Two cationic metal–organic frameworks featuring different cage-to-cage connections: syntheses, crystal structures, photoluminescence and gas sorption properties

Abstract

Solvothermal reactions of a neutral rigid tripodal ligand, 1,3,5-tris(1-imidazolyl)benzene (tib), with Zn(NO₃)₂·6H₂O and Zn(ClO₄)₂·6H₂O produced two metal–organic frameworks, [Zn₃(tib)₄·6NO₃·6H₂O·3DMA]_n (1) and [Zn₂(tib)₃·4ClO₄·6H₂O·6DMA]_n (2) (DMA = N,N-dimethylacetamide), respectively, which feature different cage-to-cage connections. Both compounds 1 and 2 exhibit 3D cationic frameworks with charge-balancing extraframework anions. Interestingly, the counterions NO₃⁻ and ClO₄⁻ play an important role in the fabrication of structures of 1 and 2. Compound 1 shows 2-fold interpenetration while 2 gives a non-interpenetrated 3D porous framework. Adjacent M₆L₄ octahedral cages in compound 1 are further linked via vertex sharing to construct a 3D framework. For compound 2, neighboring octahedral cages connect with each other through sharing four metal ions which are in the equatorial plane to generate a 2D layer. Furthermore, the 2D layers are linked by other layers via sharing the other two metal ions which are in the axial position, finally forming a 3D supramolecular framework. The photoluminescence of compounds 1 and 2 has been investigated in the solid state. Gas sorption measurements were conducted on the activated 1, showing a H₂ uptake of 1.2 wt% at 77 K and 1 bar with high initial adsorption enthalpy of 8.4 kJ mol⁻¹.