The formation of· a red-purple iron (III) ethylenediaminetetraacetate
mono hydrogen peroxide species obeys the kinetic express ion
rate= k LFe(EDTA) (OH)2 3-J [H 202] over the range pH 9.0-11.0.
The Arrhenius equation, k = 1.9 X 10 7 exp (-6,700/RT) holds and
at 25°, k = 2.5 X 10 2 M - 1 sec-1• An enhanced rate above pH = 11 .0
may be associated with enhanced reactivity of [Fe(EDTA) (0H)3] 4-
or H02-. A brief compar ison is made of the results with data for
pe roxidase and catalase reactions with hydrogen peroxide

M . Orhanović

R. G. Wilkins

English

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CROATICA CHEMICA ACTA 39 (1967) 149 CCA-465 541.127 .1 :546.723 Oriainal Scientific Paver Kinetic Studies of the Formation of Iron (III) Ethylenediaminetetraacetate-Hydrogen Peroxide Complex M. Orhanovic* and R. G. Wilkins Department of Chemistry, State University of New York at Buffalo, N. Y. 14214, U.S. A. R eceived October 11, 1966 The formation of· a red-purple iron (III) ethylenediaminetetra-acetate mono hydrogen peroxide species obeys the kinetic expression rate= k LFe(EDTA) (OH)23-J [H202] over the range pH 9.0-11.0. The Arrhenius equation, k = 1.9 X 107 exp (-6,700/RT) holds and at 25°, k = 2.5 X 102 M - 1 sec-1• An enhanced rate above pH = 11 .0 may be associated with enhanced reactivity of [Fe(EDTA) (0H)3] 4-or H02-. A brief comparison is made of the results with data for peroxidase and catalase reactions with hydrogen peroxide. INTRODUCTION We have previously examined the kinetics of formation of peroxy species of chromium (VI), vanadium (V) and titanium (IV) in acid media1• These rapid reactions which involved replacement of oxy by peroxy groups are first-order in transition metal ion concentration and first-order in hydrogen peroxide concentration and are usually acid-catalysed, involving, we believe, protonated oxy-ions as effective reactants, e. g., HCr04- + H+ ~ H2Cr04 H2Cr04 + H202 -+ H2Cr0s -!- 2 H20 H2Cr0s + H202-+ CrOs + H:,O rapid, pre-equilibrium slow, rate determining fast We now turn our attention to the peroxy compounds of metal chelates, where the major portion of the coordination sphere is occupied by a firmly attached multidentate ligand and a water or hydroxy group is replaced by the entering hydrogen peroxide. Since an increasing amount of data is be-coming available concerning the water exchange rate of metal aqua and coordinated ions2 we can attempt to relate the water exchange and hydrogen peroxide entry rates. Present trends in this area are towards identifying the role of these peroxy complexes in the catalysed decomposition of hydrogen peroxide, a large variety of metal chelates** having been studied, including the biological important iron (III) enzymes, catalase and peroxidases.5 In this paper we study the formation of a red-purple peroxy species of the iron (III)-ethylenediaminetetraacetate (EDTA) complex in alkaline solution. * On leave from »Ruder Boskovic« Institute, Zagreb, Yugoslavia. ** Of recent interest in this context are Copper (II) complexes (See Ref. 3) and Nickel (II) complexes (See Ref. 4). 150 M. ORHANOVIC AND R. G. WILKINS EXPERIMENTAL Materials A stock solution of the iron (III) EDTA complex was prepared from Fe(Cl04)3 · · 6 H20 (G. F. Smith Chemical Co.) and the disodium salt of EDTA (Fisher). Concen-trations of iron (III)r. and the EDTA complex7 were determined spectrophotometrically. An excess of EDTA was used to ensure complete complex formation. Both sodium stannate-stabilized and non-stabilized hydrogen peroxide (Fisher) were used with similar results. Sodium hydroxide and perchloric acid solutions were used to obtain the appropriate pH and ionic strength. The concentration of hydrogen peroxide was determined with standard cerium (IV), immediately after the completion of a run. Equilibria Measurements A Cary 14 was used to characterise the species spectrally. The method employed for the measurements