Stefan Bahnmüller

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]

Copper(II) Nanoballs as monomers for polyurethane coatings: synthesis, urethane derivatization and kinetic stability

The self-assembly of copper(II) ions and 5-(2-hydroxyethoxy)benzene-1,3-dicarboxylate (1) leads to Nanoballs in which twelve dinuclear copper(II) paddle-wheel units are interconnected via 24 ligands. The structure of the spherical coordination compound decorated with 24 hydroxy groups has been determined by single crystal X-ray structure analysis. As a model for the integration of Nanoballs into bulk polyurethane polymers and coatings, its reaction with phenylisocyanate is investigated. The stability of Nanoballs against hydrolytic decomposition is studied under acidic conditions and compared to simple copper(II) complexes. Release of copper(II) ions from Nanoballs is much slower than from discrete copper(II) paddle-wheel complexes, suggesting the use of Nanoballs as monomers for polyurethane-based antifouling coatings.