Solvatomorphism and adsorption properties of a cyclic hexapeptoid

The development of self-assembled solid-state supramolecular architectures with nanodimensioned pores or cavities is an intensively active research area, aiming at the preparation of new materials with potential applications in nanotechnology such as molecular sieves, sensors, and gas-storage devices. Apart from metal-organic frameworks, covalent-organic frameworks and zeolites, recently porous molecular materials attracted considerable interest and cyclic peptoids revealed as promising building block for engineering porous molecular solids. Peptoids differ from peptides in the side chains, which are shifted by one position along the peptide backbone to the nitrogen atom to give N-substituted oligoglycine. The lack of the amide proton prevents the formation of NH···OC hydrogen bonds and weaker interactions, as CH···OC hydrogen bonds and CH-pi interactions, play a key role. Moreover, cyclic peptoids are very versatile building blocks for the assembly of solid state supramolecular architectures, because the desired specific interactions may be provided by a variety of functional side groups attached to a sizeable scaffold. Recently, we observed a rather peculiar solid state dynamic behaviour for the compound cyclo-(Nme-Npa2)2 (1) during acetonitrile release and uptake. In detail, it undergoes a reversible single-crystal-to-single-crystal transformation by tilting two vertical propargyl side chains by 113° and forming an unprecedented CH-pi zipper that links together the peptoid columns in the desolvated crystal form. Our efforts to characterize the chemical and physical properties of this compound lead to the discovery of a complex solvatomorphic behaviour. In particular, we obtained a new 1D porous framework and confirmed its stability by in-situ variable temperature single crystal X-ray diffraction experiments. We also demonstrated by in-situ HR X-ray powder diffraction measurements at the ESRF beamline ID22 that the crystal form 1F is able to adsorb of propyne gas. Here we will report on the observed guest release and uptake mechanisms for compound 1 in its possible crystal forms.