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Metal complexation and rotational isomerism of simple carboxylic acids—IV Tungstate-malic acid complexes studied by 1H NMR
J. inorg, nucl. Chem. Yol. 42, pp. 389-393 Pergamon Press Ltd., 1980. Printed in Great Britain
METAL COMPLEXATION AND ROTATIONAL ISOMERISM OF SIMPLE CARBOXYLIC ACIDS--IVf TUNGSTATE--MALIC ACID C O M P L E X E S STUDIED BY IH NMR VICTORM. S. GIL Department of Chemistry,University of Aveiro,Portugal and M. EMILIAT. L. SARAIVA,M. MADALENACALDEIRAand ANA M. D. PEREIRA Department of Chemistry, Universityof Coimbra, Portugal
(First received 22 September 1978; in revised[otto 25 April 1979) Abstract--Proton NMR evidence is presented on the number and relative concentrationsof complexesformed by W(VI) and L-malicacid (pH range 3-6), their composition,conformationof the ligand moiety,and (briefly)on their pH-dependence and relative rates of formation. In particular, four complexesare detected: 1:2 (two), 1:1, 2:1. A concentration effect and a slow rate of formation suggest that the I : 1 complexis binuclear. INTRODUCTION
Previous papers in this series have dealt with the use of tH NMR spectroscopy in structural studies of complexes of Zn(II)[l] and U(VI)[2] with malic acid and W(VI) with thiomalic acid[3], in aqueous solution. We now report a similar study of the system tungstate-malic acid (L) for the pH range 3-6, which provides information on the number and relative concentrations of complexes, their stoichiometry and pH dependence, the conformation of the ligand and some kynetic features. The formation of complexes between W(VI) and malic acid has been investigated earlier by conductimetric [4-6], potentiometric [4, 5] and polarimetric [5, 7] methods. Richardson[4] found evidence for a 1:3 complex formed between tungstic and malic acids in solution. Baillie and Brown [5] reported the formation of a 1:1 complex, whereas Prasad and Pandey [6] concluded for complexation in the ratio 1:2 (WO]-: acid). These authors found, however, that on treating the solution with Pb 2+ or Ag+ the tungstomalate ion present in the solid is the 1 : 1 complex ion. EXPERIMENTAL Analytical grade sodium tungstate and commerciallyavailable L-malic acid were used. In order to reduce the OH N.M.R. signal, the former was liophylized from a solution in [hO, the latter was dried at 120°Cand D~Osolutions were used throughout. The concentrations of the sodium tungstate solutions were established by weight and those of the acid solutions by potentiometry. Adjustment of pH was done by dropwise addition of dilute DCI and NaOD solutions. The spectra were run on a Varian HA-100spectrometer. RESULTSAND DISCUSSION
(1) Stoichiometry and relative abundances of the complexes plexes The 100 MHz tH NMR spectrum of the partially deuterated malic acid DO2C--CH2-CH(OD)-CO2D in (I~O) tFor previous papers in this series see Refs. [1-3].
solution is of the ABX type, both free and as a ligand, the X signal arising from the -CH(OD)- proton. Depending on the WO]-: malic acid molar ratio and on pH, several X signals are observed as well as a complex multiplet resulting from partially superimposed AB patterns. The existence of three complexes is thus immediately demonstrated, When excess of malic acid is present (molar ratios equal to or smaller than 1:2), its spectrum is also detected, distinct from the others. Figure 1 shows the X-spectra for the complexes and for free ligand when in excess. It is noted that, whereas the AB part is almost not shifted upon complexation, the X multiplet undergoes a low-field shift of about 1 ppm (1.10, 1.01, 0.91 ppm, respectively). The observation of separate spectra (particularly their X part) for the various species involving malic acid allows the application of Job's continuous variation method which provides information on the stoichiometry of the complexes. This was firstly done in Ref. [3], for the system WO~- + thiomalic acid. The method was applied at three pH* values in the range 3-6, namely 3.0, 4.3 and 5.5. It is noted that the dominant species of W(VI) for such pH values and for the concentrations used is W~20~; in equilibrium with other isopolyanions and with WO~-[8]. For aqueous solutions of malic acid, H2A, the dominant species are H2A, HA-, A2- as the pH increases over that range. The various mixtures were allowed to reach equilibrium (see below) before the NMR spectral intensities were determined by integration. Figure 2 shows the Job's curves corresponding to the X signals of each complex, for the case of parent solution (1.01 M) of pH*= 4.3. It is thus shown that the sharp quartet corresponds to a 1:1 complex, whereas the broader X signals belong to 1:2 complexes, one less stable, (1:2)_, than the other, (1:2)+. By comparing the total intensity of the visible X signals with that for the AB multiplet, in the case of
389
390
V.M.S. GIL et al.
