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51V-NMR investigation on the formation of peroxo vanadium complexes in aqueous solution: Some novel observations
JOURNAL OF
MOLECULAR CATALYSIS Journal of MolecularCatalysis94 ( 1994) 323-333
‘lV-NMR investigation on the formation of peroxo vanadium complexes in aqueous solution: some novel observations Valeria Conte, Fulvio Di Furia *, Stefano Moro Vniversit6 di Padova, Centro CNR di Studio sui Meccanismi di Reazioni Organiche, Dipartimento di Chimica Organica, Via Mar2010 I, 35131 Padova. Italy
Received6 May 1994;accepted7 June 1994
Abstract Depending on the experimental conditions either monoperoxo or diperoxo vanadium complexes are formed in acid aqueous solutions by addition of H,O, to N&V03. The 51V-NMR spectra of both species have been registered at different pH values. While the signal of the monoperoxo derivative is unchanged in the pH range o-4, the chemical shift of the diperoxo species shows a continuous variation in the same interval thus suggesting the occurrence of a protonation equilibrium leading to a neutral diperoxo vanadium complex. In the presence of bidentate ligands L, e.g., picolinic or pyrazinic acid, under otherwise identical experimental conditions, complexed peroxo vanadium species are formed. The association equilibria can be conveniently studied by ‘IV-NMR spectroscopy. Thus, different species, e.g., VO( O*)L, [ VO( O,)h] -, [ VO( 0,) 2L]z- as a function of the pH has been detected and the corresponding association constants have been measured. Keywords:
Aqueoussolutions; NMR spectroscopy;
Peroxo complexes;
Vanadium
1. Introduction The formation of peroxo vanadium derivatives in aqueous acid (pH < 2) solutions by addition of hydrogen peroxide to vanadium(V) precursors has been extensively studied [l-4]. As a result, the association processes shown in Scheme 1 have been established. Thus, together with the oxocation VO,‘, a red monoperoxo derivative 350 nm, l=610) (&I,= 455 nm, l=278) and a yellow diperoxo one (A,,= may occur depending on the excess of hydrogen peroxide [ 141. * Corresponding
author.
0304-5102/94/$07.00 0 1994 Elsevier Science SSDIO304-5102(94)00149-9
B.V. All rights reserved
324
V. Come et al. /Journal of Molecular Catalysis 94 (1994) 323-333
van+ +
-
4202
KI
[vo(o,)l+
+ Hz0
K2 w(o2)1+
+
k!o2
-
[VO(O2)2)-
+
2 H+
(1)
(2)
Scheme 1.
The situation, however, is more complicated than that depicted in Scheme 1. As an example, the general issue of the level of hydration of the various species is still a matter of debate [ $61. Therefore, the structure of the peroxo species in solution is largely unknown. Even some discrepancies concerning the equilibria of Scheme 1 are found in the literature [ l-31. These refer to the values of the association constant K2 of Table 1, measured by electronic spectroscopy. In light of the fact that K, values have been obtained at different pH and that both Eq. 1 and 2 involve charged species, which undergo acid-base equilibria, such discrepancies are not unexpected. In the present study we have reinvestigated the system vanadium( V)-hydrogen peroxide in aqueous acid solution by “V-NMR spectroscopy. Such a technique has been recently recognized as a powerful tool for the speciation of peroxo vanadium( V) derivatives in solution [ 5-81. In connection with the strongly growing interest in the biochemical role of peroxo vanadium species [ 91 we have also investigated, by the same technique, the effect of the presence of picolinic acid 1, on the formation of peroxo vanadium species under identical experimental conditions. Peroxo vanadium complexes containing the bidentate picolinato ligand have been previously isolated [ lo]. While their scope in synthetic organic chemistry as hydroxylating agents of alkanes and aromatics has been established, [ 10-121 little information is available on their oxidative ability or even on their structure in biological systems [ 9,13,14] . Interestingly, the picolinic acid moiety is present in the cofactor of some dehydrogenases such as 4,5-dioxo-12%pyrrolo[ 2,3fl quinoline-2,7,9-tricarboxylic acid (PQQ, methoxyxanthine) [ 15 1. For comparison purposes, we have also studied the behavior of pyrazinic acid 2, which is also capable of binding to vanadium. Ligands 1 and 2
1
2
The results presented in this paper provide some more information on the nature of simple peroxo-vanadium species in water, indicating the occurrence of acidbase equilibria taking place at very low pH values. In the presence of the two
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
325
Table 1 Association constants of HzOzto V(v) (seeScheme 1) No.
K,X104M-’
Acid [cont.]
Kz M
Acid [ cont. ]
Ref.
1 2 3
3.5 f 0.2 3.5+0.1 3.1 kO.4
H,SO, [0.5 M] HClO, [ 1 M] HCIO, [O.l M]
2.2 *0.2 1.2rtO.l 0.6rtO.l
H,SO, [OS M] HClO, [ 1 M] HClO, [O.l M]
Cl1 [21 r31
heteroligands considered picolinato and pyrazinato peroxo vanadium complexes are formed. Also on the structure of these species useful information has been obtained.
