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A lead-207 nuclear magnetic resonance study of the complexation of lead by carboxylic acids and aminocarboxylic acids
JOURNAL
OF MAGNETIC
RESONANCE
51,223-232
(1983)
A Lead-207 Nuclear Magnetic Resonance Study of the Complexation of Lead by Carboxylic Acids and Aminocarboxylic Acids THOMAS
T.
NARASHIMA
AND DALLAS
L. RABENSTEIN*
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Received June 24, 1982 The complexation of PbfII) by carboxylic acids and aminocarboxylic acids was studied by *O’Pb NMR. The results indicate that the *“Pb chemical shift provides a sensitive probe of the aqueous coordination chemistry of PbfII). A single, exchange-averaged resonance is observed for lead in solutions containing Pb(NOs)* and pivalic acid, acetic acid, formic acid, or chloroacetic acid, the chemical shift of which is sensitive to the Pb(NO,), to carboxylic acid ratio and to solution pH. Formation constants for the Pb(II)-carboxylate complexes were determined by fitting the chemical shift data to a model involving the complexes and ligand protonation. Chemical shift data for solutions containing Pb(NOp)2 and glycine, histidine, or glycylglycine indicate complexation of Pb(I1) by the zwitterionic forms of these ligands. Formation constants for these complexes, which are difficult to study by other methods, were also determined from the chemical shit? data. Complexation of Pb(I1) by ethylenediaminetetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, nitrilotriacetic acid, and N-methyliminodiacetic acid causes a large deshielding of the *“Pb nucleus , e.g .>the resonance for the ethylenediaminetetraacetic acid complex is deshielded by 2350 ppm. The chemical shift of the lead in these complexes is sensitive to protonation of the complex and to the formation of mixed complexes containing hydroxide ion. INTRODUCTION
Detailed kinetic and equilibrium information about the complexation of metal ions by organic ligands can be obtained by ‘H and i3C nuclear magnetic resonance spectroscopy. In addition, the ligand donor groups involved in metal binding can often be identified in metal complexes of multidentate ligands. The information comes from changes in the NMR parameters, e.g., the chemical shifts, of nuclei which are located near the metal binding sites. Since the magnitude of the change in the chemical shift decreases as the number of bonds between the metal binding site and the nucleus increases, the changes in ‘H chemical shifts are often quite small. Somewhat larger changes are usually observed in 13C chemical shifts, particularly for those carbons directly bonded to donor atoms. Also the simplicity of ‘H-decoupled 13C spectra makes possible the study of metal binding by large molecules. The main disadvantage of 13C NMR for such studies is the relatively high concentration required. More direct information about metal binding can be obtained from NMR measurements on the donor and acceptor atoms themselves. However, of the donors * To whom correspondence should be addressed. 223
0022-2364/83/020223-10$03.00/O Copyright 0 1983 by Academic Press, Inc. All rights of reproduction in any form reserved
224
NAKASHIMA
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RABENSTEIN
common in organic ligands, only nitrogen has a naturally occurring isotope (“N, Z = l/2) which gives high-resolution spectra. Because of the low natural abundance and low magnetic moment of “N, it has only recently become practical to study metal complexes by natural-abundance 15N NMR (Z-3). On the other hand, there are NMR-active isotopes of many metals with sufficiently high natural abundance and NMR sensitivity to make metal ion NMR a practical technique for studying metal complexation reactions. Metal ion NMR has been used to study metal complexes ranging from those formed with simple monodentate ligands to those formed with proteins (4-11). For the study of metal binding by proteins, metal ion NMR has an advantage, as compared to ‘H and i3C NMR, since the metal ion spectrum is much simpler. Although 207Pb has desirable properties for high-resolution NMR (I = l/2, 2 1.1% natural abundance, and 9.13 X lop3 as sensitive as ‘H for equal numbers of nuclei) and has been used extensively in the study of organometallic compounds of lead (Z2), there have been surprisingly few reports of its use for the study of complexes of Pb(I1) in aqueous solution (13). As part of a program to determine the usefulness of 207Pb NMR for characterizing the solution chemistry of Pb(I1) complexes, we have studied the complexation of Pb(I1) by monocarboxylic acids and by aminocarboxylic acids. The objective of the carboxylic acid study was to determine if formation constants can be evaluated from 207Pb chemical shift data. The carboxylic acids were chosen to cover a range of acid strengths so that the formation constants would cover a range of values. The aminocarboxylic acids were chosen to have different numbers of amino and carboxylic acid groups to determine if the 207Pb chemical shift depends on the number and type of donor groups bonded to the lead. EXPERIMENTAL
Chemicals
The carboxylic acids and aminocarboxylic acids were of the highest-grade commercially available and were used without further purification. Sodium formate and sodium acetate were used as the source of the formate and acetate ligands. Pb(NO3)3 was obtained from Fisher Scientific. Stock solutions of -0.4 M Pb(NO3)2 were standardized by EDTA titration. Sample Preparation
Samples used for the 207Pb NMR measurements were prepared from the stock solution of Pb(N03)2 and the requisite amount of the l&and. The procedure involved first preparing a 200-ml solution containing Pb(N03)2, ligand, and NaN03 at the appropriate concentrations. As the pH of this solution was varied by addition of concentrated HN03 or NaOH, NMR samples were withdrawn. Samples were generally taken at 0.5 pH unit increments over the pH range 0.5 up to the pH at which Pb(OH)2 precipitated. The Pb(I1) concentration was generally 0.100 M, and Pb(I1) to ligand ratios of 1: 1, 1:2, and 1:4 were used. To keep the ionic strength reasonably constant in the formation constant studies, NaN03 was added such that the sum of the NaN03 and ligand concentrations was 0.40 M. By adjusting the pH to -0.5 initially and then withdrawing samples as the pH was increased, the ionic strength was in the range 1.4 to 1.8, depending on the pH and the extent of complex formation. A solvent mixture of 5% D20/95% Hz0 was used to provide an 2H fieldlfrequency lock.