lOHz
OH (REDUCED) SIGNAL
X SIGNALS OF COMPLEXES
X SIGNAL OF FREE MALIC ACID
Fig. I. X parts of the ~H NMR spectra (ABX type) of three complexes of WO42- with malic acid and of free acid in excess (l : 4 solution).
molar ratios 3: l, 2: 1, 1.5:1 and I: 1, it is noted that a fourth complex must exist in these solutions, having an X spectrum either too broad and weak to be observed or, more likely, under the residual OH signal; its concentration being maximum, although small, for the 2:1 solution, it is concluded that a 2:1 complex is also formed. It is noted that the 1 : 1 complex is always accompanied by the !:2 ones; however, for molar ratios equal to or smaller than i:3 practically only the 1:2 complexes are present.
Similar conclusions on the stoichiometry of the complexes are obtained for pH* = 3.0 and pH* = 5.5, but relative abundancies of the complexes vary. For example, at pH* = 3.0, malic acid is predominantly involved in the formation of the 1:2 complexes even in an equimolar solution of WO~- and acid, only a small concentration of 1 : 1 being present. At pH* = 4.5 it is the 1 : 1 complex that is the most abundant one. On the other hand, the concentration of the 2:1 complex increases with pH over the range studied. Table 1 shows the concentration of the malic acid moiety in the various complexes at the three
/ 1:1
1:2 .",~.
"-- ..~3
[
l:~
Q.
E o.:~
1:1.
~.
.I, ' / o.,
I
./
?
.,f
/
//
"...
...'J. . . . . . . .
/
/
~ \
\,,1:3
% \x,e1:l*
\ /
A',. . . . "" 0.2
\
\2:?L.,, o.~
o.e
ol
[ Malic acid]tot
IM'"o,%~[~o,'Jt= Fig. 2. Job's curves for the system WO~-+malic acid(L), in D~O and at pH*= 4.3, based on NMR spectral intensities.
Metal complexationand rotational isomerismof simple carboxylicacids--IV
391
Table 1. Molarities of malic acid in complexes as function of pH and WO~-: Malic acid molar ratios (Concentrationof parent solutions: 1.0 M) Molar ratio 2:1 1:1 1:2 pH* Complexes 2:1 1:1 (1:2)_(1:2)+ 2:1 1:1 (1:2)_(1:2)+ 2:1 1:1 (1:2)_(1:2)+ 3 4.5 5.5
0.06 0.1)8 0.04 0.15 0.04 0.08 0.08 0.30 ~0 n0 0.10 0.41 0.09 0.15 0.02 0.08 0.06 0.23 0.06 0.15 ~0 0.11 0.12 0.31 0.13 0.05 0.03 0.12 0.13 0.20 0.05 0.11 ~0 0.07 0.07 0.29
pH values studied for typical molar ratios. The estimate accuracy of those values is ___0.015, except perhaps those for the 2:1 complex which are likely to be slightly less accurate. Complexes having the composition 2: 1, 1:1 and 1:2 have been reported earlier for the similar system MoO~--malic acid [9] on the basis of potentiometric and polarimetric studies. The effect of concentration on the relative abundances of the complexes has also been briefly investigated in the hope of obtaining indications of possible polymerization. The spectra of 1:1 solutions, at pH*=4.5, where the molarities of the two components are 0.25 M and 1.0 M show that the molarities of malic acid in the various complexes are 0.05 (2:1), 0.09 (1:1), 0.03 [(1:2)_], 0.08 [(1:2)+] for the former (total concentration 0.25 M) and 0.12 (2:1), 0.49 (1:1), 0.07 [(I :2)_], 0.32 [(1:2)+] for the latter (total concentration 1.0 M). It is noted that the proportion of the 1:1 complex increases on increasing concentration, with relative decrease of 2:1 and (i :2)_. This suggests that the 1:1 complex is polynuclear, presumably 2:2 as is found for UO~+ with malic acid [2]. The ! : 2 complexes are probably cis-trans isomers. The dependence of the concentrations of the complexes on temperature for the range 40-5°C, is found to be insignificant. Only a slight increase of (1:2)_ concentration at 5°C seems to occur (this could suggest a lower entropy for the isomer (1:2)_ with respect to (1:2)+). At higher temperatures, the spectra become too broad to give accurate relative concentrations of the various complexes,
Table 2. Couplingconstants and internal shifts (Hz) for the 1:1 complex and malic acid (pH* = 4.5) lAB
l:l complex (-) 17.7 Malic acid (-) 15.5
Jsx
v~ - ~'A
~'x - ~'B
1.9 8.9
5.8 3.5
18.9 22.0
251.3 158.2
