Selection of a Pentameric Host in the Host–Guest Complexes {[{[P(μ‐NtBu)]2(μ‐NH)}5]⋅I}−[Li(thf)4]+ and [{[P(μ‐NtBu)]2(μ‐NH)}5]⋅HBr⋅THF

The structures of the host-guest complexes {[{[P(μ-NtBu)] 2 (μ-NH)} 5 ]I} - .[Li(thf) 4 ] + [2.I{Li(thf) 4 }] and [{[P(μ-NtBu)] 2 (μ-NH)} 5 ].HBr.THF (2.HBr.THF) show that increased distortion of the framework of the pentameric macrocycle [{[P(μ-NtBu)] 2 (μ-NH)} 5 ] (2) occurs with the larger halide ions. Theoretical studies show that the thermodynamic stabilities of the model host-guest anions [2.X] - (X=Cl, Br, I) are in the order Cl - Br - >I - , that is, the reverse of the templating trend observed experimentally. These studies support the view that the selection of the pentamer 2 over the tetramer [([P(μ-NtBu)] 2 (μ-NH)} 4 ] (1) is kinetically controlled, a conclusion which is also consistent with the previous observation that the frameworks of 1 and 2 are not in dynamic equilibrium with each other.