The sulfidation of metal oxides is critical to the creation of catalyst active sites for industrially-relevant reactions, but occurs at high temperatures for refractory oxides like zirconia. Here, we investigate sulfidation of the structurally-well-defined, Zr-based metal-organic framework (MOF) NU-1000 as a model system. Sulfidation with H2S and alkane thiols occurred readily at room temperature after thermal distortion of the Zr6 clusters in NU-1000. The sulfidation occurred via heterolytic S-H bond cleavage over frustrated Lewis-acid/Lewis-base pairs in the form of under-coordinated Zr(IV) sites and a terminal O(-II) site formed during the thermal distortion. In situ synchrotron based structural analysis, spectroscopic characterization, and computational studies provide evidence that thiols react with distorted Zr6 nodes to form stable bridging Zr-S-Zr bonds and a terminal hydroxo. This study provides insight into the surface structures responsible for the sulfidation of high-valent metal oxides and also suggests a promising strategy for introducing S-bearing moieties to otherwise sulfur-resistant MOF nodes.

The ditopic macrocycle H-8L2 comprising two 3,3 '-dihydroxy-4,4 '-bis(iminomethyl)-biphenyl and two calix[4]arene units was synthesized and its ability to bind lanthanide ions has been studied. H-8L2 reacts readily with Ln(iii) nitrate salts in the presence of NEt3 to support dinuclear neutral complexes of composition [Ln(2)(H-2L2)(H2O)(2)] (Ln = La3+ (1), Eu3+ (2), Tb3+ (3), and Yb3+ (4)). The crystal structure of the ditopic calix[4]arene ligand reveals a centrosymmetric molecule with an overall "S-shape" structure and a cone-conformation. The molecular structure of the Yb complex 4 is also "S-shape" indicative of some degree of preorganization imposed by the ligand constraints. The Yb3+ ions are seven-coordinated (distorted monocapped trigonal prismatic coordination environment (NO6 donor set)). DFT calculations for the La complex 1 (at the r(2)SCAN-3c level of computation) implies an isostructural series of compounds. The ligand and complexes exhibit strong absorptions (epsilon > 10(4) M-1 cm(-1)) in the 250-440 nm range attributed to ligand-based pi-pi* and n-pi* transitions. The bis(iminomethyl)biphenyl linkers were found to sensitize Eu-III and Tb-III emission (lambda(ex) = 370 nm and 405 nm) in the solid state at 295 K. A material containing statistically distributed Tb-III and Eu-III ions reveals upon 370 nm excitation only Eu3+ emission lines indicative of intermolecular energy transfer processes between the Ln(3+) ions.

Schiff-base units in homo- and heterodinuclear lanthanide complexes can act as antennas for sensitization of Eu and Tb luminescence.

Metal-organic frameworks (MOFs) that contain open metal sites have the potential for storing hydrogen (H2) at ambient temperatures. In particular, Cu(I)-based MOFs demonstrate very high isosteric heats of adsorption for hydrogen relative to other reported MOFs with open metal sites. However, most of these Cu(I)-based MOFs are not stable in ambient conditions since the Cu(I) species display sensitivity toward moisture and can rapidly oxidize in air. As a result, researchers have focused on the synthesis of new air-stable Cu(I)-based materials for H2 storage. Here, we have developed a de novo synthetic strategy to generate a robust Cu(I)-based MOF, denoted as NU-2100, using a mixture of Cu/Zn precursors in which zinc acts as a catalyst to transform an intermediate MOF into NU-2100 without getting incorporated into the final MOF structure. NU-2100 is air-stable and displays one of the initial highest isosteric heats of adsorption (32 kJ/mol) with good hydrogen storage capability under ambient conditions (10.4 g/L, 233 K/100 bar to 296 K/5 bar). We further elucidated the H2 storage performance of NU-2100 using a combination of spectroscopic analysis and computational modeling studies. Overall, this new synthetic route may enable the design of additional stable Cu(I)-MOFs for next-generation hydrogen storage adsorbents at ambient temperatures.

The direct and selective conversion of methane to methanol can be considered a holy grail for catalysis research. In this work, we study a metal-organic framework known as MFU-4l, modified by design to include highly reactive iron-oxo species for the catalytic C-H bond activation of methane. We investigate the oxidation of methane and the further potential oxidation of the product methanol using N₂O as an oxygen source and map the potential energy landscape of these reactions using density functional theory calculations. We show that the highest energy barrier encountered during the methane oxidation process is not the C-H bond breaking, but the activation of the iron center by N₂O. Furthermore, the potential energy landscape for the C-H bond activation exhibits a large, high-energy plateau region instead of a sharp transition state, thus differing from the traditional radical rebound mechanism. This insight offers interesting potential routes to enhance the catalytic activity of the catalyst, to hinder unwanted deactivation pathways, and to reduce the activity towards the over-oxidation of the product.

Atomically precise cerium oxo clusters offer a platform to investigate structure-property relationships that are much more complex in the ill-defined bulk material cerium dioxide. We investigated the activity of the MCe70 torus family (M = Cd, Ce, Co, Cu, Fe, Ni, and Zn), a family of discrete oxysulfate-based Ce70 rings linked by monomeric cation units, for CO oxidation. CuCe70 emerged as the best performing MCe70 catalyst among those tested, prompting our exploration of the role of the interfacial unit on catalytic activity. Temperature-programmed reduction (TPR) studies of the catalysts indicated a lower temperature reduction in CuCe70 as compared to CeCe70. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that CuCe70 exhibited a faster formation of Ce3+ and contained CO bridging sites absent in CeCe70. Isothermal CO adsorption measurements demonstrated a greater uptake of CO by CuCe70 as compared to CeCe70. The calculated energies for the formation of a single oxygen defect in the structure significantly decreased with the presence of Cu at the linkage site as opposed to Ce. This study revealed that atomic-level changes in the interfacial unit can change the reducibility, CO binding/uptake, and oxygen vacancy defect formation energetics in the MCe70 family to thus tune their catalytic activity.

