A Nesting Conformation of the 5'-bromo-1' ,3'-xylyl-18-crown-5*tert-butylammonium Hexafluorophosphate Complex; the Correlation of the Structures of Crown Ether Complexes in the Solid State and in Solution

Single-crystal X-ray analysis of the 5'-bromo-l',3'-xyly~-l8-crown-5* tert-butylammonium hexafluorophosphate complex shows that the complex is of the "nesting" type in which the cation and the aryl group are on the same face of the macroring and that the macroring has a (ag'a)(ag-a)lag'a)(ag-a)(ag'a)lag-a) eonformation. Among crown ethers with aryl-sub-units, 1' ,3'-xylyl crown ethers (1) have been studied in great detail. Our continuing interest in these crown ethers stems from structural factors 1 : their preparation by the reaction of 1,3-bis(halomethyl)benzenes with polyethylene qlycols 2 allows for 1' ,3' substitution in the aromatic ring and their 1 H NMR spectra are very useful for studying the thermodynamic and the kinetic stabilities of the crown ether complexes with ammonium salts4. Firstly the benzylic protons of the crown ethers 1 become non-equivalent when the exchange of salt and crown ether becomes slow on the 1 H NMR time scale and the rate of exchange can be determined from line-broadening studies 4 . Secondly the aryl group in 1. shields the alkyl protons in alkylammonium complexes of 1 and the up-field shift of these protons can be used for determining (relative) association constants under conditions of fast exchange 4,s . By comparison of the thermodynamic stabilities of different complexes it was found that the replacement of a CH2CH2 -0-CH2CH2-group in l&crown-6 by a 1',3'-xylyl moiety decreases the thermodynamic stability by a factor of 10 2 Cram et a1.6 in Ka -values4'5'7. estimated this decrease in stability to be 3.19 kcal.mol -1 at 24OC and attributed this difference to the loss of one favourable N+...O interaction in 1. (CH3J3CNH3+Xcomplexes compared with the corresponding 18-crown-6 complex. Because of the up-field chemical shift of the tert-butyl_protons in the‘lH NP~R spectra of the complexes of 1 in chloroform solution Cram et al.' proposed a structure in which