Contact Ion Association of Perchlorate Ion
255 1
Contact Ion Association of Perchlorate Ion. A Chlorine-35 Nuclear Magnetic Resonance Study. II. Solutions in Mixed Solvents Harvey Alan Berman, Herman J. C. Yeh, and Thomas R. Stengle' Department of Chemlstry, Universityof Massachusetts, Amherst, Massachusetts 0 1002 (Received April 25, 1975)
The NMR relaxation time (Tz) of the 35C1 nucleus has been used to study the ion association behavior of the perchlorate ion with Li+ and Mg2+ in a variety of solvents. When the clod- is in contact with a cation, efficient quadrupolar relaxation leads to a short T2, and a relatively wide NMR line. Contact ion association is promoted in solvents of low dielectric constant and low base strength. A small amount of a second solvent may have little effect on the ion aggregates, or it may be extremely effective in disrupting contact ion association, depending on the base strength of the cosolvent. For Mg(C104)2 in acetonitrile a combination of NMR and conductance data shows that addition of a small amount of water causes a transformation from contact to solvent separated ion association. The results obtained in mixed solvents confirm the earlier interpretation of line width data in terms of contact ion association.
Introduction The nature of electrolyte solutions is a long standing problem of physical chemistry. An important part of this problem is the formation of ion aggregates in solution. Electrolytes in solution often exist as complex mixtures of several species: solvated ion, solvent separated ion pairs, contact ion pairs, etc. Ion aggregation has been studied for many years, and a wide variety of experimental techniques has been developed for its investigation.' Recently, spectroscopic methods have received much attention; among these, vibrational spectra2 and nuclear magnetic resonance3 are the most important. We have developed an NMR technique which reveals the formation of contact ion aggregates involving the perchlorate The method is based on the sensitivity of the 35Cl resonance to the electronic environment of the chlorine nucleus. The electric quadrupole moment of the 35Cl nucleus can interact with an electric field gradient and provide an efficient mechanism for nuclear magnetic relaxation. Rapid relaxation is readily observed, since it causes a broadening of the NMR lines. In the liquid state the relaxation rates of the 35Clnucleus are given by (ignoring the asymmetry parameter): 1/T1= 1/T2 = K(e2Qq)27,
(1)
where K is a collection of fundamental constants, Q is the nuclear electric quadrupole moment of 35Cl, q is the electric field gradient at the nucleus, and T~ is the correlation time for molecular re~rientation.~ The relaxation time is related to the shape of the NMR line by AV = l/aT2
(2)
where AV is the full width of the line at half-height. In any ion or molecule, the chlorine nucleus is immersed in an electric field generated by the surrounding electrons and nuclei. If the field does not possess a high degree of symmetry, it will have a nonvanishing gradient, q, which can lead to a rapid relaxation and a large NMR line width. In the free Clod- the field gradient is zero because of the tetrahedral symmetry of the ion. When the ion is solvated, the solvation shell will be approximately symmetrical, and only a small field gradient will be generated. The .NMR signal of the solvated clod- ion is only a few hertz wide in
most solvents. If the C104- comes into contact with a counterion, a large field gradient will be developed, and the line width will increase by a factor of 10-100. Hence, the line width is extremely sensitive to contact with counterions; however, it is only slightly affected when the ions are separated by a layer of solvent. A more detailed treatment of these principles is given in our earlier report4 and also in a recent paper by Popov and his coworkers.6 The first applications of the 35Cl line width method used solutions of perchlorate salts in pure solvents. It was shown that contact ion association was promoted by solvents of low dielectric constant and low base strength. Here we report results obtained in mixed solvents. We are particularly concerned with the effect of adding small amounts of a second solvent to solutions which already contain contact ion pairs (and higher aggregates). The results confirm our earlier interpretation of line widths in terms of contact ion association. We also show that a small amount of a second solvent can have a drastic effect on the state of the solute in certain cases.
