Electrochemical studies of molybdenum-ethylenediaminetetraacetic acid complexes

ELECTROANALYTICALCHEMISTRYAND INTERFACIALELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne - Printed ill The Netherlands

207

E L E C T R O C H E M I C A L STUDIES OF M O L Y B D E N U M E T H Y L E N E D I A M I N E T E T R A A C E T I C ACID COMPLEXES

FRANKLIN A. SCHULTZ* AND DONALD T. SAWYER Department of Chemistry, Universily of California, Riverside, Calif., 92.502 (U.S.A .)

(Received July I7th, 1967)

Among the chelates formed b y ethylenediaminetetraacetic acid (EDTA), those with Mo(VI) and Mo(V) are unusual because the metal-to-ligand ratio is two 1. The structures of these two compounds are shown in Fig. I. For the Mo(VI) complex (Mo20~Y4-), the structure originally proposed by PECSOK AND SAWYER1 has been confirmed b y proton NMR studies 2-4 and an X - r a y structural determination 5.

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Fig. i. Structures of the 2 : I molybdenum-EDTA complexes, Mo2OeY4- and Mo204Y2-. MITCHELL6 has suggested the indicated structure for the Mo(V) compound (Mo204Y 2-) to explain the observed diamagnetism of the compoundL The basic features of this structure have also been verified b y a recent X - r a y determination 8. The unique structural features are of interest in terms of their effect on the electrochemistry of the m o l y b d e n u m - E D T A complexes. This is particularly true for Mo(VI)2-EDTA, a molecule containing two essentially independent metal atoms. Apart from structural considerations, however, the electrochemistry of m o l y b d e n u m E D T A complexes is sufficiently complicated that there has been little agreement in the literature. Two early polarographic investigations of Mo(VI)-EDTA 9,1° made no mention of possible reduction products. In a comprehensive polarographic survey 1, two reduction waves are observed for Mo(VI)-EDTA between p H 2.5 and p H 4.5, and a single wave between p H 4.5 and p H 7- The height of each of these waves decreases with increasing pH. This fact, and a linear relationship between E~ and p H led to a proposed mechanism whereby two hydrolytic species of Mo(VI)2-EDTA are reduced to Mo(V)2-EDTA. Controlled-potential electrolysis experiments 11, however, have shown that at p H 5 between 2 and 3 electrons are consumed/molybdenum atom in the reduction of Mo(VI) in o.I F EDTA. SINIAKOVAAND GLINKINA12 also support the 3-electron reduction of Mo(VI) in EDTA, but their conclusion is based * Present address: Beckman Instruments, Inc., Fullerton, Calif. j . Electroanal. Chem., x7 (1968) 207-225

208

F. A. SCHULTZ, D. T. SAWYER

solely on polarographic diffusion current measurements. These same authors report an n-value of 5-6 from coulometric experiments. A review of the polarographic investigations demonstrates the surprising fact that thele is not a single instance of agreement as to the half-wave potential of Mo(VI) reduction in solutions of comparable composition. The purpose of the present investigation has been to use modern electrochemical techniques, including chronopotentiometry and cyclic voltammetry, to elucidate the reduction mechanisms for Mo(VI) and Mo(V) in EDTA solutions. A further goal has been to ascertain whether correlations c a n be made between the electrochemical results and the structural characteristics of the various molybdenum-EDTA species. A relationship between structure and electrochemical behaviour would be very useful when applied to metal-chelate systems of biological importance. For example, several of the molybdenum-containing enzymes, including aldehyde oxidase 13, xanthine oxidase 14 and nitrate reductase 15,16, require molybdenum in certain of their enzymatic functions. The Mo(VI)/Mo(V) couple has been widely suggested 17-2° for the electron transfer sequence in such systems. EXPERIMENTAL All electrochemical experiments were performed using an instrument based on Philbrick operational amplifiers described by DEFORD 21. Chronopotentiometric potential-time patterns were recorded with a Sargent model SR-2 strip-chart recorder or, for fast times, with a Tektronix model 564 storage oscilloscope. Cyclic voltammetric curves were recorded with a Moseley model 3 X-Y recorder. The components of a Leeds and Northrup Coulometric Titration Cell Kit (No. 7961) were used as an electrochemical ceil. For coulometric studies, a tungsten wire was fused into the bottom of the cell to make contact with the mercury pool cathode. Three-electrode circuitry was employed for all electrochemical experiments. A stationary mercury working electrode was prepared by amalgamating a platinum inlay electrode (Beckman No. 39273) following the procedure of MOROS22. The geometric area of this electrode was 0.203 cm ~. The reference electrode was a saturated calomel electrode. The auxiliary electrode, a 2-cm 2 platinum foil, was isolated from the test solution in a glass tube closed with a glass frit. Visible and ultraviolet spectra were obtained using a Cary model 14 spectrophotometer with matched I-cm silica cells. Electron spin resonance measurements were made with a Varian V-45o2 spectrometer equipped with a Varian V-4548 aqueous solution sample cell. Solutions of Mo(V)2-EDTA were made directly from the prepared chelate, Na.oMo204Y .H201. Solutions of Mo(VI)-EDTA were made either from the prepared chelate, Na41Vfo206Y.8H201, or determinately from the disodium salt of EDTA and Na2MoO4.2H~O. Potassium hexachloromolybdate(III) was used as received from Alfa Inorganics. Although Mo(III) compounds are susceptible to air-oxidation, satisfactory results were obtained by working rapidly with degassed solutions. Concentrated H2S04 and NaOH were used to adjust the p H of sample solutions. Sample solutions were degassed with pre-purified grade nitrogen. All other chemicals were reagent-grade materials. The results for voltammetric experiments with Mo(VI)-EDTA and Mo(V)2J. Electroanal. Chem., 17 (1968) 2o7-225

1V[o--EDTA COMPLEXES

209

EDTA were highly dependent upon how long the electrode had been allowed to stand in contact with the solution, and whether or not the electrode had been used in a previous trial. As standard procedure, the mercury surface was renewed and allowed a 6o-sec period of equilibration prior to each trial. Despite this attempt at standardization, reproducible data were difficult to obtain, especially in terms of peak potential measurements. For chronopotentiometric experiments, however, renewal of the electrode surface was unnecessary. Reproducible transition times were obtained over a considerable period of time if the electrodes were used frequently enough to prevent the build-up of adsorbed materials. RESULTS