geometry. This conclusion is drawn from the fact that JAx and Jsx are both closer to the expected values for gauche arrangements of the H atoms than for trans. If we note that the OH group has a decreasing effect on JAx and an increasing effect on Jsx [1], then we conclude that only a small distorsion of (c) (with opening of the Hx-C--C-HA dihedral angle), if any, is required to conform to the observed values. This conclusion is based on the hypothesis that the sign of the chemical shift between H,4 and H8 is not changed upon complexation. This seems most likely but a clear proof is not available because average spectra for the malic acid and complex are not obtained. With the contrary hypothesis, a bigger distorsion of form (c) in the opposite sense (with the CO2H groups closer to each other) would have to be proposed as a result of complexation. For each of the 1 : 2 complexes, due to coincidence of peaks in the respective ABX spectra, only JAx + JBx could be obtained at pH* = 4.5. These values are
(JAx + Jax)- = 13.4 Hz (JAx + JBx)+ = 12.4 respectively for the less and the more stable complexes. The corresponding sum for malic acid is 12.5 Hz. At pH* = 5.5, however, the spectra show more lines and an approximate complete analysis is possible for (1:2)+. The results are shown in Table 3.
(2) Ligand conformation in the complexes The conformation of the malic acid moiety in the complexes is reflected in the vicinal HH coupling constants. Table 2 shows the various parameters obtained from an ABX analysis of the spectrum of the 1 : 1 complex, at pH* = 4.5, in comparison with those for malic acid in the same conditions. These results show that, whereas the conformation of malic acid can be taken as a weighted average of the three staggered conformations, (a) and (c) being the most important contributing ones [10, 1], in the 1 : 1 complex it is (c) or a conformation close to it that is the prefered
COzH
JAX
Table 3. Couplingconstants and internal shifts (Hz) for the (1:2)+ complex and malic acid (pH* = 5.5) JAB
JAX
I COzH
"~HA)
Malic acid ( - ) 15.4
C02H
H
(B;
9.7
H
I
"[A,
OH
"
(b)
3.3
~A
26.5
CO:,H
H ,e; 1 H
(X)
(a)
vB-
Vx -
vB
(l:2)+com- (-) 15.6+-0.7 ]0.5+0.22.4+-0.2 4.]+0.2 249.0 plex
HO~~ H m ,~
JBx
(c)
"cO2H H ""
163.5
392
V.M.S. GIL et al.
It is therefore concluded that the conformation of malic acid in this complex is basically (a), chelation involving OH and the vicinal CO2H group. The involvement of OH in chelation in the 1 : 1 and l : 2 complexes is in accordance with the big low-field shift of proton X on complexation. It is verified that a similar shift occurs in the complex of WO~- with the hydroxybutanioc acid CH3CH(OH)CH2CO2H where chelation necessarily involves OH and CO2H. For structures of the 2:2 and (1:2)+ complexes in solution we can thus propose the following:
In this manner it can be understood why the 1: 2 complexes is predominant at pH = 3 whereas the 1:1 complex is favoured at higher values (pH = 4-5). (3) Kynetic features The 1:2 complexes give rise to room temperature spectra broader than those obtained for the 2: 2 complex. This would seem to be just the opposite of expectation in view of the molecular size. However, it is found that, on lowering the temperature, the 1:2 spectra become appreciably narrower. We must then conclude that such
~ C O /~CO
2. . . . . . z
®
0
O
-.=I
~ ..- I ~" I 0
O
and
o
~W"
HOzC
®
•I ~ ' 1 / / 0 ~ OaC~COzH /
,,
0 (or a cis-isomer). The following tentative equations (for pH -- 4.5) can be written:
WI20~9+12C4HSO5-= 6(WO2)2(OHh(C4H3OD~+ 3H20 + 6H ÷ W,20~9 + 24C4Hs05- + 6H ÷ = 12WO2(C4H405),2- + 15H20
broadening at room temperature is due to an exchange process involving the two 1: 2 complexes. On increasing the temperature such process becomes more rapid and, accordingly, the spectra get broader. They do so up to the maximum temperature reached (95°C). Another kynetic aspect relates to the relative rate of formation of the complexes. Figure 3 shows the X part
( pH'--/,.5)
16Hi
J Fig. 3. Time-dependenceof the X spectrum of a 11 WO2--malicacid solution (pH* -- 4.5).
