Metal-organic frameworks based on magnesium(ii): adsorption and luminescence properties

Abstract

Two new metal-organic frameworks based on magnesium(ii) ions, [Mg₄(bdc)₃(dma)₆(NO₃)₂]•DMA (1; bdc is 1,4-benzenedicarboxylate; dma, DMA is N,N-dimethylacetamide) and [Mg₃(bpdc)₃(nmp)₄] (2; bpdc is 4,4´-biphenyldicarboxylate; nmp is N-methylpyrrolidone), were synthesized in DMA or NMP, respectively. According to the single-crystal X-ray diffraction data, compound 1 is built of binuclear carboxylate units {Mg₂(O₂CR)₃}, which were previously not observed in metal-organic frameworks. Compound 2 is based on trinuclear carb oxylate units {Mg₃(O₂CR)₆}. Compound 1 has a layered structure (hcb topology), whereas compound 2 has a distorted three-dimensional cubic structure (pcu topology). The phase purity and chemical identity of the compounds were confirmed by IR spectroscopy, thermogravimetry, elemental analysis, and powder X-ray diffraction. Photoluminescence properties of both compounds are indicative of intraligand charge-transfer transitions of one (for 1) or two (for 2) types. Textural characteristics of an activated sample of 1 were measured by nitrogen and hydrogen adsorption at 77 K and by carbon dioxide adsorption at 195 K.