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.