Reduction of m o l y b d e n u m ( V I ) - E D T A

The cyclic voltammetric reduction of Mo(VI) in EDTA solutions is complicated by a variety of surface effects. These are illustrated by the traces shown in Fig.2. Curve A is obtained at a "fresh" mercury surface (one that has been equilibrated with the solution for only 60 see). If instead, an electrode is allowed to stand in the solution for prolonged periods of time, a trace such as curve B results. The most

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line shows adsorption peak established following prolonged equilibration between electrode and

soln.). notable effect of extended equilibration is a sharply-spiked adsorption peak. The potential of this peak, which is highly reproducible, is a linear function of pH which can be expressed as E p = - 0 . 2 2 3 - 0 . 0 5 7 (pH) V vs. S.C.E. However, the peak current increases as a function of equilibration time and is not reproducible, & reversal of j . Electroanal. Chem., 17 (1968) 207-225

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the voltammetric scan just past the adsorption peak (inset, Fig. 2) causes limited current to flow during the anodic cycle. The effects of prolonged equilibration are less noticeable on the major reduction wave of Mo(VI). In general, equilibration decreases the peak current of the wave, simultaneously shifting the peak potential in a cathodic direction, and the potential of solvent reduction in an anodic direction; this effect is illustrated b y curve C of Fig. 2. At low pH, the v o l t a m m e t r y of Mo(VI)-EDTA is complicated by the presence of two additional reduction peaks (Fig. 3). The first (least cathodic) peak is established rapidly during the 60 sec allowed for the solution to quiesce; the peak current decreases sharply with increasing p H and vanishes at approximately p H 3.5. This rapidly established adsorption peak is distinct from the slowly established adsorption peak shown in Fig. 2. The potential at which the latter peak occurs for a solution at p H 2. 4 is indicated by the broken line in Fig. 3. An adsorption-like peak also is observed prior to the major reduction wave of Mo(VI)-EDT&. As the p H is increased, these two peaks coalesce, and at approximately p H 3.5 or above, only a single wave is observed (Fig. 2). Similar behaviour is observed in chronopotentiometry. At sufficiently high molybdenum concentrations and sufficient acidity, two distinct steps appear in the 2OO

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Fig. 4' Chronopotentiolnetric reduction of Mo(VI)-EDTA in o.I F EDTA at pH 3.6 with a mercury electrode. (A), 4.85 mF Mo(VI), i = IOO/~A; (B), (with current reversal) o.548 mF Mo(VI), i = io//A. Fig. 5. Plot of iz~/C vs. i for the chronopotentiometric reduction of Mo(VI) in o.I F EDTA at a mercury electrode. pH 3.6: (×), o.194; (El), o.548; (ZX),1.62; (O), 4.85 mF Mo(VI). pH 4.4: (I), o.548; (O), 4.85 mF Mo(VI). pH 5.2: (V1),o.548; (Q), 4.85 mF Mo(VI), j . Electroanal. Chem., 17 (1968) 207-225

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reduction wave for Mo(VI)-EDTA (curve A, Fig. 4). The first potential inflection is not well-defined which precludes accurate measurements; with increasing p H and decreasing molybdenum concentration, this wave decreases and eventually vanishes. The chronopotentiometric constant, iz~/C, for the total transition time varies drastically over a wide range of current. Figure 5 indicates that the value of iz½/C decreases with increasing current and increasing pH. At neither high nor low currents does iz*/C approach a limiting value. For solutions with higher pH-values the decrease in i~/C is so marked that a transition time is not observed above a certain current level. Thus, for a molybdenum concentration of 4.85 m F , the transition time is zero for currents above 800 #A at p H 4-4, and above 250 #A at p H 5.2. At p H 3.6, however, transition times are observed at currents as high as 4.0 mA; the value of i~/C does decrease with increasing current, but never falls to zero. Stirring during electrolysis at high current has little effect on the chronopotentiometric wave. For 4.85 m F Mo(VI) in o.I F E D T A at p H 3.6 a transition time of only 14. 5 msec is observed if stirring is carried out during electrolysis at 2.00 mA; the transition time is 12.o msec in unstirred solution for the same current. A m a x i m u m transition time of 18 msec is obtained if the electrode is equilibrated for 2-5 min with the solution, followed by stirring during electrolysis. The present voltammetric and chronopotentiometric data parallel earlier polarographic results 1. The most striking similarity is the change in the major reduction process of Mo(VI)-EDTA from a two-wave process below p H 4 to a one-wave process above p H 4. For the present studies, the reduction potentials also become more negative monotonically with increasing pI-I, but a quantitative relationship has not been established. A corresponding decrease in voltammetric peak currents and chronopotentiometric transition times occurs with an increase in pH. Varying the E D T A concentration from 4 m F to o.I F, however, produces no noticeable change in the electrochemical results. The slowly established adsorption peak (Fig. 2) has no polarographic counterpart because of frequent renewal of the electrode surface in polarography. The rapidly established adsorption peak (Fig. 3) m a y have a connection with a third polarographic wave observed at very low pH-values 1. Data for the coulometric reduction of Mo(VI)-EDTA at a mercury pool electrode are summarized in Table I. The reduction approaches, but never quite attains, an n-value of three. Even after an extended period of electrolysis (27 h) a value of only 2.90 is obtained, and most of this (950} accumulates in the first IO h. Reduction of Mo(VI) proceeds more rapidly at lower pH-values and more cathodic TABLE

1

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j . Electroanal. Chem., 17 (1968) 2 o 7 - 2 2 5