2. Experimental
section
2. I. Reagents Anhydrous NT-L,V03 (99.9% Fluka puriss. p.a.) , H202 (30% w/v Carlo Erba), picolinic acid (99% Aldrich) pyrazinic acid (99% Aldrich) were used without further purification. Deionized water was passed trough a Milli-Q/Organex-Q system (Millipore) . The hydrogen peroxide solutions were prepared by dissolving the appropriate amount of 30% w/v H202 in water. The peroxide content was determined by standard iodometric titration. pH measurements ( kO.02) were obtained with a Metrohm 632 pH-meter. 2.2. NMR sample preparation The peroxo vanadium solutions were prepared by dissolving a weighted amount of ammonium vanadate in 10 ml of water in the presence of the appropriate amount of HClO, both in the absence or in the presence of the heteroligands. H202 was added immediately before the registration of the spectra in order to minimize possible decomposition reactions. No electrolytes were added in this study in order to avoid potential coordination of anions to vanadium. At any rate, for every set of data the chemical shifts and A, ,* for all species were measured either in the presence or in the absence of NaClO, ( 1 M) or NaN03 ( 1 M). The values thus obtained were identical within the experimental error. This also indicates little or no effect of the variation of the ionic strength on the ‘IV-NMR signals. 2.3. Spectroscopy The ‘H-NMR and ‘lV-NMR spectra were recorded on a 4.69 Tesla Bruker AC, 200 MHz spectrometer for ‘H. The ‘IV-NMR spectra were obtained from the accumulation of about 1CKK&15000 transients with a 3500 Hz spectral window,
326
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
an accumulation time of 0.03 s, and a relaxation delay of 0.01 s. The probe temperature was maintained at 21°C. Data sets were obtained with a 2 Kb time domain and were zero filled to 4 Kb before Fourier transformation. Exponential linebroadening (20 Hz) was applied to FID before Fourier transformation. Chemical shifts are referred to external VOC& (0 ppm) . The line widths were measured by using the standard Bruker software for Lorentian fitting of the peaks.
3. Results and discussion Throughout this paper the water molecules coordinated to the various vanadium( V) species have been purposely omitted. As mentioned above, their number is still undefined [ $61, An exception has been made for those cases in which water molecules are required for a correct mass or charge balance. The peroxo vanadium complexes are formed in water by addition of hydrogen peroxide to a 0.005 M solution of NH,V03 and by adjusting the pH to the desired value with HC104 or NaOH. The monoperoxo species may be obtained by employing an equimolar amount of NH4V03 and H202 whereas the diperoxo one is formed by using at least a two-fold excess of Hz02 over NH4V03. The 51V-NMR spectra of solutions of the monoperoxo and of the diperoxo derivatives were recorded in the pH range 0.33-5.10. The attribution of the signals to the mono- and diperoxo derivatives was made by comparison with the available electronic spectral data ( [ VO( O,] +, A,,, = 455 nm, l=278, [VO(O,),] -, A,,,,,= 350 nm, E = 610). Such an assignment also agrees with the results of previous [ 5-81 “V-NMR studies. The salient data are collected in Table 2. Control experiments show that in comparison with the variation of 51V-NMR signals, those of electronic spectra of both peroxo species by changing the acidity in the same pH interval are almost negligible. This finding underlines the usefulness of the ‘lV-NMR spectroscopy technique which definitely is more sensitive than the electronic one. From the data of Table 2 chemical processes involving the monoperoxo species, in the entire region of pH explored, are not revealed. This is somehow unexpected since it is known that an exchange process with the diperoxo derivative occurs [ 2,4,16]. This aspect has not been further investigated. It may be noted that the chemical shift of the monoperoxo derivative ( - 540.5 ppm) is very similar to that of the oxocation VOZ ( - 544.8 ppm) [ 5,6]. This hampers an accurate determination of K1 value by ‘IV-NMR spectroscopy. We have confirmed by using the classical electronic spectroscopy method, [ 2,4] that such a value agrees very well with those reported in the literature and shown in Table 1. By contrast, both the chemical shift and the band width of the signal of the diperoxo derivative are affected by the acidity of the medium. We are not in the position to rationalize the modifications of the band width which are likely related to the occurrence of the
V. Come et al. /Journal of Molecular Catalysis 94 (1994) 323-333
321
Table 2 5’V-NMR data for mono and diperoxo vanadium species in water at 21°C as a function of the pH of the solution (HClO,) a No.
pH
WO(W+l
1 2 3 4 5 6 I 8 9 10 11 12 13 14 15 16 17 18 19 20
0.33 0.42 0.61 0.99 1.21 1.89 2.66 4.01 5.10 5.75 6.61 7.04 7.24 7.38 7.65 7.88 8.11 8.34 9.14 9.51
-
’ [V(v)],
4,#‘O(Od+l
mm
540.7 540.5 540.5 540.6 540.5
198 197 196 196 198
_
-
Hz
6WXOd;l
wm
-697.5 - 696.5 - 695.5 -694.1 -693.1 -692.1 -691.9 -691.9 -691.8 -691.6 -695.1 - 699.4 - 703.8 - 708.4 -718.4 - 728.3 - 737.8 - 746.9 -762.1 - 765.6
AdVO(Od;l
Hz
243 206 175 144 132 124 117 115 115 108 125 133 141 150 160 163 143 121 114 112
= 0.005 M, [HzOZlo=O.O1 M.
exchange equilibria mentioned above [ 2,4,16] . Other effects such as a modification of the quadrupole relaxation may also play a role. Therefore we limit our analysis to the observed variation of the chemical shifts of the diperoxo species as a function of pH. Such variation is attributed [ 5,6] to the occurrence of an acid-base equilibrium involving the diperoxo vanadium derivative, even though other processes cannot be completely ruled out. A likely formulation of such an equilibrium [ 171 is that of EQ. 3. WWzh1-
+
I-I+
W
KA
W’W’2)21
(3)
Therefore, as suggested by other authors, though without firm evidence, at very low pH values the anionic diperoxo vanadium species undergoes protonation yielding a neutral derivative [ 6,7]. No information can be given on the protonation site which could be either the oxo- or peroxo-oxygen [ 181. In order to provide further support to the occurrence of such a process we have carried out some ancillary experiments. Firstly, we have checked the reliability of our experimental procedure by investigating the equilibrium of eq.4 which had already been studied so that the corresponding pK,.,, value is available [ 5,6,19]. b’O(O2)2W20)1-
KA’
[VO(02)2(OH)]2-
+
H+
(4)
V. Come et al. /Journal of Molecular Catalysis 94 (1994) 323-333
328
Table 3 Association
constants K, calculated
No.
