Cerium-Based Coordination Network Formation: An In Situ X-ray Absorption Spectroscopy and Powder X-ray Diffraction Study

The Ce(NO$_3$)$_3$·6H$_2$O/H$_2$TDC/CH$_3$COOH/CH$_3$CN (H$_2$TDC = 2,5-thiophenedicarboxylic acid) chemical system was studied under solvothermal reaction conditions. Four different phases that successively crystallized as a function of time were observed. Three coordination networks, [Ce$^{IV}$(TDC)(CH$_3$COO)$_2$] ($\bf1$), [Ce$^{III}$$_4$(TDC)$_3$(CH$_3$COO)$_6$] ($\bf{2a}$), and [Ce$^{III}$(TDC)(CH$_3$COO)] ($\bf{3}$), could be isolated as orange, beige, and white phase pure products, respectively, and their crystal structures were resolved from powder X-ray diffraction data. Another crystalline compound ($\bf{2b}$) was observed in situ, which seems to be structurally related to compound $\bf{2a}$. Compound $\bf{2a}$ is a metal–organic framework (MOF) with a pore size of ∼3 Å. The use of Ce$^{III}$(NO$_3$)$_3$ as the starting material, the different colors of the products, and the crystal structures indicated a peculiar redox behavior with a Ce(III)–Ce(IV)–Ce(III) redox transformation during product formation of $\bf1$, $\bf{2a}$, and $\bf3$. The oxidation states of $\bf1$ and $\bf3$ were confirmed by ex situ X-ray absorption near-edge structure (XANES) measurements, and the crystallization process was followed using quasi-simultaneous in situ powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS) measurements. During the reaction, the consecutive crystallization in the order $\bf1$–$\bf{2b}$–$\bf3$ was clearly observed. Linear combination fitting (LCF) of the in situ XAS data also affirmed the formation of the title compounds.