LEAD-CARBOXYLIC
ACID COMPLEXES
225
pH Measurements pH measurements were made at 25°C with an Orion model 70 1 pH meter equipped with a standard glass electrode and a fiber-tip saturated calomel electrode. It was found that the pH response was unstable when this electrode pair was placed directly into the Pb(II)-containing solutions, possibly due to the precipitation of PbCIZ in the glass fiit of the reference electrode. To avoid this problem, the reference electrode was separated from the Pb(I1) solution by using a double junction arrangement. The reference electrode was placed in a 0.3 M NaN03 solution which in turn was placed in the Pb(I1) solution. The NaN03 solution was contained in a glass cylinder of - 1.8 cm diameter onto the end of which was attached the bottom 2 cm of the barrel of a standard reference electrode. The glass electrode-double junction reference electrode arrangement was calibrated with pH 4.00, 7.00, and 10.00 buffers. NMR Measurements “‘Pb NMR spectra were measured at 4 1.86 MHz on a Bruker WH-200 spectrometer operating in the pulsed Fourier transform mode. Samples of approximately 15 ml were contained in 20-mm NMR tubes. A spectral width of 10,000 Hz was used, with a digital resolution of 4.88 Hz/pt. Generally, 500 transients were accumulated. A 60-psec pulse width (-45” flip angle) was used, with 0.25 set between pulses. The T1 value for 207Pb in a pH 5.41 solution containing 0.10 M Pb(N03)2 and 0.40 A4 acetylglycine was found to be -0.22 sec. Chemical shifts are reported relative to the chemical shift of 0.10 A4 Pb(NOJ), at pH 1.73. Positive chemical shifts correspond to less shielding. The chemical shift of 1.0 M Pb(N03)2 in H20, which is reported to have a chemical shift of -2961.2 ppm relative to (CH&Pb (13) is -5 1.2 ppm relative to the pH 1.73 solution of 0.1 MPb(N03)2. Spectra were measured at ambient temperature (25 f 1’C). RESULTS
Carboxylate Complexes To determine the sensitivity of the 207Pb chemical shift to complexation by carboxylate ligands, chemical shifts were measured as a function of the Pb(N0s)2 to ligand mol ratio and the solution pH for the ligands pivalic acid, acetic acid, formic acid, and chloroacetic acid. Representative chemical shift data are shown in Fig. 1 for the Pb(NOj)2/acetic acid system at ligand to Pb(N03)2 mol ratios of 1, 2, and 4. Also shown in Fig. 1 is the chemical shift of Pb(NO& over the same pH range. For the Pb(N0,)2/acetic acid system and for the other systems studied, a single exchangeaveraged 207Pb resonance was always observed. A qualitative examination of the data in Fig. 1 indicates that the exchange-averaged chemical shift depends on the extent of complex formation. At low pH, the chemical shift for all three acetic acid to Pb(NO& mol ratios approaches a value near 0, indicating little complexation due to protonation of the l&and. As the pH is increased, the “‘Pb chemical shift changes, leveling off at a constant value at pH 5 to 6. At this pH, there is little competition from protonation of the ligand (p& = 4.65 (14)). The difference between the chemical shift at pH 5 to 6 and that at low pH is greater the larger the acetic acid to Pb(N03)2 mol ratio, indicating more complexation at the higher mol ratios. In the pH 1.5 to 5 region, the chemical shift depends on pH indicating that ligand protonation competes effectively with metal complexation in this pH region.
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NAKASHIMA
AND RABENSTEIN
E a ,200. t ,000. 6 2 600. ” z 600. -5 400. zoo0. 2 1 3F+f4 5 6 RG. 1. pH dependence of the *O’Pb chemical shift of Pb(I1) in solutions containing (A) 0.10 M Pb(NOp)z; (B) 0.10 M Pb(NO,),, 0.10 M sodium acetate, and 0.3 M NaN03; (C) 0.10 M Pb(NO&, 0.20 M sodium acetate, and 0.20 M NaNO,; and (D) 0.10 M Pb(NO& and 0.40 M sodium acetate. The curves drawn through the points for ED are theoretical curves calculated with the formation constants and “‘Pb chemical shifts determined in this work.
Formation constants for the Pb(II)-carboxylate complexes, as defined by Eqs. [I] and [2], were determined by fitting the exchange-averaged chemical shift data by nonlinear least-squares calculations: [PbL+] Kfl = [p,,z+][L-]
Pb2+ + L- * PbL+;
[ll
’
F’bL21
Pb2+ + 2L- * PbL2;
PI
p2 = [p,,2+][L-12 ’
The exchange-averaged chemical shift, Bobs,is the weighted average of the chemical shifts of the various species present, 6ok4
=
Pf6f
+
Pl:lh
+
131
P1:2&:2,
where Pf, PI:, , and PIE2represent, respectively, the fractional in the free, PbL+, and PbL2 forms
concentrations
p = [Pb2+l
f
[41
WI, ’
p, = P’bL+l 1.1
D’bl,
p
1.2
of lead
’
= PbJhl
WI, ’
with [Pb], = [Pb2’] + [PbL+] + [PbL2]. The fractional concentrations in terms of Kn and a, which leads to
151 I61 can be expressed
LEAD-CARBOXYLIC
6ohs =
ACID
6f + KfJLl&:,
227
COMPLEXES
+ B*[L12~1:2 .
1 + KflW
+ 82[L12
171
’
Kfi, and, for acetic acid, formic acid, and chloroacetic acid, p2 were obtained by fitting the observed chemical shift data to Eq. [7]. Since the chemical shifts of PbL+ and PbL2 cannot be obtained directly from the NMR data these were treated as unknowns in the curve fitting calculations. The value 6r was also treated as an unknown to account for any dependence on solution composition. The calculations involved first estimating Kf, , /3*, af, 6i:,, and 8iZ2, and then using these values and Eq. [7] to predict the chemical shift for the concentration and pH of each experimental point. The value of [L] in Eq. [7] was calculated using an equation derived by considering ligand and metal balances. The sum of the squares of the differences between the predicted and observed chemical shifts was calculated, and then the nonlinear least squares curve fitting program KINET (15) was used to refine the estimates of the parameters to minimize the sum of the squares of the residuals. When doing the calculations for a particular system, the data for the three ligand to metal ratios was fitted simultaneously. The results are summarized in Table 1. Also given are literature values where available for comparison. The solid curves drawn through the points of B-D in Fig. 1 are the theoretical curves calculated from the constants listed in Table 1, and are typical of the fits obtained. The free Pb(I1) chemical shifts listed in Table 1 are different from that of the pH 1.73 reference solution due to the higher concentration of NaN03 in the carboxylic acid solutions. The *07Pb chemical shift was found to be quite sensitive to nitrate concentration and to Pb(N03)* concentration, e.g., dilution of a 1.00 M Pb(N09* solution to 0.5 M caused the resonance to shift by +27 ppm. Although Pb(II) can form complexes of ligand to Pb(I1) ratio greater than 2 with carboxylic acids, their inclusion in the model did not result in a significant improve1
TABLE
FORMATION CONSTANTS FOR CARBOXYLATE COMPLEXES OF Pb(I1)
Pivalic acid Acetic acid
4.95 4.65
2.60 k 0.04 2.11 It 0.03 2.18’ 2.16’
Formic acid Chlomacetic acid Acetylglycine
3.55 2.75 3.44
1.56 ? 0.05 1.76 + 0.06
Glycylglycine’
3.13f
1.49 f 0.04 f 0.17f 1.48 * 0.08 1.04 f 0.07
1.81 k 0.03 1.3gd
1,090 f 1,010 f
34 30
508 f 37 340 f 14 565 f 15 569
k
23
3.06 k 2.92’ 2.99d 2.74 k 2.71 f 2.51 k 2.5Sd
0.09
1,850
f
60
-9f -14+
5 9
0.07 0.12 0.07
815 848 1,580
k + +
58 90 46
-18’ -15f -26
4 5 4
1.97 f
0.06
1,720
+ 110
-2lf
+
-38 -5lf
+
4
1.30
Glycit@ Histidine’
2.386 1.82h
210 f 17 k 21
2.08
f 0.10
856
367
a From Ref. [ 141unlessotherwise noted. b In ppm from 0. I h4 Pb(NO,)Z at pH 1.73. ‘E. A. Bums and D. N. Hume, J. Am. Chem. Sot. 78, 3958 (1956). 25”C, p = d D. L. Rabenstein, Cnn. J. Chem. 50, 1036 (1972). 25”C, p - 1.4 M. ’ Complexation by the carboxylate group of the zwitterionic form. ‘Reference [16]. 25”C, p - 1.8 M. g D. L. Rabenstein and G. Blakney, Inorg. Chem. 12, 128 (1973). *D. D. Petin, J. Chem. Sot., 3125 (1958).
