as the diprotonated species in 2M perchloric acidp would also lower the reaction rate since the lone-pairs of electrons on the nitrogen atoms would not be available for reduction of cerium(IV). Table II shows that for [HsS04] > 0*75M,a constant is obtained when the secondorder rate constant, k,, is multiplied by [H$OJ. These results show that, in this range of acidity, the rate varies inversely with KSOJ. It was expected that at [H,SOJ < @75M the observed rate constant for the consumption of cerium(IV) and consequently k,, would be aEected by the prior formation of the Ce(IV)-DTPA complex. The dependemx of the rate on the acidity iu that acid range was calculated from the results for the decay of the complex. Table III shows that a constant was obtained when ks for the complex-decay was divided by [H$OJs. T-IL-b------_ ACID

OF urn Q co-N coNcmmAlloN

AT

k,. htJlw.sec-a

[H&o,1

OF >

cd(n?

ON

Q7SM

k. x tWQ1. see-’

43.3 339

32.5 33-O

28.9 21.0 18.2 Il.2 15.8

369 31.5 31-9 28.0 31.5

0.65

23.1

84.0

Et 04

11.1 6.9 5.1

88.6 769 80.0

Rates of complex formation

Table IV lists the rate constants for the formation of the Ce(IV)-DTPA complex for various initial concentrations of reactants. Log plots of degree of reaction u,r. time are linear over 80% of the reaction. These results indicate that the rate of complex formation is first order with respect to each reactant: d {complex] F kKe(W1 IDTPAI dt The reaction rate is inversely proportional to the acidity of the medium, which is to be expected because protonation of the chelating agent would render it inactive towards compkxation with cerium(IV). Complexation of quadrivalent metal ions with polyaminocarboxylic acids probably involves octadentate chelation.” For example, the Ce(IV)-DTPA complex may be represented as shown in Fig. 2. If the transition state for the complex formation involves the release of protons, the rate

Dccarboxylation of

TABLEIV.-RAN

OF FORMA’ITON

po+alin~boxylic

OF

Cc(IV)-DTPA AT 25°C

1103

FL&is COMPLEX m 00311aIH,SO, k

ICe(n lo-‘M

[DTPA], lo-‘M

tm, msec

1.0 1.0 1.0 1.0 025 0.75 19

0.25 050 0.75 1.0 1-o 1.0 I.0

220 120 80 60

FIQ. 2.-Strwzure

z 60

lo” I.mo~-?m-1 1.26 1.15 1.15 1.15 1.15 1.15 1.15

of octadentate Ce(IV&DTPA complex.

of the process should be inverselyproportional to the hydrogen ion con~tration.~ The inverse dependence on the acidity of the medium is similar to that observed in the oxidation of hydrochloricacid by cerium(IV).M It was reportedthat the rate of oxidation de~!4~& with incrcGng [IfSO,-] owing to the co-ordinationof Ce(IV) with this species (releasingH+) which interferedwith the formation of a ‘Ce(IV)-Clcomplex requiredfor electrontransfer. Mehrot# has also sugge8tedthat increasing [H$O&kcrea& the activeCe(OH),* speciesbelievedto be neccssaryforcomplexation with citric acid. Rao explained his failure to detect a Ce(IV)-mTA complex in sulphuric acid as due to the strong Ce(IV)-SO, co-ordination.1 So far we have observed c&urn(W) complexes with EDTA,‘ DTPA‘ and NTAm in sulphuric acid media.

BDii Oxidation volt Di&bylea&minpsmtacssigs&rc (DTPA) durch Cer(IV) in Schw&ls&m wurdc spoktrophotomctrisch mit Hilfe des “sty-flow”tmtersucht. Die Rcaktionsgeschwindi eit wmd van der S&mkomc&ation bccinflu&, kano aber d UrJF ein vcreinfachtcs Gmchwindigkeiitsgesetz

wiedqqcben we&n. Bci [H,s04J u&r 0,75M verl&uft die Rcaktion stufcmcisc; das zcigt sicb in der Bildung eines 1: I-IComplexes ausCe(IV) und DTPA mit meSbam Bildungs- und Zerfallsgcschwindigkeit. In Mrkcrcr S&m ist die Bildung tines Zwis&qmdukts nicht z.u be&a&ten. In 475M Schwefels&ureist die Geschwindigkeit am @5&n. Die Brutto-Stochiomctrie ist zcitabhllngig.

