Combined study of structural properties on metal-organic frameworks with same topology but different linkers or metal

The recently discovered UiO-66/67/68 class of isostructural metallorganic frameworks (MOFs) [2008 J. Am. Chem. Soc. 130 13850] has attracted great interest because of its remarkable stability at high temperatures, high pressures and in presence of different solvents acids and bases [2011 Chem. Mater. 23 1700]. UiO-66 is obtained connecting Zr₆O ₄(OH)₄ inorganic cornerstones with 1,4-benzene- dicarboxylate (BDC) linkers, while for the isostructural UiO-67, the longer 4,4' biphenyl-dicarboxylate (BPDC) is used as linker [2012 Phys. Chem. Chem. Phys. 14 1614]. Hf-UiO-66 is built with the same linker of UiO-66 but, in this case, Hf6O4(OH)4 blocks substitute the Zr₆O₄(OH)₄ ones [2012, Phys. Rev. B 86 125429]. In all cases isostructural cubic MOFs are obtained. Here we report a systematic EXAFS investigation of UiO-66 and UiO-67 (at Zr L₃-edge) and of Hf-UiO-66 (at Hf L3-edge) before and after the removal of solvent in their pores, needed to make the channel volume available. The study reveals that, for the three systems, the perfect M₆O ₄(OH)₄ (M Zr or Hf) octahedron forming the cornerstones of the as prepared material undergoes a remarkable local rearrangement into a distorted M₆O₆ octahedron, with the loss of two water molecules under the desolvation process. Notwithstanding the high crystallinity of the materials, this rearrangement, moving M atoms from 8-fold to 7-fold coordination and distorting the M-M distance of the octahedron side by more than 0.2 Å, escape XRPD detection (because not ordered on the long range scale) but is clearly observed by EXAFS owing to its local sensitivity.