Experimental Section Details of the materials, apparatus, and procedures used in the present work have been given earlier.4 The experiments in mixed solvents were performed by taking a 1.0 M solution of the salt in a pure solvent and adding small amounts of cosolvent. A measurement was made after each addition of cosolvent. This procedure caused some dilution, and the overall concentration of the salt was reduced somewhat during the course of a run. In most cases the effect of dilution is small compared with the influence of the cosolvent on the state of aggregation of the salt. In two systems (1-propanol and 1-butanol) the initial salt concentration was less than 1.0 M because of limited salt solubility. Results and Discussion Our earlier studies of pure solvent systems indicate that there is a sizable amount of contact ion association in solutions of Mg(C104)~in solvents of relatively low dielectric constant and base strength. A typical solvent in this category is acetonitrile with a dielectric constant of 38 and a base strength of 14 as measured by the donor number scale of Gutmann.' Contact ion association of Clod- with Mg2+ 7 % Journal ~ of Physical Chemlstry, Vol. 79, No. 23, 1975
2552
H. A. Berman, H. J. C.Yeh. and T. R. Stengle
( H20/cation
ratio
Figure 1. The effect of adding water to acetonitrile solutions of perchlorates. The line width of the 35CIresonance divided by viscosity Is plotted vs. the mole ratio [H20]/[cation]. The initial concentration of the salts was 1.0 M.
(DMSO / Mg" ) Figure 3. The effect of adding dimethyl sulfoxide to an acetonitrile solution of Mg(CIO&. The line width of the 35CIresonance divided by viscosity is plotted vs. the mole ratio [DMSO]/[Mg2+]. The initial concentration of the salt was 1.0 M. A precipitate formed at the concentrations indicated by X's. No data were taken at these points.
(DMF/ Mg**)
Flgure 2. The effect of adding dimethylformamide to an acetonitrile solution of Mg(CI04)*.The line width of the 35CIresonance divided by viscosity is plotted vs. the mole ratio [DMF] / [Mg2+]. The initial concentration of the salt was 1.O M.
i s revealed by t h e l i n e w i d t h of t h e 35C1 NMR signal f r o m C104-. This parameter i s m u c h larger t h a n would b e expected for a free C104- in solution. The Journal of Physical Chemistry, Vol. 79, No. 23, I975
( H,O / Mg**)
Figure 4. The effect of adding water to a 1-propanol solution of Mg(C104)2.The line width of the 35CI resonance divided by viscosity is plotted vs. the mole ratio [H20]/[Mg2+]. The initial concentration of the salt was 0.5 M.
I f t h i s viewpoint is correct, one m i g h t expect t h a t the presence of a small a m o u n t o f a second solvent o f h i g h basi s i t y would produce a large change in t h e state o f t h e per-
Contact ion Association of Perchlorate Ion chlorate ion. The strongly basic solvent would solvate the Mg2+ and displace the C104-. The perchlorate ion would be found as a free ion, or as a partner in a solvent separated ion pair. In either case, the line width of the NMR signal would be drastically reduced. The results of a test of this prediction are shown in Figure 1. (The experimental data which form the basis for the figures have been recorded elsewhere?) In this experiment small amounts of water (DN = 18) have been added to acetonitrile solutions of LiClO4 and Mg(C104)2. The 35Cl line width (corrected for viscosity effects) is displayed as a function of water concentration, which is expressed as the mole ratio H20/Mg2+. A small amount of water has a profound effect on the line width. Over half of the contact ion association is disrupted when the mole ratio HzO/Mg2+ = 2. At a ratio of six molecules of water to each Mg2+,the line width reaches its minimum value, and the further addition of water produces no effect. It is tempting to take this point as evidence for a Mg2+ hydration number of six, and to assume that all of the magnesium exists as the hydrated ion, Mg(OH2)e2+. This model is possibly correct, although it cannot be absolutely proven on the basis of these data. The behavior of LiC104 is similar to that of Mg(C104)2. In an acetonitrile solution of LE104 it is likely that only a fraction of the clod- is in contact with the lithium ion, so that the initial line width is small compared with solutions of Mg(C104)~.Addition of water produces the expected reduction in the line width which reaches its minimum at a value of H20/Li+ between 4 and 6. The effects