Effect of Polymorphism on the Sorption Properties of a Flexible Square-Lattice Topology Coordination Network

The stimulus-responsive behavior of coordination networks
(CNs),
which switch between closed (nonporous) and open (porous) phases, is
of interest because of its potential utility in gas storage and
separation. Herein, we report two polymorphs of a new square-lattice
(sql) topology CN, X-sql-1-Cu, of formula
[Cu(Imibz)2]n (HImibz = {[4-(1H-imidazol-1-yl)phenylimino]methyl}benzoic acid), isolated
from the as-synthesized CN X-sql-1-Cu-(MeOH)2·2MeOH, which subsequently transformed
to a narrow pore solvate, X-sql-1-Cu-A·MeOH, upon
mild activation (drying in air or heating at 333 K under nitrogen). X-sql-1-Cu-A·MeOH contains MeOH in cavities, which was
removed through exposure to vacuum for 2 h, yielding the nonporous
(closed) phase X-sql-1-Cu-A. In contrast, a more dense
polymorph, X-sql-1-Cu-B, was obtained by exposing X-sql-1-Cu-(MeOH)2·2MeOH directly to vacuum for 2 h. Gas sorption studies conducted on X-sql-1-Cu-A and X-sql-1-Cu-B revealed different
switching behaviors to two open phases (X-sql-1-Cu·CO2 and X-sql-1-Cu·C2H2), with
different gate-opening threshold pressures for CO2 at 195
K and C2H2 at 278 K. Coincident CO2 sorption and in situ powder X-ray diffraction studies
at 195 K revealed that X-sql-1-Cu-A transformed to X-sql-1-Cu-B after the first sorption cycle and that the CO2-induced switching transformation was thereafter reversible.
The results presented herein provide insights into the relationship
between two polymorphs of a CN and the effect of polymorphism upon
gas sorption properties. To the best of our knowledge, whereas sql networks such as X-sql-1-Cu are widely studied
in terms of their structural and sorption properties, this study represents
only the second example of an in-depth study of the sorption properties
of polymorphic sql networks.

配位网络（coordination networks, CNs）在闭合（无孔）相与开放（多孔）相之间切换的刺激响应行为，因其在气体存储与分离领域的潜在应用价值而受到广泛关注。本文报道了一种新型方格点阵（square-lattice, sql）拓扑结构配位网络的两种多晶型物：X-sql-1-Cu，其分子式为[Cu(Imibz)₂]ₙ（配体HImibz = {[4-(1H-咪唑-1-基)苯基亚氨基]甲基}苯甲酸）。该样品从初始合成的配位网络X-sql-1-Cu-(MeOH)₂·2MeOH中分离得到，经温和活化（空气中干燥或333 K下氮气氛围加热）后，可转变为窄孔溶剂化物X-sql-1-Cu-A·MeOH。X-sql-1-Cu-A·MeOH的空腔中含有甲醇（MeOH），经2小时真空脱除溶剂后，得到无孔（闭合）相X-sql-1-Cu-A。与之相反，将X-sql-1-Cu-(MeOH)₂·2MeOH直接置于真空环境中2小时，可得到更为致密的多晶型物X-sql-1-Cu-B。针对X-sql-1-Cu-A与X-sql-1-Cu-B开展的气体吸附研究显示，二者可分别转变为两种开放相（X-sql-1-Cu·CO₂与X-sql-1-Cu·C₂H₂），且在195 K下的CO₂吸附以及278 K下的C₂H₂吸附过程中展现出不同的开门阈值压力。195 K下同步开展的CO₂吸附与原位粉末X射线衍射研究表明，X-sql-1-Cu-A在首次吸附循环后会转变为X-sql-1-Cu-B，此后CO₂诱导的相转变过程具备可逆性。本文所呈现的研究结果，为配位网络的两种多晶型物之间的构效关系，以及多晶型性对气体吸附性能的影响提供了新的认知。据我们所知，尽管诸如X-sql-1-Cu这类sql拓扑网络的结构与吸附性能已被广泛研究，但本研究仅为第二项针对多晶型sql拓扑网络吸附性能的深入探究案例。