1.98
M.
91
f
14 6
228
NAKASHIMA
AND
RABENSTEIN
ment in the theoretical fit to the experimental data. Also the stepwise formation constants calculated from Kn and p2 in Table 1 for the addition of a second ligand to PbL are generally quite small, suggesting that the stepwise formation constant for the addition of a third ligand to PbLz will be even smaller and can be ignored under these experimental conditions. Complexation
of Pb(II) by Aminocarboxylate
Ligands
The complexation of Pb(I1) by glycine, glycylglycine (glygly), histidine, methyliminodiacetic acid (MIDA), nitrilotriacetic acid (NTA), N-hydroxyethylethylenediaminetriacetic acid (HEDTA), and ethylenediaminetetraacetic acid (EDTA) was studied by measuring the 207Pb chemical shift as a function of solution conditions. Chemical shift data for the Pb(NOS)2/glygly system are shown in Fig. 2. Qualitatively, the dependence of the chemical shift on ligand-to-metal ratio and pH is similar to that shown for the Pb(NOs)2/acetic acid system in Fig. 1. A major difference chemically, however, is that glygly has two potential metal binding sites, the carboxylate oxygen and the amino nitrogen. At pH 1, both of these groups are protonated to give predominantly H$lCH2CONHCH2C02H, and the Pb(I1) is free in solution. The pKA of the carboxylate group of this form of glygly is 3.13, while that of the ammonium group ofthe zwitterion H3fiCH2CONHCH2C02is 8.21(16). The pH range over which the 207Pb chemical shift changes in Fig. 2 indicates complexation by the deprotonated carboxylate group while the amino group is protonated. As compared to the data in Fig. 1, the chemical shift begins to change at a lower pH, and as the pH is increased it levels off to a constant value at a lower pH. This is as expected since, the lower the pKA, the greater the availability of the carboxylate group for Pb(I1) complexation at low pH. At pH - 5, the chemical shift starts to change again, indicating the start of complexation at the amino group, in agreement with results
FIG. 2. pH dependence of the zo7Pb chemical shift of P(U) in solutions containing (A) (B) 0.10 M Pb(NO&, 0.10 M glygly, and 0.30 M NaNOp; (C) 0.10 M Pb(NO&, 0.20 M NaNOJ; and (D) 0.10 M Pb(NO& and 0.40 M glygly. The curves drawn through are theoretical curves calculated with the formation constants and 20’Pb chemical shifts work.
0.10 M Pb(NO&; M glygly, and 0.20 the points for B-D determined in this
LEAD-CARBOXYLIC
ACID
COMPLEXES
229
obtained by ‘H NMR (16). Unfortunately, however, it was not possible to study this reaction due to precipitation above pH - 5-6. The dependence of the *“Pb chemical shift on pH for the Pb(NOs)2/histidine and Pb(NOs)2/glycine systems is similar to that shown in Fig. 2, indicating complexation of Pb(I1) by the zwitterionic forms of both ligands. In the case of histidine, the imidazole group is also protonatcd. The lack of complexation of Pb(I1) by the imidazole group was further substantiated by measuring the *“Pb chemical shift for solutions containing 0.10 A4 Pb(N03)* and 0.20 M or 0.40 M imidazole. Over the pH range 0.5 to 4.0, the chemical shift did not differ significantly from that for Pb(N03)* itself over the same pH range. Formation constants for the complexation of Pb(I1) by the carboxylate groups of the zwitterion forms of glycine, glygly, and histidine were calculated using the method described above. The results are tabulated in Table 1. Chemical shift data for the other aminocarboxylate systems are presented in Fig. 3 and are interpreted in the discussion section. DISCUSSION
The results in Figs. 1 to 3 indicate that the *“Pb chemical shift provides a sensitive probe of the complexation of Pb(I1) by carboxylate and aminocarboxylate ligands. Qualitative information can be derived from the pH dependence of the chemical shift as well as quantitative formation constants. The formation constants listed in Table 1 were determined by taking advantage of the pH dependence of the extent of complexation, which is due to the competitive protonation of the ligand. This approach should be particularly useful for measuring formation constants for Pb(I1) complexes with carboxylic acids having small p& values, e.g., p& < 3 to 4. Such formation constants are difficult to measure by some of the other methods commonly used, e.g., the pH titration method, as evidenced by the lack of literature values available for comparison with most of the formation
FIG. 3. pH dependence of the chemical shift of Pb(I1) in solutions containing (B) 0.10 M MIDA; (C) 0.30 M MIDA; (D) 0.10 M NTA; (E) 0.10 M EDTA The dashed line for (C) indicates precipitation.