212

F . A . SCHULTZ, D. T. SAWYER

potentials, but the resultant n-value is unchanged. The final product, Mo(III), is green in color, but during electrolysis the solution changes from colorless to yellow, to brown and finally to green. The unusual shape of the curve for accumulated coulombs as a function of time also indicates that the reduction process is not straightforward. When the coulometric product from Mo(VI)-EDTA reduction is reoxidized at - o . 2 0 V vs. S.C.E. an n-value of 1.8 is obtained. Further studies of the reduction products of Mo(VI)-EDTA have been made using cyclic voltammetry and chronopotentiometry. Figures 2 and 3 indicate that only a limited current flows when the potential is cycled anodically following the reduction of Mo(VI)-EDTA. However, faintly discernible anodic peaks are apparent in the vicinity of - 0 . 6 to - o . 3 V vs. S.C.E. Similar oxidation processes appear to occur in reverse current chronopotentiometric experiments (curve B, Fig. 4). However, the ratio of the forward to reverse transition time varies from about 8-to-I at high currents to much larger values at low currents. At low currents, the reverse wave is composed of two waves, the more anodic being by far the longer. Figure 6 illustrates a cyclic voltammetric trace for the Mo(III) species that is produced coulometrically from Mo(VI)-EDTA. The curve is composed of two redox couples, designated as Couple I (the more anodic) and Couple II (the more cathodic). The potentials of these two couples are close to the potentials of the anodic processes observed in cyclic voltammetric and reverse-current chronopotentiometric experiments with Mo(VI)-EDTA. Both couples are considered reversible in the gross sense, although the difference of approximately 12o mV between Ep, a and Ep,e for Couple I is greater than the value of 6 o / n mV for a thermodynamically-reversible system 2s. The curve in Fig. 6 is obtained by scanning in an anodic direction from - 0 . 6 0 V vs. S.C.E. If instead, one scans cathodically from this point, only a very small cathodic peak is observed at the potential of EH~,e. This is believed to result from the presence of oxidized material or from incomplete reduction to the + 3 I

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Fig. 6. Cyclic voltammetry of Mo(III) species produced by coulometric reduction of M o ( V I ) E D T A . 2.11 m F Mo(III), 0.0 5 F E D T A , p H 3.7, scan rate = lOO mV/sec. Fig. 7. Voltammetry of Mo(VI)-EDTA coulometrically reduced by o, I, 2 a n d 3 equiv./mole Mo(VI); o.I F E D T A at pH 3.6. (A), (dotted line, right-hand ordinate) unreduced, 0.508 m F Mo(VI), scan rate = 5 ° m V / s e c ; (]3 a n d B'), (broken line, left-hand ordinate) reduced 1-equiv., 4.86 m F Mo(VI), scan rate = IO m V / s e c ; (C a n d C'), (solid line, left-hand ordinate) reduced 2-equiv., 4.86 m F Mo(VI), scan rate = IO m V / s e c ; (D a n d D'), (solid line, left-hand ordinate) reduced 3-equiv., 4.86 m F Mo(VI), scan rate ~ IO mV/sec.

.T. Electroanal. Chem., 17 (1968) 207-225

~V~o-EDTA COMPLEXES

213

state. Thus, the anodic half of Couple I must comprise the majority of electroactive material following coulometric reduction of Mo(VI), and the species in Couple II must be produced only through the oxidation of this material. A particularly informative experiment is illustrated by Fig. 7, where a series of voltammetric traces is shown for a Mo(VI)-EDTA solution that has been reduced coulometrically by o, I, 2 and 3 equivalents/mole of Mo(VI). Each curve represents a separate voltammetric trial, scanned anodically and cathodically from the rest potential of the particular solution. Essentially, the same results are obtained by chemically reducing Mo(VI) with Mo(III) species produced by coulometric reduction. Curve A in Fig. 7 is a normal cathodic trace for unreduced Mo(VI). After the solution is reduced by I.O equivalent, the rest potential falls to - 0 . 3 4 V vs. S.C.E., the approximate potential of Couple I. Curves B and/3' indicate that a small amount of this couple is present. The majority of material in a 1-equivalent reduced solution contributes to two major cathodic waves. These waves overlap, but the potential of the more cathodic wave is almost the same as that for Mo(VI) reduction. When the solution is reduced by 2.0 equivalents the rest potential falls to - 0 . 6 0 V vs. S.C.E. At this stage a single peak is observed when the electrode is scanned cathodically from - 0 . 6 0 V (curve C), and a single peak when the electrode is scanned anodically from - 0 . 6 0 V (curve C'). The potentials of these peaks correspond to the potentials of Couples I and II in Fig. 6, and, in a two-equivalent reduced solution, the respective peak heights are virtually equal. As the solution is reduced further (curves D and D'), the height of the anodic peak increases at the expense of the cathodic peak. Although the cathodic peak diminishes noticeably, it never disappears completely. This is consistent with the coulometric results, which indicate that the reduction of Mo(VI)EDTA approaches but never attains an n-value of three. Chronopotentiometric experiments conducted during the coulometric reduction of Mo(VI)-EDTA gave virtually the same results as voltammetric studies. For a two-equivalent reduced solution, an anodic and cathodic wave of equal length are observed. Current reversal studies indicate that the ratio zf/zr is only slightly larger than three. For the cathodic wave (Couple II, Fig. 6), plots of E vs. log [(z½-t~)/t½3 are linear with a reciprocal slope of 65 mV, indicating a reversible Ielectron process 24. The anodic wave (Couple I, Fig. 6) is an irreversible process on the basis of linear E vs. log [I - (t/z) ½3plots z4. Reduction of m o l y b d e n u m ( V ) 2 - E D T A

Controlled-potential coulometry of the dimeric form of the molybdenum(V)EDTA complex, Mo(V)2-EDTA, results in a change of 2.25 +0.05 electrons/molybdenum atom when reduction is carried out at - 1 . 1 5 V vs. S.C.E. at pH 4.0. As with the reduction of Mo(VI)-EDTA, several hours are required to complete a coulometric electrolysis. Although the 2-electron reduction product of Mo(V)2-EDTA is a green material, a striking series of color changes is not observed, which is in contrast to the reduction of Mo(VI). When the coulometric product is reoxidized at - 0 . 2 0 V vs. S.C.E. an n-value of 1. 9 is obtained. The visible-ultraviolet spectrum of the final product, which now has passed through a complete reduction-oxidation cycle, agrees with that of the original material, Mo(V)2-EDTA. As with Mo(VI)-EDTA, the voltammetry of Mo(V)2-EDTA is complicated by the surface-active.properties of the electroactive species. This is illustrated by the ]. Electroanal. Chem., 17 (1968) 207-225

