Exploring the Solution Formation of UiO Family Hf Metal-Organic Framework clusters 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 which 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 Xray 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 M6 cluster (as in fcu to the hcp-characteristic M12 double cluster, and following this, the crystalline hcp framework forms. These insights pave the way towards rationally designing syntheses of as-yet unknown MOF structures, via tuning the synthesis conditions to select different cluster species.* Cambridge Crystallographic Database, search performed July 2020 affected in particular by the presence of water in the synthesis. In aqueous, and especially aqueous acidic conditions, the distorted-square tetrameric species [M4(OH)8•16H2O] 8+ is dominant. 25,[27][28][29][30][31][32] This cluster is surrounded by a structured coordination sphere of water molecules 33,34 similar to other nanoparticles in solution. 35 This water coordination sphere is potentially the source of further hydroxide bridges between metal centres during condensation of the clusters. 33,34,[36][37][38] The degree of hydrolysis, and subsequent aggregation, can be modulated by the use of additional reagents, 37 such as acid, 39 which in some MOF syntheses is required to form the crystalline framework, 40,41 and in others is used to control particle morphology. 15,40,42,43 Monocarboxylic acids are often chosen for this purpose; since they can possess a wide range of metal-ligand binding strengths, a judicious choice of carboxylic acid can "block" coordination sites on the cluster to further nucleophilic attack preventing, in the case of water/hydroxides, cluster aggregation, or for linkers, the formation of multi-cluster framework species. 13,34,44 However, while it is now clear that water and modulating ligands are critical to the identity and stability of the resultant metal clusters (from ligand-terminated molecular clusters 30 through to MOFs 12,34,[45][46][47][48] ), the exact interplay of these factors, especially in multi-species reaction mixtures, is not well understood.The degree of cluster aggregation is also directly affected by the temperature and duration of the synthesis. Both heating acidic aqueous zirconium solutions, fo...