Water-Stable In(III)-Based Metal–Organic Frameworks with Rod-Shaped Secondary Building Units: Single-Crystal to Single-Crystal Transformation and Selective Sorption of C₂H₂ over CO₂ and CH₄

Three new water-stable In(III)-based metal–organic frameworks, namely, [In 3 (TTTA) 2 (OH) 3 (H 2 O)]·(DMA) 3 ( BUT-70, DMA = N, N -dimethylacetamide), [In 3 (TTTA) 2 (CH 3 O) 3 ] ( BUT-70A ), and [In 3 (TTTA) 2 (OH) 3 ] ( BUT-70B ), with rod-shaped secondary building units (SBUs) and an new acrylate-based ligand, (2 E,2′ E,2″ E )-3,3′,3″-(2,4,6-trimethylbenzene-1,3,5-triyl)-triacrylate (TTTA 3– ) were obtained and structurally characterized. BUT-70A and -70B were generated in a single-crystal to single-crystal transformation fashion from BUT-70 through guest exchange followed by their removal. The solvents used for guest exchange were methanol and dichloromethane, respectively. Single-crystal structure analyses show that the guest exchange and removal process is accompanied by the substitution of coordinated water molecules of In(III) centers with uncoordinated carboxylate O atoms of TTTA 3– ligands. Moreover, hydroxyl groups bridging two In(III) centers are also replaced by methoxyl groups in the transformation from BUT-70 to -70A . Overall, three metal–organic frameworks (MOFs) are constructed by infinite chains consisting of corner-sharing InO 4 (OR) 2 (R = H or Me) octahedral entities, which are interconnected by TTTA 3– ligands to form three-dimensional frameworks. Unlike most reported MOFs with infinite chains as SBUs, such as well-known MIL-53 and M-MOF-74, which have one-dimensional channels along the chain direction, the BUT-70 series contain two-dimensional intersecting channels. The Brunauer–Emmett–Teller surface area and pore volume of BUT-70A were estimated to be 460 m 2 g –1 and 0.18 cm 3 g –1, respectively, which are obviously lower than those of BUT-70B (695 m 2 g –1 and 0.29 cm 3 g –1 ). Gas adsorption experiments demonstrated that BUT-70A and -70B are able to selectively adsorb C 2 H 2 over CO 2 and CH 4 . At 1 atm and 298 K, BUT-70A uptakes 3.1 mmol g –1 C 2 H 2, which is 3.6 times that of the CO 2 uptake and 7.2 times that of the CH 4 uptake. Compared with BUT-70A, BUT-70B presents an even higher C 2 H 2 uptake of 3.9 mmol g –1 at the same conditions, but slightly lower Ideal Adsorbed Solution Theory C 2 H 2 /CO 2 and C 2 H 2 /CH 4 selectivities.