214

F.A. SCHULTZ, D. T. SAWYER

cyclic voltammetric curve in Fig. 8 ; the major reduction wave is preceded b y a sharp, adsorption-like peak, similar to the case for M o ( V I ) - E D T A (Fig. 3)- At high p H values the adsorption peak either vanishes or merges with the second wave. The peak potential for the reduction of Mo(V)2-EDTA is independent of p H from p H 2. 4 to p H 5.6, with a value of - 1 . o 5 + o . l o V vs. S.C.E. Consequently, at higher pH-values the voltammetric wave becomes more distinct from the background of solvent reduction. At p H 4.5 or above, the voltammetric peak current also is independent of pH, and the quantity, ip/v~, appears to be constant within this region

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1V[O-EDTA COMPLEXES

215

observed. Coincidental to the disappearance of the more anodic reverse wave, iz~ approaches a constant value, and the ratio Zf/Zr approaches a value of three.

Molybdenum (III)-EDTA species Because coulometric reduction of Mo(VI)-EDTA and Mo(V)2-EDTA leads in both cases to Mo(III), a more complete characterization of the product species is desirable. The present experiments indicate that a variety of species can be obtained for a single oxidation state of molybdenum. Therefore, to avoid confusion, each species is identified b y a subscript which refers to the compound from which it has been derived. Species derived from Mo(VI)-EDTA are denoted b y the subscript A, species from Mo(V)~-EDTA b y the subscript B, and the species from K3MoCI6

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Fig. I I . Cyclic v o l t a m m e t r i c c u r v e s a t p H 3.6 of M o ( I I I ) A - E D T A , M o ( I I I ) B - E D T A a n d K~MoC16 in E D T A . (A), M o ( I I I ) a - E D T A (from c o u l o m e t r i c r e d u c t i o n of M o ( V I ) - E D T A ) 2.1I m F Mo(III), 0.05 F E D T A , s c a n r a t e = i o o m V / s e c ; (B), M o ( I I I ) ~ r - E D T A (from coulometric r e d u c t i o n of Mo(V)~-EDTA) 1.o 7 m F Mo(III), o.1 F E D T A , s c a n r a t e = 50 m V / s e c ; (C), M o ( I I I ) c - E D T A (from KsMoC16) 1.31 m F K3MoC16, o.oi F E D T A , o.I F K2SO4, s c a n r a t e = 20 m V / s e c .

b y the subscript C. Even so, more than one species for a single oxidation state m a y be derived from one of the original compounds; in this case the various species are denoted b y primes. Thus, for a certain oxidation state derived from Mo(VI)-EDTA, one m a y have Mo(x)A, IV[O'(X)A and MO"(X)A. This notation will be used hereafter to avoid ambiguity. The coulometric reduction products of Mo(VI)-EDTA and Mo(V)~-EDTA are j . Electroanal. Chem., 17 (1968) 2o7-225

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F.A.

S C H U L T Z , D. T. S A W Y E R

clearly two distinct complexes, Mo(III)A-EDTA and Mo(III)B-EDTA. This is evident from the cyclic voltammetric curves for the two compounds which are illustrated by curves A and B in Fig. II. The potentials at which Mo(III)A-EDTA and Mo(III)B-EDTA are oxidized are not greatly different, but the variety of chemical reactions following the initial electron transfer step differentiates the two species. Following the oxidation of Mo(III)A-EDTA, two reversible or "quasi"reversible redox couples are established. Following the oxidation of Mo(III)B-EDTA, only a single cathodic wave is observed, presumably due to the reduction of 1V[o(V)2EDTA. After this cathodic peak is scanned and cycled back in an anodic reaction, a second anodic peak appears at intermediate potentials. Visible spectrophotometry further establishes the individuality of Mo(III)AEDTA and Mo(III)B-EDTA. In 0.05 F EDTA, Mo(III)A-EDTA exhibits maxima at 615 m# and 383 m#; Mo(III)B-EDTA at 605 m# and 363 m/z. Mo(III)A-EDTA and Mo(III)B-EDTA also differ in their reactions when mixed with Mo(VI)-EDTA in de-aerated solution. Combination of Mo(III)B-EDTA with Mo(VI)-EDTA yields essentially no reaction, but Mo(III)A-EDTA reacts rapidly to yield a yellow product. This product, tentatively identified as Mo(V)A-EDTA, is rapidly air-oxidized to a colorless material. Thus, Mo(V)A-EDTA is distinct from the dimeric species of Mo(V)2EDTA, which is inert to atmospheric oxidation. Curve C of Fig. I I indicates the cyclic voltammetric trace of a third lVio(III) species, Mo(III)c-EDTA, obtained upon addition of K3MoC16 to a solution of EDTA. In a non-complexing supporting electrolyte such as sodium acetate, erratic and irreproducible traces are obtained. The reproducible voltammetric peaks obtained in EDTA are, therefore, indicative of complexation. When K3MoC16 is mixed with EDTA, the solution is initially pale red, but fades to yellow-orange within an hour. This change in color, however, is not accompanied by a change in electrochemical behavior, and is probably due to the replacement of the remaining chloride ligands by solvent molecules. If the solution is allowed to stand for a prolonged period of time in the absence of oxygen, a further color change to green is observed. The new species, Mo'(III)c-EDTA, may well be identical with Mo(III)A-EDTA. A cyclic voltammetric curve of K3MoC16 in EDTA after prolonged standing is the same as curve A in Fig. II. Although desirable, isolation and characterization of the various molybdenum(III)-EDTA chelates has not been successful. A major complication is the sensitivity of the compounds to atmospheric oxidation. Also, attempts to produce Mo(III)aEDTA and Mo'(III)c-EDTA at o.I M concentrations have resulted in the formation of a green precipitate. ARCHER2s reported an insoluble green diamagnetic product upon mixing K3MoC16 with Na2H2EDTA in air-free water, but did not characterize the material further. The coulometric product from Mo(VI)-EDTA reduction (4.85 m F Mo(VI), n = 2.75 ) gives a small E S R signal immediately after being placed in an ESR cavity. However, oxidation of this material by shaking the solution in the atmosphere produces a new signal ten times more intense than that of the original sample. Therefore, the majority of Mo(III)A-EDTA is concluded to be diamagnetic; the original signal must have resulted from a minor constituent in the solution. This minor constituent may be due to incomplete reduction of Mo(VI), air-oxidized impurities, or a small percentage of paramagnetic Mo(III). J. Electroanal. Chem., 17 (1968) 207-225