of other basic cosolvents are shown in Figures 2 and 3. Dimethylformamide has a dielectric constant about half that of water, but a greater base strength (DN = 26.6). Its effect on the 35C1 line width of an acetonitrile solution of Mg(C104)~is the same as water. The line width is sharply reduced, reaching a minimum at DMF/Mg2+ = 6. The addition of dimethyl sulfoxide (DN = 29.8) to an acetonitrile solution of Mg(C104)~gives a similar result; it is shown in Figure 3. A t two DMSO concentrations a precipitate formed which redissolved upon further addition of DMSO. Line width data were not taken a t these points; they are indicated by X's in Figure 3. Besides acetonitrile, two other primary solvents have been investigated. These are the alcohols 1-propanol and 1-butanol. Donor numbers for these materials have not been determined, but they have low dielectric constants (20.1 and 17.7, respectively), and our earlier work indicated that extensive contact ion aggregation exists in these media. The effects of water on the 35Cl line width of Mg(C104)2 in these solvents are shown in Figures 4 and 5. The behavior in these alcoholic solutions exactly parallels that observed for the acetonitrile solution in Figure 1. It is important to consider the effects of cosolvents which are only slightly basic. The addition of a slightly basic substance to an acetonitrile solution of Mg(C104)z should not produce a great change in the S5C1 line width. The cosolvent would have little tendency to solvate the Mg2+and replace C104-. Three such cosolvents were chosen: ethyl acetate, tetramethylene sulfone, and nitromethane (DN = 17, 15, 3). The results of all three experiments are shown in Figure 6. A small amount of cosolvent produces only a slight change in the 35Cl line width indicating that the state of aggregation of the ions is not greatly affected. Up to a concentration of cosolvent equivalent to cosolvent/Mg2+ = 3, the curves are coincident. It is not clear if this behavior is fortuitous, or if it reflects some effect on the clod- which is
2553
(H20/ Mg**)
The effect of adding water to a I-butanol solution of Mg(C104)2. The line width of the 35CI resonance divided by viscosity is plotted vs. the mole ratio [H20]/[Mg2+].The initial concentration of the salt was 0.9 M. Figure 5.
r-----t 300
It
0
9 I
I'
EA
I I
I
I I I
5
IO
15
(cosolvent /Mg*) Flgure 6. The effect of adding weakly basic cosolvents to an acetonitrile solution of Mg(CIO&. The cosolvents are ethyl acetate (EA), nitromethane (NM), and tetramethylene sulfone (TMS). The line width of the 35CI resonance divided by viscosity is plotted vs. the mole ratio [cosolvent]/[Mg2+].The initial concentration of the salt was
1.0 M.
identical in all three systems. At high cosolvent concentrations the composition of the liquid is significantly changed. The line width begins to approach the value it would have in the pure cosolvent. The Journal of Physical Chemistry, Vol. 79, No. 23, 1975
2554
H. A.
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Berman, H. J. C. Yeh, and T. I?. Stengle
In order to obtain more information about the state of the perchlorate ion in mixed solvents, one system was selected for conductance studies. The conductance of several perchlorate salts in acetonitrile was measured as small amounts of water were added to the solutions. The results are shown in Figure 7. Here the equivalent conductance is presented as a function of the mole ratio HzO/cation for solutions of NaC104, LiC104, and Mg(C104)z. The effect of dilution by the cosolvent has been taken into account in the calculation of the conductances. Solutions of NaC104 and LiC104 show similar behavior. Addition of water increases the conductance, first at a rapid rate, then more slowly. This indicates that free perchlorate ions are released by the cosolvent. The effect of water on the Mg(C104)z solution is quite different. There is a hiatus in the conductance as the first 3 mol of water are added, then the conductance increases, albeit more slowly than in solutions of NaC104 and LiC104. The effect is more apparent in Figure 8 where the data for low water concentrations are plotted on a larger scale. In this plot the conductance has been multiplied by the viscosity; this compensates for the effect of the viscosity of the medium on the translational correlational time of the ions. The effect of low water concentrations on the conductance contrasts sharply with the NMR line width data. In the region