Investigation of the effect of polar functional groups on the crystal structures of indium MOFs

Abstract

Three new In-MOFs with –NH₂ and/or –NO₂ functionalities are reported using 2-amino- (H₂BDC-NH₂), 2-nitro- (H₂BDC-NO₂) and 2-amino-5-nitroterephthalic acid (H₂BDC-NH₂/NO₂). Their structures were determined from single crystal X-ray diffraction data. The structure of the first In-MOF of composition [In(BDC-NH₃)(BDC-NH₂)]·1.6DMF·1.9H₂O (In-BDC-NH₂; DMF is dimethylformamide; the space group is P6₂22, a = b = 14.738(2) Å and c = 12.257(3) Å) is built up by two interpenetrating nets of InO₈ polyhedra interconnected by BDC-NH₂²⁻ ions to form a framework with qtz topology. Charge balance is accomplished by partial protonation of the amino group, which was confirmed by IR spectroscopy. The interpenetration leads to a decrease of the pore dimension (4.4 Å in diameter). Thermogravimetric analysis revealed stability up to 300 °C. Replacement of H₂BDC-NH₂ by H₂BDC-NO₂ in the reaction mixture led to the isoreticular MOF containing no –NO₂ groups but exclusively un- and amino-functionalized linkers. Hence an indium mediated reduction of the H₂BDC-NO₂ linker molecule during solvothermal synthesis has occurred. The use of H₂BDC-NH₂/NO₂ under exactly the same reaction conditions did not result in the formation of a MOF, but by changing the synthesis parameters, a new –NH₂/–NO₂ bifunctionalised In-MOF of composition (DMA)₂[In₃(μ₃-O)(BDC-NH₂/NO₂)_4.5]·DMF (DMA is dimethylammonium), denoted In-BDC-NH₂/NO₂, was obtained. The crystal morphology can be altered from cubic to truncated octahedral crystals by varying the DMF/ethanol volume ratio during synthesis. The compound crystallises in the cubic space group I3m, a = 24.8947(1) Å, and the framework contains trinuclear {In₃(μ₃-O)} clusters which are interlinked by the BDC-NH₂/NO₂²⁻ ions to form super-tetrahedra. Four face-sharing super-tetrahedra form ultra-tetrahedra which are connected to form the final cubic framework with an ncb topology and isolated inaccessible pores.