0.10 M Pb(NO& (A) and and (F) 0.10 M HEDTA.
230
NAKASHIMA
AND
RABENSTEIN
constants listed in Table 1. Where literature values are available, the agreement with the formation constants determined in this work is excellent. The only exception is Kr, for the acetylglycine complex. The literature value was determined by ‘H NMR. The value determined by 207Pb NMR is considered the more reliable since the 207Pb chemical shift is much more sensitive to the complexation. The results for complexes of the aminocarboxylate ligands indicate the formation of two different types of complexes: those which under the conditions of this study involve monodentate carboxylate binding and those in which the ligand is coordinated to the Pb(I1) in a multidentate fashion. In the first group are the complexes of glycine, glygly, histidine and, in the pH region 1 to 5, MIDA. The complexes with glycine and histidine are of particular interest. Although these ligands are potentially multidentate, the results indicate that at pH < 5 the amino group of the complexed glycine is protonated while in the histidine complex both the amino and imidazole groups are protonated. Such complexes have been detected previously for other metal ions, e.g., from shifts in the infrared asymmetric carboxylate frequencies of alanine and histidine in low pH solutions containing Mn(II), Fe(II), Co(II), Ni(II), Zn(II), and Cd(I1) (17), however few formation constants have been reported for such complexes. We were unable to find any literature values for the Pb(I1) complexes of glycine and histidine, and in fact it has been reported that glycine does not complex Pb(I1) at 4.68 (18). The results of the present study show that ‘07Pb NMR is a sensitive method for their detection and quantitative characterization. It also is of interest to mention that such complexes are generally ignored in the measurement of formation constants for Pb(I1) complexes of deprotonated amino acids. The assumption is made that any Pb(I1) not complexed by the fully deprotonated amino acid, e.g., the H2NCH2C02form of glycine, is present as the “free” aquo species. The results here indicate this is not the case, even at the lower concentrations often used in formation constant determinations. The 207Pb chemical shifts for the complexes of EDTA, HEDTA, NTA, and, at pH > 7, MIDA are significantly different from those for free Pb(I1) and the carboxylate complexes. The chemical shifts show some pH dependence, however, they are generally constant over those pH ranges where equilibrium constant calculations indicate the Pb(I1) is essentially all complexed by the fully deprotonated ligands. For example, the 207Pb chemical shift for the Pb/EDTA system (curve E) is constant at 2352 ppm over the pH range 4 to 12. The conditional formation constant for the Pb(EDTA)‘complex varies from 109.4 to 1O18.0over this pH range (19), indicating that essentially all the Pb(I1) will be complexed by EDTA. The chemical shift changes.with decreasing pH below pH 4, where the complex is known to undergo protonation to form the species HPb(EDTA)- (19). ‘H NMR and infrared data indicate the protonation occurs at a carboxylate group (19). Carboxylic acid proton exchange is rapid on the NMR time scale, and thus an exchange-averaged 207Pb resonance is observed, having a chemical shift intermediate between those of Pb(EDTA)2- and HPb(EDTA)-. The change in the chemical shift upon protonation of Pb(EDTA)2- is in the direction of the chemical shift for free Pb(II), consistent with the replacement of a coordinated carboxylate group by a water molecule. For the Pb/HEDTA system, the 207Pb chemical shift (curve P) is constant at 2320 ppm over the pH range 4 to 9, also as expected from the formation constant of the complex (19). HEDTA has five potential donor groups, as compared to the six of EDTA, and the difference between the 207Pb chemical shifts of the Pb(HEDTA)- and
LEAD-CARBOXYLIC
ACID COMPLEXES
231
Pb(EDTA)2- complexes, although small, is in the direction expected for the replacement of a coordinated carboxylate group of EDTA with a water molecule. At pH < 4 the *O’Pb chemical shift shows behavior similar to that observed for the Pb(EDTA)‘- complex. Over this pH region, the Pb(HEDTA)- complex is also known to protonate at a carboxylate group (20). The change in the chemical shift at pH > 9 was not predicted by the formation constants which have been reported for the Pb/ HEDTA system. The shift is presumably due to complexation of hydroxide ion to form P~J(HEDTA)(OH)~- or, possibly, deprotonation of the ligand hydroxyl group with subsequent metal binding. The change in the 207Pb chemical shift for the Pb/ MIDA system over the same pH region indicates that it is more likely due to complexation of hydroxide ion. If Pb(I1) has a coordination number of six in these complexes, there is at least one coordination position in the Pb(HEDTA)complex at which hydroxide could bind without displacing a ligand donor group. The lack of any change in the 207Pb chemical shift of the Pb(EDTA)2- complex in this pH region suggests that EDTA is hexadentate in Pb(EDTA)2-. The “‘Pb chemical shift of the Pb/NTA system is constant at 2097 ppm over the pH range 4 to 8. Formation constants indicate the complex Pb(NTA)- is present over this pH range (19). NTA has four potential donor groups, three carboxylate oxygen, and one amino nitrogen. The 207Pb resonance for Pb(NTA)- is shifted by 355 ppm to lower frequency from that for Pb(EDTA)‘-, consistent with more H20 molecules in the inner coordination sphere of the Pb(I1) in Pb(NTA)-. However, if the ligand is tetradentate in Pb(NTA)-, pentadentate in Pb(HEDTA)-, and hexadentate in Pb(EDTA)2-, the chemical shifts for these complexes indicate that substitution of H20 in the inner coordination sphere by a carboxylate group, or by an amino group, does not have a constant, incremental effect on the 207Pb chemical shift. More likely, the effect of binding a carboxylate or amino group on the “‘Pb chemical shift depends on the nature and number of donor groups already in the inner coordination sphere. The Pb/MIDA system behaves somewhat differently from the EDTA, HEDTA, and NTA systems. At pH < 3.5, the 207Pb chemical shift data indicate carboxylate complexation while the amino group is still protonated. At pH > 3.5, the shift starts to increase when the ligand-to-metal ratio is 3, however precipitate forms between pH 3.75 and 7.5, thus preventing the characterization of the Pb/MIDA complexes in this pH region. As the pH is increased from 7.5, the chemical shift levels off at 2050 ppm. In this pH region, the complex Pb(MIDA)22- forms. At pH > 9.5, the chemical shift changes again, presumably due to the formation of hydroxide complexes such as Pb(MIDA)(OH)-, which have been detected in pH studies of the complexation of Pb(I1) by MIDA (21). In conclusion, “‘Pb chemical shifts provide a sensitive probe of the aqueous coordination chemistry of Pb(I1). From chemical shift data measured as a function of solution conditions, e.g., ligand-to-metal ratio and solution pH, qualitative and quantitative information can be obtained. This approach should be particularly useful for the study of complexes having small formation constants. One group of complexes which can be studied in this way, and which are difficult to study by other methods, are those formed with the zwitterionic amino acids. Exchange-averaged 207Pb chemical shifts also can provide information about the solution chemistry of more stable complexes with multidentate ligands, e.g., information about protonation of the complexed ligand or the formation of hydroxide complexes.
232
NAKASHIMA
AND
RABENSTEIN
ACKNOWLEDGMENT This research was supported by Operating Grants Research Council of Canada and by the University
to D.L.R. of Alberta.
from
the Natural
Sciences
and Engineering
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Il. 12. 13. 14.
