Thermally Robust 3‑D Co-DpyDtolP-MOF with Hexagonally Oriented Micropores: Formation of Polyiodine Chains in a MOF Single Crystal

A new
porphyrin-based Co-MOF, [Co­(DpyDtolP)]6·12H2O (I), composed of DpyDtolP (5,15-di­(4-pyridyl)-10,20-di­(4-methylphenyl)­porphyrin)
was prepared in a high yield and structurally characterized by X-ray
crystallography. DpyDtolP is a ditopic N-donor ligand with a large
space or gap between the two pyridyl groups at the 5- and 15-positions
of the porphyrin backbone. Unlike the pyridyl groups, the 4-tolyl
groups in DpyDtolP could not be involved in coordination toward the
metal ion. Nevertheless, the presence of these two 4-tolyl groups
led to a new infinite three-dimensional framework: Co-MOF (I) with exceptionally high thermal stability at elevated temperature.
The single crystals of I maintained their crystallinity
even after vacuum drying at 250 °C. The resulting dried single
crystals diffracted X-ray to give the same structural solution as
the as-prepared crystals. The robust framework of I contained
micropores that were periodically arranged in a hexagonal symmetry.
While the evacuated I moderately sorbed N2 at 77 K, it sorbed 142.8 cm3 g–1 (6.37
mmol g–1) of CO2 at 196 K. The CO2 sorption isotherms exhibited a very clear step in both the
adsorption and desorption branches. A slight hysteretic behavior was
observed between the two branches. Furthermore, the crystal structure
of CO2-captured I (I_CO2) revealed that the linear arrangement of the CO2 molecules occupied the inside of micropores, thereby indicating
the effective CO2 capture by evacuated I.
The evacuated I was also found to be ideal for the encapsulation
of iodine molecules in cyclohexane to provide iodine-captured I (I_I2), which was also
characterized by X-ray crystallography. The linear arrangement of
polyiodine chains in the micropores was observed, and a single crystal
of I_I2 exhibited electrically
conducting behavior. The encapsulation amount of iodine was dependent
on the crystal sizes of I. Additionally, the separately
prepared microscale sample, micro-I, with a much reduced
particle dimension than the bulk I exhibited an enhanced
uptake of iodine under the same conditions.