A series of defect-engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultra-high-vacuum conditions. Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties, including band gap, magnetism, and porosity, with hierarchical micro/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework.
The metal−organic framework (MOF) compound Cu3(BTC)2(H2O)3·xH2O (BTC = benzene 1,3,5-tricarboxylate) was prepared by solvothermal synthesis under ambient pressure and structurally characterized by
powder X-ray diffraction and nitrogen adsorption at 77 K. X- and Q-band CW electron spin resonance and
hyperfine sublevel correlation spectroscopies were used to explore the coordination state and location of the
Cu(II) ions in the porous coordination polymer. Cupric ions were found to be present in two different chemical
environments: (a) Cu(II)2 clusters in the paddle-wheel building blocks of the Cu3(BTC)2 network, giving rise
to an antiferromagnetically coupled spin state in accordance with previous susceptibility measurements
(J. Appl. Phys.
2000, 87, 6007). However, the cross-linking of the paddle wheels by the BTC linker leads to
an additional spin exchange among dimers as evidenced by the characteristics of the S = 1 ESR signal of
their excited spin state. (b) In addition, paramagnetic monomer Cu(II) species are accommodated in the pore
system of Cu3(BTC)2. They coordinate to adsorbed water molecules and form [Cu(H2O)6]2+ complexes, which
are inhomogeneously distributed over the Cu3(BTC)2 pore system.
A practical strategy is outlined for the determination of proton hyperfine parameters in paramagnetic transition metal ion complexes in disordered systems from a single two-dimensional hyperfine sublevel correlation spectroscopy (2D HYSCORE) electron spin resonance experiment. Both dipolar and isotropic hyperfine interaction parameters can directly be determined from the cross peak ridges in the HYSCORE spectrum in the limit of the point dipole approximation. This approach is justified by spectral simulations for isotropic and axially symmetric g tensors. If the HYSCORE spectrum is measured at the g ⊥ spectral region of the electron spin resonance powder spectrum, the orientation of the hyperfine interaction tensor with respect to the g tensor frame can also be deduced from the shape of the cross peak ridges in many cases. Two experimental examples are presented. Using this approach, the hyperfine interaction parameters for protons in [Cu(H 2 O) 6 ] 2+ and in [Cu(C 5 H 5 N) 4 ] 2+ complexes, both incorporated into mesoporous (L)Cu-MCM-41 silica tube material, are determined from a single 2D HYSCORE spectrum. The parameters are in good agreement with independent measurements by electron spin echo modulation spectroscopy.X
In the parent metal-organic framework Cu 3 (btc) 2 material the Cu(II) pairs in the paddle wheel building blocks of the framework give rise to an antiferromagnetic spin state with an electron spin resonance (ESR) silent S ) 0 ground state. The thermally excited S ) 1 state of the Cu(II) pairs can be observed for temperatures above 80 K by ESR spectroscopy but give rise to an exchanged narrowed resonance line preventing the exploration of any structural details in the environment of the paddle wheel units. However, magnetically diluted paramagnetic binuclear Cu-Zn clusters can be formed by substitution of Cu(II) ions by Zn(II) at low doping levels, as already known for zinc-doped copper acetate monohydrate. Indeed, ESR, hyperfine sublevel correlation spectroscopy (HYSCORE) and pulsed electron nuclear double resonance (ENDOR) verify the successful incorporation of zinc ions at cupric ion sites into the framework of the resulting Cu 3-x Zn x (btc) 2 coordination polymer. The formation of such paramagnetic binuclear Cu-Zn paddle wheel building blocks allows the investigation of the interaction between the Cu(II) ions and various adsorbates by advanced pulsed ESR methods with high accuracy. As a first example we present the adsorption of methanol over Cu 3-x Zn x (btc) 2 , which was found to coordinate directly to the Cu(II) ions via their open axial binding site.
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