15. 16.
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OF MAGNETIC
RESONANCE
51,223-232
(1983)
A Lead-207 Nuclear Magnetic Resonance Study of the Complexation of Lead by Carboxylic Acids and Aminocarboxylic Acids THOMAS
T.
NARASHIMA
AND DALLAS
L. RABENSTEIN*
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Received June 24, 1982 The complexation of PbfII) by carboxylic acids and aminocarboxylic acids was studied by *O’Pb NMR. The results indicate that the *“Pb chemical shift provides a sensitive probe of the aqueous coordination chemistry of PbfII). A single, exchange-averaged resonance is observed for lead in solutions containing Pb(NOs)* and pivalic acid, acetic acid, formic acid, or chloroacetic acid, the chemical shift of which is sensitive to the Pb(NO,), to carboxylic acid ratio and to solution pH. Formation constants for the Pb(II)-carboxylate complexes were determined by fitting the chemical shift data to a model involving the complexes and ligand protonation. Chemical shift data for solutions containing Pb(NOp)2 and glycine, histidine, or glycylglycine indicate complexation of Pb(I1) by the zwitterionic forms of these ligands. Formation constants for these complexes, which are difficult to study by other methods, were also determined from the chemical shit? data. Complexation of Pb(I1) by ethylenediaminetetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, nitrilotriacetic acid, and N-methyliminodiacetic acid causes a large deshielding of the *“Pb nucleus , e.g .>the resonance for the ethylenediaminetetraacetic acid complex is deshielded by 2350 ppm. The chemical shift of the lead in these complexes is sensitive to protonation of the complex and to the formation of mixed complexes containing hydroxide ion. INTRODUCTION
Detailed kinetic and equilibrium information about the complexation of metal ions by organic ligands can be obtained by ‘H and i3C nuclear magnetic resonance spectroscopy. In addition, the ligand donor groups involved in metal binding can often be identified in metal complexes of multidentate ligands. The information comes from changes in the NMR parameters, e.g., the chemical shifts, of nuclei which are located near the metal binding sites. Since the magnitude of the change in the chemical shift decreases as the number of bonds between the metal binding site and the nucleus increases, the changes in ‘H chemical shifts are often quite small. Somewhat larger changes are usually observed in 13C chemical shifts, particularly for those carbons directly bonded to donor atoms. Also the simplicity of ‘H-decoupled 13C spectra makes possible the study of metal binding by large molecules. The main disadvantage of 13C NMR for such studies is the relatively high concentration required. More direct information about metal binding can be obtained from NMR measurements on the donor and acceptor atoms themselves. However, of the donors * To whom correspondence should be addressed. 223
0022-2364/83/020223-10$03.00/O Copyright 0 1983 by Academic Press, Inc. All rights of reproduction in any form reserved
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common in organic ligands, only nitrogen has a naturally occurring isotope (“N, Z = l/2) which gives high-resolution spectra. Because of the low natural abundance and low magnetic moment of “N, it has only recently become practical to study metal complexes by natural-abundance 15N NMR (Z-3). On the other hand, there are NMR-active isotopes of many metals with sufficiently high natural abundance and NMR sensitivity to make metal ion NMR a practical technique for studying metal complexation reactions. Metal ion NMR has been used to study metal complexes ranging from those formed with simple monodentate ligands to those formed with proteins (4-11). For the study of metal binding by proteins, metal ion NMR has an advantage, as compared to ‘H and i3C NMR, since the metal ion spectrum is much simpler. Although 207Pb has desirable properties for high-resolution NMR (I = l/2, 2 1.1% natural abundance, and 9.13 X lop3 as sensitive as ‘H for equal numbers of nuclei) and has been used extensively in the study of organometallic compounds of lead (Z2), there have been surprisingly few reports of its use for the study of complexes of Pb(I1) in aqueous solution (13). As part of a program to determine the usefulness of 207Pb NMR for characterizing the solution chemistry of Pb(I1) complexes, we have studied the complexation of Pb(I1) by monocarboxylic acids and by aminocarboxylic acids. The objective of the carboxylic acid study was to determine if formation constants can be evaluated from 207Pb chemical shift data. The carboxylic acids were chosen to cover a range of acid strengths so that the formation constants would cover a range of values. The aminocarboxylic acids were chosen to have different numbers of amino and carboxylic acid groups to determine if the 207Pb chemical shift depends on the number and type of donor groups bonded to the lead. EXPERIMENTAL
Chemicals
The carboxylic acids and aminocarboxylic acids were of the highest-grade commercially available and were used without further purification. Sodium formate and sodium acetate were used as the source of the formate and acetate ligands. Pb(NO3)3 was obtained from Fisher Scientific. Stock solutions of -0.4 M Pb(NO3)2 were standardized by EDTA titration. Sample Preparation
Samples used for the 207Pb NMR measurements were prepared from the stock solution of Pb(N03)2 and the requisite amount of the l&and. The procedure involved first preparing a 200-ml solution containing Pb(N03)2, ligand, and NaN03 at the appropriate concentrations. As the pH of this solution was varied by addition of concentrated HN03 or NaOH, NMR samples were withdrawn. Samples were generally taken at 0.5 pH unit increments over the pH range 0.5 up to the pH at which Pb(OH)2 precipitated. The Pb(I1) concentration was generally 0.100 M, and Pb(I1) to ligand ratios of 1: 1, 1:2, and 1:4 were used. To keep the ionic strength reasonably constant in the formation constant studies, NaN03 was added such that the sum of the NaN03 and ligand concentrations was 0.40 M. By adjusting the pH to -0.5 initially and then withdrawing samples as the pH was increased, the ionic strength was in the range 1.4 to 1.8, depending on the pH and the extent of complex formation. A solvent mixture of 5% D20/95% Hz0 was used to provide an 2H fieldlfrequency lock.
