Electroanalytical chemistry of vanadium complexes

143

J. Electroanal. Chem., 251 (1988) 143-150 Elsevier Sequoia S.A., Lausanne - Pnnted

in The Netherlands

Electroara~ytical chemistry of vanadium comptexes Part I. Voltammetric investigation of the vanadium( III)/ vanadium( II)-nitrilotriacetato redox system Roland Meier Department of Natural Scrences, Technical Unruersrty of Leipzig, P.O. Box 66, Knri-Lrebknecht-Strasse L.eipziq DDR-7030 (G.D.R.)

132,

Gerhard Werner Department of Chemistty, Karl-Marx-Umverstr

of Letpzrg, Talstrasse 35, Letpq,

DDR-7OIO (G.D.R.)

Matthias Otto Department of Chemutry. Bergakademre of Frelberg, Leipzrger Strasse, Fredberg, DDR-9200 (G.D. Ii.) (Received

18 June 1987; in revised form 28 Apnl

1988)

ABSTRACT The reduction of V(IIQ-nitrilotriacetato (NTA) was studied in aqueous media m the pH range 3-7. Changing the pH value and variation of the NTA concentration gives rise to typical changes of the voltammetric signals. These may be attributed to ligand exchange reacttons coupled to the electron-transfer processes in the system under study. Because the complex formation constant of V(III)NTA(H,O) is known from the literature, it was possible, with the appropriate Et/a value and the Nernst equation, to evaluate the formation constant for V(II)NTA(H,O)-; this was found to be log & = 3.46.

Vanadium complexes are of importance in both mammalian and plant biochemistry [l]. Thus, the sea-squirt Ascidia nigra contains high concentrations of V(III) ions in specialized blood cells 121.To obtain a clearer insight into the bi~he~ca~ role of, for example, vanadium ions in the blood cells of ascidians, the study of the coordination and redox properties of model complexes in aqueous solutions is of value. Electroanalytical methods are well suited for this purpose. Surprisingly, the aqueous electrochemistry of V(II1) complexes has been rather poorly investigated ]3]. 0022-0728,‘88,‘$03.50

0 1988 Eisevier Sequoia

S.A.

144

We report here for the first time on the electrochemical properties of the V(III)/V(II)-NTA system. The results obtained in our study reflect new aspects of the different coordination properties of NTA to the vanadium oxidation states + 3 and + 2, respectively.

EXPERIMENTAL

Synthesis of Na3V(ZZZ)(NTA)_, NaV(IV)ONTA(H,O) .2.5 H,O served as the starting material. It was prepared as described by Nishizawa and Saito [4]. A 0.5 M solution of this complex was made up in 50 ml of water. The solution was stirred magnetically and was degassed continuously with argon. Thereafter, an equimolar amount of NTA was added. After the additional ligand had completely dissolved, sodium amalgam was added to the solution in small portions. This was done until the solution became grass-green. Then the solution was separated from the mercury by decantation and its volume was concentrated to 10 ml under reduced pressure. The resulting solution was placed immediately in a vacuum desiccator and stored over Mg(ClO,),. Removal of the remaining water was supported by continuous pumping. After 4-5 days, grass-green crystals were obtained. These were filtered by suction, washed with a small amount of ice-cold, deaerated water, and then dried again over Mg(ClO,),. The yield was 30%. Anal. Calculated for (Na,V0i2N,C,,H,,) in %: V, 10.3; N, 5.6; C, 29.0; H, 2.4. Found: V. 10.2; N, 5.6; C, 28.9; H, 2.5.