PH
1 2 3 4 5
1.89 1.21 0.99 0.61 0.36
at various pH (see Eq. 2)
0.30*0.04 0.51 f0.04 0.61 f 0.04 0.86*0.04 0.97 f0.04
According to Eq. 4, the monoanionic diperoxo vanadium species is transformed at higher pH values into the corresponding dianion. The value of pK,, = 6.9 f 0.2, determined by our experiments (see Table 2) is in good agreement with those, 6.98 f 0.23 and 7.2 + 0.1, reported in the literature [ 6,191 and obtained by the same technique under slightly different experimental conditions. Once confirmed the reliability of our measurements we return to the diperoxo vanadium derivative under acid conditions. If Eq. 2 were the only equilibrium involving such a derivative, we should of course find that the association constant K2 is independent of the pH. On the contrary, as shown by the data of Table 3, we observe that K2 values decrease with increasing the pH. Such K2 values are calculated from Eq. 2 [3] taking into account the mass balances on vanadium and hydrogen peroxide. This leads to Eq. 5. [W2)2-] [vo@,)+]
=K
[H2021,
+‘I,
_K
[H30+12
2
[vo(02)2-]
2
[H,O+r
=
[H2021,-[Vl,-[V0(02)2-] ]
=&[ 3
If Eq. 5 holds, plots of the ratio [ VO( O,),] - / [ VO( 0,) + 1, provided directly by the integration of the corresponding ‘IV-NMR signals, vs. different values of where [ H202] 0 and [V] ,, are the initial concentraIU-LWo- Wl,WW+12, tions of the two reagents, should give straight lines. Indeed in all cases the linear correlation was observed (r 2 0.99) so that K2 values of Table 3 are obtained as slopes of such lines. The dependence of K2 on the pH has some important implications, because it clearly indicates that K2, as obtained here, is not a simple constant and that acidbase equilibria involving the peroxo species operate. This explains why different values of K2 have been reported in the literature [ l-31 since they have been obtained at different pH values. In the light of the occurrence of a protonation equilibrium involving the diperoxo derivative, its formation from the vanadium precursor and hydrogen peroxide should be described not only by Eq. 2 but also by Eq. 6. Kt [VO(O,)]+
+
I+02
2_
WW2)21
+
H+
(6)
V. Conte et al. /Journal of MolecularCatalysis94 (1994) 323-333
329
-691 -
-703 -3.5
-1.5
0.5
2.5
4.5
PH Fig. 1. (0) Experimental and (W) calculated variation of the 5’V-NMR chemical shift for the [VO(O,),] signal as a function of the pH. Experimental conditions were 5.0 mM total vanadate, 40.0 mM total hydrogen peroxide, 5.0 mM total picolinic acid. The calculated values were obtained by a standard mathematical procedure (see [201).
From the ‘IV-NMR data we may establish the pH range at which either Eq. 2 or Eq. 6 are operating and, by mathematical methods, calculate both K2 and K2’ values from the appropriate equations. Such a calculation is straightforward as far as the K2 value is concerned, giving 0.30 M - ’ ( + 0.04) at the plateau value corresponding to pH > 2. For K2’ a more sophisticated treatment of the experimental data must be used owing to the limited range of occurrence of HV0(02)2 in the pH region, as shown in Fig. 1 (see experimental points (0) ) . In particular, it may be observed that complete protonation of the diperoxo anion would occur at pH well below zero. Given the experimental difficulty to explore such a region we are forced to employ a mathematical simulation procedure [ 201. Fig. 1 shows that a satisfactory agreement is observed between the calculated and the measured parts of the curve. From the data of Fig. 1 the pK, of the neutral diperoxo species is obtained. Its value of 0.43 ( + 0.04) confirms the strongly acidic character of the species. Once that the pK, value of the protonated diperoxo species is known it is possible to obtain the K,’ value of Eq. 6. In fact such equation is the sum of Eq. 2 and Eq. 3, so that K2’ = K2 X KA.The value of K2'thus calculated is 0.81 ( f 0.05). In the course of our investigation we discovered a unique feature of the system which also may contribute to the variability of the K2 values reported in the literature. In fact, as shown by the data presented in Table 4, it is observed that both the ‘IV-NMR chemical shifts and particularly the band width of the monoperoxo vanadium species at a fixed pH value do not depend on the nature of the strong acid employed, with the exception of HzS04. Although we have not examined this aspect in detail it is likely that the unique behavior of HzS04 is related to the
330 Table 4 “V-NMR acid
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
data for monoperoxo
No.
