Systematic Investigations of the Transition between Framework Topologies in Ce/Zr-MOFs.

1- H-Pyrazole-3,5-dicarboxylic acid (HPZDC), a small, strongly bent linker molecule with an angle of 147.4° between the carboxylate groups, was used in the synthesis of metal organic frameworks (MOFs) with fcu, bcu and reo topology. In systematic studies of the chemical system Ce/Zr/HPZDC/HCOOH, their fields of formations were established. The decisive factors for the product formation and hence the transition between the framework topologies are the HCOOH/metal ratio and the molar ratio of Ce/Zr employed in the synthesis. All title compounds crystallize with the well-known hexanuclear cluster {M(μ-O)(μ-OH)(-CO) }, with n = 8 or 12 and M = Ce and Zr, as the inorganic building unit (IBU). Connection through 12 or eight linker molecules leads to three framework topologies: fcu, bcu, and reo, respectively. The dominant phase observed in this system crystallizes with reo topology and is known as DUT-67. The pure Zr-MOF of composition [Zr(μ-O)(μ-OH)(PZDC)(OH)(HO)] (Zr-DUT-67-PZDC) as well as the mixed-metal compounds Ce/Zr-DUT-67-PZDC are accessible and the molar ratio Ce/Zr can be adjusted between 0 and 1. At low HCOOH/metal ratios, surprisingly, the UiO-66 type structure with fcu topology is formed despite the nonlinear geometry of the linker. Thus, using exclusively Zr ions in the starting mixture the pure Zr-MOF with ideal composition [Zr(μ-O)(μ-OH)(PZDC)] (Zr-UiO-66-PZDC) was obtained. Variation of the Ce/Zr molar ratio leads to a continuous increase in linker defects with increasing Ce content in the MOF. At a Ce/Zr value of ∼ 1:1 a transition from the fcu to the reo framework topology takes place. Using high HCOOH/metal ratios, a transition from the reo to the bcu topology is observed when a molar ratio of Ce/Zr ≥ 1:5 is employed. Irrespective of the molar ratio used in the reaction mixture, the mixed-metal MOF of composition [CeZr(μ-O)(μ-OH)(PZDC)(OH)(HO)] (Ce/Zr-CAU-38-PZDC) is always formed as confirmed by comprehensive EDX analyses. Rietveld refinement strongly indicates the presence of exclusively hexanuclear {CeZr(μ-O)(μ-OH)} clusters and thus CAU-38 is the first Ce/Zr-MOF which solely occurs at a specific metal stoichiometry. In addition to the detailed synthetic study, the compounds were thoroughly characterized regarding their composition, lattice parameters, and porosity, as well as thermal and chemical stability.