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.

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.

The process of water adsorption on a dehydrated Cu(3)(BTC)(2) (copper (II) benzene 1,3,5-tricarboxylate) metal-organic framework (MOF) was studied with (1)H and (13)C solid-state NMR. Different relative amounts of water (0.5, 0.75, 1, 1.5, 2, and 5 mole equivalents with respect to copper) were adsorbed via the gas phase. (1)H and (13)C MAS NMR spectra of dehydrated and water-loaded Cu(3)(BTC)(2) samples gave evidence on the structural changes due to water adsorption within the MOF material as well as information on water dynamics. The analysis of (1)H spinning sideband intensities reveals differences in the (1)H-(63/65)Cu hyperfine coupling between dehydrated and water-loaded samples. The investigation was continued for 60 days to follow the stability of the Cu(3)(BTC)(2) network under humid conditions. NMR data reveal that Cu(3)(BTC)(2) decomposes quite fast with the decomposition being different for different water contents.

The dehydrated aluminum form of the metal−organic framework compound MIL-53 shows a temperature-driven phase transition with pronounced structural hysteresis as recently shown by neutron diffraction and scattering experiments (J. Am. Chem. Soc.200813011813). Thereby, the structure of the corner-sharing metal AlO4(OH)2 octahedra differs in terms of local symmetry for the dehydrated MIL-53(Al) material in its high- and low-temperature form with open or closed pore structure, respectively. In this work, some of the framework aluminum ions were exchanged by chromium(III) to introduce an electron spin resonance (ESR) active probe ion. The resulting material was investigated by means of ESR and electron nuclear double resonance (ENDOR) spectroscopy to verify the incorporation of chromium at the octahedral framework sites. In addition, variable-temperature ESR measurements were performed to analyze the temperature-dependent phase behavior of the doped MIL-53 material. The Cr(III) ions have an electron spin S = 3/2 which shows a characteristic fine structure interaction depending very sensitively on the local symmetry of the chromium site. Therefore, the fine structure splitting of the Cr(III) probe ions allows for a concise elucidation of changes in the local symmetry at the CrO4(OH)2 octahedra which occur upon structural transitions. In agreement with results from neutron experiments, the transformation from the open to the closed pore structure was found to occur in the temperature range between 150 and 60 K whereas the back transformation is taking place within a smaller temperature interval between 330 and 375 K.