5418
J. Phys. Chem. 1983, 87, 5418-5421
Multinuclear Magnetic Resonance Study of Ion Pairing and Solvation in the System Sodium Tetraethylaluminate/Pyridine/rn-Xylenet Joel L. Gray and Gary E. Maclel' Department of Chemlstty, Colorado State University, Fort Collins, Colorado 80523 (Recelved: December 20, 1982)
23Na,27Al,and lH NMR spectra have been recorded for solutions of sodium tetraethylaluminate (NaTEA) in solvent mixtures of pyridine (Py) and m-xylene. The pyridine content of the solutions was varied, the NaTEA concentration being held constant. NMR parameters were interpreted in terms of ion pairing of NaTEA, NaTEASPy, and NaTEA.3Py and aggregation of these species.
Introduction Recently we reported multinuclear NMR datal on the nuclides 23Na,27Al,and 'H in sodium tetraethylaluminate (NaTEA) in m-xylene and in mixtures of tetrahydrofuran (THF) and m-xylene. The observed variations in the 23Na chemical shift and line width were interpreted as the result of changing NaTEA aggregation with changing NaTEA concentration in pure m-xylene. In mixtures of NaTEA (at fixed molal concentration), THF, and m-xylene evidence of discrete complexes of NaTEA-2THF and NaTEAdTHF arose from examination of variations of the 23Naand a-THF lH chemical shifts with the THF:NaTEA mole ratio. From the 23Nachemical shift the maximum THF-to-Na+ coordination was found to be four T H F molecules per Na+ for this system. Because of the apparent absence of 27Al-1H spin-pin coupling in the lH and 27AlNMR spectra the ionic species present a t all THF: NaTEA mole ratios studied were concluded to be ion paired. On the basis of reasonable evidence2 indicating that the maximum THF-to-Na+ coordination number is 6, we concluded that these THF-NaTEA complexes are various forms of solvent-associated ion pairs (SAIP), but not solvent-separated ion pairs (SSIP). We report here similar multinuclear NMR studies on mixtures of NaTEA, pyridine (Py), and m-xylene. The similarities of this system and that described in the preceding paragraph are obvious, except that pyridine differs from THF in the nature of its coordination with metal ions and the magnitude of its electric dipole moment and dielectric constant. Experimental Section Materials. The preparation and purification of NaTEA and the purification of m-xylene were described previous1y.l Pyridine (Aldrich) was stirred with anhydrous BaO for several days and fractionally distilled onto molecular sieves (Linde 4A), and the fraction distilling a t 100-101 OC was collected a t ambient pressure. Water was not detected by gas chromatography; this provides an upper limit on the water of about 2 X 10-2-3 X (v/v), the detectability limit established by the method of standard additions. All materials were stored in capped bottles in a dry N2 atmosphere in a drybox. All solution preparations and material manipulations were carried out in the drybox. Before any transfer of NaTEA, the atmosphere in the drybox was completely flushed with dry ND Solutions were transferred to 10-mm (0.d.) NMR tubes, and tightly capped. 'Taken from the Ph.D. Dissertation of J. L. Gray, submitted June 1980, at Colorado State University.
