Synthesis, Structure, and Characterization of Defect‐free [Hf 6 (μ 3 ‐O) 4 (μ 3 ‐OH) 4 (C 4 H 2 O 4 ) 6 ] (Hf‐UiO‐66‐Fum)

The reaction of HfCl 4 with fumaric acid (C 4 H 4 O 4 , H 2 Fum) 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 [Hf 6 (μ 3 ‐O) 4 (μ 3 ‐OH) 4 (Fum) 6 ] · 18H 2 O · 2EtOH. In the crystal structure the hexanuclear clusters {Hf 6 (μ 3 ‐O) 4 (μ 3 ‐OH) 4 } are twelve‐fold connected by C 4 H 2 O 4 2– 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, N 2 sorption as well as 1 H‐NMR spectroscopic and thermogravimetric measurements. The MOF shows a thermal stability of T = 390 °C, a specific BET surface area of 389 m 2 · 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 Pn 3 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.