Ligand-Alkali Metal Ion Complexes A. Gupta and C. N. R. Rao, J. Phys. Chem., 77,2888 (1973). K. G. Breitschwerdt and H. Kistenmacher, Chem. Phys. Lett., 14, 288
(1972). P. Schuster and H. W. Preuss, Chem. Phys. Lett., l l , 3 5 (1971). (a) G. H. F. Diercksen and W. P. Kraemer, Theor. Chim. Acta (Berlin), 23, 387 (1972); (b) W. P. Kraemer and G. H. F. Diercksen, ibid., 23, 393 (1972). (a) E. Clernenti and H. Popkie, J. Chem. Phys., 57, 1077 (1972); (b) H.
679 Kistenmacher, H. Popkie, and E. Clementi, ibid., 58, 1689 (1973). (17) (a) P. A. Kollman and I. D.Kuntz, J. Am. Chem. Soc., 94, 9236 (1972); (b) ibid., 96, 4766 (1974). (18) K. G. Spears and F. C. Fehsenfeld, J. Chem. Phys., 56, 5698 (1972). (19) D. Hankins, J. Moskowitz, and F. Stillinger, J. Chem. Phys., 53, 4544 (1970). (20) G. Diercksen, Chem. Phys. Lett., 4,373 (1970). (21) S. H. Kim and B. T. Rubin, J. Phys. Chem., 77, 1245 (1973).
Association of Triethanolamine and Related Ligands with Alkali Metal Ions and Ion Pairs in Tetrahydrofuran at 25 O C 1 H. 6. Flora, II, and W. R. Gilkerson" Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received November 2 1. 1975)
In a continuing study of ion-molecule interaction in solution, the effects of added triethanolamine (TEA) on the conductances of dilute solutions of lithium, sodium, potassium, rubidium, and tetra-n-butylammonium 2,4-dinitrophenolate (DNP) in tetrahydrofuran solvent have been measured at 25 "C. The effects of added TEA on the spectra of LiDNP and KDNP in T H F were recorded. The conductances of dilute solutions of LiDNP in T H F have also been measured as a function of added N,N-dimethylethanolamine,diethanolamine, a series of related diols and triols and several acyclic polyethers. The observed increases in conductivity in the presence of the ligands have been interpreted as due to formation of cation-ligand and ion pair-ligand complexes and equilibrium constants are obtained for these processes. Values of the equilibrium constants for ion pair-ligand complex formation derived from the observed changes in spectra of LiDNP and KDNP in T H F upon the addition of TEA compare well with those obtained conductometrically. Trends in the equilibrium constants for cation-ligand association of TEA with the alkali metal ions are compared with those observed earlier with triphenylphosphine oxide as ligand. Lithium cation does not have as strong an affinity for TEA as one would expect based on these latter results and the trend indicated with rubidium and potassium. This is explained in terms of ligand-solvent competition for the cations and the strong steric requirements of the chelating groups on the triethanolamine. Although quinuclidine is a weak ligand toward Li+ in THF, replacement of a methylene or an ether group by an amino group in a ligand results in a significant enhancement of ligand association with Li+. Replacement of an ether group by a hydroxy group also results in an enhanced ligand association in THF, presumably due to hydrogen bonding of the hydroxy group to solvent molecules.