Metal complexation and rotational isomerism of simple carboxylic acids--IV of the room temperature spectrum of an equimolar solution, at pH* = 4.5, just after addition of the parent solutions of WO~- and malic acid and after 24hr. The spectra are the same whatever the order the two solutions are added to each other. It is thus found that, whereas the 1:2 complexes are almost instantaneously formed (with no free malic acid being left), 1:1 has a smaller rate of formation; with time the concentration of 1:1 increases whereas that of (1:2)+ decreases (there is also a slight increase of (1:2)_ with time). This result corroborates the previous indication that the !:1 complex is binuclear.
Acknowledgements--This work is a contribution of the Centro de Investiga~fio em Quimica, Coimbra, (QC-I) supported by the
393
lnstituto Nacional de Investiga9% Cientifica (Portugese Ministry of Education). REFERENCES
1. J. S. Mariano and V. M. S. Gil, Molec. Phys. 13, 313 (1969). 2. J. D. Pedrosa and V. M. S. Gil, J, Inorg. Nucl. Chem. 36, 1803 0974). 3. Ana M. D. Pereira and V. M. S. Gil, 3'. Inorg. Nucl. Chem. 39, 857 (1977). 4. E. Richardson, J. lnorg. Ni~cl. Chem. 13, 84 (1960). 5. M. J. Baillie and D. H. Brown, 3. Chem. Soc. (lII), 3691 (1961). 6. S. Prasad and L. P. Pandey, J. Proc. Inst. Chemists, 37, 129 0965) C. A. 63, 7870 b 0965). 7. D. H. Brown and D. Neumann, J. Inorg. Nucl. Chem. 37, 330 (1975). 8. D. L. Kepert, Progress in Inorganic Chemistry (Edited by Willey), p. 267, 1962, J. Aveston, Inorg. Chem. 3, 981 (1964). 9. M. Cadiot and B. Viossat, Rev. Chim. Mindrale, 6, 727, 0%9).
METAL COMPLEXATION AND ROTATIONAL ISOMERISM OF SIMPLE CARBOXYLIC ACIDS--IVf TUNGSTATE--MALIC ACID C O M P L E X E S STUDIED BY IH NMR VICTORM. S. GIL Department of Chemistry,University of Aveiro,Portugal and M. EMILIAT. L. SARAIVA,M. MADALENACALDEIRAand ANA M. D. PEREIRA Department of Chemistry, Universityof Coimbra, Portugal
(First received 22 September 1978; in revised[otto 25 April 1979) Abstract--Proton NMR evidence is presented on the number and relative concentrationsof complexesformed by W(VI) and L-malicacid (pH range 3-6), their composition,conformationof the ligand moiety,and (briefly)on their pH-dependence and relative rates of formation. In particular, four complexesare detected: 1:2 (two), 1:1, 2:1. A concentration effect and a slow rate of formation suggest that the I : 1 complexis binuclear. INTRODUCTION
Previous papers in this series have dealt with the use of tH NMR spectroscopy in structural studies of complexes of Zn(II)[l] and U(VI)[2] with malic acid and W(VI) with thiomalic acid[3], in aqueous solution. We now report a similar study of the system tungstate-malic acid (L) for the pH range 3-6, which provides information on the number and relative concentrations of complexes, their stoichiometry and pH dependence, the conformation of the ligand and some kynetic features. The formation of complexes between W(VI) and malic acid has been investigated earlier by conductimetric [4-6], potentiometric [4, 5] and polarimetric [5, 7] methods. Richardson[4] found evidence for a 1:3 complex formed between tungstic and malic acids in solution. Baillie and Brown [5] reported the formation of a 1:1 complex, whereas Prasad and Pandey [6] concluded for complexation in the ratio 1:2 (WO]-: acid). These authors found, however, that on treating the solution with Pb 2+ or Ag+ the tungstomalate ion present in the solid is the 1 : 1 complex ion. EXPERIMENTAL Analytical grade sodium tungstate and commerciallyavailable L-malic acid were used. In order to reduce the OH N.M.R. signal, the former was liophylized from a solution in [hO, the latter was dried at 120°Cand D~Osolutions were used throughout. The concentrations of the sodium tungstate solutions were established by weight and those of the acid solutions by potentiometry. Adjustment of pH was done by dropwise addition of dilute DCI and NaOD solutions. The spectra were run on a Varian HA-100spectrometer. RESULTSAND DISCUSSION