LEAD-CARBOXYLIC
ACID COMPLEXES
225
pH Measurements pH measurements were made at 25°C with an Orion model 70 1 pH meter equipped with a standard glass electrode and a fiber-tip saturated calomel electrode. It was found that the pH response was unstable when this electrode pair was placed directly into the Pb(II)-containing solutions, possibly due to the precipitation of PbCIZ in the glass fiit of the reference electrode. To avoid this problem, the reference electrode was separated from the Pb(I1) solution by using a double junction arrangement. The reference electrode was placed in a 0.3 M NaN03 solution which in turn was placed in the Pb(I1) solution. The NaN03 solution was contained in a glass cylinder of - 1.8 cm diameter onto the end of which was attached the bottom 2 cm of the barrel of a standard reference electrode. The glass electrode-double junction reference electrode arrangement was calibrated with pH 4.00, 7.00, and 10.00 buffers. NMR Measurements “‘Pb NMR spectra were measured at 4 1.86 MHz on a Bruker WH-200 spectrometer operating in the pulsed Fourier transform mode. Samples of approximately 15 ml were contained in 20-mm NMR tubes. A spectral width of 10,000 Hz was used, with a digital resolution of 4.88 Hz/pt. Generally, 500 transients were accumulated. A 60-psec pulse width (-45” flip angle) was used, with 0.25 set between pulses. The T1 value for 207Pb in a pH 5.41 solution containing 0.10 M Pb(N03)2 and 0.40 A4 acetylglycine was found to be -0.22 sec. Chemical shifts are reported relative to the chemical shift of 0.10 A4 Pb(NOJ), at pH 1.73. Positive chemical shifts correspond to less shielding. The chemical shift of 1.0 M Pb(N03)2 in H20, which is reported to have a chemical shift of -2961.2 ppm relative to (CH&Pb (13) is -5 1.2 ppm relative to the pH 1.73 solution of 0.1 MPb(N03)2. Spectra were measured at ambient temperature (25 f 1’C). RESULTS
Carboxylate Complexes To determine the sensitivity of the 207Pb chemical shift to complexation by carboxylate ligands, chemical shifts were measured as a function of the Pb(N0s)2 to ligand mol ratio and the solution pH for the ligands pivalic acid, acetic acid, formic acid, and chloroacetic acid. Representative chemical shift data are shown in Fig. 1 for the Pb(NOj)2/acetic acid system at ligand to Pb(N03)2 mol ratios of 1, 2, and 4. Also shown in Fig. 1 is the chemical shift of Pb(NO& over the same pH range. For the Pb(N0,)2/acetic acid system and for the other systems studied, a single exchangeaveraged 207Pb resonance was always observed. A qualitative examination of the data in Fig. 1 indicates that the exchange-averaged chemical shift depends on the extent of complex formation. At low pH, the chemical shift for all three acetic acid to Pb(NO& mol ratios approaches a value near 0, indicating little complexation due to protonation of the l&and. As the pH is increased, the “‘Pb chemical shift changes, leveling off at a constant value at pH 5 to 6. At this pH, there is little competition from protonation of the ligand (p& = 4.65 (14)). The difference between the chemical shift at pH 5 to 6 and that at low pH is greater the larger the acetic acid to Pb(N03)2 mol ratio, indicating more complexation at the higher mol ratios. In the pH 1.5 to 5 region, the chemical shift depends on pH indicating that ligand protonation competes effectively with metal complexation in this pH region.
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E a ,200. t ,000. 6 2 600. ” z 600. -5 400. zoo0. 2 1 3F+f4 5 6 RG. 1. pH dependence of the *O’Pb chemical shift of Pb(I1) in solutions containing (A) 0.10 M Pb(NOp)z; (B) 0.10 M Pb(NO,),, 0.10 M sodium acetate, and 0.3 M NaN03; (C) 0.10 M Pb(NO&, 0.20 M sodium acetate, and 0.20 M NaNO,; and (D) 0.10 M Pb(NO& and 0.40 M sodium acetate. The curves drawn through the points for ED are theoretical curves calculated with the formation constants and “‘Pb chemical shifts determined in this work.
Formation constants for the Pb(II)-carboxylate complexes, as defined by Eqs. [I] and [2], were determined by fitting the exchange-averaged chemical shift data by nonlinear least-squares calculations: [PbL+] Kfl = [p,,z+][L-]
Pb2+ + L- * PbL+;
[ll
’
F’bL21
Pb2+ + 2L- * PbL2;
PI
p2 = [p,,2+][L-12 ’
The exchange-averaged chemical shift, Bobs,is the weighted average of the chemical shifts of the various species present, 6ok4
=
Pf6f
+
Pl:lh
+
131
P1:2&:2,
where Pf, PI:, , and PIE2represent, respectively, the fractional in the free, PbL+, and PbL2 forms
concentrations
p = [Pb2+l
f
[41
WI, ’
p, = P’bL+l 1.1
D’bl,
p
1.2
of lead
’
= PbJhl
WI, ’
with [Pb], = [Pb2’] + [PbL+] + [PbL2]. The fractional concentrations in terms of Kn and a, which leads to
151 I61 can be expressed
LEAD-CARBOXYLIC
6ohs =
ACID
6f + KfJLl&:,
227
COMPLEXES
+ B*[L12~1:2 .
1 + KflW
+ 82[L12
171
’
Kfi, and, for acetic acid, formic acid, and chloroacetic acid, p2 were obtained by fitting the observed chemical shift data to Eq. [7]. Since the chemical shifts of PbL+ and PbL2 cannot be obtained directly from the NMR data these were treated as unknowns in the curve fitting calculations. The value 6r was also treated as an unknown to account for any dependence on solution composition. The calculations involved first estimating Kf, , /3*, af, 6i:,, and 8iZ2, and then using these values and Eq. [7] to predict the chemical shift for the concentration and pH of each experimental point. The value of [L] in Eq. [7] was calculated using an equation derived by considering ligand and metal balances. The sum of the squares of the differences between the predicted and observed chemical shifts was calculated, and then the nonlinear least squares curve fitting program KINET (15) was used to refine the estimates of the parameters to minimize the sum of the squares of the residuals. When doing the calculations for a particular system, the data for the three ligand to metal ratios was fitted simultaneously. The results are summarized in Table 1. Also given are literature values where available for comparison. The solid curves drawn through the points of B-D in Fig. 1 are the theoretical curves calculated from the constants listed in Table 1, and are typical of the fits obtained. The free Pb(I1) chemical shifts listed in Table 1 are different from that of the pH 1.73 reference solution due to the higher concentration of NaN03 in the carboxylic acid solutions. The *07Pb chemical shift was found to be quite sensitive to nitrate concentration and to Pb(N03)* concentration, e.g., dilution of a 1.00 M Pb(N09* solution to 0.5 M caused the resonance to shift by +27 ppm. Although Pb(II) can form complexes of ligand to Pb(I1) ratio greater than 2 with carboxylic acids, their inclusion in the model did not result in a significant improve1
TABLE
FORMATION CONSTANTS FOR CARBOXYLATE COMPLEXES OF Pb(I1)
Pivalic acid Acetic acid
4.95 4.65
2.60 k 0.04 2.11 It 0.03 2.18’ 2.16’
Formic acid Chlomacetic acid Acetylglycine
3.55 2.75 3.44
1.56 ? 0.05 1.76 + 0.06
Glycylglycine’
3.13f
1.49 f 0.04 f 0.17f 1.48 * 0.08 1.04 f 0.07
1.81 k 0.03 1.3gd
1,090 f 1,010 f
34 30
508 f 37 340 f 14 565 f 15 569
k
23
3.06 k 2.92’ 2.99d 2.74 k 2.71 f 2.51 k 2.5Sd
0.09
1,850
f
60
-9f -14+
5 9
0.07 0.12 0.07
815 848 1,580
k + +
58 90 46
-18’ -15f -26
4 5 4
1.97 f
0.06
1,720
+ 110
-2lf
+
-38 -5lf
+
4
1.30
Glycit@ Histidine’
2.386 1.82h
210 f 17 k 21
2.08
f 0.10
856
367
a From Ref. [ 141unlessotherwise noted. b In ppm from 0. I h4 Pb(NO,)Z at pH 1.73. ‘E. A. Bums and D. N. Hume, J. Am. Chem. Sot. 78, 3958 (1956). 25”C, p = d D. L. Rabenstein, Cnn. J. Chem. 50, 1036 (1972). 25”C, p - 1.4 M. ’ Complexation by the carboxylate group of the zwitterionic form. ‘Reference [16]. 25”C, p - 1.8 M. g D. L. Rabenstein and G. Blakney, Inorg. Chem. 12, 128 (1973). *D. D. Petin, J. Chem. Sot., 3125 (1958).