~O-EDTA COMPLEXES

217

DISCUSSION AND CONCLUSIONS

Reduction of molybdenum(VI)-EDTA The electrochemical reduction of Mo(VI)-EDTA is complicated by both physical and chemical processes. The physical processes include surface interactions and other phenomena which manifest themselves as voltammetric adsorption peaks. The chemical processes include reactions that occur either before or after the initial electrode reaction. Although the experimental conditions generally are such that complications of both types occur simultaneously, the two types of processes will be treated separately. Physical processes in the reduction of Mo(VI)-EDTA. Figure 3 illustrates the three adsorption peaks observed in the voltammetric reduction of Mo(VI)-EDTA. The first, or least cathodic peak, occurs only in strongly acid solution and results from the reduction of surface material supplied by a rapidly established equilibrium between the electrode and the golution. The second adsorption peak is established only after a rather long equilibration period. For these two adsorption peaks, the molecular composition of the species that is reduced has not been determined. All that is certain is that an adsorbed layer or film is involved. Because both the slowly and rapidly established peaks occur at potentials much more anodic than the major reduction wave, these peaks are probably not due to the reduction of a simple Mo(VI)EDTA species. The adsorption peaks may be due to the reduction of polymeric molybdate species or to Hg-Mo-EDTA compounds formed by chemical oxidation of the electrode. The third adsorption peak occurs at the potential of the maior reduction wave for Mo(VI)-EDTA. For this reason the adsorbed material is believed to be a molybdenum(VI)-EDTA chelate species; this material and the bulk solution component are probably reduced via similar electrochemical routes. The specifically adsorbed material, however, is present only at p H 4 and below. The non-adsorbed material appears to be reduced both above and below pH 4. When adsorbed and diffusive materials are reduced concomitantly, the chronopotentiometric product, its, increases with increasing current 2~. However, Fig. 5 indicates that iT ~decreases with increasing current. Such behaviour is indicative of a system involving a chemical reaction prior to the electrode reaction, Y ~ O , O ~ R +ne-28. That such a reaction is not dominant below pH 4, can be established by the chronopotentiometric experiments in which the solution is stirred during electrolysis. Only a small enhancement of the transition time is observed. If a prior chemical step, Y ~ O , were involved, there would still be sufficient O in solution to increase enormously the transition time during stirring. The occurrence of a finite transition time during stirring is also indicative of the presence of an adsorbed species. The adsorption of this material must therefore be the kinetically-limiting factor. At a low level of current, the transition time is not inordinately short because the rate of adsorption is sufficiently fast to be competitive with the electrochemical reduction of Mo(VI)-EDTA. At a high current, however, even stirring of the solution does not supply material fast enough to alter significantly the rate of adsorption, and the transition time approaches that for purely adsorbed material. Although no adsorption of Mo(VI)-EDTA is evident above pH 4, a similar j . Electroanal. Chem., 17 (1968) 2o7-225

218

F. A. SCHULTZ, D. T. SAWYER

variation of iz~ with current occurs. A possible explanation has been offered by SCHMID AND REILLEY 27. In a polarographic study of c a d m i u m - E D T A they found that the wave height for CdY 2- reduction is dependent upon the concentration of "inert" or supporting electrolyte cation. This effect is attributed to either ion-pair formation between CdY 2- and the cations, or formation of an activated bridge complex involving the electrode surface, the cation, and CdY 2-. Because PECSOK AND SAWYER had observed a similai increase with supporting electrolyte concentration of the polarographic wave height for Mo(VI)-EDTA reduction 1, SCHMIDAND REILLEY proposed that ion-pair or activated bridge participation m a y also occur in this system. Chronopotentiometric data have not been obtained at varying cation concentrations, but the present evidence indicates that of the two mechanisms, activated bridge formation is more likely to be involved in the reduction of Mo(VI)-EDTA. Ion-pair formation would be expected to behave in the manner of a prior chemical reaction, but, being purely electrostatic in nature, its rate would most likely be too fast to be detected b y chronopotentiometry. Furthermore, at p H 4.4 and p H 5.2 a chronopotentiometric transition time is not observed above a certain level of current. If ion-pair formation were the controlling process there would be a sufficient concentration of ion-pairs in the bulk solution to yield a detectable transition time even at the high currents. On the other hand, if activated bridge formation is the kinetically-limiting factor, the electrode can be charged to the potential of solvent reduction before bridge formation occurs. If bridge formation is assumed not to occur prior to the experiment, a transition time would not be observed. In summary, below p H 4, two hydrolytic species of Mo(VI)-EDTA are apparently reduced b y independent but parallel paths. One species, which occurs only under acidic conditions, can be reduced only if it is first adsorbed on the electrode surface (PARRY AND YAKUBIKzs also have demonstrated the adsorption of Mo(VI) in the presence of tartaric acid). A second species occurring both above and below p H 4, is apparently reduced b y means of an activated bridge complex. For both forms of the complex, an interaction with the electrode surface is the ratecontrolling step in their reduction. Consequently, a decrease in the product, iz½, is observed with increasing current. For the more basic species, iz½ also decreases with increasing pH. The rate of formation of the activated bridge complex appears to be dependent on hydrogen ion concentration, and the activated bridge m a y specifically involve hydrogen ion. Chemical processes in the reduction of Mo ( V I ) - E D T A . The controlled-potential coulometric results indicate that the overall reduction of Mo(VI)-EDTA involves three electrons. Given this fact, however, there are a variety of paths to be considered. The initial step could be a 3-electron reduction.

