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

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Abstract
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.