Co-Ligand Dependent Formation and Phase Transformation of Four Porphyrin-Based Cerium Metal–Organic Frameworks

The four porphyrin-based metal–organic frameworks (MOFs) containing Ce3+ ions, [Ce4(H2TCPP)3­(DMF)2­(H2O)4] (CAU-18), [Ce4­(H2TCPP)3]­·22H2O (CAU-18a), [Ce3(H2TCPP)2­(BA-X)­(HBA-X/H2O)2]­·2HBA-X­·nH2O (CAU-19-X with X = H, 2Cl, 3Cl, 4Cl, 3CO2H, 4NH2, 4NO2, HBA = C7H4O2), and [Ce2(H2TCPP)­(C7H4O2NO2)2]­·2DMF (Ce-PMOF-4NO2 ) were synthesized using the linker 4-tetracarboxyphenylporphyrin (H6TCPP). The formation of the respective MOFs depends mainly on the presence of a coligand in the synthesis mixture. CAU-18 was obtained in the absence of a coligand, while CAU-19-X was observed when the benzoic acid derivative HBA-X (X = H, 2Cl, 3Cl, 4Cl, 3CO2H, 4NH2) was added. In the case that HBA-4NO2 was used as a coligand, yet another compound Ce-PMOF-4NO2 is obtained. The structures of CAU-18 and CAU-19-H were determined from single crystal X-ray diffraction data, while the structure of Ce-PMOF-4NO2 was refined from powder X-ray diffraction data by the Rietveld method. Activation of CAU-18 and Ce-PMOF-4NO2 resulted in phase transformations. Thermal treatment of CAU-18 at 250 °C leads to CAU-18a, which is porous toward N2 and H2O, while treatment of Ce-PMOF-4NO2 in organic solvents at 70 °C leads to the formation of CAU-19-4NO 2 , which cannot be synthesized directly. All CAU-19-X compounds are porous toward N2 and H2O, and the specific surface areas vary between 330 and 600 m2 g–1 depending on the size of the incorporated coligand. CAU-18, CAU-18a, and CAU-19-X are thermally stable in air up to 330 °C and chemically stable in H2O and all tested organic solvents. Ce L3-edge X-ray absorption near edge structure measurements revealed that exclusively Ce3+ ions are present in the title compounds, despite the use of (NH4)2­[Ce­(NO3)6] in all syntheses. In addition, the crystallization of CAU-18 and CAU-19-H was investigated in situ by synchrotron powder X-ray diffraction at DESY, Hamburg, using reaction temperatures between 110 and 130 °C. The data were evaluated using the approach by Gualtieri to determine the probability of nucleation (P n) and the Arrhenius activation energy for nucleation (k n) and crystal growth (k g). The Arrhenius activation energies for the nucleation were determined as 47(2) and 56(3) kJ mol–1 and for crystal growth 45(4) and 58(5) kJ mol–1 for CAU-18 and CAU-19-H, respectively. The induction time (t ind), in which no crystalline products are detected, and the total reaction time to achieve full conversion (t com) are shortened at higher temperat