Isomorphous Substitution in a Flexible Metal–Organic Framework: Mixed-Metal, Mixed-Valent MIL-53 Type Materials

Mixed-metal iron–vanadium analogues of the 1,4-benzenedicarboxylate (BDC) metal–organic framework MIL-53 have been synthesized solvothermally in N,N′-dimethylformamide (DMF) from metal chlorides using initial Fe:V ratios of 2:1 and 1:1. At 200 °C and short reaction time (1 h), materials (Fe,V)II/IIIBDC(DMF1–x F x ) crystallize directly, whereas the use of longer reaction times (3 days) at 170 °C yields phases of composition [(Fe,V)III 0.5(Fe,V)0.5 II(BDC)(OH,F)]0.5–·0.5DMA+ (DMA = dimethylammonium). The identity of the materials is confirmed using high-resolution powder X-ray diffraction, with refined unit cell parameters compared to known pure iron analogues of the same phases. The oxidation states of iron and vanadium in all samples are verified using X-ray absorption near edge structure (XANES) spectroscopy at the metal K-edges. This shows that in the two sets of materials each of the vanadium and the iron centers are present in both +2 and +3 oxidation states. The local environment and oxidation state of iron is confirmed by 57Fe Mössbauer spectrometry. Infrared and Raman spectroscopies as a function of temperature allowed the conditions for removal of extra-framework species to be identified, and the evolution of μ2-hydroxyls to be monitored. Thus calcination of the mixed-valent, mixed-metal phases [(Fe,V)III 0.5(Fe,V)0.5 II(BDC)(OH,F)]0.5–·0.5DMA+ yields single-phase MIL-53-type materials, (Fe,V)III(BDC)(OH,F). The iron-rich, mixed-metal MIL-53 shows structural flexibility that is distinct from either the pure Fe material or the pure V material, with a thermally induced pore opening upon heating that is reversible upon cooling. In contrast, the material with a Fe:V content of 1:1 shows an irreversible expansion upon heating, akin to the pure vanadium analogue, suggesting the presence of some domains of vanadium-rich regions that can be permanently oxidized to V(IV).