1.98
M.
91
f
14 6
228
NAKASHIMA
AND
RABENSTEIN
ment in the theoretical fit to the experimental data. Also the stepwise formation constants calculated from Kn and p2 in Table 1 for the addition of a second ligand to PbL are generally quite small, suggesting that the stepwise formation constant for the addition of a third ligand to PbLz will be even smaller and can be ignored under these experimental conditions. Complexation
of Pb(II) by Aminocarboxylate
Ligands
The complexation of Pb(I1) by glycine, glycylglycine (glygly), histidine, methyliminodiacetic acid (MIDA), nitrilotriacetic acid (NTA), N-hydroxyethylethylenediaminetriacetic acid (HEDTA), and ethylenediaminetetraacetic acid (EDTA) was studied by measuring the 207Pb chemical shift as a function of solution conditions. Chemical shift data for the Pb(NOS)2/glygly system are shown in Fig. 2. Qualitatively, the dependence of the chemical shift on ligand-to-metal ratio and pH is similar to that shown for the Pb(NOs)2/acetic acid system in Fig. 1. A major difference chemically, however, is that glygly has two potential metal binding sites, the carboxylate oxygen and the amino nitrogen. At pH 1, both of these groups are protonated to give predominantly H$lCH2CONHCH2C02H, and the Pb(I1) is free in solution. The pKA of the carboxylate group of this form of glygly is 3.13, while that of the ammonium group ofthe zwitterion H3fiCH2CONHCH2C02is 8.21(16). The pH range over which the 207Pb chemical shift changes in Fig. 2 indicates complexation by the deprotonated carboxylate group while the amino group is protonated. As compared to the data in Fig. 1, the chemical shift begins to change at a lower pH, and as the pH is increased it levels off to a constant value at a lower pH. This is as expected since, the lower the pKA, the greater the availability of the carboxylate group for Pb(I1) complexation at low pH. At pH - 5, the chemical shift starts to change again, indicating the start of complexation at the amino group, in agreement with results
FIG. 2. pH dependence of the zo7Pb chemical shift of P(U) in solutions containing (A) (B) 0.10 M Pb(NO&, 0.10 M glygly, and 0.30 M NaNOp; (C) 0.10 M Pb(NO&, 0.20 M NaNOJ; and (D) 0.10 M Pb(NO& and 0.40 M glygly. The curves drawn through are theoretical curves calculated with the formation constants and 20’Pb chemical shifts work.
0.10 M Pb(NO&; M glygly, and 0.20 the points for B-D determined in this
LEAD-CARBOXYLIC
ACID
COMPLEXES
229
obtained by ‘H NMR (16). Unfortunately, however, it was not possible to study this reaction due to precipitation above pH - 5-6. The dependence of the *“Pb chemical shift on pH for the Pb(NOs)2/histidine and Pb(NOs)2/glycine systems is similar to that shown in Fig. 2, indicating complexation of Pb(I1) by the zwitterionic forms of both ligands. In the case of histidine, the imidazole group is also protonatcd. The lack of complexation of Pb(I1) by the imidazole group was further substantiated by measuring the *“Pb chemical shift for solutions containing 0.10 A4 Pb(N03)* and 0.20 M or 0.40 M imidazole. Over the pH range 0.5 to 4.0, the chemical shift did not differ significantly from that for Pb(N03)* itself over the same pH range. Formation constants for the complexation of Pb(I1) by the carboxylate groups of the zwitterion forms of glycine, glygly, and histidine were calculated using the method described above. The results are tabulated in Table 1. Chemical shift data for the other aminocarboxylate systems are presented in Fig. 3 and are interpreted in the discussion section. DISCUSSION
The results in Figs. 1 to 3 indicate that the *“Pb chemical shift provides a sensitive probe of the complexation of Pb(I1) by carboxylate and aminocarboxylate ligands. Qualitative information can be derived from the pH dependence of the chemical shift as well as quantitative formation constants. The formation constants listed in Table 1 were determined by taking advantage of the pH dependence of the extent of complexation, which is due to the competitive protonation of the ligand. This approach should be particularly useful for measuring formation constants for Pb(I1) complexes with carboxylic acids having small p& values, e.g., p& < 3 to 4. Such formation constants are difficult to measure by some of the other methods commonly used, e.g., the pH titration method, as evidenced by the lack of literature values available for comparison with most of the formation
FIG. 3. pH dependence of the chemical shift of Pb(I1) in solutions containing (B) 0.10 M MIDA; (C) 0.30 M MIDA; (D) 0.10 M NTA; (E) 0.10 M EDTA The dashed line for (C) indicates precipitation.
