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

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)₂(NCS)₂]_n, sql-1-Ni-NCS (1 = bpy = 4,4'-bipyridine), using coincident gas sorption and in situ powder X-ray diffraction (PXRD) under low-temperature conditions. Gas adsorption measurements revealed that C₂H₄ (169 K) and C₂H₆ (185 K) exhibited single-step type F-IV^s sorption isotherms with sorption uptakes of around 180-185 cm³ g^-1, equivalent to four sorbate molecules per formula unit. Furthermore, parallel in situ PXRD experiments provided insight into sorbate-dependent phase switching during the sorption process. Specifically, CO₂ sorption induced single-step phase switching (path I) solely between cp and lp phases consistent with the observed single-step type F-IV^s sorption isotherm. By contrast, intermediate pore (ip) phases emerged during C₂H₄ and C₂H₆ desorption as well as C₃H₆ adsorption, although they remained undetectable in the sorption isotherms. To our knowledge, such a cp-lp-ip-cp transformation (path II) induced by C₂H_4/6 and accompanied by single-step type F-IV^s 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.