Apparatus and procedures All voltammetric measurements were conducted on mercury as the electrode material. Cyclic voltammograms (CVs) and differential pulse polarograms (DPPs) were recorded with a PA 3 polarographic analyser (Laboratorni pristroje, Prague, CSSR). This device consists of a polarograph, an X-Y recorder, and a static mercury drop electrode, SMDE 1. Opening times of 160 ms for the needle valve of the SMDE were used throughout. Fast sweep rate CVs were recorded with an ECM 700 computer-controlled system (ZWG, Academy of Sciences of the G.D.R., Berlin). All voltammetric and pH measurements were made at 22 + lo C. The pH values were measured with an EGA 801 N micro-combination glass electrode (Forschungsinstitut “Kurt Schwabe”, Meinsberg, G.D.R.) coupled to an MV 88 pH-meter (VEB Praecitronik Dresden, G.D.R.). All potentials given in this work were measured vs. a Ag/AgCl/Cl;. reference electrode. The pH-meter was calibrated with commercial buffer solutions (e.g. National Bureau of Standards). The chemicals used were of analytical-grade purity. The complex was added to the sample solutions as a solid aliquot of NaY(NTA),, which was weighed precisely in all cases.

145 RESULTS

Numerous cases of electrode reactions in the electrochemistry of coordination complexes are known which are coupled to ligand exchange reactions. These occur as steps preceding or following the electron-transfer process that takes place at the electrode surface [5]. Therefore, it is not surprising that such reactions accompany the electron-transfer reactions in the V(III)/V(II)-NTA system, too. A typical CV for the reduction of V(III)-NTA in slightly acidic supporting electrolytes and without excess ligand is shown in Fig. 1. A double reduction wave of complexed V(III)-NTA at - 1.0 to - 1.1 V has no corresponding re-oxidation wave in the same potential range. When the return scan is continued to potentials more positive than the initial potential, an anodic wave close to - 0.45 V becomes visible. The redox couple responsible for the peaks at this potential is V(H,O)i+“+, a conclusion based on its standard potential of E” = - 0.453 V [3]. The situation reflected by the CVs can be depicted in the framework of a “square scheme”: - NTA3 V(III)NTA(H~~) + V(H,O):+ +NTA+ +e-

+e-

Jr -e-

Jr -e-

- NTA3 -

V(II)NTA(H,O)-

c\F-- V(I%O):+ + NT&’ ‘Ilk scheme is somewhat simplified because the formation of complexes with a 1: 2 metal to ligand ratio is not shown for both oxidation states, despite the fact that V(III)(NTA)iwas added to all the solutions. The simplification was made because, as shown below, a 1: 2 V(II)-NTA complex does not exist.

Fig. 1. Cyclic voltammogram

of 2 mM

V(III)(NTA)z-

m 1 M KCl. pH 3.0; sweep rate = 0.1 V s-l.

146

-r@ ,

-1il

-l.Z

-I.4

E/V

,

Fig. 2. Differential pulse polarograms of 6 mM V(III)(NTA)iin 0.5 M phosphate buffer. pH 5.6. at mcreasing excessive NTA concentrations: (1) 0 mM; (2) 1.3 mM: (3) 4.0 mM. SMDE: drop time = 1 s, sweep rate = 2 mV s-l, pulse amplitude = - 25 mV.

On the other hand, the occurrence of 1: 1 and 1: 2 complexes in the V(II1) state causes the double reduction wave in Fig. 1. The validity of this suggestion can be proved by the dependence of the currents for the reduction of both species on the NTA concentration at a constant pH value. The DP polarograms in Fig. 2 show that, with increasing ligand concentration, the current of the peak at the more positive potential decreases while that for the more negatively located peak increases. Thus, it is obvious that the latter peak is caused by V(III)(NTA):while the former peak corresponds to the reduction of V(III)NTA(H,O). The CVs in Fig. 3 were recorded in a solution where the pH was 2.7 units higher

Fig. 3. Cyclic volt~o~~s rate/V s-‘: (1) 0.2; (2) 0.02.

of 6 mM

V(lIt~NTA)~-

m 0.5 N

phosphate

buffer,

pH 5.7. Sweep

i OS/AA

-1p

-12

-l$

-lJ

EV

Fig. 4. Cyclic voltammograms of 5 mM V(III)(NTA)sin 0.5 M phosphate buffer, pH 6.0. NTA concentration = 0.05 M. Sweep rate/V s -‘: (1) 0.2; (2) 0.1; (3) 0.05; (4) 0.02.

