STRUCTURAL ASPECTS OF RUBIDIUM ION SELECTIVITY BY TRIBENZO-21-CROWN-7

The structural origins of the selectivity of rubidium ion over other a lkali metal ions by tribenzo-21-crown-7 is investigated from single-cr ystal X-ray diffraction: data for Cs(tribenzo-21-crown-7)(NO3), monocl inic, P2(1)/c, a = 9.598(2) Angstrom, b = 23.466(3) Angstrom, c = 23.9 73(5) Angstrom, beta = 93.31(1)degrees, V = 5390(2) Angstrom(3), Z = 8 ; data for [Rb(4,4'-bis-tert-butylbenzo,benzo-21-crown-7) (dioxane)(1. 5)(H2O)(0.18)][Cl] . (dioxane)1.82(H2O), triclinic, P (1) over bar, a = 11.687(3) Angstrom, b = 12.800(4) Angstrom, c = 17.680(3) Angstrom, alpha = 75.31(2)degrees, beta = 80.01(2)degrees, gamma = 69.09(2)degre es, V = 2379.8(10) Angstrom(3), Z = 2; data for Na(4,4'-bis-tert-butyl benzo,benzo-21-crown-7) ReO4 . 0.5(i-PrOH), monoclinic, P2(1)/c, a = 2 4.300(5) Angstrom, b = 14.066(3) Angstrom, c = 22.676(5) Angstrom, bet a = 108.06(3)degrees, V = 7369(3) Angstrom(3), Z = 8; data for and 4,4 '-bis-tert-butylbenzo,benzo-21-crown-7, monoclinic, P2(1)/n, a = 16.42 7(2) Angstrom, b = 11.3675(9) Angstrom, c = 33.137(3) Angstrom, beta = 94.469(8)degrees, V = 6169.0(10) Angstrom(3), Z = 8. The structures r eported here are the first reported for a tribenzo-21-crown-7, and the alkali metal ion complexes are the first reported structures of these ions with any 21-crown-7 ether. Different crown conformations are obs erved for each structure. Molecular mechanics calculations were perfor med on all conformers, and the results are related to the observed ext raction selectivity for rubidium.