Oxovanadium(iv) complexes of malic, succinic, and 2-mercaptosuccinic acids

Oxovanadium(IV) Complexes of Malic, Succinic, and 2-Mercaptosuccinic Acids Giovanni Micera, and Alessandro Dessi GM. Dipartimento di Chimica, Universita’ di Sassari, Sassuri, Italy.-AD. Istituto per I ‘Applicazione delle Tecniche Chimiche Avanzate ai Probiemi Agrobiologici, C.N.R., Sassari, Italy

ABSTRACT The formation of oxovanadium(IV) complexes with D- and L-malic, succinic and D, L-2-mercaptosuccinic acids has been investigated in aqueous solution by means of spectroscopic (ESR and electronic absorption) and titration data. Evidence is presented on the coordination behaviour of the ligands and on the nature of the formed complexes.

INTRODUCTION Among the hydroxycarboxylic acids that are found in the exudates of plant roots [ 1, 21, malic acid is often the predominant aliphatic component. Therefore, the knowledge of the complex formation properties of this ligand may contribute to the understanding of the distribution of micronutrients in plant roots. On the other hand, oxovanadium(IV) is an essential ion present in soil due also to the occurrence of reduction reactions involving vanadium(V) [3, 41. Recent findings indicate that certain N2-fixing bacteria having vanadium as the prosthetic group could play a relevant role in the uptake of this element in soil [5, 61. However, ESR studies have shown that the partitioning of sorbed vanadium within the plant roots produces soluble oxovanadium(IV) complexes with organic components of low molecular weight [7]. As a part of research aimed to the identification of ligands involved in VO(IV) chelation in plants, we report here spectroscopic data for the complexes formed in aqueous solution by D- and L-malic acids. Comparison with the behaviour of related ligands, such as succinic and 2-mercaptosuccinic acids, is made.

Address reprint requests to Professor G. Micera, Dipartimento di Chimica, Universita’ di Sassari, Via Vienna 2, 07100 Sassari, Italy. Journal of Inorganic Biochemistry 33, 99-109 (1988) 0 1988 Elsevier Science Publishing Co., Inc., 52 Vanderbilt Ave., New York, NY 10017

99 0162-0134/88/$3.50

100

G. Micera and A. Dessi

EXPERIMENTAL Materials Anhydrous L- and D-malic. succinic and D, L-2-mercaptosuccinic acids were purchased from Fluka and used without purification. VOS04 *3H20 (Aldrich) was the metal source. Fresh V02+ solutions were prepared in twice distilled water just prior to the commencement of any experiment. The pH was adjusted by addition to H2S04 or NaOH. The titration data were obtained on solutions containing ligafids in the absence or presence of 2.5 x 10e2 M oxovanadium(IV), at the metal ion ligand ratios of 1: 1, 1:2 or 1:3. The ionic strength was adjusted to 0.4 M with KCl. KOH solution of known concentrations was used as titrant. In order to minimize air oxidation of oxovanadium(IV), nitrogen was run through the solutions during preparation and titrations. Measurements ESR measurements were carried out at room temperature on a Varian E-9 spectrometer operating at the X-band frequency (-9.15 GHz). Flat quartz cells, sealed under a nitrogen atmosphere, were used. The electronic spectra were obtained on a Uvidec Jasco 610 spectrophotometer. The pH was monitored by a Radiometer PHM 84 pH-meter. RESULTS

AND DISCUSSION

VO(IV)-D-

or L-Malic

Acid System

First of all, it must be observed that identical results were obtained with the two enantiomeric forms of ligand. The titration curve of the ligand (Fig. 1) shows only two distinguishable deprotonation processes corresponding to the successive ionization of two carboxyl groups (pK = 3.22 and 4.78, respectively, according to literature data, see refs. 8 and 9). Instead, in the presence of VO*+, a third inflection due to the deprotonation of the hydroxyl group occurs below pH 7. An inflection at three equivalents of base per mole of ligand is observed around pH 5 in the VO*+ :ligand 1: 1 system. This can be consistent with the coordination of a fully deprotonated ligand. Instead, with an increase in the amount of acid, the presence of free ligand molecules shifts this inflection point toward minor amounts of base and higher pH values. A buffer region, the extent of which is inversely proportional to the ligand-to-metal ratio, then occurs, followed by further base-consuming processes due to hydrolysis of vanadium. ESR spectra recorded at room temperature on the VO(IV)-malic acid system as a function of pH are shown in Figures 2 and 3. As known, low-molecular weight oxovanadium(IV) complexes in solution exhibit eight sharp ESR lines, due to interaction of the odd electron with the W nucleus (I = 7/2), which is nearly 100% abundant. The isotropic coupling constant is sensitive to the nature of the donor groups bound to the metal ion in the equatorial plane [lo]. Examination of spectra shows that, for solutions with a ligand-to-VO*+ molar ratio of 2: 1, a first complex species (I) is observed around pH 3. The spectral parameters indicate a metal coordination only slightly different from that of the free aquaion (see Table 1) and strongly suggest the interaction of the metal ion with carboxylate groups. A second complex species (II) is present at pH 4. The measured A, value, which is smaller than that of I (Table l), supports the insertion of further donor groups into the metal coordination. Two

