Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed‐Metal Metal–Organic Frameworks

Abstract

Metal-organic frameworksc ontaining multiple metals distributed over crystallographically equivalent framework positions (mixed-metal MOFs) represent an interesting class of materials, since the closev icinity of isolated metal centerso ften gives rise to synergistic effects. However,a ppropriate characterization techniques for detailed investigations of these mixed-metal metal-organicf ramework materials, particularlya ddressing the distribution of metalsw ithin the lattice,a re rarely available.T he synthesis of mixed-metal FeCuBTC materials in direct syntheses provedt ob ed ifficult and only at horough characterizationu sing varioust echniques,l ike powder X-ray diffraction, X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy,u nambiguously evidencedt he formationof am ixed-metalF eCuBTC materialw ith HKUST-1s tructure, which containedb imetallic FeÀCu paddlewheels as wella sm onometallic CuÀCu andF e ÀFe units under optimizeds ynthesis conditions. The in-depthc haracterization showedt hat other synthetic procedures led to impurities, whichc ontained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this studys hows the necessity to characterize mixed-metal MOFs extensively to unambiguously prove the incorporationo fb oth metals at the desiredp ositions. The controlled positioning of metal centers in mixed-metal metal-organicf ramework materials and the thorough characterization thereof is particularly important to derive structure-property or structure-activity correlations.Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.Figure 1. Schematic framework structure (a) andthe characteristic paddlewheel unit (b) of amixed-metal FeCuBTCmaterial with HKUST-1 structure.Figure 5. Best fit calculations and experimentally obtained EXAFS spectra(a+ b), Fourier-transformed EXAFS spectra (c + d) and Fourier back-transformed spectra (e + f) of FeCuBTC at the Cu K-edge (a,c,e) and the Fe K-edge (b,d,f).

Monitoring the Local Structure and Magnetic Properties of the Dinuclear Cu2-Paddle Wheel Nodes in the Mesoporous Metal–Organic Framework, DUT-49(Cu), upon Adsorption-Induced Breathing Transitions

“…Experimentally derived spin Hamiltonian parameters of species A op are summarized in Table . Similar signals were reported for other MOFs containing Cu II –Cu II PWs, ,,, which support our assignment of this S = 1 signal, as well as of all other S = 1 signals, which we report in this work, to Cu II –Cu II dimers in the PW units of the MOF. Moreover, the ratio of Cu II monomer (∼11%) and Cu II –Cu II dimer (∼89%) species were found to be 0.12(2) from the EPR simulation of a single spectrum at room temperature.…”

“…During the full xenon ad/desorption cycle, the corresponding g z and A z parameters do not change significantly, indicating no significant interaction of the xenon molecules with the CUS of the Cu II ions. In contrast to that, the corresponding values adopt 2.36 > g z > 2.30 and 210 MHz < A z < 250 MHz for PW species in the ethylene loaded DUT-49(Cu) (Figures S9b and S13b, Those values are typical for a square pyramidal symmetry of the Cu II coordination environment 48,49,52,54,57,58 and give direct evidence for the coordination of the adsorbed ethylene molecules at the Cu II via their axial CUS, as already reported for CuZn PW units in HKUST-1. 59,60 We further want to comment on the peak-to-peak line width ΔB pp (B xy2 ) of the B xy2 transition, whose size might reflect the degree of structural disorder.…”

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

“…277 More effort are being made to develop Fe containing HKUST-1 materials, mainly using the mentioned Mo ¨ssbauer, 278 as well as XAS and XRD tools for the purpose of studying these metal sites. 279 Ru 3+ /Ru 2+ mixed-valence states have been also observed in the HKUST-1(Ru) analogue, reported by Fischer et al by XPS (Fig. 12a).…”

Paramagnetic species in catalysis research: a unified approach towards (the role of EPR in) heterogeneous, homogeneous and enzyme catalysis

“…Recently, Bitzer et al, 164 used EPR along with the X-ray diffraction and X-ray absorption spectroscopy to study the incorporation of Fe 31 ions into another copper-based MOF, labelled CuBTC, with paddlewheel units. The presence of Fe 31 -Cu 21 paddlewheels have been successfully confirmed from the strong magnetic interactions among the Cu 21 -Fe 31 species with a g value of 2.023.…”