Rochus Schmid

Surface chemistry of metal-organic frameworks at the liquid-solid interface.

Metal-organic frameworks (MOFs) are a fascinating class of novel inorganic-organic hybrid materials. They are essentially based on classic coordination chemistry and hold much promise for unique applications ranging from gas storage and separation to chemical sensing, catalysis, and drug release. The evolution of the full innovative potential of MOFs, in particular for nanotechnology and device integration, however requires a fundamental understanding of the formation process of MOFs. Also necessary is the ability to control the growth of thin MOF films and the positioning of size- and shape-selected crystals as well as MOF heterostructures on a given surface in a well-defined and oriented fashion. MOFs are solid-state materials typically formed by solvothermal reactions and their crystallization from the liquid phase involves the surface chemistry of their building blocks. This Review brings together various key aspects of the surface chemistry of MOFs.

Loading of porous metal–organic open frameworks with organometallic CVD precursors: inclusion compounds of the type [LnM]a@MOF-5

The highly porous coordination polymer [Zn4O(bdc)3] (bdc = benzene-1,4-dicarboxylate; MOF-5 or IRMOF-1) was loaded with typical MOCVD precursor molecules 1–10 for metals such as Fe, Pt, Pd, Au, Cu, Zn, Sn. Exposure of [Zn4O(bdc)3] to the vapour of the volatile organometallic compounds, e.g. ferrocene (3), resulted in the formation of inclusion compounds of the type [LnM]a@MOF-5, where [LnM] indicates the MOCVD precursor and a denotes the effective number of molecules per cavity of the MOF-5 lattice. The obtained inclusion compounds were characterised by C/H combustion analysis, determination of the metal content by atomic absorption spectroscopy, FT-IR and solid state NMR spectroscopy and by powder X-ray diffraction. The data prove that the host lattice and the guest molecules interact only by weak van der Waals forces without any change of the framework or the chemical nature of the included molecules. Rapid desorption is observed for small and comparably volatile compounds such as pentacarbonyliron or diethyl zinc. Less labile inclusion compounds were obtained for cyclopentadienyl complexes as guest molecules, e.g. a rather high loading of six molecules of ferrocene per cavity was observed. Careful hydrolysis/calcination of [Zn(C2H5)2]2@MOF-5 resulted in the composite (ZnO)2@MOF-5 pointing to the possibility to develop a subsequent chemistry of the embedded precursor molecules to yield novel nanocomposite materials based on MOFs as host matrices and MOCVD precursors in general.

Structural Complexity in Metal–Organic Frameworks: Simultaneous Modification of Open Metal Sites and Hierarchical Porosity by Systematic Doping with Defective Linkers

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.

Flexibility and Sorption Selectivity in Rigid Metal-Organic Frameworks: The Impact of Ether-Functionalised Linkers

The functionalisation of well-known rigid metal-organic frameworks (namely, [Zn(4)O(bdc)(3)](n), MOF-5, IRMOF-1 and [Zn(2)(bdc)(2)(dabco)](n); bdc = 1,4-benzene dicarboxylate, dabco = diazabicyclo[2.2.2]octane) with additional alkyl ether groups of the type -O-(CH(2))(n)-O-CH(3) (n = 2-4) initiates unexpected structural flexibility, as well as high sorption selectivity towards CO(2) over N(2) and CH(4) in the porous materials. These novel materials respond to the presence/absence of guest molecules with structural transformations. We found that the chain length of the alkyl ether groups and the substitution pattern of the bdc-type linker have a major impact on structural flexibility and sorption selectivity. Remarkably, our results show that a high crystalline order of the activated material is not a prerequisite to achieve significant porosity and high sorption selectivity.

Ab initio parametrized MM3 force field for the metal-organic framework MOF-5.

