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Kinetics and Equilibria of Cryptate Formation In Propylene Carbonate B. G. Cox,' J. Garcla-Rosas, and H. Schnelder" Department of Chemistty, University of Stirling, Stiriing, FK9 4LA, Scotland, and the Max-Planck-Institut fur blophys Chemie, 0-3400 Giiffingen-Nikoiausberg, West Germany (Received: March 14, 1980; In Final Form: July 22, 1980)
Stability constants and dissociation rates of alkali metal cryptates have been measured in propylene carbonate, and the corresponding formation rates obtained from a combination of the two. Qualitatively, the variation in stability constants with cation size and ligand is similar to that in water, but the complexes are up to lo7 times more stable in propylene carbonate. It is shown that for a given ligand the free energy transfer of the cryptates from water to propylene carbonate varies systematically with the cation enclosed and tends to be at a minimum for the most stable cryptate. The higher stability constants in propylene carbonate relative to water are a result of both smaller dissociation rates and, to a lesser extent, larger formation rates. Within propylene carbonate, the cryptand selectivity is reflected almost entirely in the dissociation rates. The dissociation rates are much more susceptible to acid catalysis in propylene carbonate than in water.
Introduction Selective complexation, extraction, and transportation of alkali metal ions by naturally occurring and synthetic macrocyclic ligands have raised many fundamental questions concerning the origin of the selectivity, and the rates and mechanisms of cation exchange processes involving such ligands.' An important factor in the formation of complexes of this type in particular must be the involvement of the solvent. Thus variation of the solvent might be expected to influence the overall stability, the selectivity pattern, and the kinetics of complex formation including, for example, the susceptibilities of the complexation processes to catalysis, etc. To the extent that the cation is held within the ligand cavity and shielded from the solvent, the free energies of the complexes may be much less sensitive to solvent variation than that of the uncomplexed cations. In the present paper we report the results of a study of the rate and equilibria of complex formation involving the macrobicyclic diazapolyether ligands 1-3 (cryptands, Cry)2
~ J O Q L o ,fd k N\/J?O)*N
0.J
1, a = 1 , b = c = 0 (211) 2,a=b=l,c=0(221) 3,a=b=c=1(222)
in propylene carbonate (PC). The results are compared with earlier results obtained in ~ a t e r and ~ - methanol9 ~ as solvents. Propylene carbonate (4-methyl-l,3-dioxolan-2-one) is a dipolar aprotic solvent with high dielectric constant (65 0022-3654/80/2084-3178$01.OO/O
at 25 OC)l0 and dipole moment (4.94 D).'l Unlike water and methanol, it does not exhibit any strong self-association or well-defined intramolecular structure. However, it is possible to dissolve salts in PC up to quite high concentrations, and most salts are completely dissociated in dilute solutions. An additional advantage of PC is that a considerable amount of thermodynamic data on electrolyte solutions is available from electrochemi~al~~-~~ and solubility studies.I5-l7 It is known from electrochemical studiesl6 that alkali metal cryptates are quite stable in PC, but quantitative data, other than that for Cs" complexes, are not available. Cesium-133 NMR studies of rates and equilibria of Cs" complexes in PC have been reported by Popov and coworker~.~~ Experimental and Results Materials. Cryptands 211,221,and 222 were purchased from Merck and used without further purification. Their purities have been checked by pH-metric, conductimetric, and NMR methods. Propylene carbonate wa8 dried over CaS04and distilled several times at reduced pressure under an atmosphere of Nz,through a column packed with Raschig rings. Ita purity was checked by running a background cyclic polarogram. High purity was necessary to prevent silver solutions from turning brown. Inorganic salta &C1Od, LiCIOd,NaCIOd,KClOd, RbNOs, and CiN03 were &ied under vacuum before use. Anhydrous solutions of Ca(N03)2in PC were prepared by drying solutions of the hydrated salt over molecular sieves (BDH, Type 4A). Tetraethylammonium perchlorate (TEAP)was prepared by reacting TEA hydroxide (Aldrich Chemical Co., 20% 0 1980 American Chemical Society
Cryptate Formation in Propylene Carbonate
The Journal of Physical Chemistry, Vol. 84, No. 24, 1980 3179
TABLE I : Stability Constants of Metal Cryptates in Propylene Carbonate at 25 " C ( K , in mol-' dm3) log KSa with cation
ligand
MCry+ + 2HA
Lit
Na+ K+ Rb+ Cs' Ag+ Ca2+ 211 12.4, 8.7, 3.3,
