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

The structures of the host–guest complexes {[{[P(μ‐N t Bu)] 2 (μ‐NH)} 5 ]I} − ⋅[Li(thf) 4 ] + [ 2 ⋅I{Li(thf) 4 }] and [{[P(μ‐N t Bu)] 2 (μ‐NH)} 5 ]⋅HBr⋅THF ( 2 ⋅HBr⋅THF) show that increased distortion of the framework of the pentameric macrocycle [{[P(μ‐N t Bu)] 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‐N t Bu)] 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.