The formation of complexes of alkali metal cations with ligands and, in particular, chelating ligands such as polycyclic ethers, has been the subject of intense interest recently.2 A program to study the thermodynamics of alkali metal cation-ligand interaction in nonaqueous solvents as a function of cation size and the nature and structure of the ligand has been undertaken in this laboratory in an effort to understand some of the aspects of the foregoing reactions. An earlier report dealt with the association equilibria of lithium, sodium, potassium, and rubidium cations with the monodentate ligand triphenylphosphine oxide (Ph3PO) in tetrahydrofuran (THF) as ~ o l v e n tThis . ~ report is an extension of these studies to include the interactions of these alkali metal cations with a polydentate ligand, triethanolamine (TEA), and the interactions of lithium cation with a series of diols, triols, a mono- and a diethanolamine, and several acyclic polyethers. Experimental Section T H F solvent and the alkali metal dinitrophenolate salts
were prepared and purified as reported earlier.3 Tetra-nbutylammonium 2,4-dinitrophenolate was prepared and purified (mp 108-109 "C) in the same manner as the corresponding picrate salt.4 Triethanolamine (Fisher Scientific Co.) was distilled under vacuum using a 30 X 2 cm Vigreaux column; a middle fraction was collected [bp 166 "C (4 mmHg)]. Diethanolamine (2,2'-iminodiethanol, Columbia Organic Chemicals) was distilled in vacuo as above; a middle fraction [bp 129 "C (12 mmHg)] was collected. N,N-Dimethylaminoethanol (Aldrich Chemical Co.) was distilled under reduced pressure; a middle fraction [bp 66 "C (67 mmHg)] was taken. Quinuclidine (K & K Laboratories) was sublimed before use (mp 156 "C). 1,2-Ethanediol (Baker and Adamson, Reagent Grade) was distilled in vacuo; a middle cut was taken [bp 67 "C (4 mmHg)] and stored over molecular sieve (Linde Type 4 A). It was distilled again with a middle cut being taken [bp 66.5-67 "C (4 mmHg)]. 1,5-Pentanediol (Matheson Coleman and Bell, Practical Grade) was vacuum distilled on a 25 x 2 cm Vigreaux column, a middle cut being taken [bp 109-110 "C The Journal of Physical Chemistry, Vol. 80, No. 7,1976
680
H. B. Flora and W. R. Gilkerson
(10 mmHg)]. 1,2,3-Propanetriol (Baker and Adamson, Reagent Grade) was distilled under vacuum, a middle cut being taken [bp 134 OC (4.5 mmHg)]. l,l,l-Tris(hydroxymethy1)ethane (Aldrich Chemical Co.) was dissolved in a minimum of hot THF, filtered, and precipitated with hexane; this was repeated twice. The solid was dried in vacuo, mp 200 "C. Bis(2-methoxyethyl) ether (diglyme, glyme-3, Columbia Organic Chemicals) was refluxed over sodium ribbon for 2 h and distilled using a 30 X 2 cm Vigreaux column, bp 161-162 OC. 1,2-Bis(2-methoxyethoxy)ethane (glyme-4, Eastman Kodak Co.) was distilled in vacuo, a middle fraction being taken [bp 75 OC (3 mmHg)]. Bis[2(2-methoxyethoxy)ethyl] ether (glyme-5, Eastman Kodak Co.) was distilled under reduced pressure, a middle fraction being collected [bp 106 "C (2 mm)]. Conductance measurements were carried out using cells, bridge, and thermostat already d e ~ c r i b e dSpectra .~ were recorded a t ambient temperature (24 OC in the cell compartment) on a Cary Model 14 spectrophotometer using stoppered silica cells (path length 1 cm). All spectra were recorded with solvent in a cell in the reference beam. All weighings of salts were carried out in a nitrogen atmosphere. In a typical conductance experiment, a solution of salt in T H F was prepared by weight. A weighed portion of the solution of salt was placed in the conductance cell and thermostatted. A concentrated solution of ligand in the salt solution was prepared. Weighed increments of this concentrated ligand solution were then added to the salt solution in the conductance cell.
Results Examples of the values of the equivalent conductance, A, obtained as described in the Experimental Section are plotted vs. concentration of added ligand in Figure 1 for lithium, sodium, potassium, rubidium, and tetra-n-butylammonium 2,4-dinitrophenolate (DNP). Note that the salt concentrations for the alkali metal salts are all around 0.13 mM for the solutions in Figure 1. These alkali metal salts of 2,4-dinitrophenol are all highly associated to form ion pairs in the concentration range 0.1-1.0 mM in THF.3 The ion pair dissociation equilibrium may be represented by M+,X-
* M+ + X-
K = [M+][X-]Y*~/[M+,X-](1)
where y* is the mean ionic activity coefficient, taken to be unity. The value of K has been found to be 2.3 X for LiDNP and 350 X for RbDNP in T H F at 25 0C.3 The addition of a ligand L which forms a complex with the cation results in an increased total ion concentration by mass action M+
+ L + M+,L
K1+ = [M+,L]/[M+][L]
(2)
and, thus, an increased conductivity. Similar titrations with TEA were carried out at four different concentrations of each of the alkali metal salts in the range 0.1-0.4 mM; Figure 2 shows these results for LiDNP. If eq 1 and 2 were the only processes occurring in these solutions, then it has been shown6 that the apparent ion pair dissociation constant, K , in the presence of ligand is related to the value, K O ,in the absence of ligand by the equation
+
K = Ko(1 K1+[L])
(3)
Not only are processes 1 and 2 occurring in these systems but triple ion formation and ion pair-ligand complex formation are also occurring. These data were treated as outThe Journal of Physical Chemistry, Vol. BO, No. 7, 1976
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