PH

0

1

2 3 E

4

5

FIGURE 1. Titration curves of L-malic acid in the presence and absence of V02+:2.5 x lo-* M ligand (a); 2.5 x lo-*M ligand and 2.5 x lOA2 M VO*+ (b); 5.0 x lo-* M ligand and 2.5 X lo-* M VO*+ (c); 7.5 x lo-* M ligand and 2.5 x lo-* M VO*+ (d). E = equivalents of base per mole of ligand.

4

6

8

10

6

102 G. Micera and A. Dessi

10 mT

FIGURE 2. ESR spectra of solutions of VOW) at pH 3.0 (a), 4.0 (b), 6.0 (c), and 8.5 (d).

and L-malic (1:~ metal-to-ligand

molar ratio)

COMPLEXES

OF VO(IV)

103

FIGURE 3. ESR spectra of solutions of VO(IV) and L-malic acid (1: 1 metal-to-ligand ratio) at pH 3.0 (a), 6.3 (b), 7.6 (c), and 9.0 (d).

molar

additional species are detected above pH 5. Complex III, which increases up to 4050% of total vanadium around pH 6, exhibits an A, value higher compared to that of II (Table 1). In contrast, complex IV, which shows a noticeable decrease in the A, value (Table 1) clearly indicating the involvement of deprotonated hydroxyl groups in chelation to VO(IV), becomes a single species at pH about 7. Indeed, this complex exhibits ESR parameters similar to those measured for oxovanadium bis-chelates formed by hydroxycarboxylic acids coordinating through the ionized carboxyl and

104

G. Micera and A. Dessi

TABLE 1. ESR Parameters and d-d Absorption Maxima for Oxovanadium(IV)

Ligand

A,(10-4 cm-‘)

Species

Absorption

maxima (nm)O

vo*+

1.967

106

630 (9)sh, 770 (18)

Malic acid HOOC-CH(OH)-CH2-COOH

I II III IV

1.968 1.970 1.968 1.974

103 92 98 82

600 410 600 580

Succinic acid HOOC-CHZ-CHr-COOH

la

1.967

103

600 (1 l), 780 (22)’ 590 (15), 810 (23)g

2-Mercaptosuccinic acid HOOC-CH(SH)-CHr-COOH

Ib

1.973 1.981

94 81

IIb

(1 l), (8)sh. (13). (26),

Complexes

770 (22)b 605 (14), 790 (20)’ 820 (18)d 830 (27)e

630 (14), 800 (20)” 555 (33)sh, 640 (47)’

a molar extinction coefficient (M-r cm-‘) for total vanadium in parentheses. b from c from d from Dfrom 1 from g from h from ’ from

1: 1 (ML) 1:2 (M:L) 1: 1 (ML) 1:2 (ML) 1: 1 (M:L) 1:2 (M:L) 1: 1 (ML.) 1:2 (M:L)

solution solution solution solution solution solution solution solution

at pH at pH at pH at pH at pH at pH at pH at pH

3. 5. 6. 8.5. 5. 5.5. 4.1. 7.