than that in Fig. 1. Now the anodic peak for the oxidation of V(II)NTA(H,O)appears. Comparing the two CVs in Fig. 3, it becomes evident that (i) in addition to the 1 . current ratio zPcl/ipa, the ratio i /i pc2 is also sweep rate dependent; and (ii) there is no oxidation peak coupled to %e reduction peak of V(III)(NTA)~-. One could now try to find this oxidation wave by using a high ligand excess and a very fast sweep rate. One attempt to achieve this is shown in Fig. 4, where the NTA concentration is ten times higher than that of V(III)(NTA)~-. The CVs recorded under these conditions show that in the V(II1) state only V(III)(NTA):is present. Nevertheless, the large peak separation (250 mV at the slowest sweep rate) and the position of the anodic wave indicate that again only V(II)NTA(H,O)is oxidized. CVs were recorded with sweep rates up to 20 V s -’ but the oxidation wave for V(II)(NTA)~never became visible. DISCUSSION

The CVs recorded at different pH values and Iigand concentrations give us an indication of the different cordination properties of NTA towards V(II1) and V(II), respectively. Generally, with EDTA-type ligands the formation constants of V(II1) complexes are several orders of ma~tude higher than those of the ~rr~ponding V(I1) complexes (e.g. log ,8VoIIrnDrA-= 25.9; log /?vor)nnrAz- = 12.7 [6]) and the same is true in the present case. The CVs observed in Fig. 1 show that V(II)-NTA has a tendency to decompose during the time scale of the CV sweep. The electrogenerated V(I1) complex decays, dependent on the sweep rate, the pH and the NTA concentration, into uncomplexed V(I1) and the free ligand.

148

In principle, the whole cycle represents an ECEC mechanism because the electrogenerated V(H,O)i+ is also consumed by a chemical follow-up reaction which is its re-complexation with NTA. This becomes evident from the drastically lower peak current in the negative half of the extended scan (dotted line in Fig. 1) compared with the corresponding anodic peak current. This is a consequence of the rather high formation constant of the 1 : 1 V(III)-NTA complex (log & = 13.41 [7]) and the lability of the V(III) ion [8]. From the pH dependence of the first cathodic wave it should be possible to evaluate the formation constant for V(II)(NTA)(H,O); on the basis of the Nernst equation, because a value for the formation constant of V(III)NTA(H,O), was available from the literature. The DPP and CV measurements in 1 M KC1 led to a pH-independent value of E, ,2 = - 1.040 V in the pH range 4.5-6.0 for the electrode reaction V(III)NTA(H*O)~

+ e- * V(II)NTA(H~O)-

(1)

Insertion of this value, together with the values for E1,2.v~H10);+/~-+and log PI”(nnNr*~n,O), given above, into a modified Nemst equation

(2) leads, after rearrangement, to log &voIjNTA(H20jI = 3.46. The sweep rate dependence of the ratio ipcl/ipc2 (Fig. 3) and the fact that no re-oxidation wave appears for V(II)(NTA)$- reflect a mechanistic picture which is comparable with that found for the Co(II)/Co(I) couple in the vitamin B,, system. Saveant and co-workers have studied intensively the so-called base-on/base-off coordination equi~b~um of the terminal N-atom of the nucleotide side chain with the cobalt centre by means of CV [9]. In the vitamin B,, system the base-on form in the Co(I) state does not exist, and the same is true in the present system for V(II)(NTA);-. The observations from Fig. 3 can be given in a scheme similar to that in ref. 9b: ]V(III)NTA(H,O),l +e-

\

+

-

[V(III)(NTA);-] NTA’ -

-e-

+ e ‘-

[V(II)NTA(H,O)-]

/

The sweep rate dependence of the ratio i,,,/i,,, results from the CE reaction which influences the first cathodic peak current. This peak current consists, depending on the sweep rate, of two components. One component corresponds to the equilibrium amount of V(III)NTA(H,O), present in the bulk of the solution and the other one arises from the dynamic interconversion of V(III)(NTA)~into the more easily reduced V(III)NTA(H*O)*. Thus, when the sweep rate is lowered, the first cathodic peak current increases at the expense of the second, reflecting the growing kinetic interference of the ligand exchange reaction.