0022-3654/83/2087-5418$0 1.50/0
All other materials were reagent-grade purity and used without further purification. NMR Measurements. 23Na,27Al,and lH NMR spectra were recorded by using the previously described 2.1-T Bruker spectr0meter.l Pyridine lH chemical shifts were measured with a JEOL FX-100 spectrometer and a Nicolet NT-150 spectrometer. Further aspects of NMR measurements were detailed e1sewhere.l Chemical shifts for 23Naand 27Alare reported as more positive shifts corresponding to lower shielding; 3% (w/w) cyclohexane was added to provide an internal lH reference. Results and Discussion All solutions were 0.0636 m NaTEA with respect to the combined weights of m-xylene and pyridine. Phase separation occurred when the mole ratio, Py:NaTEA, just exceeded 9.5, which was the largest Py:NaTEA mole ratio examined in this study. This NaTEA concentration, 0.0636 m, exceeded the solubility of NaTEA in neat mxylene by about 0.02 m. The addition of pyridine affected solution with the minimum required amount of pyridine being about 0.03 m i.e., a minimum Py:NaTEA mole ratio of about 0.5. 23Nachemical shifts and pyridine l€€chemical shifts are given in Figures 1 and 2, respectively, plotted vs. the Py:NaTEA mole ratio. The 27AlNMR resonance position did not vary, within experimental error, as the pyridine content was changed. In order to determine contributions that effects other than bulk magnetic susceptibility changes can have on the cyclohexane resonance and thus on the apparent pyridine lH chemical shift, the chemical shifts of the pyridine protons were measured for two solutions having Py:NaTEA mole ratios of 0.903 and 8.065, external cyclohexane being used as a reference. Interpreting such measurements ordinarily requires very accurate knowledge of the solvent magnetic susceptibility changes. This requirement (and the bulk magnetic susceptibility correction) can be eliminated by using the two different sample tube vs. Ho geometries of an iron-core magnet and a superconducting solenoid when the sample and the external reference are contained within concentric tubese3 In an iron-core magnet the sample-tube long axis is perpendicular to the H, field, whereas in a solenoid this relationship is parallel. It has been shown3that the corrected (intrinsic) chemical shifts can be derived algebraically from chemical shifts obtained relative to the concentric-tube external reference (1)J. L. Gray and G. E. Maciel, J . Phys. Chem., 87, 5290 (1983). (2) J. Smid and A. M. Grotens, J. Phys. Chem., 77, 2377 (1973). (3) J. K.Becconsall, J. Am. Chem. SOC.,92,430 (1970).
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 5419
NMR Study of Ion Pairing and Solvation
9.000
t
,
, 1.0
1
1
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1
1
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Figure 1. 23Na chemical shift (relative to an 8.0 X 10-4m NaCiO, solution in THF') vs. the Py:NaTEA mole ratio. The error bars on the points were determined from the estimates of the uncertainty in determining the maximum of the 23Naresonance. I t was found that this uncertainty was related to the line wldth. Always the reproduciblllty of two separate measurements fell within these error bars.
15 H I
Figure 3. Series of portions of proton NMR spectra indicating the varlation of the methylene proton line shape of the TEA- anion with the Py:NaTEA mole ratio (indicated directly above each spectrum).
420
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. .,7/ 6.60
.
a ,''/
P" I h T [ A
in the two different geometrical arrangements. With this approach it was determined that the contribution which the solvent effects on the resonance position of the cyclohexane internal reference make to the apparent pyridine lH chemical shifts represents about 20% of the total chemical shift range of each pyridine lH resonance. Therefore, the major contributions to the pyridine 'H chemical shifts are of chemical origin, and we will neglect solvent effects in the remainder of this discussion. Figure 3 shows the variation in the TEA- methylene proton line shape vs. the Py:NaTEA mole ratio. These spectra are consistent with spectra presented earlier1 and spectra reported by Ahmad and Day.4 Variations in the line width, Avl12 (full width at halfmaximum intensity), of the 23Naand nAl NMR resonances are given in Figures 4 and 5, respectively. In Figures 1, 4, and. 5 the points occurring at a Py:NaTEA mole ratio of zero are values extrapolated from graphs of the 23Na chemical shift and line width and the 27Alline width vs. the NaTEA molal concentration in neat m-xylene given in ref. 1. Each 23Naand 27AlNMR spectrum consisted of single peak with apparently Lorentzian line shape. No hint of (4)N. Ahmad and M. C. Day, J. Am. Chem. SOC.99, 941 (1977).
Flgure 4. Plot of the 23Naline width vs. the Py:NaTEA mole ratio. The error bars were established from the reproducibility of separate measurements on the same solution, and when they are absent the reproducibility is roughly the size of the point. 220
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180
160 2
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140
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Flgure 5. *'Ai line width variation with the Py:NaTEA mole ratio. Error bars were established by the line width reproducibility of two measurements on the same solution only for those points showing an error bar.
nAl-lH spin-spin coupling was observed at any Py:NaTEA mole ratio. A 27A1spectrum for a dilute NaTEA solution = in pyridine consisted of a nine-line multiplet with JA1-H 5.9 Hz.