~o(VI) +3 e--+~ro(III)

(~)

which in view of the chemical mixing experiments, would have to be followed b y reactions of the type:

2 Mo(III) +~o(VI)-+3 Mo(IV)

(2a)

Mo(III) +2 Mo(VI)-+3 Mo(V)

(2b)

Mo(IV) + Mo(VI)-+2 Mo(V)

(2c)

j. Electroanal. Chem., 17 (1968) 207-225

IV[O--EDTA COMPLEXES

219

and subsequent reduction of the intermediates. Various step-wise mechanisms are also possible with an initial electrode reaction involving either one, Mo(VI) + e - -~ Mo(V) or two, Mo(VI) +2 e- -+ •o(IV)

(3) (4)

electrons. These electrode reactions would be followed by reduction of the intermediates, Mo(V) +2 e--+Mo(III)

(5a)

Mo(V) + e--->Mo (IV)

(5b)

Mo(IV) +e--+Mo(III),

(5c)

and by the chemical reactions represented by eqns. (2a)-(2c). The evidence from several experiments establishes that reaction (4) represents the initial step in the electrochemical reduction of Mo(VI)-EDTA. Reaction 3 is eliminated because, after reducing Mo(VI)-EDTA by one equivalent, Mo(VI) is still present in the solution. This is illustrated by Fig. 7 where, for a one-equivalent reduced solution, a voltammetric peak due to Mo(VI)-EDTA is observed. Reaction I cannot be dismissed with the same degree of certainty. The chronopotentiometric behavior of a system involving reactions (2a)-(2c) in which the products are electroactive has not been established. The increase in iz~[C at low current (Fig. 5), however, is characteristic of a follow-up chemical reaction which regenerates electroactive material 24. A disproportionation reaction, 2 Mo(IV) ~

Mo(III) +Mo(V)

(6)

accounts for this behavior if Mo(V) is reduced more easily than Mo(VI). Curves C and D in Fig. 7 indicate that this is true, and reaction (4) is concluded to be the primary electrode reaction in the reduction of Mo(VI)-EDTA The postulated disproportionation reaction also must be reconciled with the voltammetric data in Figs. 6 and 7. The anodic process in Couple I (curves B', C' and D' in Fig. 7) is definitely the oxidation of Mo(III). The potential is close to that for the oxidation of species known to be Mo(III) (Fig. ii), and in Fig. 7 there is a continuous increase in the anodic peak current as the coulometric reduction of Mo(VI) is carried towards an n-value of three. In curves C and D of Fig. 7 the species being reduced has been identified as Mo(V), but this can be confirmed only indirectly. Assuming that the reduction of Mo(VI) proceeds by reaction (4) and the disproportionation of Mo(IV) by reaction (6) is complete, the reduction of I.OO mmole of Mo(VI) by I.OO mequiv, should give a solution that contains o.5 ° mmole Mo(VI), 0.25 mmole Mo(V) and 0.25 mmole Mo(III). Similarly, the reduction of I.OO mmole of Mo(VI) by 2.oo mequiv, should give a solution that contains 0.5o mmole Mo(V) and 0.50 mmole Mo(III). Curves B and B' and C and C' in Fig. 7 confirm these expectations if the peak for Mo(V)reduction is just to the anodic side of the peak for Mo(VI) reduction in curve B. From these arguments, the oxidation-reduction couples in Fig. 6 are concluded to be Mo(IV)/Mo(III) (Couple I) and Mo(V)/Mo(IV) (Couple II). This is a seeming paradox in that the Mo(IV)/Mo(III) couple is situated anodic of the Mo(V)/ Mo(IV) couple. However, such behaviour might intuitively be expected for a system j . Electroanal. Chem., 17 (1968) 207-225

220

F. A. SCI-IULTZ, D. T. S A W Y E R

in which the intermediate oxidation state is unstable. Conclusive proof of the identity and respective location of these two couples cannot be presented, but this conclusion represents the most satisfactory explanation of the experimental results. Another point of contention in the proposed scheme is that the reduction of Mo(V) stops at iVfo(IV) although occurring at potentials far more cathodic than the reduction of Mo(IV) in Couple I. Similarly, the oxidation of lV[o(III) stops at Mo(IV) although the potential is sufficiently anodic to oxidize the Mo(IV) produced in Couple II. The reduction of Mo(VI) to Mo(IV) occurs at the most cathodic potential of all, yet Mo(IV) is apparently reduced no further. This behaviour is difficult to explain other than by suggesting that different forms of Mo(IV) may be produced electrochemically from the (III), (v) and (VI) states of molybdenum. From reverse current chronopotentiometric and cyclic voltammetric experiments, at least two independent species of Mo(IV) appear to exist. Current reversal studies of the reduction of lV[o(VI) (Fig. 4) indicate that the majority of the reverse wave is due to the oxidation of Mo(III), with the portion of the wave due to Mo(IV) oxidation being almost too small to be observed. Thus the Mo(IV) species produced from Mo(VI) disproportionates rapidly. This is confirmed by the increase in iT~/C at low current, because disproportionation of Mo(IV) is the only means by which electroactive material is regenerated in the presence of the other processes acting simultaneously to decrease iz~/C. Current reversal experiments with the Mo(III) and Mo(V) species produced coulometrically from Mo(VI), show zf/vr to be only slightly larger than three. Cyclic voltammetric traces of these same materials (Fig. 6) show that considerable current flows on reverse scan following oxidation of Mo(III) and reduction of Mo(V). Hence, these particular forms of Mo(IV) disproportionate relatively slowly, and therefore are distinct from the initial product of Mo(VI) reduction. The other oxidation states resulting from the reduction of Mo(VI) also appear to have a variety of forms. The eoulometric product, Mo(III)A-EDTA, is a diamagnetic species. However, as originally produced from the reduction of Mo(VI) via the rapid disproportionation of Mo(IV), Mo(III)a-EDTA must almost certainly be paramagnetic. This is because the original Mo(VI)-EDTA molecule contains an isolated molybdenum atom which, on reduction to an uneven oxidation state, must have unpaired spin. During the extended duration of a coulometlic experiment, Mo(III)A-EDTA is apparently converted from a paramagnetic to a diamagnetic material. In Mo(V) compounds, diamagnetism can arise through the formation of dimers involving oxygen bridges. The most common compounds of this type, Na2[lV[o204Y.H2011,6,8,Ba[Mo2Q (C20@2(H20)21" 3H20 6,~9,30and (pyH)4[M0204 (NCS),I 6,7, contain a double oxygen bridge. A diamagnetic, singly oxygen-bridged compound, Mo203(xanthate)431 has also been reported. At the end of a prolonged coulometric experiment, these dimerization reactions could yield a family of Mo(III), (IV), and (V) species distinct from those produced transiently in the chronopotentiometric and voltammetrie reduction of Mo(VI)-EDTA. On the basis of these considerations, a more detailed mechanism can now be written for the electrochemical reduction of Mo(VI)-EDTA. The initial reduction step, Mo(VI)-EDTA +2 e- -+ Mo(IV)A-EDTA is followed by rapid disproportionation of the product,

