Abstract
Properties of mixed-metal MOFs depend on the distribution of different metals within their frameworks. Determination of this distribution is often very challenging. Using an example of aluminum- and iron-containing MIL-100, we demonstrate that 27Al NMR spectroscopy, when combined with first-principles calculations and magnetic, X-band electron paramagnetic resonance, Fe K-edge extended X-ray absorption fine structure, and Mössbauer measurements, enables one to accurately determine the arrangement of Al and Fe within the metal trimers, which are the basic building units of MIL-100. In this particular material, the incorporation of Fe and Al on the framework metal sites is random. Crucial for deciphering the arrangement is detecting NMR signals, shifted because of the strong hyperfine interaction between the 27Al nuclei and the unpaired electronic spins of Fe3+ ions, assigning the shifted signals aided by first-principles calculations of hyperfine couplings, and quantitatively evaluating the NMR intensities and the measured effective magnetic moment.
Highlights
Introduction ofFunctionality, Selection of Topology, and Enhancement of Gas Adsorption in Multivariate Metal−Organic Framework[177]
We show that quickly accessible and readable information about the arrangement of Fe3+ and Al3+ ions is offered by NMR spectroscopy
The results suggest that NMR can, even more generally, offer relatively accessible and very informative insight into the distribution of different metals within mixed-metal materials
Summary
For the 27Al nuclei of MIL100(Al,Fe), predicting the isotropic paramagnetic shifts involves only first-principles calculation of the hyperfine coupling constants. As suggested by the EPR analysis, the magnetic susceptibility of the two samples is apparently the sum of several contributions, and even at the relatively high temperature of 300 K, neither MIL-100(Al,Fe) nor MIL-100(Fe) can be described as a paramagnetic phase comprising only independent paramagnetic Fe3+ centers (Figure 3).
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