A new valence force field has been developed and validated for a particular class of coordination polymers known as nanoporous metal‐organic frameworks (MOFs), introduced recently by the group of Yaghi. The experimental, structural, and spectroscopic data in combination with density functional theory calculations on several model systems were used to parametrize the bonded terms of the force field, which explicitly treats the metal–oxygen interactions as partially covalent as well as distinguishes different types of oxygens in the framework. Both the experimental crystal structure of MOF‐5 and vibrational infrared spectrum are reproduced reasonably well. The proposed force field is believed to be useful in atomistic simulations of adsorption/diffusion of guest molecules inside the flexible pores of this important class of MOF materials.

A Cryogenically Flexible Covalent Organic Framework for Efficient Hydrogen Isotope Separation by Quantum Sieving

Conventional molecular sieves are often utilized in industryfor the purification of gas mixtures consisting of differentmolecular sizes. However, separation of hydrogen isotopesrequires special efforts because of their identical size, shape,and thermodynamic properties. Separation of isotope mix-tures is only possible with limited techniques, such ascryogenic distillation, thermal diffusion, and the Girdlersulfide process, but these methods have a low separationfactor and are excessively time- and energy-intensive.

QuickFF: a program for a quick and easy derivation of force fields for metal-organic frameworks from ab initio input

QuickFF is a software package to derive accurate force fields for isolated and complex molecular systems in a quick and easy manner. Apart from its general applicability, the program has been designed to generate force fields for metal‐organic frameworks in an automated fashion. The force field parameters for the covalent interaction are derived from ab initio data. The mathematical expression of the covalent energy is kept simple to ensure robustness and to avoid fitting deficiencies as much as possible. The user needs to produce an equilibrium structure and a Hessian matrix for one or more building units. Afterward, a force field is generated for the system using a three‐step method implemented in QuickFF. The first two steps of the methodology are designed to minimize correlations among the force field parameters. In the last step, the parameters are refined by imposing the force field parameters to reproduce the ab initio Hessian matrix in Cartesian coordinate space as accurate as possible. The method is applied on a set of 1000 organic molecules to show the easiness of the software protocol. To illustrate its application to metal‐organic frameworks (MOFs), QuickFF is used to determine force fields for MIL‐53(Al) and MOF‐5. For both materials, accurate force fields were already generated in literature but they requested a lot of manual interventions. QuickFF is a tool that can easily be used by anyone with a basic knowledge of performing ab initio calculations. As a result, accurate force fields are generated with minimal effort.

An Accurate Force Field Model for the Strain Energy Analysis of the Covalent Organic Framework COF-102

By using an accurate molecular mechanics force field, which was parametrized on the basis of first principles calculations, the network topology of the Covalent Organic Framework COF-102 could correctly be predicted without experimental input. The ctn structure is preferred thermodynamically by 11.2 kcal mol(-1) over the bor topology due to a lower steric strain. The origin for this strain is analyzed in detail.

Hypothetical 3D-periodic covalent organic frameworks: exploring the possibilities by a first principles derived force field

A scheme to predict as yet unknown, hypothetical covalent organic frameworks (COFs) from scratch by screening the possible space of supramolecular isomers is presented. This is achieved by extending our currently developed first principles derived force field MOF-FF with a parametrization for the boroxin unit. We considered four non-tetrahedral monomers with four boronic acid groups inspired by the corresponding carboxylate linkers known from metal–organic frameworks, and investigated the potential 3,4-connected topologies with edge-transitivity (ctn, bor, pto and tbo) or transitivity 32 (ofp, tfj, fjh, iab and nju). Due to the partly lower symmetry of the building blocks with respect to the vertex, beyond topological isomerism also isoreticular isomers are formed. We have used our reverse topological approach to construct the fictitious structures and employed an automated genetic algorithm based global minimum search approach to screen the vast configurational space of isoreticular isomerism and predicted a series of hypothetical 3D-COFs. All structures are completely relaxed by including the lattice parameters. From the atomistic structures, the accessible surface areas were determined, and, because of the isomer screening procedure, the question of crystallographic disorder could also be answered. Beyond the examples of hypothetical 3D-COFs serving as a lead for future synthetic investigations, this work is intended in particular to introduce the efficient predictive modeling method which can be applied to any kind of hypothetical COF system.

