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

Exceptionally strong (OC-)(2)N-I center dot center dot center dot N halogen bonding (XB) in a combination with C=O center dot center dot center dot 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]center dot[NIS](4) complexes where the observed I center dot center dot center dot N distances, ranging from 2.486 to 2.586 angstrom, were remarkable shorter (from 29.6 to 26.7%) than the sum of the vdW radii of nitrogen and iodine atoms. Multiple C=O center dot center dot center dot H-C HBs interconnected the [HMTA]center dot[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 angstrom(3)) 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 (P4(3)2(1)2, V = 4642 angstrom(3) to P4(2)/nmc, V = 2324 angstrom(3)) with halving the unit cell volume.