The dynamics of CO2 in third generation Metal-Organic Frameworks can be experimentally observed by 13C NMR spectroscopy. The obtained line shapes directly correlate with the motion of the adsorbed CO2, which in turn are readily available from classical molecular dynamics simulations. In this article, we present our publicly available implementation of an algorithm to calculate NMR line shapes from MD trajectories in a matter of minutes on any current personal computer. We apply the methodology to study an effect observed experimentally when adsorbing CO2 in different samples of the pillared layer MOF DUT-8(Ni). In 13C NMR experiments of adsorbed CO2 in this MOF, small (rigid) crystals result in narrower NMR line shapes than larger (flexible) crystals. The reasons for the higher mobility of CO2 inside the smaller crystals is unknown. Our ligand field molecular mechanics simulations provide atomistic insight into the effects visible in NMR experiments with limited computational effort.

The development of adsorbents with molecular precision offers a promising strategy to enhance storage of hydrogen and methane─considered the fuel of the future and a transitional fuel, respectively─and to realize a carbon-neutral energy cycle. Herein we employ a postsynthetic modification strategy on a robust metal-organic framework (MOF), MFU-4l, to boost its storage capacity toward these clean energy gases. MFU-4l-Li displays one of the best volumetric deliverable hydrogen capacities of 50.2 g L-1 under combined temperature and pressure swing conditions (77 K/100 bar → 160 K/5 bar) while maintaining a moderately high gravimetric capacity of 9.4 wt %. Moreover, MFU-4l-Li demonstrates impressive methane storage performance with a 5-100 bar usable capacity of 251 cm3 (STP) cm-3 (0.38 g g-1) and 220 cm3 (STP) cm-3 (0.30 g g-1) at 270 and 296 K, respectively. Notably, these hydrogen and methane storage capacities are significantly improved compared to those of its isoreticular analogue, MFU-4l, and place MFU-4l-Li among the best MOF-based materials for this application.

In this work we investigate the adsorption of chlorinated methanes (CHxCl4-x, x=0-4) in a representative layer-pillar Metal-Organic Framework (MOF), the flexible MOF Ni2(ndc)2(dabco) (ndc = 2,6-naphthalene-dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane), also known as DUT-8(Ni). The guest molecules show a systematic increase of polarizability with increasing number of chlorine atoms, while the dipole moment exceeds 2 Debye for x = 2 and 3. Our ligand field molecular mechanics (LFMM) simulations show that, counter-intuitively, the host-guest interactions are mainly characterized by London dispersion, despite the molecular dipole moments reaching magnitudes as large as water. This highlights the importance of London dispersion interactions in the description of host-guest interactions.

Flexible metal-organic Frameworks (MOFs) are an interesting class of materials due to their diverse properties. One representative of this class is the layered-pillar MOF DUT-8(Ni). This MOF consists of Ni-2 paddle wheels interconnected by naphthalene dicarboxylate linkers and dabco pillars (Ni-2(ndc)(2)(dabco), ndc = 2,6-naphthalene-dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane). DUT-8(Ni) undergoes a volume change of over 140% upon adsorption of guest molecules. Herein, a ligand field molecular mechanics (LFMM) study of the CO2-induced flexibility of DUT-8(Ni) is presented. LFMM is able to reproduce experimental and DFT structural features as well as properties that require large simulation cells. It is shown that the transformation energy from a closed to open state of the MOF is overcompensated fivefold by the host-guest interactions. Structural characteristics of the MOF explain the shape of the energy profile at different loading states and provide useful insights to the interpretation of previous experimental results.

Metal-organic frameworks (MOFs) are coordination networks with organic ligands containing potential voids. Some MOFs show pronounced structural flexibility that may result in closing and re-opening these pores. Here, we show that collective flexibility in a MOF-DUT-8(Ni) - is controlled by conformational isomerism. DUT-8(Ni), a pillared-layer MOF with Ni2 paddle-wheels, dabco pillars and naphthalene dicarboxylate (ndc) linkers, can crystallize in many conformational isomers that depend on the orientation of the non-linear ndc linkers with respect to each other. While the open form is compatible with several of these conformations, only one of them, with alternating linker orientations, is stable as the closed form. We show, by means of first principles calculations, that in the stable closed form, the appreciable lattice strain is compensated by London-dispersion forces between the ndc linkers that arrange with maximum overlap in a stacking order similar to the stacking in graphite. We substantiate these results by well-tempered metadynamics calculations on the DFT-based Born-Oppenheimer potential energy surface, by refined X-ray diffraction data and by nitrogen adsorption data obtained by experiment and grand-canonical Monte-Carlo simulations based on the DFT-optimized and PXRD-derived geometries. While the reported origin of flexibility cannot be generalized to all flexible MOFs, it offers a rational design concept of folding mechanisms in switchable MOFs by exploitation of the stabilization effect of linker stacking in the closed form.

We evaluate the performance of spin-polarized DFTB within the SCC-DFTB (also known as DFTB2) model. The method has been implemented in the ADF modeling suite. We briefly review how spin polarization is incorporated into the DFTB2 method and validate the method in terms of structural parameters and energies using the GMTKN30 test set, from which we used 288 spin-polarized systems.