Structural diversification and single-crystal-to-single-crystal transformation of alkaline earth metal-based MOFs regulated by solvent effect

Abstract

Two pairs of alkaline earth metal-based MOFs have been synthesized by spontaneous self-assembly reactions of Mg(II) or Ca(II) and two tricarboxylate ligands, H₃BTTB and H₃TCM (H₃BTTB = 4,4′,4′′-[benzene-1,3,5-triyl-tris(oxy)]tribenzoic acid; H₃TCM = 4,4′,4′′-{[(2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene)]tris(oxy)}tribenzoic acid), in different solvent media. The Mg/BTTB assemblies, {[Mg_3/2(BTTB)(H₂O)(DMF)](DMF)}_n (1) (DMF = N,N′-dimethylformamide) and {[Mg_3/2(BTTB)(DMAc)₂](DMAc)_0.5}_n (2) (DMAc = N,N′-dimethylacetamide), are prepared in DMF/H₂O and DMAc/H₂O media, respectively, and feature uniform kgd-type 2D bilayer structures but with different compactness. Meanwhile, two Ca/TCM assemblies, {[Ca_5/2(TCM)₂(DMF)₂(H₂O)](Me₂NH₂)(DMF)₂(H₂O)}_n (3) and {[Ca₃(TCM)₂(DMAc)₃(H₂O)₂](DMAc)₅}_n (4), have been achieved in DMF/H₂O and DMAc/H₂O mixed systems as well. However, compounds 3 and 4 have an enormous structural discrepancy, that is, compound 3 shows a complicated parallel 2D → 3D polycatenaned supramolecular architecture while 4 exhibits a 2-fold interpenetrating sit-type 3D framework. These various coordination patterns evidently illustrate the solvent regulation on the alkaline earth metal-based MOFs. The single-crystal-to-single-crystal structural transformation experiment reveals that the loose kgd-network compound 2 can be converted into the compact isoreticular compound 1 when immersed in DMF for 12 h. However, the reversed conversion cannot be realized. In addition, their thermal stability, gas adsorption, and solid-state fluorescence properties have also been investigated.