hydroxyl groups [ 111. With increasing pH, the signals of IV are still detected, but at pH 11 the resonances of [VO(OH)& (g, = 1.970 and A, = 87 x 1O-4 cm-‘), the final hydrolysis product, appear. The spectra of the equimolar solutions (Fig. 3) allow completion of the identification of the complex species. Below pH 7, only the resonances due to I and III are observed (single sets of signals around pH 3 and 6, respectively). Therefore, they have a major contribution from species with the 1: 1 metal-to-ligand ratio. Formation of IV also takes place above pH 7 in the equimolar solutions. However, simultaneously, the intensity of the ESR signals strongly diminishes while a broad resonance appears, indicating formation of hydrolytic species of oxovanadium(IV) which, at higher pH values, yield [VO(OH)$. The combination of titration and ESR data allow the suggestion that I, which is formed at both the ML = 1: 1 or 1:2 ratios, concomitantly to the first deprotonation process, represents complexes formed by insertion of the first dissociated carboxyl group in metal coordination. The complex II, which is formed concomitantly to the deprotonation of the second carboxyl group and only at ligand-to-metal molar ratios of at least 2, is thus a bis-chelate complex involving two ligands bound to the metal via both the carboxylate groups. Really, the ESR parameters are very similar to those of the bis-oxalate complex [lo]. The species III, which is never a major species at tbe 1:2 molar ratio while predominates in the 1: 1 system, is formed simultaneously to the deprotonation of the hydroxyl group and, thus, probably involves the coordination of a fully ionized tridentate ligand molecule. The observed behavior supports the theory that the bischelate complex IV may be formed also in the 1: 1 system in basic solution, but excess oxovanadium(IV) ions are in uncomplexed form and, as such, yield polynuclear species or [VO(OH),] - , depending on the pH range [ 101.

COMPLEXES OF VO(IV)

105

The electronic absorption data (Table 1) assist the ESR interpretation. Formation of I is indicated by d-d absorption maxima at 600 and 770 nm, assigned as the 2B2 + 2Bl and 2B2 -+ 2E transitions, respectively, in C,, symmetry [ll]. These absorptions correspond well to those for carboxylate complexes (e.g., 610 and 780 nm for VO(IV)-acetate, see ref. 12). The insertion of two ionized hydroxyl groups into metal coordination (IV) shifts the ?B2 + 2BI and 2B2 + 2E absorptions to lower and higher energy values, respectively, conforming to literature data [IO]. The absorption energy values for complexes II and III agree with intermediate structures between those of I and IV. Formation of ligand-free vanadium species in the M:L = 1: 1 solution above pH 7 results in the appearance of strong UV absorptions which obscure the bands in the visible region. W(N)-Succinic

Acid System

Titration curves and ESR spectra are shown in Figures 4 and 5, respectively, while the spin-hamiltonian and electronic data are listed in Table 1. The titration curves in the presence of metal are very close, in the pH range 3-4, while the aquaion is detected by the ESR and electronic spectra (Fig. 5a). A buffer region (the extent of which decreases as the ligand-to-metal ratio increases), is then observed, and it can be attributed to the hydrolysis of vanadium aquaion. Only in the pH range 4.5-6, in solutions with either 1: 1 and 1:2 ligand-to-metal molar ratios, are the ESR signals of species Ia, with parameters comparable to those of I, again supporting the formation of carboxyl complexes of monodentate ligands. Correspondingly, the electronic spectra show maxima in the ranges 600-590 and 780-810 nm. However, the carboxyl species are not stable enough, because the further addition of base results in spectroscopic features indicating the formation of a hydrolytic species of oxovanadium(IV).

FIGURE 4. Titration curves of succinic acid in the presence and absence of V02+ :2.5 X 10e2M ligand (a); 2.5 x 10e2 M ligand and 2.5 x 10e2 M V02+ (b); 5.0 x 10e2 M ligand and 2.5 x 10e2 M V02+ (c); 7.5 x lo-* M ligand and 2.5 x low2M VO*+ (d). E = equivalents of base per mole of ligand. 12

106

G. Micera and A. Dessi

10mT -

+-xY -

H

FIGURE 5. ESR spectra of solutions of VO(IV) and succinic acid (1:2 metal-to-ligand at pH 4.0 (a), 5.2 (b), and 6.1 (c).

VO(IV)-2-Mercaptosuccinic

ratio)

Acid System

As for malic acid, in the titration data, (Fig. 6) the ionization of carboxyl groups is observed for the ligand (pK = 3.14 and 3.94, see refs. 8 and 9) only below pH 7. Instead, the deprotonation of the mercapto group also occurs in the presence of oxovanadium(IV) . The ESR spectra of the solutions at the 1:2 metal-to-ligand molar ratio (Fig. 7) indicate that the free aquaion predominates below pH 3, and the species, Ib having parameters similar to II (see Table 1)) is formed over the pH range 3-3.5. Above pH 4 IIb also appears, becoming a single species at about pH 6. Based on the spectral parameters (Table l), IIb is identified as the bis-chelate complex involving coordination by deprotonated carboxyl and mercapto groups. The electronic spectra, exhibiting d-d bands at 555 and 640 nm, are in agreement with the literature data for this type of coordination [ 121. In the equimolar solutions (Fig. 7), the species Ib and IIb are again observed, around pH 4 (after addition of two equivalents of base per mole of ligand) and pH 4.6, respectively. However, the presence of free vanadium is again supported by the buffer region in the titration curve and by the spectral measurements.