J. Electroanal. Chem., 17 (1968) 207-225

(7)

]tV[O--EDTA COMPLEXES

221

2 Mo(IV)A-EDTA -+ Mo(III)A-EDTA +Mo(V)A-EDTA.

(8)

Reaction (8) regenerates an electroactive material, Mo(V)a-EDTA, which can be further reduced, Mo(V)A-EDTA + e - --> 1V[o(IV)A-EDTA,

(9)

to continue a cyclic process which leads via reaction (8) to the formation of Mo(III)AEDTA. This species is apparently convert ed to a diamagnetic form, Mo' (I I I) A-EDTA, which leads to a new redox couple Mo'(III)A-EDTA~Mo'(IV)A-EDTA + e -

(IO)

This couple actually represents Couple I in Fig. 6. The species Mo'(IV)A-EDTA also is unstable, but the disproportionation reaction, 2 1V[o'(IV)A-EDTA~Mo(III)A-EDTA+Mo(V)A-EDTA,

(II)

is significantly slower than that for Mo(IV)A-EDTA. Reaction (9) represents Couple II in Fig. 6 and involves a species, Mo(IV)A-EDTA, which is not electrochemically reduced. The oxidation of Mo(III)c-EDTA (curve C, Fig. II) appears to be equivalent to that for the species Mo(III)A-EDTA Mo(III)A-EDTA-~Mo(IV) A-EDTA + e -

(12)

which is produced transiently by reaction (8). Thus, Mo(III)c-EDTA is probably equivalent to Mo(III)a-EDTA and with time is converted to Mo'(III)A-EDTA (the actual species represented by curve A, Fig. ii). Likewise, Mo(V)A-EDTA appears to be converted in time to Mo'(V)A-EDTA. Thus, the primed species represent more stable forms of the complex and result in solutions on standing. These may be formed by dimerization reactions which involve oxo-bridges between the molybdenum atoms. In writing these reactions only the change in oxidation state and the designation of the various species are noted. The molecular formulae of the chelates, and the pH and EDTA concentration-dependence of the reactions have not been determined. As a solid chelate, Mo(VI)-EDTA has the formula Na4(MoOa)2Y, but the dimeric form of the complex is probably not present to a great extent in solution. In a recent NMR study, KULA4 has detected appreciable quantities of a I : I chelate in aqueous solution, and reports a value of about 13 for the equilibrium constant of the reaction

(MoOa)2Y 4- + H 2 Y ~ - ~ 2 MoO3HYa-

(13)

Under the experimental conditions in the present work Fo.5-5 m F Mo(VI) in the presence of a IO- to Ioo-fold excess EDTA], at p H > 4 1V[o(VI) is apparently present in solution almost exclusively as MoOaHY a-. As the acidity is increased, greater quantities of the 2 : I chelate may be formed because of the decrease in concentration of the species H2Y 2-, but no unique electrochemical behavior is observed that can be attributed to the presence of the dimeric form. KULA suggests the existence of (MoOa)2HY a- and polymeric Mo(VI)-EDTA below p H 4, but the more highly protonated forms of the i :I chelate, MoOaH2Y 2- and MoOaHaY-, are likely to exist in this pH range also. Which of these compounds is the specifically adsorbed material that is reduced for solutions below pH 4, has not been determined, however. J. Electroanal. Chem., 17 (1968) 2o7-225

222

F . A . SCI-IULTZ, D. T. SAWYER

The proposed reduction mechanism has not been suggested previously for Mo(VI)-EDTA but has been mentioned for Mo(VI) in other supporting electrolytes. In hydrochloric acid, the polarographic reduction of Mo(VI) to No(IV) is proposed by HAIGHT32 and by WITTICK AND RECHNITZ33. Earlier work 34,a5, which postulated the reduction of Mo(V) to Mo(III), apparently is in error. No(IV) has also been mentioned as a possible product in the polarographic reduction of molybdenum(VI)gluconate complexes a6. The disproportionation of Mo(IV) is well-established a7 and is supported by the work of RECHNITZ33. In I F HCI04, the rate of disproportionation of Mo(IV) has been estimated to be approximately 10 41 mole -1 sec -1as. Mo(IV) also has been identified as the species responsible for the molybdenum-catalyzed reduction of nitrate and perchlorate ions 3s-40. Reduction of m o l y b d e n u m ( V ) . ~ - E D T A

For the dimeric, diamagnetic chelate, Mo(V)2-EDTA, coulometric reduction yields an n-value of two electrons/molybdenum atom. Reoxidation of the product indicates that the overall process is reversible in the gross sense. Also significant is the impossibility of electrochemically oxidizing Mo(V)2-EDTA. These and other characteristics establish that the electrochemistry of Mo(V)2-EDTA is independent of that of Mo(VI)-EDTA. Such a result clearly is the consequence of the unique structural features of the molybdenum(V)2-EDTA compound. Chronopotentiometric results (Fig. I0) reveal an increase in iz½ at both high and low currents. The increase at high current is characteristic of an adsorption process 26. Although a variety of methods 41 43 have been proposed for treating chronopotentiometric data complicated by adsorption, chronopotentiometry itself is considered a poor method for quantitative measurement of adsorbed electroactive material 44. Nevertheless, plots of iT vs. z ½and (iT) ½vs. (I/i) ½are linear and have small positive intercepts42, 43. This is qualitative evidence of an electroactive adsorbed component coupled with the diffusion-controlled reduction of an electroactive species from the bulk solution. In contrast to No(VI)-EDTA, No(V)~-EDTA appears to be adsorbed rapidly. In Fig. I0 the increase in iz~ at low current is parallelled by a similar rise in the ratio of z~/Tr. This behaviour is consistent with an electrode process that regenerates electroactive material~4,45. Also for low currents, a second wave becomes apparent in the current reversal experiments (Fig. 9). The potential of this new wave is the same as that for the oxidation of No(III)~-EDTA. Therefore, the initial reduction step of Mo(V)2-EDTA is concluded to be a 2-electron process (one electron/molybdenum atom), Mo(V)2-EDTA +2 e-~[Mo(IV),~]B-EDTA