Acid
WO(O,)
1 2 3 4 5
HClO., HCl HBr HNG H,SQ
- 540.7 -541.0 - 540.4 - 540.5 - 544.0
vanadium
species in water (pH = 1.2) at 21°C as a function of the nature of the
+ 1 mm
A,,,[VO(W + 1 Hz 197 205 199 204 550
possibility of HSO, anion to coordinate to vanadium. As a consequence, it is expected that the K, value of the association of the second molecule of H,Oz to form the diperoxo anion should be different when measured in HzS04 solutions from that obtained in solution of the other strong acids. In fact, by repeating in HzS04 solutions the same measurements described above, we obtain (at pH = 1.2) a Kz value of 1.2 + 0.05 M- ’ which should be compared with the value of 0.5 + 0.04 M- ’ measured in HClO,. The finding that K2 is larger for sulfate complexed peroxo vanadium derivatives deserves further attention outside, however, the aim of the present investigation. We have also investigated the complexation equilibria of the vanadium peroxo complexes with picolinic and pyrazinic acid. In order to take advantage of the information gained for the aqueous complexes we have considered a system very similar to the previous one, involving a constant concentration of NH,VO, (0.005 M) , H,Oz (0.04 M) in the presence of picolinic (0.005 M) or pyrazinic (0.005 M) acid and we have recorded the ‘IV-NMR spectra at different pH values (HCIO,). Although, at such low pH values one might expect the occurrence of protonated ligands unable to coordinate to vanadium, ‘H-NMR data indicate that no free ligand is present in solution. Taking the view that a competition between the proton and the vanadium center for the association to the nitrogen and to the carboxylate occurs, it may be concluded that the ligands prefer the metal, perhaps for entropic reasons. Fig. 2 shows the various signals observed and the attributions made in the case of picolinic acid. Some of these are rather straightforward as in the case of [ VO( O,),] -, based on the results presented above, and also of VO( 0,) PIC which can be independently prepared by a reported procedure, [ 91 so that the spectrum of the isolated compound can be compared with that of the complex formed in situ. We have confirmed by ‘H-NMR spectroscopy that VO( 0,) PIC does not undergo ligand dissociation under the same conditions. On the basis of the observation that the association of the ligand causes an upfield shift of the ‘IV-NMR signal of the hydrated species also the identification of [ VO( O,),PIC] *- is relatively safe. By contrast the attribution of the signal at - 632.6 ppm requires more information. The key for the identification of such a species is provided by ‘H-NMR spectroscopy which shows the occurrence of two different signals for the Hs proton of the picolinato ligand in the
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
331
Fig. 2. 5’V-NMR spectra of the system: NI&VO,, H202, picolinic acid, in Hz0 as a function of pH. Experimental conditions are as in Fig. 1. The signals at -600.6, -632.6, -691.8, -744.8 ppm, refer respectively to: [VO(Oz)PIC], [VO(O,)(PIC),]-, [VO(O,),]and [VO(O&PIC]-’
complex. These are found at 9.34 and 9.80 ppm respectively. In all the other complexes only one signal for the I& proton is observed. Moreover, by combining the information provided by ‘H- and ‘IVXMR spectra, it may be seen that the disappearance of the vanadium signals of VO( 0,) PIC on increasing the pH parallels the appearance of the signals of the two protons. On this basis, the signal at - 632.6 ppm has been attributed to the monoperoxo vanadium complex containing two molecules of the picolinato ligand, [ VO( 0,) (PIC) *] -.
Having identified all the vanadium peroxo complexes present at different pH values it is possible to write the complexation equilibria taking place in solution as in Scheme 2. Note that exactly the same scheme applies to pyrazinic acid whose “V-NMR behavior has been found to be similar to that of picolinic acid, even though the chemical shifts values of the peroxo vanadium species containing the two ligands are different. In particular [ VO( 0,) PYR] , [ VO( 0,) (PYR) J and [VO( 0z),PYR12are found at - 585.4 ppm, -600.3 ppm and - 733.8 ppm
V. Conte et al. / Joumal of Molecular Catalysis 94 (1994) 323-333
332
[VO(O,]+
+
[VO(02)L]
+
LH
-
[VO(O,),]-
+
LH
-
LH
-
Kass
Kass
rass
w(o2)u
+
I++
ww2ml-
+
H+
[WO2)2W
+
l-l+
(7)
LH = picolinic or pyrazinic acid Scheme 2. Table 5 Association
constants of picolinic and pyrazinic acid to mono and diperoxo vanadium species in water, at 21°C
No. =
Picolinic acid
Pyrazinic acid
K ass
5.8. lo3 1.4 5.7. 1o-3
2.5. IO3 0.8 1.7.10-*
K’,,, K”ass
’ See Scheme 2 for definition of the constants.
respectively. Scheme 2 displays the equations defining the various association constants which have been determined from experiments in which one of the three parameters, i.e., hydrogen peroxide, heteroligand concentration or pH, is changed by maintaining fixed the other two. Typical spectra are those shown in Fig. 2. Table 5 collects the association constants values, K,,,, K,,’ and K,,,” thus determined. An important feature disclosed by the data of Table 5 is the rather large equilibrium constants found for the association of the two heteroligands to the monoperoxo vanadium species. It should be noted that such values are comparable with those measured for the association of hydrogen peroxide. This finding has biochemical implications that should be taken into account because it suggests that formation of complexed peroxo vanadium species even under very acidic conditions, is a likely process [ 191. A further information provided by the data of Table 5 is the similarity of the behavior of picolinic and pyrazinic acid which suggests that the major role in determining the binding ability is played by the presence of a carboxylato and amino group in the appropriate position [ 8,191.
4. Conclusions The investigation presented here concerning the chemistry of peroxo vanadium species in water, either in the absence or in the presence of heteroligands, has revealed the occurrence of several species which exist at different pH values. Taking into account that all these species are, at least potentially, oxidants of organic
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
333
substrates in biological reactions it is expected that, depending on their nature, different oxidative behaviors can be observed. Experiments aimed at exploring this aspect are currently being carried out in our laboratory.
Acknowledgements This work has been carried out with the financial support from MURST, Italian Ministry of Research, and from Italian National Research Council.