COMPLEXES

OF VO(IV)

107

10

8 PH 0

4

01 0

1

I

I

I

1

1

2

3

4

5

5

E

FIGURE 6. Titration curves of 2-mercaptosuccinic acid in the presence and absence of VO*+:2.5 x lo-* M ligand (a); 2.5 x lo-* M ligand and 2.5 x lo-* M VO*+ (b); 5.0 X lo-* M ligand and 2.5 x lo-* M VO*+ (c); 7.5 x 10m2M ligand and 2.5 x lo-* M VO*+ (d). E = equivalents of base per mole of ligand.

DISCUSSION

AND CONCLUSIONS

The results of this work provide evidence of the major species involved in the complex equilbria in the studied systems. The main conclusions are summarized below. Malic acid is a potentially tridentate ligand. The dissociation of the first carboxyl group allows the formation of complex species, where the ligand behaves as monodentate. The ionization of the second carboxyl group yields a stable ML = 1:2 complex, where both carboxylate groups of the ligands are bound to metal. This complex is probably stabilized by the additional metal coordination from undissociated OH groups. However, at higher pH values, the hydroxyls of the coordinate ligands undergo deprotonation and bind oxovanadium(IV), yielding species with the 1: 1 or 1:2 metal-to-ligand ratios, where the ligand molecules basically bind the metal ion through oxygens from ionized carboxyl and hydroxyl groups. 2-Mercaptosuccinic acid, which apparently does not form species involving ligand coordinating through only a carboxyl groups, exhibits behaviour similar to that of malic acid, due to the presence of a SH instead of a OH group and behaves as {O-, O- > (ionized carboxyl groups) or { O- , S - } (ionized carboxyl and mercapto groups) donor. Thus, complexes having the ML = 1:2composition and involving the coordination of chelate ligand molecules are detected as major species.

108 C. Micera and A. Dem

7 10 mT

-

H

FIGURE 7. ESR spectra of solutions of VO(IV) and 2-mercaptosuccinic acid: AU pH 4.0 (a); M:L = 1:1, pH 4.6 (b); IWL = 1:2, pH 3 (c); and ML

= 1: 1, = 1:2, pH 6.8 (d).

In contrast, with succinic acid a chelate behaviour is not observed, probably due to the absence of additional binding groups that could favor simultaneous binding of two carboxyl groups to the metal ion.

REFERENCES 1. F. J. Stevenson, in Soil Biochemistry, A. D. McLaren and G. H. Peterson, Eds., Marcel Dekker, New York, 1967, Vol. I, Chap. 5.

COMPLEXES

OF VO(IV)

109

2. M. F. Kovacs, Jr., Plant Soil 34, 441 (1971). 3. C. Gessa, M. L. De Cherchi, A. Dessi, S. Deiana, and G. Micera, Inorg. Chim. Acta 80, L.53 (1983). 4. G. Micera, S. Deiana, A. Dessi, A. Pusino, and C. Gessa, Inorg. Chim. Acta 100,49 (1986) and references therein. 5. R. L. Robson, R. R. Eady, T. H. Richardson, R. W. Miller, M. Hawkins, and J. R. Postgate, Nature 322, 388 (1986). 6. B. J. Hales, E. E. Case, J. E. Morningstar, M. F. Dzeda, and L. A. Mauterer, Biochemistry 25, 7251 (1986). 7. G. Micera and A. Dessi, in preparation. 8. L. G. Sillen and A. E. Martell, Chem. Sot. Spec. Pubf., 17, (1964). 9. L. G. Sillen and A. E. Martell, Chem. Sot. Spec. Publ., 25, (1971). 10. N. D. Chasteen, in BiologicaI Magnetic Resonance, L. J. Berliner and J. Reuben, Eds., Plenum Press, New York, 1981, Vol. 3, Chap. 2. 11. R. R. Reeder and P. H. Rieger, Inorg. Chem. 10,1258 (1971). 12. N. D. Chasteen, R. J. DeKoch, B. L. Rogers, and M. W. Hanna, J. Am. Chem. Sot. 95, 1301 (1973). Received September 22, 1987; accepted January 14, 1988