was essentially that described by Ringbom, Siitonen and Saxen8 and later used by Mader0. The absorbancy a t 520 mµ was measured at room temperature (23° C) of a series of solutions (at various pH values) containing different concentrations of iron (III)-EDTA and H202. The ionic strength was maintained at 0.1 M using sodium perchlorate and the pH was adjusted after all components h ad been added, to prevent precipitation of ferric hydroxide. The equilibrium constant K was calculated on the basis of the expression (1)8 [FeEDTA-H202] {[H+] + KR,o, } K = [FeEDTA] KH,o, {[H202] - [FeEDTA-H20 2l} where [H202] = total peroxide added, [FeEDTA-H202] = concentration of the colored peroxy species which is the only contributor to the optical density at 520 m~L (E = 530 cm.-1 M-1) , and K R20, = 2.2 X 10·12, [FeEDTA] represent the concentration of all Fe (IIl)-EDTA species present. Charges of the complex species have been omitted for clarity. The values of K are accurate to ± 80/o. The spectra of completely formed peroxy species have been taken at pH = 11-12. The complex obeys Beer's law in the range of concentrations used, 9 X 10- 6 - 1.7 X 10- 4 M. At lower pH (8 .0-9.5) the high H20 2 concentrations, needed to ensure complete complex formation , promote a subsequent appearance of a yellow color that interferes with spectral m easurements, but not with the kinetic measurements of the red species. At higl, pH ( ~ 12.5) a yellow solution appears which then becomes turbid, probably due to decomposition to ferric hydroxide. By working quickly however it was possible to measure the spectra of the peroxy species from pH = 8.5 to 12.0. An additional excess of EDTA had very small effect on the absorbancy. Kinetic Runs The formation of the red violet iron EDTA-H202 species was studied at 520 m~t, using a glass-lucite stopped-flow apparatus. The concentrations of iron EDTA were 6.4-8.6 X 10-s M , with excess EDTA 6.1X10- 5 - 4.0 X 10-5 M . Hydrogen peroxide was always present in large excess, ensuring pseudo first-order conditions, but its concentration range was limited at the lower end by requirements for completion of reaction and at higher concentrations with sufficiently long half-lives (t y, ~ 5 msec). High H20 2 concentrations produced oxygen bubbles in the observation chamber which prevented measurements. In the majority of the kinetic runs, peroxy formation was substantially complete, but in the two runs at pH = 9.5 and 9.0, the extent of formation was ""' 90 and ""' 800/o respectively, so that k obs (= 0.93/ t y2 ) probably includes a small contribution from the back reaction. Excellent traces and first-order kinetics were shown. Attempts were made to follow the rates of formation of [Fe(EDTA) (OH)]2- from [Fe(EDTA)J- and of [Fe(EDTA) (OH)2p - from [Fe(EDTA) (0H)]2- by reacting a solution of [Fe(EDTA)]- at pH = 6-7 with an alkaline solution so as to obtain a final pH of 00 8.5 or 00 10.0, using phenolphthalein or thymolphthalein as indicators. There was no indication of any slow processes in the production of the hydroxy species, indicator color adjustment being complete in the time of mixing. In the IRON (III) EDTA - H202 COMPLEX 151 event that a coordinated carboxylate (rather than an aqua species) was being replaced by hydroxide ion a measurable rate may have resulted. A rapid hydroxylation is however inconclusive in dis tinguishing between the two possibilities. RESULTS AND DISCUSSION Iron (III) ethylenediaminetetraacetate complex forms hydroxy species in alkaline solution, [Fe(EDTA)OH]2- pK = 7.5, [Fe(EDTA) (OH2p- pK = 9.5 and [Fe(EDTA) (OH) 3] 4- pK ~ 12.510. At the low concentrations of complex used, binuclear species are unimportant11 . In most our kinetic studies, the major species in solution is the dihydroxy complex. Our results, where they overlap, are in substantial agreement with previous workers. The red-violet peroxy species has a m aximum at 520 m~t12 , E = 