Low‐Temperature CO Oxidation over Cu‐Based Metal–Organic Frameworks Monitored by using FTIR Spectroscopy

Low-temperature CO oxidation is among the most interesting reactions in heterogeneous catalysis. It is well known that this reaction can be catalyzed by gold deposited as nanoparticles on metal oxide surfaces (e.g. TiO2, MgO, and ZnO). [1] Developing highly efficient and low-cost catalysts for low-temperature CO oxidation represents a major challenge. Most recently, Cubased metal–organic frameworks (MOFs) have been identified for efficient CO oxidation at ambient pressure and elevated temperatures, namely [Cu3btc2] (btc = benzene-1,3,5-tricarboxylate ) and [Cu5(OH)2(nip)4(H2O)6] (H2O)4.25 (nip = 5-nitroisophthalate) with 100 % conversion at 240 8C and 200 8C, respectively. 3] However, the origin of the catalytic activity of such CuMOFs is not well understood. Coordinatively unsaturated metal ion sites (CUS) at the backbone of MOFs are known to play an important role for catalysis, gas storage, chemical sensing, and other applications. For catalysis, Schlichte et al. reported on the cyanosilylation of benzaldehyde and acetone with [Cu3btc2] as catalyst (HKUST-1, Figure S1) for which its activity was assigned to the Lewis acid properties of intrinsic Cu CUS at the copper-carboxylate paddle-wheel building unit (CuPW) of this prototypical MOF in its dehydrated form. The characterization of reactive CUS of MOFs with infrared spectroscopic techniques and suitable probe molecules, quite similar to classic heterogeneous catalysts, is expected to provide most valuable information for mechanistic understanding and fine tuning of the MOF materials. Herein, we present our studies on the in situ monitoring of the co-adsorption of CO and O2 at [Cu3btc2] (HKUST-1) [9] and its congener [Cu3btb2] (MOF-14, [11] btb = benzene-1,3,5-tribenzoate) by using an ultrahigh vacuum infrared spectroscopy tool (UHV-FTIRS). The high-quality IR data provide unambiguous spectroscopic evidence for the surprisingly high catalytic activity of both Cu-MOFs samples for CO oxidation at temperatures as low as 105 K, according to Scheme 1. Presented in Figure 1 are the UHV-FTIR spectra obtained after exposing activated [Cu3btc2] to CO (1 10 4 mbar) in the temperature range of 105 K to 135 K. Prior to CO adsorption, the thermal stability of the sample was confirmed up to 650 K (Figure S2). The H2O-related bands at ñ= 3647 (O H stretching mode) and 1617 cm 1 (d(H2O) scissoring mode) disappeared completely by heating the sample above 400 K, indicating the formation of Cu + CUS. The adsorption of CO at 105 K leads to the appearance of four IR bands at ñ= 2198, 2175, 2152, and 2128 cm . The dominant CO band at ñ= 2175 cm 1 exhibits a blue shift of 32 cm 1 with respect to the gas-phase value and is assigned to CO adsorbed at the intrinsic Cu + CUS of the CuPW in the axial position. After evacuation to 10 10 mbar, this band shifts to ñ= 2179 cm 1 and decreases in intensity (Figure 1 c). It is removed completely upon heating to 130 K, revealing that the majority CO species is weakly bound to intrinsic Cu + CUS sites. Density functional theory (DFT) calculations were performed in order to support the assignment of the IR bands. Based on our previous studies on copper paddle-wheel (CuPW) MOFs, we found it to be sufficient to use a non-periodic formate model of the inorganic unit to represent the carboxylate-linked Scheme 1. Low-temperature oxidation of CO to CO2 over activated Cu-based MOFs.