Synthesis, Structure, and Characterization of Defect‐free [Hf6(μ3‐O)4(μ3‐OH)4(C4H2O4)6] (Hf‐UiO‐66‐Fum)

Abstract

The reaction of HfCl4 with fumaric acid (C4H4O4, H2Fum) in a DMF/HCOOH solvent mixture at 120 °C for 24 h results in the formation of a defect‐free sample of Hf‐UiO‐66‐Fum with UiO‐66 structure type. Treatment of the (as)‐synthesized sample in boiling ethanol followed by a thermal treatment at 70 °C leads to a sample of composition [Hf6(μ3‐O)4(μ3‐OH)4(Fum)6]·18H2O·2EtOH. In the crystal structure the hexanuclear clusters {Hf6(μ3‐O)4(μ3‐OH)4} are twelve‐fold connected by C4H2O42– ions to form the framework with the fcu topology. This cluster connectivity was confirmed by a combination of characterization techniques methods, i.e. CHN analyses, N2 sorption as well as 1H‐NMR spectroscopic and thermogravimetric measurements. The MOF shows a thermal stability of T = 390 °C, a specific BET surface area of 389 m2·g–1 and a water uptake of 204 mg·g–1. The crystal structure was determined from powder X‐ray diffraction (PXRD) data, employing the Rietveld method. The compound crystallizes in the cubic space group Pn3 and is isostructural to Zr‐UiO‐66‐Fum. Only a small increase of the lattice parameter a from 17.8309(4) to 17.87978(14) Å is observed.