6760
Complexes of Macrocyclic Polyethers and Ion Pairs U. Takaki, T. E. Hogen Esch, and J. Smid*
Contribution from the Chemistry Department, College of Forestry, State University of New York, Syracuse, New York 13210. Received March 29, 1971 Abstract: The complexation of 4’-methylbenzo-l5-crown-5(C5), 4’-methylbenzo-l8-crown-6 (C6), and the two isomers of dicyclohexyl-18-crown-6(DC6) to the ion pairs of fluorenylsodium and -potassium was studied in tetrahydrofuran and tetrahydropyran by optical spectroscopy. The binding of a C5 molecule to F-,Na+ was shown to produce a mixture of crown-complexed contact ion pairs and crown-separated ion pairs, the proportion of the latter species being higher in more polar solvents. The same complexation mechanism was found to describe the interaction of C6 and DC6 with the F-,K+ contact ion pair. Interaction of C5 with F-,K+ produced separated ion pairs with two C5 molecules bound to the potassium salt. The stability of the respective ion-pair complexes was found to be a sensitivefunction of the temperature, solvent, size of the cation, and structure of the crown compound, especially the size of the polyether ring relative to that of the cation diameter.
R
ecent investigations on macrocyclic polyethers to study the exchange process between complexed and have generated considerable interest in the free crown species. cation-binding properties of these The In the present work we report in detail the results of so-called crown compounds considerably increase the studies of the interaction of fluorenylsodium and solubility of inorganic salts in nonpolar media’ and fluorenylpotassium ion pairs with two crown comcan form crystalline stoichiometric complexes with a pounds, Le., 4’-methylbenzo-l5-crown-5(I) and 4’variety of salts. 1 , 2 When added to ethereal solutions methylbenzo- 18-crown-6 (11) (referred to in Pedersen’s of alkali or alkaline earth salts of carbanions, the work as compounds IV and X, with methyl being the crown ethers often modify the structure of the respective substituent in the aromatic ring). We specifically carbanion pairs. Potentiometric and calorimetric measurements prove the existence of stable crown complexes of free alkali ions in water and methanol.1~4~5 The macrocyclic polyethers are also found to strongly affect the reaction rate and stereospecific course of reactions involving ion pairs.6 Of particular significance are recent studies by Eisenman, et ai., and others: who established that crown compounds can exert specific effects on cation transport across biological membranes. This property makes them useful as model compounds of neutral ion carriers and resembles the behavior of the more complex macrocyclic antiI biotics monactin and valinomycin which are known to make phospholipid bilayer membranes selectively permeable to cations. We showed recently that complexation of certain crown compounds to fluorenyl salts in ethereal media produces crown-separated ion pairs. The difference 0 o m in optical spectra between contact and separated fluorenyl ion pairs was used as an analytical tool to I establish the complexation order of dimethyldibenzo18-crown-6 with respect to the different alkali fluorenyl ion pairs. Also, the strong upfield nuclear magnetic resonance shifts of the protons of the crown compound looked for evidence for the presence of crown-comwhen complexed with the fluorenyl ion pair allowed us plexed contact ion pairs, in addition to crown-separated ( I ) (a) C. J. Pedersen, J . Amer. Chem. SOC.,89, 7017 (1967); (b) ion pairs, and also were concerned with the possible ibid., 92, 386, 391 (1970). existence of 2 : 1 crown-ion pair complexes in the (2) (a) D. Bright and M. R. Truter, Nature (London), 225, 177 (1970); ethereal fluorenyl salt solutions, Species of a similar ( b ) J . Chem. SOC.,1545 (1970). (3) I
