Structural isomerism of an anionic nanoporous In-MOF with interpenetrated diamond-like topology

Abstract

Topologically interesting new In-MOFs were prepared by using a 4,4′-biphenyldicarboxylate bridging ligand and In(NO₃)₃·xH₂O. Two isomeric forms of In-MOFs were independently prepared and characterized by X-ray crystallography, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), elemental analysis, and gas sorption analysis. By simply using different solvents, N,N-dimethylformamide or N,N-diethylformamide, we obtained two new In-MOFs. Despite the same coordination mode of an In^III node, these two anionic MOFs adopted distinct crystal structures. One isomer (1) showed an overall 4-fold interpenetrated uninodal 4-connected diamond network with Z_t = 1 and Z_n = 2 (class IIa). The other (2) showed a 4-fold interpenetrated uninodal 4-connected diamond network with Z_t = 2 and Z_n = 2 (class IIIa). Thus, despite the same metal–ligand connectivity, 1 and 2 are unusual isomeric MOFs based on the ways of catenation. This may be accounted for by the flexible bonding nature of the In-centered pseudotetrahedral motif in the presence of counter-cations with different dimensions. The In-MOFs 1 and 2 exhibited different counter ion distributions and porosity properties. Interestingly, the In-MOF 1 showed a reversible framework transformation from crystalline form → amorphized form (after activation) → crystalline state (after resolvation). This reversible transformation can also be explained by the flexible bonding nature of the In^III ion with four η²-coordinated 4,4′-BPDC ligands.