149

Fig. 5. Plots of the shifts of the cathodic and anodic peak potentials from Fig. 4 vs. the logarithm of the sweep rate (log 0).

When the NTA excess in the bulk of the solution is very high (Fig. 4), all V(III)-NTA is present as V(III)(NTA):and is reduced through the path V(III)(NTA):- -+ V(II)NTA{H,O);. In Fig. 5, the sweep rate dependence of the peak potentials from the CVs of Fig. 4 is shown. The cathodic peak potential is shifted 96 mV in the negative direction per decade of A log u. This confirms that an irreversible process occurs (the electron-transfer reaction is accompanied by a bond-breaking process, the ligand expulsion). The positive shift of the anodic peak potentials (31 mV per decade of A log u) can be explained in terms of a reversible electron transfer followed by an irreversible reaction [lo]. This is the ligand exchange in the V(II1) state shown in the above scheme from left to right.

The most surprising finding of the present investigation is the non-existence of V(II)(NTA);-. One reason is very probably its highly negative charge. Interestingly, the bis-complexes with the very similar ligands iminodiacetic acid (IDA) and methyli~no~acetic acid (MIDA) and V(H) exist 1111. Therefore, stereoche~~al requirements might also play a role because NTA is a terdentate ligand while IDA and MIDA are only tridentate. The following is added from the viewpoint of the biochemical relevance of the present paper. Up to now, contradictory results have been given in the literature concerning whether complexed or uncomplexed V(II1) occurs in ascidian blood cells [12,13]. Only very recently, Hawkins and co-workers have shown that it is complexed V(II1) which is present in the Vanadocytes 1141.The identity of the ligands

150

coordinating to the V(III) ions (different Iigands seem to be present in different types of these animals) is still unclear at present. Nevertheless, we believe that voltammetric techniques would be worthwhile tools for the characterization of V(II1) species in ascidian blood cell extracts. REFERENCES 1 N.D. Chasteen, Struct. Bonding (Berlin), 53 (1983) 105. 2 D.W. Boyd and K. Kustin, Adv. Inorg. B&hem.. 6 (1985) 311. 3 Y. Israel and L. Meites m A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, Vol. 6. Mar& Dekker, New York, 1976, pp. 293-466. 4 M. Nishizawa and K. Saito, Inorg. Chem.. 19 (1980) 2284. 5 (a) D.R. Crow, Polarography of Metal Complexes, Academic Press, New York, 1969; (b) W.E. Geiger in J. Zuckerman (Ed.), inorganic Reactions and Methods, VCH Publishers, Weinheim, 1985, Ch. 12.3. 6 G. Schwarzenbach and J. Sandera, Helv. Chim. Acta, 36 (1953) 1089. 7 J. PodIaha and P. Petras, J. Inorg. Nucl. Chem., 32 (1970) 1963. 8 F. Basolo and R.G. Pearson, Mechanism of Inorganic Reactions: A Study of Metal Complexes in Solution, Wiley, New York, 1967. 9 (a) D. Lexa and J.M. Savtant, Act. Chem. Res.. 16 (1983) 235; (b) D. Fame, D. Lexa and J.M. Sav&mt. J. Electroanal. Chem., 140 (1982) 269. 10 I.. Nadjo and J.M. Saveant, J. Electroanal. Chem., 48 (1973) 113. 11 R. Meier, H. Frank, R. Kirmse, R. Salzer, J. Stach, G. Werner and M. Otto, Collect. Czech. Chem. Commun., m press. 12 MI. Agudelo, K. Kustin and G.C. McLeod, Comp. Biochem. Physiol. A, 75 (1983) 211. 13 P. Frank, R.M.K. Carlson and K.O. Hodgson, Inorg. Chem., 25 (1986) 470. 14 S.G. Brand, C.J. Hawkins and D.L. Parry, Inorg. Chem.. 26 (1987) 627.