References [l] G.A. DeanCan. J. Chem.,39 (1961) 1174. [2] M. Orhanovich and R.G. Wilkins, J. Am. Chem. Sot., 89 ( 1967) 278. [3] F. Secco, Inorg. Chem., 19 (1980) 2722. [4] S. Funahashi, K. Ishihara, M. Inamo and M. Tanaka, Inorg. Chim. Acta, 157 (1989) 65. [5] O.W. Howarth and J.R. Hunt, J. Chem. Sot., Dalton Trans., (1979) 1388. [6] O.W. Howarth and A.T. Harrison, J. Chem. Sot., Dalton Trans., (1985) 1173. [7] N.J. Campbell, A.C. Dengel, W.P. Griffith, Polyhedron., 8 (1989) 1379. [S] AS. Tracey and J.S. Jaswal, J. Am. Chem. Sot., 114 (1992) 3835. [9] A. Shaver, J.B. Ng, D.A. Hall, B.S. Lum, and B.I. Posner, Inorg. Chem., 32 (1993) 3109. [IO] H. Mimoun, L. Saussine, E. Daire, M. Postel, J. Fischer and R. Weiss, J. Am. Chem. Sot., 105 ( 1983) 3101. [ 111 M. Bonchio, V. Conte, F. Di Furia and G. Modena, J. Org. Chem., 54 ( 1989) 4368. [ 121 M. Bianchi, M. Bonchio, V. Conte, F. Coppa, F. Di Furia, G. Modena, S. Moro and S. Standen, J. Mol. Catal., 83 (1993) 107. [ 131 (a)A. Butler, in N.D. Chasteen (Ed.), Vanadium in Biological Systems: Physiology and Biochemistry, Kluwer, Dordrecht, 1990, Chap. 2, p. 25-49; (b) A. Butler and C.J. Carrano, Coord. Chem. Rev., 109 (1991) 61. [ 141 (a) A. Butler, Bioinorganic Chemistry Symposium 205th ACS Meeting, Denver, 1993; (b) A. Butler and J.V. Walker, Chem. Rev., 93 (1993) 1937. [ 151 S. Suzuki, T. Sakurai, S. Itoh and Y. Ohshiro, Inorg. Chem., 27 ( 1988) 591, and references cited therein. [ 161 V. Conte and D.C. Crans, unpublished results. [ 171 K. Wieghardt, Inorg. Chem., 17 (1978) 57. [ 181 U. Quilitzsch and K. Wieghardt, Inorg. Chem., 18 (1979) 869. [ 191 AS. Tracey and J.S. Jaswal, Inorg. Chem., 30 (1991) 3718. [20] F.L. Slejko, R.S. Drago and D.G. Brown, J. Am. Chem. Sot., 94 (1972) 9210.
MOLECULAR CATALYSIS Journal of MolecularCatalysis94 ( 1994) 323-333
‘lV-NMR investigation on the formation of peroxo vanadium complexes in aqueous solution: some novel observations Valeria Conte, Fulvio Di Furia *, Stefano Moro Vniversit6 di Padova, Centro CNR di Studio sui Meccanismi di Reazioni Organiche, Dipartimento di Chimica Organica, Via Mar2010 I, 35131 Padova. Italy
Received6 May 1994;accepted7 June 1994
Abstract Depending on the experimental conditions either monoperoxo or diperoxo vanadium complexes are formed in acid aqueous solutions by addition of H,O, to N&V03. The 51V-NMR spectra of both species have been registered at different pH values. While the signal of the monoperoxo derivative is unchanged in the pH range o-4, the chemical shift of the diperoxo species shows a continuous variation in the same interval thus suggesting the occurrence of a protonation equilibrium leading to a neutral diperoxo vanadium complex. In the presence of bidentate ligands L, e.g., picolinic or pyrazinic acid, under otherwise identical experimental conditions, complexed peroxo vanadium species are formed. The association equilibria can be conveniently studied by ‘IV-NMR spectroscopy. Thus, different species, e.g., VO( O*)L, [ VO( O,)h] -, [ VO( 0,) 2L]z- as a function of the pH has been detected and the corresponding association constants have been measured. Keywords:
Aqueoussolutions; NMR spectroscopy;
Peroxo complexes;
Vanadium
1. Introduction The formation of peroxo vanadium derivatives in aqueous acid (pH < 2) solutions by addition of hydrogen peroxide to vanadium(V) precursors has been extensively studied [l-4]. As a result, the association processes shown in Scheme 1 have been established. Thus, together with the oxocation VO,‘, a red monoperoxo derivative 350 nm, l=610) (&I,= 455 nm, l=278) and a yellow diperoxo one (A,,= may occur depending on the excess of hydrogen peroxide [ 141. * Corresponding
author.
0304-5102/94/$07.00 0 1994 Elsevier Science SSDIO304-5102(94)00149-9
B.V. All rights reserved
324
V. Come et al. /Journal of Molecular Catalysis 94 (1994) 323-333
van+ +
-
4202
KI
[vo(o,)l+
+ Hz0
K2 w(o2)1+
+
k!o2
-
[VO(O2)2)-
+
2 H+
(1)
(2)
Scheme 1.
The situation, however, is more complicated than that depicted in Scheme 1. As an example, the general issue of the level of hydration of the various species is still a matter of debate [ $61. Therefore, the structure of the peroxo species in solution is largely unknown. Even some discrepancies concerning the equilibria of Scheme 1 are found in the literature [ l-31. These refer to the values of the association constant K2 of Table 1, measured by electronic spectroscopy. In light of the fact that K, values have been obtained at different pH and that both Eq. 1 and 2 involve charged species, which undergo acid-base equilibria, such discrepancies are not unexpected. In the present study we have reinvestigated the system vanadium( V)-hydrogen peroxide in aqueous acid solution by “V-NMR spectroscopy. Such a technique has been recently recognized as a powerful tool for the speciation of peroxo vanadium( V) derivatives in solution [ 5-81. In connection with the strongly growing interest in the biochemical role of peroxo vanadium species [ 91 we have also investigated, by the same technique, the effect of the presence of picolinic acid 1, on the formation of peroxo vanadium species under identical experimental conditions. Peroxo vanadium complexes containing the bidentate picolinato ligand have been previously isolated [ lo]. While their scope in synthetic organic chemistry as hydroxylating agents of alkanes and aromatics has been established, [ 10-121 little information is available on their oxidative ability or even on their structure in biological systems [ 9,13,14] . Interestingly, the picolinic acid moiety is present in the cofactor of some dehydrogenases such as 4,5-dioxo-12%pyrrolo[ 2,3fl quinoline-2,7,9-tricarboxylic acid (PQQ, methoxyxanthine) [ 15 1. For comparison purposes, we have also studied the behavior of pyrazinic acid 2, which is also capable of binding to vanadium. Ligands 1 and 2
1
2
The results presented in this paper provide some more information on the nature of simple peroxo-vanadium species in water, indicating the occurrence of acidbase equilibria taking place at very low pH values. In the presence of the two
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
325
Table 1 Association constants of HzOzto V(v) (seeScheme 1) No.
K,X104M-’
Acid [cont.]
Kz M
Acid [ cont. ]
Ref.