(1) Stoichiometry and relative abundances of the complexes plexes The 100 MHz tH NMR spectrum of the partially deuterated malic acid DO2C--CH2-CH(OD)-CO2D in (I~O) tFor previous papers in this series see Refs. [1-3].
solution is of the ABX type, both free and as a ligand, the X signal arising from the -CH(OD)- proton. Depending on the WO]-: malic acid molar ratio and on pH, several X signals are observed as well as a complex multiplet resulting from partially superimposed AB patterns. The existence of three complexes is thus immediately demonstrated, When excess of malic acid is present (molar ratios equal to or smaller than 1:2), its spectrum is also detected, distinct from the others. Figure 1 shows the X-spectra for the complexes and for free ligand when in excess. It is noted that, whereas the AB part is almost not shifted upon complexation, the X multiplet undergoes a low-field shift of about 1 ppm (1.10, 1.01, 0.91 ppm, respectively). The observation of separate spectra (particularly their X part) for the various species involving malic acid allows the application of Job's continuous variation method which provides information on the stoichiometry of the complexes. This was firstly done in Ref. [3], for the system WO~- + thiomalic acid. The method was applied at three pH* values in the range 3-6, namely 3.0, 4.3 and 5.5. It is noted that the dominant species of W(VI) for such pH values and for the concentrations used is W~20~; in equilibrium with other isopolyanions and with WO~-[8]. For aqueous solutions of malic acid, H2A, the dominant species are H2A, HA-, A2- as the pH increases over that range. The various mixtures were allowed to reach equilibrium (see below) before the NMR spectral intensities were determined by integration. Figure 2 shows the Job's curves corresponding to the X signals of each complex, for the case of parent solution (1.01 M) of pH*= 4.3. It is thus shown that the sharp quartet corresponds to a 1:1 complex, whereas the broader X signals belong to 1:2 complexes, one less stable, (1:2)_, than the other, (1:2)+. By comparing the total intensity of the visible X signals with that for the AB multiplet, in the case of
389
390
V.M.S. GIL et al.
lOHz
OH (REDUCED) SIGNAL
X SIGNALS OF COMPLEXES
X SIGNAL OF FREE MALIC ACID
Fig. I. X parts of the ~H NMR spectra (ABX type) of three complexes of WO42- with malic acid and of free acid in excess (l : 4 solution).
molar ratios 3: l, 2: 1, 1.5:1 and I: 1, it is noted that a fourth complex must exist in these solutions, having an X spectrum either too broad and weak to be observed or, more likely, under the residual OH signal; its concentration being maximum, although small, for the 2:1 solution, it is concluded that a 2:1 complex is also formed. It is noted that the 1 : 1 complex is always accompanied by the !:2 ones; however, for molar ratios equal to or smaller than i:3 practically only the 1:2 complexes are present.
Similar conclusions on the stoichiometry of the complexes are obtained for pH* = 3.0 and pH* = 5.5, but relative abundancies of the complexes vary. For example, at pH* = 3.0, malic acid is predominantly involved in the formation of the 1:2 complexes even in an equimolar solution of WO~- and acid, only a small concentration of 1 : 1 being present. At pH* = 4.5 it is the 1 : 1 complex that is the most abundant one. On the other hand, the concentration of the 2:1 complex increases with pH over the range studied. Table 1 shows the concentration of the malic acid moiety in the various complexes at the three
/ 1:1
1:2 .",~.
