Björn Bredenkötter

Heterogeneous Catalytic Oxidation by MFU‐1: A Cobalt(II)‐Containing Metal–Organic Framework

Porous metal–organic frameworks (MOFs) are a rapidly emerging class of multifunctional hybrid materials that might be useful for diverse technical applications, such as gas or liquid adsorption and separation, molecular recognition, or catalysis. Combining polycarboxylate ligands and (transition) metal ions, moderately robust MOFs can be prepared; 1,4-benzenedicarboxylate (bdc, terephthalic acid) and 4,4’biphenyldicarboxylate (bpdc) are often used as linkers. Highly porous non-interpenetrated frameworks, such as the well-known MOF-5 ([Zn4O(bdc)3]) [2] or IRMOF-9 ([Zn4O(bpdc)3]) [3] then form. These microporous MOFs generally show good thermal stabilities (decomposition occurs at T> 350 8C). A fundamental disadvantage, however, is their low hydrolytic stability: Decomposition of the framework occurs rapidly when the gas or liquid phase contains a small amount of water, which imposes severe limitations on their usage in catalytic oxygenation reactions, in which water is a major reaction product. Preliminary attempts to use MOF-5 as a photocatalyst have been reported recently; however, the fact that these frameworks contain Lewis acidic zinc(II) ions only imposes severe limitations on their use in redox catalytic applications in general. Conceptually different approaches have been reported to circumvent the intrinsic disadvantages of MOF-5-type frameworks. Fischer et al. reported the gas-phase deposition of volatile organometallic complexes in the open cavities of MOF-5. Subsequent photolytic or reductive cleavage of the precursors led to catalytically active metal clusters (Cu, Pd, Au) that are finely dispersed in the MOF-5 framework. Nguyen, Hupp et al. were among the first to present oxidations using a MOF catalyst. They used an enantiomerically pure manganese complex of a modified salen ligand as a building block to construct a three-dimensional porous framework. A distinct approach towards heterogeneous asymmetric catalysis based on a homochiral metal–organic framework was recently proposed by Lin et al. However, industrial oxidation or oxygenation reactions typically require very high turn-over numbers (TONs) and frequencies (TOFs), which have not been achieved to date by current MOF catalysts. To produce thermally and hydrolytically stable redoxactive MOFs, our initial efforts focused on the isostructural replacement of a single zinc ion by an open-shell transition metal ion M within the tetranuclear {Zn4O} coordination unit of MOF-5. However, all attempts in this direction led to heteronuclear MOFs containing trinuclear coordination units (for example, [MZn2(bpdc)3(dmf)2], M = Co , Ni, Cd), which are structurally different from MOF-5. A search of the CSD database, however, led to the tetranuclear complex [Co4O(3,5-dmpz)6] (3,5-dmpz = 3,5-dimethylpyrazolate), [10]

Almost Enclosed Buckyball Joints: Synthesis, Complex Formation, and Computational Simulations of Pentypticene‐Extended Tribenzotriquinacene

We report the synthesis of a tribenzotriquinacene-based (TBTQ) receptor (3) for C60 fullerene, which is extended by pentiptycene moieties to provide an almost enclosed concave ball bearing. The system serves as a model for a self-assembling molecular rotor with a flexible and adapting stator. Unexpectedly, nuclear magnetic resonance spectroscopic investigations reveal a surprisingly low complex stability constant of K1 =213±37 M(-1) for [C60 ⊂3], seemingly inconsistent with the previously reported TBTQ systems. Molecular dynamics (MD) simulations have been conducted for three different [C60 ⊂TBTQ] complexes to resolve this. Because of the dominating dispersive interactions, the binding energies increase with the contact area between guest and host, however, only for rigid host structures. By means of free-energy calculations with an explicit solvent model it can be shown that the novel flexible TBTQ receptor 3 binds weakly because of hampering entropic contributions.

Isolated and linear arrays of surfactant-encapsulated polyoxometalate clusters on graphite

We report on the self-assembly of several surfactant-encapsulated clusters (SECs) on the basal plane of graphite consisting of the doughnut-shaped tungstophosphate anion [Na(H2O)P5W30O110] covered by a hydrophobic shell of surfactants. Well-ordered rodlike structures are observed using scanning force microscopy. No such ordering is observed if the surfactant methyltrioctadecylammonium is used for encapsulation, suggesting that the density of alkyl chains around the polyoxometalate cluster is an important factor in determining the order of SEC assemblies on graphite. Coadsorption of tetratetracontane (n-C44H90) and (DODA)14[Na(H2O)P5W30O110] results in single, isolated SECs on a buffer layer of tetratetracontane, as determined by scanning tunneling microscopy.

Magnetodielectric coupling in a non-perovskite metal–organic framework

Multiferroicity and magnetodielectric coupling in metal–organic frameworks (MOFs) are rare and so far restricted mainly to formate-based systems with a perovskite structure. In the course of this work we designed a tetragonal framework [Co(C16H15N5O2)], exhibiting spin-chains of Co2+ ions, which are bridged by an organic linker containing a dipolar nitrobenzene moiety. This compound shows relaxor-like ferroelectricity at 100 K, which is followed by the onset of complex magnetic order at 15 K, indicative of weak ferromagnetism. The clear anomaly of the dielectric constant at the magnetic ordering transition indicates magnetodielectric coupling, which is also confirmed by magnetic-field dependent dielectric measurements. Both weak ferromagnetism and magnetodielectric couplingprobably result from a significant Dzyaloshinskii–Moriya interaction, which cants the spin structure and locally breaks inversion symmetry. We document that the introduction of dipolar nitrobenzene as a building block into the crystal structure paves the way to designing new multiferroic and magnetodielectric MOFs.

A supramolecular compound mimicking the Cu-containing active site of pMMO enzyme

In the last few years we have been investigating structural and physical properties of complexes of a wide range of polycarboxylic acids with many main-group, transition and lanthanide metals. Ligands have 2–6 carboxylate groups and some have other functional groups that can coordinate to metal centres. Since most of the structures are polymeric coordination networks, solubilities are generally low and it has been necessary to develop synthetic methods that generate the products slowly, including solvothermal and ionothermal synthesis and slow diffusion through solid, liquid and gel media. While some crystals can be satisfactorily studied by single-crystal diffraction with standard laboratory equipment, many are very small and require synchrotron facilities, with which we have had considerable success. Structures include 1D (chain), 2D (sheet) and 3D assemblies, some of them with pores or channels containing counterions and/or solvent molecules. The metal-organic frameworks may be charged or neutral. Along with structural characterization by singlecrystal diffraction, we have also carried out powder diffraction, thermal and adsorption experiments, and some spectroscopic studies. A selection of results will be presented, illustrating the range of structures and properties found, and trends and relationships that have emerged.