Exploring the Role of Cluster Formation in UiO Family Hf Metal–Organic Frameworks with in Situ X-ray Pair Distribution Function Analysis

Abstract

The structures of Zr and Hf metal−organic frameworks (MOFs) are very sensitive to small changes in synthetic conditions. One key difference affecting the structure of UiO MOF phases is the shape and nuclearity of Zr or Hf metal clusters acting as nodes in the framework; although these clusters are crucial, their evolution during MOF synthesis is not fully understood. In this paper, we explore the nature of Hf metal clusters that form in different reaction solutions, including in a mixture of DMF, formic acid, and water. We show that the choice of solvent and reaction temperature in UiO MOF syntheses determines the cluster identity and hence the MOF structure. Using in situ X-ray pair distribution function measurements, we demonstrate that the evolution of different Hf cluster species can be tracked during UiO MOF synthesis, from solution stages to the full crystalline framework, and use our understanding to propose a formation mechanism for the hcp UiO-66(Hf) MOF, in which first the metal clusters aggregate from the M 6 cluster (as in fcu UiO-66) to the hcp-characteristic M 12 double cluster and, following this, the crystalline hcp framework forms. These insights pave the way toward rationally designing syntheses of as-yet unknown MOF structures, via tuning the synthesis conditions to select different cluster species.

Fatty acid capped, metal oxo clusters as the smallest conceivable nanocrystal prototypes

“…Recently, oxo clusters were modelled via PDF analysis without organic ligands in MOF reaction mixtures. 47,49 On the other hand, X-ray PDF has been oen used to study the structure of nanocrystals. In this case, the organic ligands have usually been neglected in the structural models because the organic ligands scatter very little compared to the inorganic core.…”

Quick and robust PDF data acquisition using a laboratory single-crystal X-ray diffractometer for study of polynuclear lanthanide complexes in solid form and in solution

“…So far, PDF studies on colloidal systems have almost exclusively been carried out at synchrotron sources to boost the signal-to-noise ratio via the high available flux. Recently, real-time synchrotron PDF analysis was applied to in situ monitoring of selfassembly of polynuclear Bi (Szczerba et al, 2021) and HfO 2 (Christensen et al, 2021) clusters during hydrolysis, formation of UiO-66 MOFs in solvothermal conditions from hexanuclear Zr 6 (Xu et al, 2019) and Hf 6 (Firth et al, 2021) clusters, and formation of Zn clusters and the nucleation process during the in situ reaction of Zn(NO 3 ) 2 with 2-methylimidazolate, resulting in a zeolitic imidazolate framework (ZIF-8) (Terban et al, 2018).…”

The chemistry behind room temperature synthesis of hafnium and cerium UiO-66 derivatives

“…The formation of the hexanuclear metal( iv ) cluster is critical in the assembly of UiO-66 structures, so it is important to account for the many variables that impact this process, including solvent, solution acidity, and temperature. 42 Recently, Serre and co-workers demonstrated the formation of Zr UiO-66-NH 2 and Zr UiO-66-COOH derivatives in aqueous media in the presence of a formic acid modulator without any energy input. 30…”

“…17 The formation of the hexanuclear metal(IV) cluster is critical in the assembly of UiO-66 structures, so it is important to account for the many variables that impact this process, including solvent, solution acidity, and temperature. 42 Recently, Serre and co-workers demonstrated the formation of Zr UiO-66-NH 2 and Zr UiO-66-COOH derivatives in aqueous media in the presence of a formic acid modulator without any energy input. 30 To probe whether it is possible to isoreticularly synthesize Hf UiO-66 analogues under similar synthetic conditions, we reacted equimolar amounts of HfCl 4 precursor and functionalized terephthalic acid linker in the presence of formic acid.…”