530, slightly lower than the value reported by Mader9 (E = 547). The position of the absorption maximum is identical from pH - 8.5 to 12.0 and so if in this range there exists different peroxy species with respect to the number of hydroxy groups (which is the case with ferric-EDTA itself) they appear to have identical spectral characteristics. TABLE I Formation Constants (Expression (1)) for Iron Ethylenediaminetetraacetate Peroxy Species at 23°, I = 0.1 M [FeIII complex] pH 10-3 K ~tM M-1 75 8.5 9.4 75 9.0 9.9 75 10.0 9.8 75 11.0 8.3 86 11.0 7.8 170 11.0 7.2 340 11.0 7.8 75 11.5 7.1 The constancy of K (Table I) on the basis of expression (1) indicates8 that H"O is being replaced by H022 - or that 0 22- is taking the place of Off in the coordination sphere of iron(III). The magnitude of K (9.8 X 103) is naturally sensi-tive to the chosen value of K u,0 , : For our conditions, K H,o, = 2.2 X 10-12 appears reasonable13•14• The values for K p reviously reported are 3.6 X 103 at 22° (conditions unspecified)8 and 7.1 X 10a at 250 (I = 0.38)9• The latter value would be modified to 1.3 X 10( if our value for K H,o, was used. Our constant shows a small trend toward decreasing values with increase in pH which, if real, possibly means a small direct participation of H 00 2 in the reaction. Two typical kinetic oscilloscope traces are shown in Fig. 1. These demon-trate, together with the data in Table II, the insensitivity to pH from 9 to 11 of a second-order rate constant k 2 for which: d/dt [Fe(EDTA) - H 20 2] = k, [Fe(EDTA) (OHL3-] [H"0 2] with k 2 = 1.9 X 101 exp (-6,700/RT) and at 25°, k 2 = 2.5 X 102 M-1 sec.-1 The rate law does not, of course, distinguish between a replacement mechanism controlled by a water exchange process2 and a simple bimolecular reaction 152 M. ORHANOVIC AND R. G. WILKINS ~ ---Ai , '/ -~ ,,,,_ / -Fig. 1. Stopped-flow trace showing formation of red-violet [FeEDTA-H202] complex from [FeEDTA(OH)2]3-, [FeEDTA] = 6.4 X 10-s M, [H202] = 0.09 M, I = 0.1 M, t = 230 C. Upper curve at pH = 11.5, lower at pH = 10. Each major division represents 0.05 sec. Increasing optical -density is represented upwards. TABLE II Kinetic Data for the Formation of Iron (III) Ethylenediaminetetraacetate Peroxy Species, I = 0·1 M Temp. oc 23 23 23 23 23 23 23 23 23 23 23 23 23 23 14.5 6.5 1.0 pH 9.0 9.5 10.0 10.0 10.0 10.0 11.0 11.0 11.0 11.0 11.5 12.0 12.0 12,0 11.0 11.0 11.0 ! I 203 144 123 lOlc lOld 101 96 56 20 6 90 80 40 21 60 60 60 t y,, sec. 0.018 0.020 0.026 0.036 0.037 0.034 0.031 0.061 0.20 0.58 0.025 0.027 0.058 0.12 0,072 0.097 0.12 10- 2 kobsf [H202]b M - 1 sec.-1 1.9 2.4 2.2 2.0 1.9 2.1 2.8 2.4 2.2 2,5 5.2 10.3 9.6 9.2 1.6 1.2 1.0 •Total hydrogen perox ide u sed. "Free [H202]. c1ron (III) complexes 1.3 X 10-• M. "Excess [EDTA] = 3.2 X 10-• M. between [Fe(EDTA) (OHL] 3- and H 20 2 • Certainly the second order rate constant is not an unreasonable value for the water exchange for iron (III) coordinated water should this be present in the complex or alternatively for possible carboxylate-iron (III) bond breakage prior to H 20 2 attack. On the other hand a coordination number exceeding six is present in solid structures of Fe(III)-EDTA complexes and so a direct SN2 reaction, unusual in replacement IRON (III) EDTA - H 202 COMPLEX 153 reactions generally, may be plausible here. We consider the equivalent process (in terms of the composition of the transition state) between [Fe(EDTA) (OH)]2-and Ho2- unlikely in view of the constancy of a computed k 2 at pH= 9.0, where the monohydroxy species dominates. It appears that H 20 2 reacts equally easily with the mono- and di-hydroxy iron (III) chelate. At pH above 11.0 there is an increase of rate which may be associated with increasing amounts and reactivity of the Ho2- species. Alternatively the reaction of [Fe(EDTA) (OH3)] 4-, which appears at these alkalinities, with H20 2 or Ho2- may play a role in the enhanced rates. The