The metallocene-based linker molecule
1,1′-ferrocenedicarboxylic acid (H2FcDC) was used
to synthesize four different polymorphs of composition [In(OH)(FeC12H8O4)]. Using conventional solvent-based
synthesis methods and varying the synthetic parameters such as metal
source, reaction temperature, and solvent, two different MOFs and
one 1D-coordination polymer denoted as CAU-43 (1), In-MIL-53-FcDC_a
(2), and In-FcDC (3) were obtained. Furthermore,
thermal treatment of CAU-43 (1) at 190 °C under
vacuum yielded a new polymorph of 2, In-MIL-53-FcDC_b
(4). Both MOFs 2 and 4 crystallize
in a MIL-53 type structure, but in different space groups C2/m for 2 and P1̅ for 4. The structures of the four title compounds
were determined by single-crystal X-ray diffraction (SCXRD), powder
X-ray diffraction (PXRD), or a combination of three-dimensional electron
diffraction measurements (3D ED) and PXRD. N2 sorption
experiments of 1, 2, and 4 showed
specific surface areas of 355 m2 g–1,
110 m2 g–1, and 140 m2 g–1, respectively. Furthermore, the electronic properties
of the title compounds were characterized via Mössbauer and
EPR spectroscopy. All Mössbauer spectra showed the characteristic
doublet, proving the persistence of the ferrocene moiety. In the cases
of 1, 3, and 4, appreciable
impurities of ferrocenium ions could be detected by electron paramagnetic
resonance spectroscopy. Cyclovoltammetric experiments were performed
to demonstrate the accessible redox activity of the linker molecule
of the title compounds. A redox process of FcDC2– with oxidation (between 0.86 and 0.97 V) and reduction wave (between
0.69 and 0.80 V) was observed.
A new gallium based metal-organic framework, denoted as Ga-MIL-53-FcDC, with the chemical formula [Ga(OH)(FeC 12 H 8 O 4)] was synthesized using the ferrocene containing linker molecule 1,1'-ferrocenedicarboxylic acid (H 2 FcDC, FeC 12 H 10 O 4). The porous nature of the compound could be confirmed by nitrogen sorption and a specific surface area of 270 m 2 /g was determined. The persistence of the ferrocene complex inside the structure was confirmed by Mössbauer-, EPR and UV/VIS-spectroscopy. Ga-MIL-53-FcDC shows structural flexibility depending on which guest molecule is located in the pores of the compound. The mechanism of structural flexibility was analyzed by means of powder X-ray diffraction adsorbing pyrazine or iodine. The flexibility of the crystal structure can be attributed to the torsion of the GaO 6 octahedra in the IBU respective to each other and the torsion of the carboxylate groups of FcDC 2À relative to the aromatic ring.
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