where the conductance is constant, the line width changes by a factor of 7. These contrasting results indicate that c104- is associated with Mg2+ in this medium, but the ions are not in contact. The perchlorate ion is contained in solvent separated ion pairs (or higher aggregates). The effect of the cation on the field gradient a t the chlorine nucleus falls off rapidly with distance, and it is essentially nil for ions which are separated by a layer of solvent. Addition41 evidence for a negligible field gradient in separated ion pairs has been reported recently.9 The results divide the cosolvents into two distinct groups, strongly and weakly basic. Water (DN = 18) belongs to the strongly basic class, although its donor number is only slightly greater than ethyl acetate (DN = 17) which behaves as a weak base. It is unlikely that such a small difference in donor number could explain the great difference in the tendency of these solvents to solvate Mg2+.Other factors must be taken into account in explaining the solvating power of these materials. Possibly the steric factor plays a role. The small size of the water molecule compared with ethyl acetate could enhance the ability of water to solvate Mg2+.Another factor lies in the thermodynamics of the water-acetonitrile system. These solutions show large positive deviations from Raoult's law, and the activity coefficient of water is much greater than unity even in the presence of a large excess of acetonitrile.1° Water is an exceptionally effective solvating agent toward halide ions in such mixtures,ll and it is reasonable to expect similar behavior toward cations.
i , , , , , . 8
24
16
40
32
( H20/cation)
48
ratio
Figure 7. The effect of adding water on the conductance of acetonitrile solutions of NaC104, LiC104, and Mg(CIO&. The equivalent conductance is plotted vs. the mole ratio [H20]/[cation].The initial salt concentration was 2.0 M for NaCIO4 and 1.0 M for LiC104 and Mg(C104)2.
F -
2 5 ~ 5
IO
15
20
(H,O/Mg'*)
Figure 8. The effect of adding water on the conductance of acetonitrile solutions of Mg(C104)2. The equivalent conductance times the viscosity is plotted vs. the mole ratio [H20]/[Mg2+].The initial salt concentration was 1.O M. All of the line width data reported here have been corrected for viscosity effects, that is, they are recorded as Au/q where q is the bulk viscosity of the solution. In making comparisons among different solutions we are primarily concerned with changes in the field gradient, q. However according to eq 1 and 2, Au will also be affected by changes in the correlation time T ~ If. T~ is governed by the same factors which influence a sphere turning in a viscous liquid, it will be proportional to the viscosity of the s ~ l u t i o nThe .~ effect of 7c should be removed by dividing the line width by the viscosity. This simple model is not rigorously correct, but it seems to be a fair approximation. In certain solvents, particularly ethyl acetate, the model appears to fail. This is probably due to the formation of large ion aggregates as discussed in our earlier report." The Journal of Physical Chemistry, Vol. 79, No. 23, 7975
Acknowledgments. This work was supported by a grant from the Directorate of Chemical Sciences of the Air Force Office of Scientific Research. We are grateful to Professor Cooper H. Langford for his encouragement and for many stimulating discussions. Supplementary Material Available. Tables I-XI which form the basis of the figures will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper
2555
Adsorption of Cyclohexane and Benzene on Silica
2449 (1973). (4)H. A. Berman and T. R. Stengle, J. Phys. Chem., 79, 1001 (1975). (5) J. A. Pople, W. G. Schnelder, and H. J. Bernstein, “High Resolution Nuclear Magnetic Resonance”, McGraw-Hill, New York, N.Y., 1959,Chapter 9. (6)Y. M. Cahen, P. R. Handy, E. T. Roach, and A. I. Popov, J. Phys. Chem., 79, EO (1975). (7)The donor number is the negative of the enthalpy of reaction of the donor solvent and SbC15 in the “inert” medium 1.2dichloroethane. V. Gutmann, “Coordination Chemistry in Nonaqueous Solvents”, SpringerVerlag, Vienna, 1968,p 19. (8)H. A. Berman, Ph.D. Thesis, University of Massachusetts, Amherst, Mass., 1974,and in the supplementary material. (9)K. L. Craigheadand R. G. Bryant, J. Phys. Chem., 79, 1602 (1975). (IO)V. de Landsberg, Bull. SOC. Chim. Belg., 49, 59 (1940). (11) T. R. Stengle, Y. E. Pan, and C. H. Langford, J. Am. Chem. SOC.,94, 9037 (1972).