5420
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983
The results obtained from the NaTEAlpyridinelrnxylene system are similar to those of the previously discussed NaTEA/THF/m-xylene system.l Therefore, many of the considerations used to interpret the data of the NaTEAITHFlrn-xylene system will be used to interpret the data presented here. Each of the plots showing variations in the pyridine a-, P-, and y-proton chemical shift curves with changing Py:NaTEA mole ratios (Figure 2) contains a distinct discontinuity at a mole ratio value of 1, suggesting a discrete NaTEAePy complex. As the Py:NaTEA mole ratio increases from 1, these proton chemical shifts increase in a smooth, nonlinear fashion. The 23Nachemical shift variation of Figure 1 changes from a nonlinear, steeply increasing dependence upon the Py:NaTEA mole ratio to an apparently linear variation with small slope, as the Py:NaTEA mole ratio increases from 0.6 to 3. Since the 23Nachemical shifts of discrete NaTEA-Py complexes are probably sufficiently different, and because the 23Na chemical shift variation shows little increase beyond Py:NaTEA mole ratio of 3, then these 23Nachemical shift data indicate that the maximum Py-to-Na+ coordination number is 3. (Shortly we will examine 23Naand 27Alline width data that support this coordination number.) Because only single, broad 23NaNMR peaks with variable resonance position were detected, chemical exchange is probably occurring between the various sodium-containing species. We postulate the following two equilibria:
+ Py NaTEAaPy NaTEA-Py + 2Py + NaTEA.3Py NaTEA
(la)
Ob)
The species in equilibria 1are shown as monomeric species, but one should realize that various degree of aggregation of these complexes and NaTEA monomers exist. Since the 23Nachemical shift rises steeply as the Py: NaTEA mole ratio increases from 0, one would suspect that the concentration of free pyridine is small, relative to the concentration of bound Py, below a Py:NaTEA mole ratio of 3. The break in the pyridine proton chemical shift curves at a Py:NaTEA mole ratio of 1 indicates the presence of a discrete NaTEA-Py complex; thus, one would expect to see nearly constant values for the pyridine proton chemical shifts between Py:NaTEA mole ratios of 0.6 and 1. This is not observed in Figure 2; rather, the pyridine proton chemical shifts increase significantly as the Py: NaTEA mole ratio increases from 0.6 to 1. Since the extent of aggregation (i.e., the number of monomers in the aggregate) depends on the pyridine concentration and since aggregation influences the overall nature of Py-NaTEA complexation (in an indirect manner involving the sharing of pyridine molecules between Na+ ions in the aggregate), then one would suspect that varying degrees of aggregation could influence the pyridine proton shielding. Thus, the increase in the pyridine proton chemical shift between Py:NaTEA mole ratios of 0.6 and 1 is interpreted as the result of changes in the extent of aggregation. (The reader may consult ref 1, where a similar a-THF proton chemical shift variation was seen for THF:NaTEA mole ratios less than 2 for the NaTEA/THF/m-xylene system.) The 23Nachemical shift ranges from about -12 to about 6 ppm for Py:NaTEA mole ratios increasing from 0 to 4; this is due to the combined influence of Py-NaTEA complexation and aggregation. One cannot justifably consider separately the effects of aggregation and Py-NaTEA complexation on the steep initial rise in the 23Nachemical shift. For the NaTEA/THF/m-xylene system1 the 23Na chemical shift range is somewhat smaller, -12 to about 1.5 ppm. This difference in 23Nachemical shift ranges in these
Gray and Macle1
two systems is expected, since the 23Nachemical shifts of various sodium salts have different values in THF and pyridine? which would reflect different solvating properties of pyridine and T H F (e.g., Lewis basicities). In Figure 3 there is no evidence in any TEA- methylene proton spectra of a nine-line multiplet structure that would indicate dissociated ions, or perhaps solvent-separated ion pairs (SSIP), if the criterion of Ahmad and Dafl is correct. Therefore, all ionic species exist as ion pairs throughout the entire range of Py:NaTEA mole ratios under study. From this conclusion, it follows that uncomplexed NaTEA exists as aggregates of contact ion