1 2 3
3.5 f 0.2 3.5+0.1 3.1 kO.4
H,SO, [0.5 M] HClO, [ 1 M] HCIO, [O.l M]
2.2 *0.2 1.2rtO.l 0.6rtO.l
H,SO, [OS M] HClO, [ 1 M] HClO, [O.l M]
Cl1 [21 r31
heteroligands considered picolinato and pyrazinato peroxo vanadium complexes are formed. Also on the structure of these species useful information has been obtained.
2. Experimental
section
2. I. Reagents Anhydrous NT-L,V03 (99.9% Fluka puriss. p.a.) , H202 (30% w/v Carlo Erba), picolinic acid (99% Aldrich) pyrazinic acid (99% Aldrich) were used without further purification. Deionized water was passed trough a Milli-Q/Organex-Q system (Millipore) . The hydrogen peroxide solutions were prepared by dissolving the appropriate amount of 30% w/v H202 in water. The peroxide content was determined by standard iodometric titration. pH measurements ( kO.02) were obtained with a Metrohm 632 pH-meter. 2.2. NMR sample preparation The peroxo vanadium solutions were prepared by dissolving a weighted amount of ammonium vanadate in 10 ml of water in the presence of the appropriate amount of HClO, both in the absence or in the presence of the heteroligands. H202 was added immediately before the registration of the spectra in order to minimize possible decomposition reactions. No electrolytes were added in this study in order to avoid potential coordination of anions to vanadium. At any rate, for every set of data the chemical shifts and A, ,* for all species were measured either in the presence or in the absence of NaClO, ( 1 M) or NaN03 ( 1 M). The values thus obtained were identical within the experimental error. This also indicates little or no effect of the variation of the ionic strength on the ‘IV-NMR signals. 2.3. Spectroscopy The ‘H-NMR and ‘lV-NMR spectra were recorded on a 4.69 Tesla Bruker AC, 200 MHz spectrometer for ‘H. The ‘IV-NMR spectra were obtained from the accumulation of about 1CKK&15000 transients with a 3500 Hz spectral window,
326
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
an accumulation time of 0.03 s, and a relaxation delay of 0.01 s. The probe temperature was maintained at 21°C. Data sets were obtained with a 2 Kb time domain and were zero filled to 4 Kb before Fourier transformation. Exponential linebroadening (20 Hz) was applied to FID before Fourier transformation. Chemical shifts are referred to external VOC& (0 ppm) . The line widths were measured by using the standard Bruker software for Lorentian fitting of the peaks.
3. Results and discussion Throughout this paper the water molecules coordinated to the various vanadium( V) species have been purposely omitted. As mentioned above, their number is still undefined [ $61, An exception has been made for those cases in which water molecules are required for a correct mass or charge balance. The peroxo vanadium complexes are formed in water by addition of hydrogen peroxide to a 0.005 M solution of NH,V03 and by adjusting the pH to the desired value with HC104 or NaOH. The monoperoxo species may be obtained by employing an equimolar amount of NH4V03 and H202 whereas the diperoxo one is formed by using at least a two-fold excess of Hz02 over NH4V03. The 51V-NMR spectra of solutions of the monoperoxo and of the diperoxo derivatives were recorded in the pH range 0.33-5.10. The attribution of the signals to the mono- and diperoxo derivatives was made by comparison with the available electronic spectral data ( [ VO( O,] +, A,,, = 455 nm, l=278, [VO(O,),] -, A,,,,,= 350 nm, E = 610). Such an assignment also agrees with the results of previous [ 5-81 “V-NMR studies. The salient data are collected in Table 2. Control experiments show that in comparison with the variation of 51V-NMR signals, those of electronic spectra of both peroxo species by changing the acidity in the same pH interval are almost negligible. This finding underlines the usefulness of the ‘lV-NMR spectroscopy technique which definitely is more sensitive than the electronic one. From the data of Table 2 chemical processes involving the monoperoxo species, in the entire region of pH explored, are not revealed. This is somehow unexpected since it is known that an exchange process with the diperoxo derivative occurs [ 2,4,16]. This aspect has not been further investigated. It may be noted that the chemical shift of the monoperoxo derivative ( - 540.5 ppm) is very similar to that of the oxocation VOZ ( - 544.8 ppm) [ 5,6]. This hampers an accurate determination of K1 value by ‘IV-NMR spectroscopy. We have confirmed by using the classical electronic spectroscopy method, [ 2,4] that such a value agrees very well with those reported in the literature and shown in Table 1. By contrast, both the chemical shift and the band width of the signal of the diperoxo derivative are affected by the acidity of the medium. We are not in the position to rationalize the modifications of the band width which are likely related to the occurrence of the
V. Come et al. /Journal of Molecular Catalysis 94 (1994) 323-333
321
Table 2 5’V-NMR data for mono and diperoxo vanadium species in water at 21°C as a function of the pH of the solution (HClO,) a No.
pH
WO(W+l
1 2 3 4 5 6 I 8 9 10 11 12 13 14 15 16 17 18 19 20
0.33 0.42 0.61 0.99 1.21 1.89 2.66 4.01 5.10 5.75 6.61 7.04 7.24 7.38 7.65 7.88 8.11 8.34 9.14 9.51
-
’ [V(v)],
4,#‘O(Od+l
mm
540.7 540.5 540.5 540.6 540.5
198 197 196 196 198
_
-
Hz
6WXOd;l
wm
-697.5 - 696.5 - 695.5 -694.1 -693.1 -692.1 -691.9 -691.9 -691.8 -691.6 -695.1 - 699.4 - 703.8 - 708.4 -718.4 - 728.3 - 737.8 - 746.9 -762.1 - 765.6
AdVO(Od;l
Hz
243 206 175 144 132 124 117 115 115 108 125 133 141 150 160 163 143 121 114 112
= 0.005 M, [HzOZlo=O.O1 M.
