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[(mu-NtBu)](2)(mu-NH)}(5)]I}-(.)[Li(thf)(4)](+) [2(.)I{Li(thf)(4)}] and [{[P(mu-NtBu)](2)(mu-NH)}(5)](HBrTHF)-H-.-T-. (2(.)HBr(.)THF) show that increased distortion of the framework of the pentameric macrocycle [{[P(mu-NtBu)](2)(mu- 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- approximate to 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(mu-NtBu)](2)(mu-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.