pairs (CIP). As a result of the 23Nachemical shift data, which show that the Na+ ions are mainly complexes at Py:NaTEA mole ratios of 1 and above, one concludes that CIP have significant concentrations only below a Py:NaTEA mole ratio of 1. The NaTEA-Py complex exists in aggregates of ion-paired species and represents a form of solvent-associated ion pairs (SAIP). Since the maximum solvent-to-Na+ coordination number is probably at least 4,6 then the NaTEA-3Py complex is probably another form of SAIP. The 27Alline width variation vs. the Py:NaTEA mole ratio is shown in Figure 5. Because the 27Alresonance position has no dependence on the pyridine content, chemical exchange contributions to the 27Alline width variation are negligible. The reduction in the 27Alline width with increasing pyridine content can be explained by Py-Na+ complexation. Pyridine complexing to the Na+ in a SAIP reduces the effective charge on the Na+ ion, which consequently would reduce the principal electric field gradient, V,,, at the 27Alnucleus relative to that in CIP. Also, reduction in the Na+ effective charge would allow an increase in the cation-anion separation distance, which would also reduce V,,. Once the primary coordination sphere of the Na+ is filled, additional pyridine (i.e,, at Py:NaTEA mole ratios greater than 3) begins filling primary anion solvation spheres and secondary cation solvaion spheres of the SAIP. This has the effect of increasing the dielectric constant in the immediate SAIP environment and the bulk solvent, which further reduces the ionic interaction, Le., reducing V,,. Thse arguments explain the change in the 27Alline width variation from a rather steep decline to an apparently linear, less steep decline near a Py:NaTEA mole ratio of 3, and support the 23Nachemical shift data that indicate a maximum Py-toNa+ coordination number of 3 in this system. The influence of the quadrupolar reorientation correlation time, T ~ , is judged to be less important in determining the 27Al quadrupolar relaxation rate than electric field gradient influences, because the 27A1 line width decreases throughout the Py:NaTEA mole ratio range, despite an expected increase in 7,. It is of interest to compare the 27Alline width variations between the previous results for the NaTEA/THF/rnxylene system1 and this pyridine case. At a so1vent:NaTEA mole ratio of 4 the 27Alline width has approximately the same value (115 Hz) in both systems. At so1vent:NaTEA mole ratios values greater than 4 the decline in the 27Al line width is steeper in the pyridine case than in the THF case. Since the solvent dielectric constant is thought to be a major factor in establishing the magnitude of the 27Al V,, via the reduction in ionic interaction, then the solvent with the largest dielectric constant should be more effective in reducing the 27AlV,, value. The dielectric constant for (5) M. S. Greenberg, R. L. Bodner, and A. I. Popov, J.Phys. Chern., 77,2449 (1973). (6) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemistry”, 3rd ed., Interscience, New York, 1972, p 198.
NMR Study of Ion Pairing and Solvation
pyridine is 12.3 and that for T H F is 7.6.5 These dielectric constants and the above line width comparisons support this thesis. The 23Naline width dependence on the Py:NaTEA mole ratio is shown in Figure 4. The overall variation in Figure 4 is qualitatively similar to the 23Naline width change seen in the NaTEA/THF/m-xylene system,' where chemical exchange was shown not to contribute to the 23Naline width. On the basis of this correlation between 23Naline width data in these two systems one might assume that the chemical exchange contribution to the 23Naline width in the pyridine case is negligible. Thus, we will assume that the primary factors governing the 23Naline width are the 23Naquadrupolar relaxation rates of the individual species in solution; the population-weighted average of each line width yields the observed line width. Model electric field gradient calculations were conducted for pyridine dipole moments arranged near Na+ in a manner exactly analogous to the calculations described in ref 1 and are detailed in an Appendix to this discussion. The magnitudes of the computed electric field gradients have the following order: V,,(NaTEA) > V,,(NaTEA.Py) = V2,(NaTEA.2Py) > Vz,(NaTEA.3Py) > V,,(NaTEA4Py). Thus, pyridine