Photon Factory Activity Report 2022 #40 (2023)
BL-5A/2021G046

Long time CO2 storage under ambient condition in isolated voids of a porous
coordination network facilitated by the “magic door” mechanism
Terumasa Shimada,1 Pavel M. Usov1, *, Yuki Wada1, Hiroyoshi Ohtsu1, Takaya Matsumoto1,2, *, andMasaki Kawano1, *1
Department of Chemistry, School of Science, Tokyo Institute of Technology,
2-12-1, Ookayama, Meguro-ku, Tokyo 152-8550, Japan.
2
Central Technical Research Laboratory, ENEOS Corporation,
8 Chidoricho, Naka-ku, Yokohama, Kanagawa, 231-0815, Japan.
Despite the lack of accessibility, CO2 was selectively adsorbed into these pores at 298 K and then retained for more than
one week while exposed to atmosphere. CO2 entered the isolated voids through transient channels, termed “magic doors”,
which can momentarily appear within the structure. Once inside the voids, CO2 remained locked in limiting its escape.
This mechanism was facilitated by the flexibility of organic ligands and the pivot motion of cluster units. The uncovered
CO2 sorption and retention ability paves the way for the design of sorbents based on isolated voids.

1 Introduction
To achieve a carbon-neutral society, there is an urgent
need to develop materials for CO2 capture and storage.
Porous coordination networks (PCNs) have attracted
attention as promising candidates because of their readily
modifiable structures and tunable pore environments.
Although there has been a considerable progress in
improving CO2 uptake capacities and selectivity, thepossibility of its long-term storage inside PCNs underambient conditions has received comparatively lessattention since the interactions.
2 Experiment
A ligand (L) based on a pseudo-tetrahedral bimesityl
skeleton decorated with four pyrimidine groups wassynthesized by Suzuki-Miyaura coupling reaction (Fig 1a).
Reaction of L with CuI in the presence of KI and PPh3 in
the CH3CN/H2O/EtOH solvent mixture yielded two
distinct crystal morphologies, yellow plates and prisms.
The prisms were {[(Cu4I4)(L)]·solvent}n (1), with a three
dimensional interpenetrated structure (network 1). The
pure 1 could be obtained by the density separation method.

Fig 1. (a) Single crystal structure of L. (b) The structure ofnetwork 1, showing the voids outlined in dark yellow. C –grey, N – blue, Cu – orange, and I – purple, hydrogen atoms
were omitted for clarity.
3 Results and Discussion
1 possessed closed pores with dimensions of 5.2 × 4.9 ×
4.9 Å. However, the size of maximum aperture between
each pore was only 2.4 × 1.5 Å, which was too small forany molecule to pass through. Nevertheless, acetonitrile
molecules were present in the pores of the as-synthesized
crystal of 1, which could be removed by heating at 473 K
for 12 h under vacuum without loss of crystallinity,

suggesting that some diffusion pathways could exist for theguest escape to occur. This fact prompted us to investigate
the possibility of encapsulating different gasses into theactivated crystals of 1 (activated 1). First, N2 isotherms
measured at 298 K showed no uptake. In contrast, the CO2
adsorption isotherm showed hysteresis between adsorption
and desorption branches, suggesting some degree of guest
trapping. To test this hypothesis, IR spectroscopy
measurement was performed. The IR spectrum of CO2@1
showed a strong absorption band at 2340 cm-1, indicating
the presence of physiosorbed CO2. CO2@1 were left in theair while monitoring changes in the IR peak over time.
Remarkably, the IR signal was still present after 1 week,
only gradually decreasing in intensity (Figure 2b). To
elucidate the structural changes accompanying the CO2
adsorption and visualize the encapsulated gas molecules,
SCXRD analysis was performed.

Fig 2. (a) Adsorption and desorption isotherms of N2 andCO2 for activated 1 measured at 298 K. (b) Timedependent IR spectra of CO2@1 left in the air, (c) crystal
structure of CO2@1 (d) CO2 orientation inside the pore.
Rearch Achievements
The crystallographic Society of Japan poster award in 2021
* mkawano@chem.titech.ac.jp