Breathing molecular crystals: halogen- and hydrogen-bonded porous molecular crystals with solvent induced adaptation of the nanosized channels

Abstract

Exceptionally strong (OC–)₂N–I⋯N halogen bonding (XB) in a combination with CO⋯H–C hydrogen bonds (HB) between N-iodosuccinimide (NIS) and hexamethylenetetramine (HMTA) yielded a series of molecular crystals possessing large 1D channels. In each structure, HMTA was tetra-coordinated by four NIS molecules resulting in robust [HMTA]·[NIS]₄ complexes where the observed I⋯N distances, ranging from 2.486 to 2.586 Å, were remarkable shorter (from 29.6 to 26.7%) than the sum of the vdW radii of nitrogen and iodine atoms. Multiple CO⋯H–C HBs interconnected the [HMTA]·[NIS]₄ complexes into the structures with flexible “breathing” host-channels. Three different host-channel structures, either oval or cylindrically shaped with volumes of about 20, 28 or 38% (700–1800 ų) of the unit cell volume, were obtained to match the selected guest. This adaptability was taken further by guest molecule exchange experiments, where CH₂Cl₂ were exchanged with CCl₄ in solution and from gas-to-solid reaction. Both experiments lead to the same single-crystal to single-crystal transformation (P4₃2₁2, V = 4642 ų to P4₂/nmc, V = 2324 ų) with halving the unit cell volume.