Accessing Postsynthetic Modification in a Series of Metal-Organic Frameworks and the Influence of Framework Topology on Reactivity

Abstract

2-Amino-1,4-benzenedicarboxylic acid (NH(2)-BDC) has been found to be a compatible building block for the construction of two new metal-organic frameworks (MOFs) that have structures isoreticular to reported MOFs that use 1,4-benzenedicarboxylic acid (BDC) as a building block. DMOF-1-NH(2) (DABCO MOF-1-NH(2)) is a derivative of a previously studied MOF that contains two-dimensional square grids based on NH(2)-BDC and zinc(II) paddle-wheel units; the grid layers are connected by DABCO (1,4-diazabicyclo[2.2.2]octane) molecules that coordinate in the axial positions of the paddlewheel secondary-building units (SBUs). UMCM-1-NH(2) is an NH(2)-BDC derivative of UMCM-1 (University of Michigan Crystalline Material-1), a highly porous MOF reported by Matzger et al., and consists of both NH(2)-BDC and BTB (BTB = 4,4',4''-benzene-1,3,5-triyl-tribenzoate) linkers with Zn(4)O SBUs. The structure of UMCM-1-NH(2) was confirmed by single-crystal X-ray diffraction. By using NH(2)-BDC to generate these MOFs, the pendant amino groups can serve as a chemical handle that can be manipulated via postsynthetic modification with alkyl anhydrides. Reactions of each MOF and different anhydrides have been performed to compare the extent of conversion, thermal and structural stability, and Brunauer-Emmett-Teller surface areas afforded by the resulting materials. Under comparable reaction conditions, (1)H NMR of digested samples show that UMCM-1-NH(2) has conversions comparable to that of IRMOF-3, while DMOF-1-NH(2) only shows high conversions with smaller anhydrides. Under specific reaction conditions, higher conversions were obtained with complete retention of crystallinity, as verified by single-crystal X-ray diffraction experiments. The results presented here demonstrate three important findings: (a) NH(2)-BDC can be used as a surrogate for BDC in a number of MOFs thereby providing a handle for postsynthetic modification, (b) postsynthetic modification is a general strategy to functionalizing MOFs that can be applied to a variety of MOF structures, and (c) the topology and chemical/thermal stability of a MOF can influence the type of chemical reactions and reagents that can be used for postsynthetic modification.