SAMIR B. HANNA et al.

1104

Rdimd-On a Ctudi6 s lkcidedi&hyltnsftiaminopen sulfurlquc par la technique d’&o rtactlon cst inlluencda par I’aciditC,mais pcut &rc cxprk de vitcssc slmpli&: - F

par unc loi

= k’[Cc(IV)][DTPA]

Pour [H,SO,] infhicur &0,75M la r&actionpruc&dc par &g&s commc Ie montm la formation d’un complexo 1:l C&V)-DTPA avcc une vitesac muwrabla de formation et de d&gradation. A des forces dkclde plus Clcvks, la formation d’un intermkliak n’cst as hidente. E, stoechioLavitssseestmaximale en acldo sulfurique -0,75M. m6trie global0 varic avcc le tcmps. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

G. N. Rae, hiian J. Chem., 1970,8,328. M. B. Hafez and R. Gulllmumnt. &II. Sot. Chim. Fence, 1%9,1047. Indian J. C&m., 1%5,3,51. T. R. Bhat and R. Radhwna, S. B. Haana, S. Al-Ha&hi, W. W. Webb and W. R. Cpmou, Z. Ad. Chem., 1%9,246,231. S. B. Hamu+, R. K. He&y. W. II. Webb and W. R. Carroll, Ibid, 1971,255,30. S. B. Haana and R. K. Hcsslcy, brag. Nucl Chun. Lerretz, 1971,7,83. A. I. Vo@, Qnanti#&ue hnganic AmIysis, 3rd Ed., pp. 305,318. Longmana, London, 1961. and D. T. Sawyer, &pWnente for Ztutrumwd Methoak, pp. 176-181. McGrawC. N. Hi&New 7 ark, l%l. H. A. F&cl&a, EDTA lWatitnw; An Intro&t&m to 77wot-yandPractice, 2nd Ed.. Chap. 19. Pergamon Prew New York 1964. S. Al-Ha&in& MS. ?Re&, Unive&y of Missouri. Rolla, Missouri, 1968. M. Joaea aad D. 0. Lag&art, J. Am. C&L Sot., 1966,88,4615. S. B. Hanna, L. M. Niin and R. K. Heshy, Z. And. C%am.,1972,2S8,126. R. F&her a&l D. Putw, QuraHrcrriecChemhxl Anolysi, 3rd Ed., p. 523. Saunders, Phila-

19. L. T. v and Hauu Kh-Xh, Rws. J. Inog. Chem., 1%3,8,1299. 20. T. F. Young and L. A. BIatz, Chem. Rev., 1949,44,98. 21. S. K. It&i&mandY. K. Gupta,J. Ckm. Sk. A, 1970,2918. 22. B. Kxirhre and K. C. Tewari, I; Chem. Sot?., 1961.3097. 23. A. N. Exmakov, N. B. Kahichenko and I. N. Marov. Russ. J. Inog. Chem., 1%8,13,1622; see also E. Lin, A. A. Sandoval and K. L. Chcng, J. Mqn. Resonance, 1971,4,301. 24. R.F. and A. B. M&rteU, J. Am. Ckm. Sot., 1?58,8@, 4170. 25. R P. &uroa k ckmwy, Chapt. XIV. Cornell univ. Pm3& Ithaca, 1959. 26. A. Dulz and N. S&a, Zmwg. Chem.. 1963,2,917. 27. R. N. Mehrotra and S. Gosh, Z. Phys. Chem., Leipz&, 1%3,224,57. 28. S. B. Iianna, Unppbtthed obaewntmn.