Katz Group

The modern-day alchemists in Alexander Katz' UC Berkeley laboratory are concentrating on the engineering of catalysts—those chemical substances that, like enzymes in plants, trigger biological transformations necessary in the conversion of biomass to fuel. It's a tricky balancing act of organic and inorganic substances, but an essential one if the biofuel process is going to be economically viable.
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Building on my research group's prior use of calixarene ligands to prevent active site aggregation in systems involving coordinated metal cations (see J. Am. Chem. Soc. 2004, 126, 16478-16486 and Chem. Mater. 2009, 21, 1852-1860) and nanoparticle active sites (see reference 7 in manuscript), this manuscript describes the first calixarene-bound metal polyhedra consisting of an Ir₄ cluster. It also demonstrates patterning of the polyhedra with synthesis of compounds consisting of a pair of Ir₄ clusters coordinated to a single calixarene phosphine ligand.
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Modifying the surface of metal nanoparticles with organic ligands can enhance the particles' catalytic properties. Ligands can stabilize neighboring particles against agglomerating into large clumps and burying active sites, and they may also serve as electron donors to activate metal catalysts. But finding suitable ligands and ensuring that they do not render the metal surface inaccessible to would-be reagents are formidable challenges.
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Calixarenes have been successfully anchored onto solid surfaces for the synthesis of chromatographic stationary phases during the past ten years. However, calixarene immobilization has never been accomplished previously without the use of an organic tether, which requires organic synthesis for attachment between calixarene and silica. Our method of calixarene anchoring obviates the need for organic tether and thereby significantly decreases the difficulty and cost of immobilizing calixarene macrocycles on silica. Importantly, from the perspective of functional application, our method of calixarene anchoring selectively produces the cone conformer of the calixarene, which is the most desired for binding neutral organic guests via non-covalent interactions, and metal ions via strategically organized phenolic oxygens on the lower rim. The former capability is used for demonstrating our anchored calixarenes as adsorption active sites.
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Because of the absence of organic tether between calixarene and inorganic oxide support in Cal silica, our method of calixarene immobilization is unique in that it offers intimate electronic and steric contact between calixarene and support surface via a single metal atom, which is not possible to accomplish with conventional calixarene-on-silica materials. We wished to exploit this feature in implementing the calixarene in Cal silica as a surface organometallic ligand for catalytically active transition metal atoms. Because titanium is known to be isostructural with silicon in a tetrahedral oxo environment, we hypothesized that the silicon atom connecting calixarene to silica in Cal silica can readily be substituted with titanium.
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