Coordination Network That Reversibly Switches between Two Nonporous Polymorphs and a High Surface Area Porous Phase

We report a 2-fold interpenetrated
primitive cubic (pcu) network X-pcu-5-Zn, [Zn2­(DMTDC)2­(dpe)] (H2DMTDC = 3,4-di­meth­yl­thieno­[2,3-b]­thio­phene-2,5-di­carb­oxylic acid,
dpe = 1,2-di­(4-pyridyl)­ethylene), that exhibits reversible switching
between an as-synthesized “open” phase, X-pcu-5-Zn-α, and two nonporous or “closed” polymorphs, X-pcu-5-Zn-β and X-pcu-5-Zn-γ. There are two unusual features
of X-pcu-5-Zn. The first relates to its sorption properties,
which reveal that the α form exhibits high CO2 uptake (ca. 255 cm3/g at 195 K) via reversible
closed-to-open switching (type F-IV isotherm) of the type desirable
for gas and vapor storage; there are only three other reports of porous
materials that combine these two features. Second, we could only isolate
the β form by activation of the CO2 loaded α form and it persists through multiple CO2 adsorption/desorption cycles. We are unaware of a new polymorph
having been isolated in such a manner. That the observed phase changes
of X-pcu-5-Zn-α occur in single-crystal-to-single-crystal
fashion enabled structural characterization of the three forms; γ is a coordination isomer of α and β, both of which are based upon “paddlewheel”
clusters.