Gas Sorption Properties of a New Three-Dimensional In-ABDC MOF With a Diamond Net

The pseudotetrahedral node, [In(O₂CR)₄]⁻, often found in In^III-based metal-organic frameworks (MOFs) without a cluster-based secondary building unit (SBU) is a negatively charged center due to charge mismatch between an 8-coordinate In^III ion and four anionic carboxylate bridging ligands. Thus, In-MOFs with this pseudotetrahedral node tend to bear a counter-cation near each In^III center in the frameworks. Generally, dialkylammonium-based cations such as Me₂NH⁺₂ and Et₂NH⁺₂ directly derived from N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF) solvents during MOF formation reactions play a significant role to form a stable framework through charge matching. If these cations thermally derived from DMF or DEF were not suitable for crystal growth of In-MOFs, it becomes very challenging to obtain high quality single crystals for X-ray structure determination of the frameworks. In this context, high quality crystals of In-ABDC MOF were not easily prepared from a ditopic azobenzene-4,4'-dicarboxylic acid (H₂ABDC) through a thermal reaction in DMF or DEF. We successfully overcome this problem by employing a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), and the resulting three-dimensional (3D) In-ABDC MOF, [EMIM][In(ABDC)₂]·DEF·H₂O (I), was structurally characterized by X-ray diffraction. The 3D framework indicates a 4-connected uninodal net with Schläfli symbol of 6⁶ (dia). The gas sorption properties of solvent-free I were also investigated in detail.