Structural Aspects of Rubidium Ion Selectivity by Tribenzo-21-crown-7(1)(a)

The structural origins of the selectivity of rubidium ion over other alkali metal ions by tribenzo-21-crown-7 is investigated from single-crystal X-ray diffraction: data for Cs(tribenzo-21-crown-7)(NO(3)), monoclinic, P2(1)/c, a = 9.598(2) Å, b = 23.466(3) Å, c = 23.973(5) Å, beta = 93.31(1) degrees, V = 5390(2) Å(3), Z = 8; data for [Rb(4,4'-bis-tert-butylbenzo,benzo-21-crown-7)(dioxane)(1.5)(H(2)O)(0.18)][Cl].(dioxane)1.82(H(2)O), triclinic, P&onemacr;, a = 11.687(3) Å, b = 12.800(4) Å, c = 17.680(3) Å, alpha = 75.31(2) degrees, beta = 80.01(2) degrees, gamma = 69.09(2) degrees, V = 2379.8(10) Å(3), Z = 2; data for Na(4,4'-bis-tert-butylbenzo,benzo-21-crown-7)ReO(4).0.5(i-PrOH), monoclinic, P2(1)/c, a = 24.300(5) Å, b = 14.066(3) Å, c = 22.676(5) Å, beta = 108.06(3) degrees, V = 7369(3) Å(3), Z = 8; data for and 4,4'-bis-tert-butylbenzo,benzo-21-crown-7, monoclinic, P2(1)/n, a = 16.427(2) Å, b = 11.3675(9) Å, c = 33.137(3) Å, beta = 94.469(8) degrees, V = 6169.0(10) Å(3), Z = 8. The structures reported 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 observed for each structure. Molecular mechanics calculations were performed on all conformers, and the results are related to the observed extraction selectivity for rubidium.