(~4)

which is followed by a disproportionation reaction to provide additional Mo(V). 2 EMo(I V) 21B-EDTA -+ EMo(V) ~]B-E DTA + ENo(I I I) 21B-E DTA

(~5)

In the reoxidation of Mo(III)B-EDTA (Fig. I I ) , no intermediate state is observed comparable to that observed from the reduction of Mo(V)2-EDTA (Fig. 8). Therefore, the oxidation of INo(III)~]B-EDTA must be assumed to proceed directly to

Mo(V), ar. E l e c t r o a n a l . C h e m . , 17 (1968) 2o7-225

~V[o-E DTA COMPLEXES

223

[Mo(III)2]B-EDTA~[Mo(V)2]~EDTA + 4 e-

(I6)

In reactions (14)-(16) the complexes are represented as dimers because the dioxo-bridged structure of the chelate is presumed to be maintained in all oxidation states. This statement cannot be verified, but the bridged configuration of Mo(V)2EDTA exhibits unusual stability in many other respects. If this dimeric structural unit is retained throughout, then all the species represented in reactions (I4)-(I6) have a Mo/EDTA ratio of 2. A comparison of the relative ratios of "rr/'rrdemonstrates that reaction (15) is much slower than the disproportionation of Mo(IV)A-EDTA and somewhat slower than the disproportionation of Mo'(IV)a-EDTA. STRUCTURAL IMPLICATIONS FROM THE ELECTROCHEMICAL REDUCTION OF MOLYBDENUM-EDTA CHELATES

One aim of this work has been to establish a correlation between electrochemical parameters and the structural characteristics of the electroactive species. The electrochemical behavior of dimeric molybdenum(VI)-EDTA has been of particular interest. For example, an interesting possibility is the intramolecular disproportionation of a 2:1 Mo(IV)-EDTA compound produced electrochemically from Mo(VI)2-EDTA. Such a disproportionation, which would lead to a mixed valence compound, is purely a hypothetical case because Mo(VI)-EDTA appears to exist in solution primarily as a i : I chelate in the presence of excess EDTA. Perhaps the most significant electrochemical observation is the difference in rates of disproportionation of the various molybdenum(IV)-EDTA complexes. Although only qualitative in nature, these differences are surprisingly consistent with structural features of the original compounds. The most reactive Mo(1V) species, Mo(IV)A-EDTA, is produced transiently by the reduction of Mo(VI)-EDTA, and its reactivity can be rationalized by the "open-ended" structure of the Mo(VI) chelate which places the molybdenum atom in a highly exposed position. On the other hand, the most stable Mo(IV) entity, [Mo(IV)2]~-EDTA, is produced from Mo(V)~-EDTA, a compound which derives unusual stability from its dioxo-bridged structure. Of intermediate reactivity is the species, Mo'(IV)A-EDTA, which is produced over a period of time following the coulometric reduction of Mo(VI)-EDTA. This material is believed to result from a slow dimerization (which accounts for its diamagnetism) and may contain a single oxygen bridge to give a linear dimer. The relative disproportionation rates can be accounted for in two ways. In one sense, the rates may signify an inverse relationship between reactivity and the amount of "shielding" provided to the molybdenum atoms by the ligand molecule. Such shielding would make the molybdenum atoms less accessible and thus reduce the effectiveness of bimolecular collisions. An alternative consideration is that reactivity may be directly related to the number of unpaired electrons in the various molecules. However, magnetic data are not available for these compounds, which precludes a meaningful comparison. A final important point is that the chemical and electrochemical pathways for the reduction of Mo(VI)-EDTA and of Mo(V)2-EDTA are entirely independent. Even for a 2 : r molybdenum(VI)-chelate, the reduced molecule is not likely to collapse end-to-end upon itself because of the unfavorable entropy considerations. This j . Electroanal. Chem., 17 (1968) 2o7-225

224

F . A . SCHULTZ, D. T. SAWYER

appears to be the primary reason for the existence of two parallel, but independent redox systems for molybdenum-EDTA chelates. ACKNOWLEDGEMENT

The authors are grateful to Mr. K. P. CALLAHANand Professor R. M. WING of the University of California, Riverside for many helpful discussions during the course of this investigation. Mr. R. K. DARLINGTON kindly performed the ESR experiments. This work was supported by the U.S. Atomic Energy Commission under Contract No. AT(II-I)-34, Project No. 45. SUMMARY

Among the chelates formed by ethylenediaminetetraacetic acid (EDTA), those with Mo(VI) and Mo(V) are unusual because the metal-to-ligand ratio is two-to-one. Coulometry, chronopotentiometry and cyclic voltammetry have been used to investigate the electrochemistry of these compounds with the intention of establishing a correlation between the electrochemical results and the unique structural features of the chelates. In the presence of EDTA, the overall reduction of Mo(VI) involves three electrons. The initial electrode reaction, however, is the reduction of Mo(VI) to Mo(IV), which is followed by a disproportionation reaction to give Mo(V) and Mo(III) The two-to-one Mo(V)-EDTA chelate (a diamagnetic, dioxo-bridged species) is reduced through a series of parallel, but entirely independent steps. A third series of compounds arises from the Mo(III) species produced coulometrically from Mo(VI)EDTA. The uniqueness of the various molybdenum-EDTA species can be seen from cyclic voltammetric experiments and the differences in the rate of Mo(IV) disproportionation. The rate of disproportionation of Mo(IV) is considerably slower for those compounds having oxygen bridges between molybdenum atoms.

REFERENCES

I 2 3 4 5 6 7 8 9 IO ii 12 13 14 15 16 17 18 19

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