Solubility and Stability of Hexanuclear Ce(IV)–O Clusters

Abstract

Stable molecular clusters are of interest for targeted deposition in porous materials. In this work, we report the discovery of two new molecular Ce–O clusters of composition [Ce6O4(OH)4(NO3)4(DMF)4(C7H4O2X)8]·(DMF)4(H2O)2 (1-X) and [Ce6O4(OH)4(H2O)6(NO3)6(C7H4O2X)6] (2-X) (X = −Cl, −CHO, and −Br). Both cluster types contain a similar hexanuclear building unit, and crystal structures were determined from single-crystal X-ray diffraction or 3D electron diffraction data and subsequent Rietveld refinements against powder X-ray diffraction (PXRD) data. The crystal structure data is complemented by results from the local structure around the cerium ions, determined by extended X-ray absorption fine structure (EXAFS) measurements in the solid state. The composition of all Ce–O clusters was confirmed by elemental analysis, NMR and IR spectroscopy. The Ce–O clusters are highly soluble, up to 101 and 136 g/L for 1-Cl and 2-Cl, respectively, in organic solvents, which strongly depends on the type of cluster and functionalization of the benzoate ligands. Moreover, the structural and compositional integrity of dissolved clusters in different solvents was established. Recrystallization of 1-Cl from dichloromethane (DCM) and Raman spectroscopy confirm the integrity of both cluster types in solution. Further examination by EXAFS measurements on the Ce K-edge of clusters containing 4-chlorobenzoate reveals that only minor changes in the cerium environment of 1-Cl are observed upon dissolution in THF, DCM, and dioxane, while the results for 2-Cl indicate a partial degradation upon dissolution. After proving the stability, a cluster solution of 1-Cl was used to impregnate the mesoporous metal–organic framework Cr-MIL-101. Extensive characterization by PXRD, inductively coupled plasma-optical emission spectroscopy, and energy-dispersive X-ray spectroscopy, as well as thermogravimetry and N2-sorption measurements, confirm the successful insertion of Ce–O clusters into the large mesoporous cages of the framework. Due to the combination of high surface area and potential catalytic activity, the Cluster@MOF materials could be of high interest for application in heterogeneous catalysis.