exchange equilibria mentioned above [ 2,4,16] . Other effects such as a modification of the quadrupole relaxation may also play a role. Therefore we limit our analysis to the observed variation of the chemical shifts of the diperoxo species as a function of pH. Such variation is attributed [ 5,6] to the occurrence of an acid-base equilibrium involving the diperoxo vanadium derivative, even though other processes cannot be completely ruled out. A likely formulation of such an equilibrium [ 171 is that of EQ. 3. WWzh1-
+
I-I+
W
KA
W’W’2)21
(3)
Therefore, as suggested by other authors, though without firm evidence, at very low pH values the anionic diperoxo vanadium species undergoes protonation yielding a neutral derivative [ 6,7]. No information can be given on the protonation site which could be either the oxo- or peroxo-oxygen [ 181. In order to provide further support to the occurrence of such a process we have carried out some ancillary experiments. Firstly, we have checked the reliability of our experimental procedure by investigating the equilibrium of eq.4 which had already been studied so that the corresponding pK,.,, value is available [ 5,6,19]. b’O(O2)2W20)1-
KA’
[VO(02)2(OH)]2-
+
H+
(4)
V. Come et al. /Journal of Molecular Catalysis 94 (1994) 323-333
328
Table 3 Association
constants K, calculated
No.
PH
1 2 3 4 5
1.89 1.21 0.99 0.61 0.36
at various pH (see Eq. 2)
0.30*0.04 0.51 f0.04 0.61 f 0.04 0.86*0.04 0.97 f0.04
According to Eq. 4, the monoanionic diperoxo vanadium species is transformed at higher pH values into the corresponding dianion. The value of pK,, = 6.9 f 0.2, determined by our experiments (see Table 2) is in good agreement with those, 6.98 f 0.23 and 7.2 + 0.1, reported in the literature [ 6,191 and obtained by the same technique under slightly different experimental conditions. Once confirmed the reliability of our measurements we return to the diperoxo vanadium derivative under acid conditions. If Eq. 2 were the only equilibrium involving such a derivative, we should of course find that the association constant K2 is independent of the pH. On the contrary, as shown by the data of Table 3, we observe that K2 values decrease with increasing the pH. Such K2 values are calculated from Eq. 2 [3] taking into account the mass balances on vanadium and hydrogen peroxide. This leads to Eq. 5. [W2)2-] [vo@,)+]
=K
[H2021,
+‘I,
_K
[H30+12
2
[vo(02)2-]
2
[H,O+r
=
[H2021,-[Vl,-[V0(02)2-] ]
=&[ 3
If Eq. 5 holds, plots of the ratio [ VO( O,),] - / [ VO( 0,) + 1, provided directly by the integration of the corresponding ‘IV-NMR signals, vs. different values of where [ H202] 0 and [V] ,, are the initial concentraIU-LWo- Wl,WW+12, tions of the two reagents, should give straight lines. Indeed in all cases the linear correlation was observed (r 2 0.99) so that K2 values of Table 3 are obtained as slopes of such lines. The dependence of K2 on the pH has some important implications, because it clearly indicates that K2, as obtained here, is not a simple constant and that acidbase equilibria involving the peroxo species operate. This explains why different values of K2 have been reported in the literature [ l-31 since they have been obtained at different pH values. In the light of the occurrence of a protonation equilibrium involving the diperoxo derivative, its formation from the vanadium precursor and hydrogen peroxide should be described not only by Eq. 2 but also by Eq. 6. Kt [VO(O,)]+
+
I+02
2_
WW2)21
+
H+
(6)
V. Conte et al. /Journal of MolecularCatalysis94 (1994) 323-333
329
-691 -
-703 -3.5
-1.5
0.5
2.5
4.5
PH Fig. 1. (0) Experimental and (W) calculated variation of the 5’V-NMR chemical shift for the [VO(O,),] signal as a function of the pH. Experimental conditions were 5.0 mM total vanadate, 40.0 mM total hydrogen peroxide, 5.0 mM total picolinic acid. The calculated values were obtained by a standard mathematical procedure (see [201).
From the ‘IV-NMR data we may establish the pH range at which either Eq. 2 or Eq. 6 are operating and, by mathematical methods, calculate both K2 and K2’ values from the appropriate equations. Such a calculation is straightforward as far as the K2 value is concerned, giving 0.30 M - ’ ( + 0.04) at the plateau value corresponding to pH > 2. For K2’ a more sophisticated treatment of the experimental data must be used owing to the limited range of occurrence of HV0(02)2 in the pH region, as shown in Fig. 1 (see experimental points (0) ) . In particular, it may be observed that complete protonation of the diperoxo anion would occur at pH well below zero. Given the experimental difficulty to explore such a region we are forced to employ a mathematical simulation procedure [ 201. Fig. 1 shows that a satisfactory agreement is observed between the calculated and the measured parts of the curve. From the data of Fig. 1 the pK, of the neutral diperoxo species is obtained. Its value of 0.43 ( + 0.04) confirms the strongly acidic character of the species. Once that the pK, value of the protonated diperoxo species is known it is possible to obtain the K,’ value of Eq. 6. In fact such equation is the sum of Eq. 2 and Eq. 3, so that K2’ = K2 X KA.The value of K2'thus calculated is 0.81 ( f 0.05). In the course of our investigation we discovered a unique feature of the system which also may contribute to the variability of the K2 values reported in the literature. In fact, as shown by the data presented in Table 4, it is observed that both the ‘IV-NMR chemical shifts and particularly the band width of the monoperoxo vanadium species at a fixed pH value do not depend on the nature of the strong acid employed, with the exception of HzS04. Although we have not examined this aspect in detail it is likely that the unique behavior of HzS04 is related to the
330 Table 4 “V-NMR acid
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
data for monoperoxo
No.
