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.