"-- ..~3
[
l:~
Q.
E o.:~
1:1.
~.
.I, ' / o.,
I
./
?
.,f
/
//
"...
...'J. . . . . . . .
/
/
~ \
\,,1:3
% \x,e1:l*
\ /
A',. . . . "" 0.2
\
\2:?L.,, o.~
o.e
ol
[ Malic acid]tot
IM'"o,%~[~o,'Jt= Fig. 2. Job's curves for the system WO~-+malic acid(L), in D~O and at pH*= 4.3, based on NMR spectral intensities.
Metal complexationand rotational isomerismof simple carboxylicacids--IV
391
Table 1. Molarities of malic acid in complexes as function of pH and WO~-: Malic acid molar ratios (Concentrationof parent solutions: 1.0 M) Molar ratio 2:1 1:1 1:2 pH* Complexes 2:1 1:1 (1:2)_(1:2)+ 2:1 1:1 (1:2)_(1:2)+ 2:1 1:1 (1:2)_(1:2)+ 3 4.5 5.5
0.06 0.1)8 0.04 0.15 0.04 0.08 0.08 0.30 ~0 n0 0.10 0.41 0.09 0.15 0.02 0.08 0.06 0.23 0.06 0.15 ~0 0.11 0.12 0.31 0.13 0.05 0.03 0.12 0.13 0.20 0.05 0.11 ~0 0.07 0.07 0.29
pH values studied for typical molar ratios. The estimate accuracy of those values is ___0.015, except perhaps those for the 2:1 complex which are likely to be slightly less accurate. Complexes having the composition 2: 1, 1:1 and 1:2 have been reported earlier for the similar system MoO~--malic acid [9] on the basis of potentiometric and polarimetric studies. The effect of concentration on the relative abundances of the complexes has also been briefly investigated in the hope of obtaining indications of possible polymerization. The spectra of 1:1 solutions, at pH*=4.5, where the molarities of the two components are 0.25 M and 1.0 M show that the molarities of malic acid in the various complexes are 0.05 (2:1), 0.09 (1:1), 0.03 [(1:2)_], 0.08 [(1:2)+] for the former (total concentration 0.25 M) and 0.12 (2:1), 0.49 (1:1), 0.07 [(I :2)_], 0.32 [(1:2)+] for the latter (total concentration 1.0 M). It is noted that the proportion of the 1:1 complex increases on increasing concentration, with relative decrease of 2:1 and (i :2)_. This suggests that the 1:1 complex is polynuclear, presumably 2:2 as is found for UO~+ with malic acid [2]. The ! : 2 complexes are probably cis-trans isomers. The dependence of the concentrations of the complexes on temperature for the range 40-5°C, is found to be insignificant. Only a slight increase of (1:2)_ concentration at 5°C seems to occur (this could suggest a lower entropy for the isomer (1:2)_ with respect to (1:2)+). At higher temperatures, the spectra become too broad to give accurate relative concentrations of the various complexes,
Table 2. Couplingconstants and internal shifts (Hz) for the 1:1 complex and malic acid (pH* = 4.5) lAB
l:l complex (-) 17.7 Malic acid (-) 15.5
Jsx
v~ - ~'A
~'x - ~'B
1.9 8.9
5.8 3.5
18.9 22.0
251.3 158.2
geometry. This conclusion is drawn from the fact that JAx and Jsx are both closer to the expected values for gauche arrangements of the H atoms than for trans. If we note that the OH group has a decreasing effect on JAx and an increasing effect on Jsx [1], then we conclude that only a small distorsion of (c) (with opening of the Hx-C--C-HA dihedral angle), if any, is required to conform to the observed values. This conclusion is based on the hypothesis that the sign of the chemical shift between H,4 and H8 is not changed upon complexation. This seems most likely but a clear proof is not available because average spectra for the malic acid and complex are not obtained. With the contrary hypothesis, a bigger distorsion of form (c) in the opposite sense (with the CO2H groups closer to each other) would have to be proposed as a result of complexation. For each of the 1 : 2 complexes, due to coincidence of peaks in the respective ABX spectra, only JAx + JBx could be obtained at pH* = 4.5. These values are
(JAx + Jax)- = 13.4 Hz (JAx + JBx)+ = 12.4 respectively for the less and the more stable complexes. The corresponding sum for malic acid is 12.5 Hz. At pH* = 5.5, however, the spectra show more lines and an approximate complete analysis is possible for (1:2)+. The results are shown in Table 3.
