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.