complexation can result in a reduction in the 23Naelectric field gradients. A reduction in the extent of NaTEA aggregation can remove the degree of symmetry of anion packing about a 23Na+in the core of an aggregate. This can result in an increase in the electric field gradients about the %Nanuclei in low-order aggregate or monomeric forms of NaTEA. Py-NaTEA complexation can reduce the extent of aggregation; i.e., SAIP aggregate to a lesser extent than NaTEA aggregates. The interrelated effects of aggregation and pyridine complexation produce conflicting influences on the 23Na electric field gradients, which can explain the maximum seen in Figure 4. The extent of aggregation probably decreases as the Py:NaTEA mole ratio increases. Between Py:NaTEA mole ratios of 0 and 1the species present are aggregate forms of uncomplexed NaTEA and NaTEA-Py. In this region of the 23Na line width curve the loss of aggregation dominates and consequently produces a majority of 23Nanuclei with larger electric field gradients. As the Py:NaTEA mole ratio increases beyond 1,the majority of 23Nanuclei experience reduced electric field gradients offered by increased Py-Na+ complexation. The 23NaAvllz curve undergoes a steep decline between Py:NaTEA mole ratios of 2 and 4, resulting from increased populations of NaTEA-Py complexes. The curve shows a minimum Av, value of about 200 Hz. This minimum is about 80 Hz iarger in the Py system than in the T H F system,l where the maximum THF-Na+ coordination is 4, thus suggesting that NaTEA-solvent complexation involves a smaller coordination number in the Py system
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 5421
than in the T H F system. The comparison of 23NaAvli2 minima between the Py and T H F systems and the rapid Avliz decline in Figure 4 between Py:NaTEA mole ratios of 2 and 4 support the conclusion that the maximum Pyto-Na+ coordination number is 3 in the Py system. The broad, shallow minimum in the 23Naline width curve centered at a Py:NaTEA mole ratio of about 7 suggests different conflicting influences on the 23Na quadrupolar relaxation rate in NaTEA.3Py SAIP. Increasing amounts of pyridine (i.e., Py:NaTEA mole ratios > 3) reduce the ionic interactions between the cation and anion that are primarily responsible for the electric field gradients at both 23Naand 27Alnuclei by increasing the dielectric constant of the immediate SAIP environment. However, additional pyridine increases the molecular reorientational correlation time, which has the opposite effect on the 23Naquadrupolar relaxation rate. Acknowledgment. We gratefully acknowledge support of this research by National Science Foundation Grant No. CHE 74-23980 and use of the Colorado State University Regional NMR Center, funded by the National Science Foundation Grant No. CHE 78-18581. Appendix Model Electrostatic Calculations. The calculations presented here are fashioned after the calculations given previously,' except for the additional cases covering the NaTEASPy and NaTEAq3Py complexes. If one assumes a linear arrangement in NaTEA-Py (i.e., the pyridine dipole moment vector is collinear with the z axis), then the calculation for NaTEAePy produces a V,, value equivalent to the V,, value for the NaTEA.2Py case, with its dipole moment vectors 60° away from the z axis. We use the equations for V,, and Ed and the geometries given in ref 1 for estimates of V,, in NaTEA-Py, NaTEA.2Py, and NaTEA.4Py; the calculation for the NaTEA.3Py model is very similar to that of the NaTEA.4Py complex, except that three dipole moments were considered with the dipole moments 60" off the z axis and axially symmetric about the z axis. If one takes the covalent radius of nitrogen to be 1.12 A and assumes pyridine to be a regular hexagon 1.35 A on a side, then ro is equal to 4.2 A, and r is equal to 2.8 A. The pyridine dipole moment is 2.2 D.7 Using these values, one calculates a V,, value of 2.5 X esu cmV3for NaTEA-Py and NaTEA-2Py, and a V,, value of 2.0 X 1013 esu cm-3 for NaTEA-4Py. These values are respectively about 14% and 30% less than the V,, value of 2.9 X 1013esu cmn3given in ref 1 for NaTEA. Registry No. Na, 7440-23-5; AI,7429-90-5; NaTEA, 2397-68-4; pyridine, 110-86-1. (7) R. C. West, Ed., "Handbook of Chemistry and Physics", 55th ed., CRC Press, Cleveland, OH,1974.