0.10 M Pb(NO& (A) and and (F) 0.10 M HEDTA.
230
NAKASHIMA
AND
RABENSTEIN
constants listed in Table 1. Where literature values are available, the agreement with the formation constants determined in this work is excellent. The only exception is Kr, for the acetylglycine complex. The literature value was determined by ‘H NMR. The value determined by 207Pb NMR is considered the more reliable since the 207Pb chemical shift is much more sensitive to the complexation. The results for complexes of the aminocarboxylate ligands indicate the formation of two different types of complexes: those which under the conditions of this study involve monodentate carboxylate binding and those in which the ligand is coordinated to the Pb(I1) in a multidentate fashion. In the first group are the complexes of glycine, glygly, histidine and, in the pH region 1 to 5, MIDA. The complexes with glycine and histidine are of particular interest. Although these ligands are potentially multidentate, the results indicate that at pH < 5 the amino group of the complexed glycine is protonated while in the histidine complex both the amino and imidazole groups are protonated. Such complexes have been detected previously for other metal ions, e.g., from shifts in the infrared asymmetric carboxylate frequencies of alanine and histidine in low pH solutions containing Mn(II), Fe(II), Co(II), Ni(II), Zn(II), and Cd(I1) (17), however few formation constants have been reported for such complexes. We were unable to find any literature values for the Pb(I1) complexes of glycine and histidine, and in fact it has been reported that glycine does not complex Pb(I1) at 4.68 (18). The results of the present study show that ‘07Pb NMR is a sensitive method for their detection and quantitative characterization. It also is of interest to mention that such complexes are generally ignored in the measurement of formation constants for Pb(I1) complexes of deprotonated amino acids. The assumption is made that any Pb(I1) not complexed by the fully deprotonated amino acid, e.g., the H2NCH2C02form of glycine, is present as the “free” aquo species. The results here indicate this is not the case, even at the lower concentrations often used in formation constant determinations. The 207Pb chemical shifts for the complexes of EDTA, HEDTA, NTA, and, at pH > 7, MIDA are significantly different from those for free Pb(I1) and the carboxylate complexes. The chemical shifts show some pH dependence, however, they are generally constant over those pH ranges where equilibrium constant calculations indicate the Pb(I1) is essentially all complexed by the fully deprotonated ligands. For example, the 207Pb chemical shift for the Pb/EDTA system (curve E) is constant at 2352 ppm over the pH range 4 to 12. The conditional formation constant for the Pb(EDTA)‘complex varies from 109.4 to 1O18.0over this pH range (19), indicating that essentially all the Pb(I1) will be complexed by EDTA. The chemical shift changes.with decreasing pH below pH 4, where the complex is known to undergo protonation to form the species HPb(EDTA)- (19). ‘H NMR and infrared data indicate the protonation occurs at a carboxylate group (19). Carboxylic acid proton exchange is rapid on the NMR time scale, and thus an exchange-averaged 207Pb resonance is observed, having a chemical shift intermediate between those of Pb(EDTA)2- and HPb(EDTA)-. The change in the chemical shift upon protonation of Pb(EDTA)2- is in the direction of the chemical shift for free Pb(II), consistent with the replacement of a coordinated carboxylate group by a water molecule. For the Pb/HEDTA system, the 207Pb chemical shift (curve P) is constant at 2320 ppm over the pH range 4 to 9, also as expected from the formation constant of the complex (19). HEDTA has five potential donor groups, as compared to the six of EDTA, and the difference between the 207Pb chemical shifts of the Pb(HEDTA)- and
LEAD-CARBOXYLIC
ACID COMPLEXES
231
Pb(EDTA)2- complexes, although small, is in the direction expected for the replacement of a coordinated carboxylate group of EDTA with a water molecule. At pH < 4 the *O’Pb chemical shift shows behavior similar to that observed for the Pb(EDTA)‘- complex. Over this pH region, the Pb(HEDTA)- complex is also known to protonate at a carboxylate group (20). The change in the chemical shift at pH > 9 was not predicted by the formation constants which have been reported for the Pb/ HEDTA system. The shift is presumably due to complexation of hydroxide ion to form P~J(HEDTA)(OH)~- or, possibly, deprotonation of the ligand hydroxyl group with subsequent metal binding. The change in the 207Pb chemical shift for the Pb/ MIDA system over the same pH region indicates that it is more likely due to complexation of hydroxide ion. If Pb(I1) has a coordination number of six in these complexes, there is at least one coordination position in the Pb(HEDTA)complex at which hydroxide could bind without displacing a ligand donor group. The lack of any change in the 207Pb chemical shift of the Pb(EDTA)2- complex in this pH region suggests that EDTA is hexadentate in Pb(EDTA)2-. The “‘Pb chemical shift of the Pb/NTA system is constant at 2097 ppm over the pH range 4 to 8. Formation constants indicate the complex Pb(NTA)- is present over this pH range (19). NTA has four potential donor groups, three carboxylate oxygen, and one amino nitrogen. The 207Pb resonance for Pb(NTA)- is shifted by 355 ppm to lower frequency from that for Pb(EDTA)‘-, consistent with more H20 molecules in the inner coordination sphere of the Pb(I1) in Pb(NTA)-. However, if the ligand is tetradentate in Pb(NTA)-, pentadentate in Pb(HEDTA)-, and hexadentate in Pb(EDTA)2-, the chemical shifts for these complexes indicate that substitution of H20 in the inner coordination sphere by a carboxylate group, or by an amino group, does not have a constant, incremental effect on the 207Pb chemical shift. More likely, the effect of binding a carboxylate or amino group on the “‘Pb chemical shift depends on the nature and number of donor groups already in the inner coordination sphere. The Pb/MIDA system behaves somewhat differently from the EDTA, HEDTA, and NTA systems. At pH < 3.5, the 207Pb chemical shift data indicate carboxylate complexation while the amino group is still protonated. At pH > 3.5, the shift starts to increase when the ligand-to-metal ratio is 3, however precipitate forms between pH 3.75 and 7.5, thus preventing the characterization of the Pb/MIDA complexes in this pH region. As the pH is increased from 7.5, the chemical shift levels off at 2050 ppm. In this pH region, the complex Pb(MIDA)22- forms. At pH > 9.5, the chemical shift changes again, presumably due to the formation of hydroxide complexes such as Pb(MIDA)(OH)-, which have been detected in pH studies of the complexation of Pb(I1) by MIDA (21). In conclusion, “‘Pb chemical shifts provide a sensitive probe of the aqueous coordination chemistry of Pb(I1). From chemical shift data measured as a function of solution conditions, e.g., ligand-to-metal ratio and solution pH, qualitative and quantitative information can be obtained. This approach should be particularly useful for the study of complexes having small formation constants. One group of complexes which can be studied in this way, and which are difficult to study by other methods, are those formed with the zwitterionic amino acids. Exchange-averaged 207Pb chemical shifts also can provide information about the solution chemistry of more stable complexes with multidentate ligands, e.g., information about protonation of the complexed ligand or the formation of hydroxide complexes.
232
NAKASHIMA
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RABENSTEIN
ACKNOWLEDGMENT This research was supported by Operating Grants Research Council of Canada and by the University
to D.L.R. of Alberta.
from
the Natural
Sciences
and Engineering
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Il. 12. 13. 14.
15. 16.
17. 18. 19. 20. 21.
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