Reversible phase transformations in a double-walled diamondoid coordination network with a stepped isotherm for methane

Flexible metal−organic materials (FMOMs) with stepped isotherms can offer enhanced working capacity in storage applications such as adsorbed natural gas (ANG) storage. Unfortunately, whereas >1000 FMOMs are known, only a handful exhibit methane uptake of >150 cm3 /cm3 at 65 atm and 298 K, conditions relevant to ANG. Here, we report a double-walled 2-fold interpenetrated diamondoid (dia) network, X-dia-6-Ni, [Ni2L4(μ-H2O)]n, comprising a new azo linker ligand, L− (L− = (E)-3-(pyridin-4-yldiazenyl)- benzoate) and 8-connected dinuclear molecular building blocks. X-dia-6-Ni exhibited gas (CO2, N2, CH4) and liquid (C8 hydrocarbons)-induced reversible transformations between its activated narrow-pore β phase and γ, a large-pore phase with ca. 33% increase in unit cell volume. Single-crystal X-ray diffraction (SCXRD) studies of the as-synthesized phase α, β, and γ revealed that structural transformations were enabled by twisting of the azo moiety and/or deformation of the MBB. Further insight into these transformations was gained from variable temperature powder XRD and in situ variable pressure powder XRD. Low-temperature N2 and CO2 sorption revealed stepped Type F−II isotherms with saturation uptakes of 422 and 401 cm3 /g, respectively. X-dia-6-Ni exhibited uptake of 200 cm3 /cm3 (65 atm, 298 K) and a high CH4 working capacity of 166 cm3 /cm3 (5−65 bar, 298 K, 33 cycles), the third highest value yet reported for an FMOM and the highest value for an FMOM with a Type F−II isotherm