Structures, Sorption Characteristics, and Nonlinear Optical Properties of a New Series of Highly Stable Aluminum MOFs

Employing high-throughput methods, the synthesis \nconditions for a series of six new MOFs based on aluminum ions and \nthe V-shaped linker molecule 1,3-benzene dicarboxylic acid, denoted \nas CAU-10-X (CAU = Christian-Albrechts-University) with the sum \nformula [Al(OH)(C \n8 \nH \n3 \nO \n4 \nX)] \n· \nsolvent, were established ( \nX \n= func- \ntional group in 5-position of the aromatic ring; \nX \n=H( \n1 \n), CH \n3 \n( \n2 \n), \nOCH \n3 \n( \n3 \n)NO \n2 \n( \n4 \n), NH \n2 \n( \n5 \n), or OH ( \n6 \n)). Because of the absence of \nmacroscopic crystals, the obtained compounds were structurally \ncharacterized employing XRPD-methods. The crystal structures of \n1 \n, \n2 \n, and \n3 \nwere re \nfi \nned using Rietveld methods. Although the described \nMOFs are isoreticular, they crystallize in several, sometimes noncentrosymmetric space groups ( \n1 \n, \n4 \n, \n6 \n), because of slight \nstructural changes induced by the functionalization. These space groups were con \nfi \nrmed with second-harmonic generation \nmeasurements. All compounds are highly stable as con \nfi \nrmed by temperature-dependent XRPD and IR experiments and \ndecompose at temperatures above 350 \n° \nC. The stabilities of all compounds in aqueous solutions of varying pH were con \nfi \nrmed \nby XRPD measurements and their sorption properties toward nitrogen, hydrogen, carbon dioxide, and water vapor at low \npressures are reported. A drastic in \nfl \nuence of the functional group on a \nffi \nnity, capacity, and accessibility of the pores for these \ngases is observed. These properties depend on the polarity and size of the functional group as well as on subtle structural \ndi \nff \nerences between the CAU-10-X compounds.