Pressure-Induced Structural Effects in the Square Lattice (sql) Topology Coordination Network Sql-1-Co-NCSmiddot4OX

A high-pressure study of a switching coordination network of square lattice topology (sql) loaded with o-xylene (OX), [Co(4,4 '-bipyridine)2(NCS)2]n center dot 4nC8H10 (sql-1-CoNCS center dot 4OX), was conducted up to approximately 1 GPa to investigate pressure-induced structural changes. Previous reports revealed that sql-1-Co-NCS exhibits multiple phases thanks to its ability to switch between closed (nonporous) and several open (porous) phases in the presence of various gases, vapors, and liquids. Networks of such properties are of topical interest because they can offer high working capacity and improved recyclability for gas adsorption. The monoclinic crystal structure of sql-1-Co-NCS center dot 4OX at 100 K was previously reported to show an increase in interlayer separation of more than 100% compared to the corresponding closed phase, sql-1-Co-NCS, when exposed to gases or vapors under ambient conditions. Herein, a tetragonal crystal form of sql-1-Co-NCS center dot 4OX (space group I4/mmm, Phase I) that exists at 0.1 MPa/303 K is reported. Exposure of Phase Ito high pressure using penetrable pressure transmitting media (OX and 1:1 vol MeOH/EtOH) did not result in further separation of the sql networks. Rather, compression of the crystals and release of adsorbed OX molecules occurred. These pressure-induced changes are discussed in terms of structural voids, framework conformation, and molecular packing of the sql layers. Although Phase I retained tetragonal symmetry throughout the investigated pressure range, the interlayer voids occupied by OX molecules were significantly reduced between 0.3 and 0.5 GPa; further compression above 0.5 GPa induced structural disorder. Additionally, analysis of the electron count present in the pores of sql-1-Co-NCS confirmed the multistep evacuation of OX molecules from the crystal, and two intermediate phases, Ia and Ib, differing in the OX loading level, are postulated.

Automated summary

This study investigates the structural changes of a square lattice coordination network under high pressure. The results show that the interlayer separation of the network does not increase under high pressure, but the crystals are compressed and the adsorbed molecules are released. The changes induced by pressure are discussed in terms of structural voids, framework conformation, and molecular packing. Additionally, analysis of the electron count in the pores confirms the multistep evacuation of the adsorbed molecules and suggests the existence of intermediate phases.