Metal-Dependent and Selective Crystallization of CAU-10 and MIL-53 Frameworks through Linker Nitration

The reaction of the V-shaped linker molecule 5-hydroxyisophthalic acid (H₂L⁰), with Al or Ga nitrate under almost identical reaction conditions leads to the nitration of the linker and subsequent formation of metal–organic frameworks (MOFs) with CAU-10 or MIL-53 type structure of composition Al(OH)(L)], denoted as Al-CAU-10-L^{0, 2, 4, 6} or Ga(OH)(L)], denoted as Ga-MIL-53-L². The Al-MOF contains the original linker L⁰ as well as three different nitration products (L², L⁴ and L^4/6), whereas the Ga-MOF mainly incorporates the linker L². The compositions were deduced by ¹H NMR spectroscopy and confirmed by Rietveld refinement. In situ and ex situ studies were carried out to follow the nitration and crystallization, as well as the composition of the MOFs. The crystal structures were refined against powder X-ray diffraction (PXRD) data. As anticipated, the use of the V-shaped linker results in the formation of the CAU-10 type structure in the Al-MOF. Unexpectedly, the Ga-MOF crystallizes in a MIL-53 type structure, which is usually observed with linear or slightly bent linker molecules. To study the structure directing effect of the in situ nitrated linker, pure 2-nitrobenzene-1,3-dicarboxylic acid (m-H₂BDC-NO₂) was employed which exclusively led to the formation of Ga(OH)(C₈H₃NO₆)] (Ga-MIL-53-m-BDC-NO₂), which is isoreticular to Ga-MIL-53-L². Density Functional Theory (DFT) calculations confirmed the higher stability of Ga-MIL-53-L² compared to Ga-CAU-10-L² and grand canonical Monte Carlo simulations (GCMC) are in agreement with the observed water adsorption isotherms of Ga-MIL-53-L².