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

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)(NO₃), monoclinic, P2₁/c, a = 9.598(2) Å, b = 23.466(3) Å, c = 23.973(5) Å, β = 93.31(1)°, V = 5390(2) ų, 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, P1̄, a = 11.687(3) Å, b = 12.800(4) Å, c = 17.680(3) Å, α = 75.31(2)°, β = 80.01(2)°, γ = 69.09(2)°, V = 2379.8(10) ų, Z = 2; data for Na(4,4‘-bis-tert-butylbenzo,benzo-21-crown-7)ReO₄·0.5(i-PrOH), monoclinic, P2₁/c, a = 24.300(5) Å, b = 14.066(3) Å, c = 22.676(5) Å, β = 108.06(3)°, V = 7369(3) ų, Z = 8; data for and 4,4‘-bis-tert-butylbenzo,benzo-21-crown-7, monoclinic, P2₁/n, a = 16.427(2) Å, b = 11.3675(9) Å, c = 33.137(3) Å, β = 94.469(8)°, V = 6169.0(10) ų, 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.