low enthalpy of activation (6.1 kcal. mole-1) and the highly negative entropy of activation (·- 28 e. u.) are characteristic of the formation of peroxy compounds so far studied1 • Peroxidase and catalase are hemo-proteins containing, like also hemoglobin and myoglobin, the same reactive group, iron (III) protoporphyrin. Peroxidase has one heme group and catalase four heme groups per molecule. The kinetics of their reactions with peroxides have been studied by Chance and his col-leagues by rapid flow methods5• The formation of the peroxidase or catalase 1 : 1 adducts with hydrogen peroxide (which probably also involves replacement of iron (III) coordinq.ted aqua or hydroxy groups) are also second-order and as in our reactions are fairly indepedent of pH over a wide range, pH 4-11. They occur however with several orders of magnitude higher second-order rate constants (9 X 10~ and 5 X 106 M-1 sec.-1) than the corresponding reaction of [Fe(EDTA) (OH)2p-, 2.5 X 102 M-1 sec.-1 This presumably reflects an enhanced !ability of coordinated water or hydroxy group in the former case caused by the strong in-plane coordination of the fused porphyrin ring to the iron (III). This would be similar to the effect of coordinated hydroxide on the rates of reaction of iron (III). With a variety of ligands Fe(OH)2+ reacts with formation rate constants of the order 104 M-1 sec.-1, whereas the corresponding reactions with the fully aquated iron (III) ion proceed much more slowly (k "' 10 M-1 sec.-1) 2• This wo·rk was supported by the National Science Foundation (Grants GP 1963 and GP 5671) to whom we are indebted. REFERENCES 1. P. Moore, S. F. A. Kett 1 e, and R. G. W i 1 kins, Inorg. Chem. 5 (1966) 466. M. Orhanovic and R. G. Wilkins, J. Am. Chem. Soc. 89 (1967) 278. 2. M. E i gen and R. G . W i 1 kin s, The Kinetics and Mechanism of Formation of Metal Complexes in Mechanisms of Inorganic Reactions, Advances in Chemistry Series 49, American Chemical Society, Washington, 1965. 3. H . Sig e 1 and H . Er 1 en me ye r, Helv. Chim. Acta 49 (1966) 1266. 4. R. Z e 11 and H. Sig e 1, Helv. Chim. Acta 49 (1966) 870. 5. B . Ch an c e, Enzyme Kinetics in the Transient State in Investigation of Rates and Mechanisms of Reactions Part II, S . L . Fri es, E. S. L ewis, and A. Weissberg er, Eds., Interscience Publ., New York, 1963. 6.W. L. Reynolds, N. Liu, andG. Mickus, J.Am.Chem. Soc.83(1961)1078. 7. R. Bastin, R. Weberling, and F . Pahla, Anal. Chem. 28 (1956) 459. 8. A . Ring b om, S. Si it one n, and B. Saxe n, Anal. Chim. Acta 16 (1957) 541. 9. P . M. M ad er, J. Am. Chem. Soc. 82 (1960) 2956. 10. G. Schwarzenbach and J . He 11 er, 'Helv. Chim. Acta 34 (1951) 576. 11. R. L. Gust a f son and A. E. Marte 11, J. Phys. Chem. 67 (1963) 576. 12· Z. P . K a ch an ova and. A. P. P u rm a 1, Zhur. fiz. khim. 38 (1964) 1342. 154 M. ORHANOVIC AND R. G . WILKINS 13. M. G . Ev ans and N. Uri, Trans. Faraday Soc. 45 (1949) 224. 14. K . Y . S a 1 n is, K. P . Mish c h e n k o, and I. E. F 1 i s, Zhur. neorg. khim. 2 (1957) 1985. IZVOD KinetiCke studije stvaranja Fe llll) etilendiamin-tetraacetat-hidrogen peroksid kompleksa M. Orhanovic i R. G. Wilkins Stvaranje purpurno crvene Fe (III) etilenediamin-tetraacetat mono hidrogenper-oksid kompleksne vrste slijedi kineticki izraz brzine = k [Fe(EDTA) (OH)23-] [H20d u podrucju pH od 9,0-11,0. Arrheniusova jednadzba, k = 1,9 X 107 exp (- 6.700/R'l') vrijedi i kod 25° C k = 2,5 X 102 M-1 sek-1• P ovecana brzina reakcije iznad pH = 11,0 mogla bi se pripisati poveeanoj reaktivnosti [Fe(EDTA) (OH)sJ 4- ili Ho2- . Rezultati su ukratko usporedeni s podacima reakcija peroksidaze i katalaze s vodikovim peroksidom. CHEMISTRY DEPARTMENT STATE UNIVERSITY OF NEW YORK AT BUFFALO, N. Y., U . S . A. Primlj eno 11. listopada 1966. 

Kinetic Studies of the Formation of Iron (III) Ethylenediaminetetraacetate-Hydrogen Peroxide Complex

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