Structure and conformational landscape of an unprecedented acyclic pentakis(cyclodiphosphazane)

During the condensation reaction of [ClP(μ-N t Bu)] 2 and [H 2 NP(μ-N t Bu)] 2 , which generates established inorganic macrocycles {[P(μ-N t Bu)] 2 (μ-NH)} 4 ( 1 ) and {[P(μ-N t Bu)] 2 (μ-NH)} 5 ( 2 ), the acyclic cyclophosph(III)azane ‘pentamer’ {( t BuNH)[{[P(μ-N t Bu)] 2 } 5 (μ-NH) 4 ](NH t Bu)} ( 3 ) was isolated as a minor product. Compound 3 comprises an unprecedented, extended [P 2 (μ-N t Bu) 2 ] 5 (open-chain) core of phosphorus‒nitrogen rings, which provides mechanistic evidence for the elusive intermediates formed during macrocyclisation to 1 and 2 . The solid-state structure of the n -pentane solvate of 3 features a folded chain conformation, arranged periodically in the lattice between cross-linked hydrophobic channels. An extensive theoretical study on the conformational landscape of 3 was performed, exploring its potential application as a supramolecular host. The presence of a large number of N‒H H-bond donors allows thermodynamically favourable halide binding in high-energy conformers of 3 which ‘wrap’ around the guests. Nonetheless, the large pre-organisation cost compares unfavourably to the rigid macrocyclic host 2 . The nature of non-covalent interactions in host‒guest complexes of 2 and 3 was examined computationally, offering insight into the effective design of future main-group supramolecular receptors.