Insight into the Gas-Induced Phase Transformations in a 2D Switching Coordination Network via Coincident Gas Sorption and PXRD.

Switching coordination networks (CNs) that reversibly transform between narrow or closed pore (cp) and large pore (lp) phases, though fewer than their rigid counterparts, offer opportunities for sorption-related applications. However, their structural transformations and switching mechanisms remain underexplored at the molecular level. In this study, we conducted a systematic investigation into a 2D switching CN, [Ni(bpy)(NCS)], sql-1-Ni-NCS (1 = bpy = 4,4'-bipyridine), using coincident gas sorption and powder X-ray diffraction (PXRD) under low-temperature conditions. Gas adsorption measurements revealed that CH (169 K) and CH (185 K) exhibited single-step type F-IV sorption isotherms with sorption uptakes of around 180-185 cm g, equivalent to four sorbate molecules per formula unit. Furthermore, parallel PXRD experiments provided insight into sorbate-dependent phase switching during the sorption process. Specifically, CO sorption induced single-step phase switching (path I) solely between cp and lp phases consistent with the observed single-step type F-IV sorption isotherm. By contrast, intermediate pore (ip) phases emerged during CH and CH desorption as well as CH adsorption, although they remained undetectable in the sorption isotherms. To our knowledge, such a cp-lp-ip-cp transformation (path II) induced by CH and accompanied by single-step type F-IV sorption isotherms represents a novel type of phase transition mechanism in switching CNs. By virtue of Rietveld refinements and molecular simulations, we elucidated that the phase transformations are governed by cooperative local and global structural changes involving NCS ligand reorientation, bpy ligand twist and rotation, cavity edge (Ni-bpy-Ni) deformation, and interlayer expansion and sliding.