(2) Ligand conformation in the complexes The conformation of the malic acid moiety in the complexes is reflected in the vicinal HH coupling constants. Table 2 shows the various parameters obtained from an ABX analysis of the spectrum of the 1 : 1 complex, at pH* = 4.5, in comparison with those for malic acid in the same conditions. These results show that, whereas the conformation of malic acid can be taken as a weighted average of the three staggered conformations, (a) and (c) being the most important contributing ones [10, 1], in the 1 : 1 complex it is (c) or a conformation close to it that is the prefered
COzH
JAX
Table 3. Couplingconstants and internal shifts (Hz) for the (1:2)+ complex and malic acid (pH* = 5.5) JAB
JAX
I COzH
"~HA)
Malic acid ( - ) 15.4
C02H
H
(B;
9.7
H
I
"[A,
OH
"
(b)
3.3
~A
26.5
CO:,H
H ,e; 1 H
(X)
(a)
vB-
Vx -
vB
(l:2)+com- (-) 15.6+-0.7 ]0.5+0.22.4+-0.2 4.]+0.2 249.0 plex
HO~~ H m ,~
JBx
(c)
"cO2H H ""
163.5
392
V.M.S. GIL et al.
It is therefore concluded that the conformation of malic acid in this complex is basically (a), chelation involving OH and the vicinal CO2H group. The involvement of OH in chelation in the 1 : 1 and l : 2 complexes is in accordance with the big low-field shift of proton X on complexation. It is verified that a similar shift occurs in the complex of WO~- with the hydroxybutanioc acid CH3CH(OH)CH2CO2H where chelation necessarily involves OH and CO2H. For structures of the 2:2 and (1:2)+ complexes in solution we can thus propose the following:
In this manner it can be understood why the 1: 2 complexes is predominant at pH = 3 whereas the 1:1 complex is favoured at higher values (pH = 4-5). (3) Kynetic features The 1:2 complexes give rise to room temperature spectra broader than those obtained for the 2: 2 complex. This would seem to be just the opposite of expectation in view of the molecular size. However, it is found that, on lowering the temperature, the 1:2 spectra become appreciably narrower. We must then conclude that such
~ C O /~CO
2. . . . . . z
®
0
O
-.=I
~ ..- I ~" I 0
O
and
o
~W"
HOzC
®
•I ~ ' 1 / / 0 ~ OaC~COzH /
,,
0 (or a cis-isomer). The following tentative equations (for pH -- 4.5) can be written:
WI20~9+12C4HSO5-= 6(WO2)2(OHh(C4H3OD~+ 3H20 + 6H ÷ W,20~9 + 24C4Hs05- + 6H ÷ = 12WO2(C4H405),2- + 15H20
broadening at room temperature is due to an exchange process involving the two 1: 2 complexes. On increasing the temperature such process becomes more rapid and, accordingly, the spectra get broader. They do so up to the maximum temperature reached (95°C). Another kynetic aspect relates to the relative rate of formation of the complexes. Figure 3 shows the X part
( pH'--/,.5)
16Hi
J Fig. 3. Time-dependenceof the X spectrum of a 11 WO2--malicacid solution (pH* -- 4.5).
Metal complexation and rotational isomerism of simple carboxylic acids--IV of the room temperature spectrum of an equimolar solution, at pH* = 4.5, just after addition of the parent solutions of WO~- and malic acid and after 24hr. The spectra are the same whatever the order the two solutions are added to each other. It is thus found that, whereas the 1:2 complexes are almost instantaneously formed (with no free malic acid being left), 1:1 has a smaller rate of formation; with time the concentration of 1:1 increases whereas that of (1:2)+ decreases (there is also a slight increase of (1:2)_ with time). This result corroborates the previous indication that the !:1 complex is binuclear.
Acknowledgements--This work is a contribution of the Centro de Investiga~fio em Quimica, Coimbra, (QC-I) supported by the
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