A Convenient Strategy for Designing a Soft Nanospace: An Atomic Exchange in a Ligand with Isostructural Frameworks

Direct observation
of gas molecules confined in the nanospace of
porous materials by single-crystal X-ray diffraction (SXRD) technique
is significant because it leads to deep insight into the adsorption
mechanism and the actual state of the adsorbents in molecular level.
A recent study revealed that flexibility is one of the important factors
to achieve periodic guest accommodation in the nanospace enabling
direct observation of gas molecules. Here, we report a convenient
strategy to tune the framework flexibility by just an atomic exchange
in a ligand, which enables us to easily construct a soft nanospace
as the best platform to study gas adsorption. Indeed, we succeeded
to observe C2H2 and CO2 molecules
confined in the pores of a flexible porous coordination polymer (PCP-N) in different configurations using SXRD measurement,
whereas gas molecules in a rigid framework (PCP-C) isostructural
to PCP-N were not seen crystallographically. The result
of the coincident in situ powder X-ray diffraction and adsorption
measurement for PCP-N unambiguously showed that the framework
could flexibly transform to trap gas molecules with a commensurate
fashion. In addition, for PCP-N, we found that the adsorbed
gas molecules induced significant structural change involving dimensional
change of the pore from one-dimensional to three-dimensional, and
subsequently, additional gas molecules formed periodic molecular clusters
in the nanospace.