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 cm 3 /cm 3 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, [Ni 2 L 4 (μ-H 2 O)] 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 (CO 2 , N 2 , CH 4 ) 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 N 2 and CO 2 sorption revealed stepped Type F-II isotherms with saturation uptakes of 422 and 401 cm 3 /g, respectively. X-dia-6-Ni exhibited uptake of 200 cm 3 /cm 3 (65 atm, 298 K) and a high CH 4 working capacity of 166 cm 3 /cm 3 (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.