only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Business Office, Books and Journals Division, American Chemical Society, 1155 16th St. N.W., Washington, D.C. 20036. Remit check or money order for $4.50 for photocopy or $2.50 for microfiche, referring to code number JPC-75-2551. References and Notes (1)C. W. Davies, ”Ion Association”, Butterworths, Washington, D.C., 1962. (2)J. P. Coetzee and W. R. Sharpe, J. SolutionChem., 1, 77 (1972). (3)M. S. Greenberg, R . L. Bodner, and A. I. Popov, J. Phys. Chem., 77,
Adsorption of Cyclohexane and Benzene on Two Modified Silica Supports Donald Barry* and Martha Cook Department of Chemistry, Universityof Houston, Houston, Texas 77004 (Received December 4, 1974; RevisedMnuscrlpt ReceivedJuly 28, 1975) Publicafloncosts assistedby The Robert A. Welch Foundation
The adsorption of cyclohexane and benzene were studied on two modified silica supports. The first support had the form of SSi-CH2CH2CH2NH2 and the second support had the form [ 3 i C H ~ C H ~ C H ~ N H ~ P ~ ([PtCL]. N H ~ ) The S ] methods of preparation of these supports are presented in the paper. The surface characteristics of these supports were studied using a constant volume system to determine adsorption isotherms. These isotherms were used to derive the isosteric heats of adsorption of both benzene and cyclohexane on the two supports. The isotherms were found to follow a Freundlich relationship and information about the surfaces is inferred from the coefficients of the Freundlich expressions and from the heats of adsorption.
Introduction The bonding of transition metal complexes to organic polymers or silica has provided an easy method for combining some of the most advantageous characteristics of homogeneous and heterogeneous catalysts. The major advantages of this process of “heterogenization” are the easy recovery of the catalyst and the extra selectivity provided by the presence of a surface.l The organic polymers which are used as supports have the major disadvantages that they tend to swell in organic solvents and that a great part of the catalytic reaction tends to occur in polymer channels where diffusion of reactants can become rate limiting. Grubbs and Krol12 successfully bonded Wilkinson’s catalyst to polystyrene beads and they showed that the bound catalyst exhibited a great deal of activity and selectivity in the hydrogenation of olefins. Other workers3v4have developed methods for attaching enzymes or peptides to silica and Schwetlich5 has recently adapted their method to bond transition metal complexes to a silica surface. This method involves the condensation of either a substituted trichloro- or triethoxysilane with a silica surface. The product of the reaction is a siloxane bond between the silica surface and the silane derivative. By varying the substituents attached to the trichloro- or triethoxysilanes he was able to bond a variety of
I
transition metal complexes to the silica surface. The principal disadvantage of this method is the ease of hydrolysis of the siloxane bond. Locke6 and coworkers have developed a method for bonding a variety of alkyl or aryl substituents to siliceous surfaces using Grignard reagents. This method has the principle advantage of leading to a hydrolytically stable silicon-carbon bond. We have extended this method so as to allow us to bind Magnus salt, [Pt(NH&][PtC14], to a modified silica surface. Our method can be extended to allow the binding of any amine containing complex to a silica surface. The crystal structure of Magnus salt has been shown to consist of metal complex units stacked in columns with the metal atoms in close enough proximity to bond to each othern7It has also been found that Magnus salt can display anisotropic semiconductor b e h a ~ i o r in ~ , the ~ direction of the metal-metal bonds. Since the structure of Magnus salt exhibits coordinative unsaturation at the surface, we felt that the salt would provide a useful link between platinum metal catalysts and their homogeneous analogs. The behavior of the bound Magnus salt was studied by determining the physical adsorption of benzene and cyclohexane on it. It has been shown that the shape of isatherms can be somewhat sensitive to the surface structure of the adsorbent.1° Additionally, the magnitude of the heats of The Journalof PhysicalChemistry, Vol. 79, No. 23, 1975
