Subtle structural variation in copper metal-organic frameworks: syntheses, structures, magnetic properties and catalytic behaviour

Abstract

Two new copper metal-organic frameworks containing 5-nitro-1,3-benzenedicarboxylate (5-nbdc) have been prepared from the reaction between Cu(NO₃)₂·3H₂O and H₂(5-nbdc) in DMF at different temperatures. Single crystal X-ray structures of {[Cu₂(5-nbdc)₂(DMF)₂]·2DMF}_∞ (1) and {[Cu₂(5-nbdc)₂(DMF)₂]·3⅓DMF}_∞ (2) revealed similar sheet structures, containing triangular and hexagonal pores, but differences in the stacking of the sheets. Magnetic measurements on 1 and 2 are consistent with antiferromagnetic dimers containing a small quantity of paramagnetic impurity. The desolvated forms of 1 and 2 were applied as Lewis acid catalysts in the acetylation of methyl 4-hydroxybenzoate. When the reaction between Cu(NO₃)₂·3H₂O and H₂(5-nbdc) was carried out in a mixture of DMF and water, the reaction gave metallomacrocycles of formula [Cu₆(5-nbdc)₆(H₂O)₁₂(DMF)₆] (3). These assemble through hydrogen-bonding interactions to form a gross structure in which the macrocycle pores align into channels. The reaction between Cu(NO₃)₂·3H₂O and 5-methylsulfanylmethyl-1,3-benzenedicarboxylic acid, H₂(5-msbdc), in DMF-water gave {[Cu₂(5-msbdc)₂(OH₂)₂]·3DMF}_∞ (4), which contains similar sheets to those in 1 and 2, whereas the reaction with 5-amino-1,3-benzenedicarboxylic acid, H₂(5-abdc), gave {[Cu₂(5-abdc)₂(DMF)₂]}_∞ (5), which has a previously reported network based on sheets containing rhombohedral pores. The reaction between Cu(NO₃)₂·3H₂O and 2-methoxy-1,3-benzenedicarboxylic acid, H₂(2-mbdc), in DMF gave [Cu₂(2-mbdc)₂(DMF)₂] (6). The presence of the substituent in the 2-position removes the co-planarity of the carboxylate groups, and the sheet structure adopted by 6 contains rhomboidal pores.