posted on 2024-01-23, 17:11authored byShi-Qiang Wang, Volodymyr Bon, Shaza Darwish, Shao-Min Wang, Qing-Yuan Yang, Zhengtao Xu, Stefan Kaskel, Michael J. Zaworotko
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–IVs 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–IVs 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 C2H4/6 and accompanied by single-step type F–IVs 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.