The Chemistry of Zirconium/Carboxylate Clustering Process: Acidic Conditions to Promote Carboxylate-Unsaturated Octahedral Hexamers and Pentanuclear Species

Clustering chemistry is a key point in the design and synthesis of the secondary building units that comprise metal-organic frameworks (MOFs) based on group IV metals. In this work, the first stages of the zirconium-carboxylate clustering process in alcohol/water mixtures are studied in detail using the monocarboxylicbenzoic and hydroxybenzoic acids to avoid the polymerization. Mass spectroscopy measurements performed on the reactions revealed the presence of hexa- andpentanuclear species even at low pH values and also evidenced the acid-base natureand pH dependence of the transformation between both species. The control on thechemistry governing the equilibria between these species has allowed us to isolate sixnew compounds in the solid state. The single-crystal X-ray diffraction analysisrevealed that they are closely related to the well-known [Zr6(O)4(OH)4(OOC)12]secondary building unit found in many MOFs by removing carboxylic ligands in thecase of the hexameric species ([Zr6(O)4(OH)4(OOC)8(H2O)8]4+) or by additionallyremoving one of the metal centers in the case of the pentameric entities([Zr5(O)2(OH)6(OOC)4(H2O)11(alcohol)]6+). Going in detail, the unsaturated hexameric clusters exhibit different dispositionsof their eight carboxylate ligands in such a way that the remaining four carboxylate-free positions are arranged according to a square planar or tetrahedral symmetry. It should be highlighted that the pentameric complexes imply an unprecedented core nuclearity inzirconium clusters and thus their isolation provides a novel building block for the design of metal-organic materials

Automated summary

This study investigates the initial stages of zirconium-carboxylate clustering process in alcohol/water mixtures. By controlling the chemistry, six new compounds have been isolated, providing a new building block for the design of metal-organic materials.