Acid
WO(O,)
1 2 3 4 5
HClO., HCl HBr HNG H,SQ
- 540.7 -541.0 - 540.4 - 540.5 - 544.0
vanadium
species in water (pH = 1.2) at 21°C as a function of the nature of the
+ 1 mm
A,,,[VO(W + 1 Hz 197 205 199 204 550
possibility of HSO, anion to coordinate to vanadium. As a consequence, it is expected that the K, value of the association of the second molecule of H,Oz to form the diperoxo anion should be different when measured in HzS04 solutions from that obtained in solution of the other strong acids. In fact, by repeating in HzS04 solutions the same measurements described above, we obtain (at pH = 1.2) a Kz value of 1.2 + 0.05 M- ’ which should be compared with the value of 0.5 + 0.04 M- ’ measured in HClO,. The finding that K2 is larger for sulfate complexed peroxo vanadium derivatives deserves further attention outside, however, the aim of the present investigation. We have also investigated the complexation equilibria of the vanadium peroxo complexes with picolinic and pyrazinic acid. In order to take advantage of the information gained for the aqueous complexes we have considered a system very similar to the previous one, involving a constant concentration of NH,VO, (0.005 M) , H,Oz (0.04 M) in the presence of picolinic (0.005 M) or pyrazinic (0.005 M) acid and we have recorded the ‘IV-NMR spectra at different pH values (HCIO,). Although, at such low pH values one might expect the occurrence of protonated ligands unable to coordinate to vanadium, ‘H-NMR data indicate that no free ligand is present in solution. Taking the view that a competition between the proton and the vanadium center for the association to the nitrogen and to the carboxylate occurs, it may be concluded that the ligands prefer the metal, perhaps for entropic reasons. Fig. 2 shows the various signals observed and the attributions made in the case of picolinic acid. Some of these are rather straightforward as in the case of [ VO( O,),] -, based on the results presented above, and also of VO( 0,) PIC which can be independently prepared by a reported procedure, [ 91 so that the spectrum of the isolated compound can be compared with that of the complex formed in situ. We have confirmed by ‘H-NMR spectroscopy that VO( 0,) PIC does not undergo ligand dissociation under the same conditions. On the basis of the observation that the association of the ligand causes an upfield shift of the ‘IV-NMR signal of the hydrated species also the identification of [ VO( O,),PIC] *- is relatively safe. By contrast the attribution of the signal at - 632.6 ppm requires more information. The key for the identification of such a species is provided by ‘H-NMR spectroscopy which shows the occurrence of two different signals for the Hs proton of the picolinato ligand in the
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
331
Fig. 2. 5’V-NMR spectra of the system: NI&VO,, H202, picolinic acid, in Hz0 as a function of pH. Experimental conditions are as in Fig. 1. The signals at -600.6, -632.6, -691.8, -744.8 ppm, refer respectively to: [VO(Oz)PIC], [VO(O,)(PIC),]-, [VO(O,),]and [VO(O&PIC]-’
complex. These are found at 9.34 and 9.80 ppm respectively. In all the other complexes only one signal for the I& proton is observed. Moreover, by combining the information provided by ‘H- and ‘IVXMR spectra, it may be seen that the disappearance of the vanadium signals of VO( 0,) PIC on increasing the pH parallels the appearance of the signals of the two protons. On this basis, the signal at - 632.6 ppm has been attributed to the monoperoxo vanadium complex containing two molecules of the picolinato ligand, [ VO( 0,) (PIC) *] -.
Having identified all the vanadium peroxo complexes present at different pH values it is possible to write the complexation equilibria taking place in solution as in Scheme 2. Note that exactly the same scheme applies to pyrazinic acid whose “V-NMR behavior has been found to be similar to that of picolinic acid, even though the chemical shifts values of the peroxo vanadium species containing the two ligands are different. In particular [ VO( 0,) PYR] , [ VO( 0,) (PYR) J and [VO( 0z),PYR12are found at - 585.4 ppm, -600.3 ppm and - 733.8 ppm
V. Conte et al. / Joumal of Molecular Catalysis 94 (1994) 323-333
332
[VO(O,]+
+
[VO(02)L]
+
LH
-
[VO(O,),]-
+
LH
-
LH
-
Kass
Kass
rass
w(o2)u
+
I++
ww2ml-
+
H+
[WO2)2W
+
l-l+
(7)
LH = picolinic or pyrazinic acid Scheme 2. Table 5 Association
constants of picolinic and pyrazinic acid to mono and diperoxo vanadium species in water, at 21°C
No. =
Picolinic acid
Pyrazinic acid
K ass
5.8. lo3 1.4 5.7. 1o-3
2.5. IO3 0.8 1.7.10-*
K’,,, K”ass
’ See Scheme 2 for definition of the constants.
respectively. Scheme 2 displays the equations defining the various association constants which have been determined from experiments in which one of the three parameters, i.e., hydrogen peroxide, heteroligand concentration or pH, is changed by maintaining fixed the other two. Typical spectra are those shown in Fig. 2. Table 5 collects the association constants values, K,,,, K,,’ and K,,,” thus determined. An important feature disclosed by the data of Table 5 is the rather large equilibrium constants found for the association of the two heteroligands to the monoperoxo vanadium species. It should be noted that such values are comparable with those measured for the association of hydrogen peroxide. This finding has biochemical implications that should be taken into account because it suggests that formation of complexed peroxo vanadium species even under very acidic conditions, is a likely process [ 191. A further information provided by the data of Table 5 is the similarity of the behavior of picolinic and pyrazinic acid which suggests that the major role in determining the binding ability is played by the presence of a carboxylato and amino group in the appropriate position [ 8,191.
4. Conclusions The investigation presented here concerning the chemistry of peroxo vanadium species in water, either in the absence or in the presence of heteroligands, has revealed the occurrence of several species which exist at different pH values. Taking into account that all these species are, at least potentially, oxidants of organic
V. Conte et al. /Journal of Molecular Catalysis 94 (1994) 323-333
333
substrates in biological reactions it is expected that, depending on their nature, different oxidative behaviors can be observed. Experiments aimed at exploring this aspect are currently being carried out in our laboratory.
Acknowledgements This work has been carried out with the financial support from MURST, Italian Ministry of Research, and from Italian National Research Council.
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