Molecular Foundry Seminar

"Calixarene-Bound Metal Clusters: Controlling Electronics,
Accessibility, and Catalysis on Metal Surfaces Using Organic Ligands"

Professor Alex Katz, Department of Chemical and Biomolecular Engineering, UC Berkeley,
Tuesday, November 9th at 1:30 pm, Bldg. 66 - Auditorium

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Abstract:

The limited stability of metal cluster catalysts towards aggregation/coalescence/sintering is known to be the most significant factor preventing practical application.  An open question that my research group aims to address is whether an organic-ligand approach can be used to synthesize stable and accessible metal cluster catalysts, while providing for the possibility of electronic tuning of the metal core. 

Using gold nanoparticles as a synthetically-relevant model system, we coordinate symmetric calix[6]arene triphosphine ligands on the surface of large (4 nm) TOAB-stabilized gold nanoparticles and study changes in the electronic properties of the metal core accompanying ligand adsorption.  Au 4f7/2 binding energies measured via XPS decrease upon calixarene coordination to the gold nanoparticle surface.  Further indirect evidence of interaction between calixarene ligands and gold nanoparticles is demonstrated via CD spectroscopy, with a 10-fold enhancement in the molar ellipticity of a calixarene diamine ligand upon adsorption.  This interaction is responsible for the appearance of CD-active SPR band, which plateaus in intensity upon saturation of the surface with chiral ligand.  Calixarene phosphine ligands adsorbed on 4 nm gold nanoparticles enhance metal colloid stability against aggregation, both in solution and when supported on the surface of TiO2.  The amount of accessibility in these systems is measured using a steady-state fluorescence chemisorption technique, which relies on the probe molecule 2-naphthalenethiol (2NT).  Up to approximately 5% of the surface atoms in 4 nm gold nanoparticles are able to bind 2NT, which in turn corresponds to approximately 19% of the gold surface area, when considering the adsorbed 2NT footprint.  Using comparative synthetic studies of various calix[6]arene and calix[8]arene tri- and tetra-phosphine ligands adsorbed on 4 nm gold nanoparticles, we elucidate factors that control accessibility to the metal surface in these systems.  Coupling molecular modeling with experimental observations of accessibility trends suggests an induced-fit mechanism of 2NT binding.  According to this mechanism, calixarene ligand flexibility in the bound state is a key attribute that controls accessibility.  Building on this understanding, a new paradigm of accessibility is demonstrated with the synthesis of subnanometer gold clusters bound with calixarene phosphine ligands.  Up to 25% of the gold atoms are able to bind 2NT in these clusters, which compares with no binding in similarly-sized (coordinatively saturated) conventional Au11 clusters lacking calixarene ligand.

The paradigm of synthesizing robust, accessible, and electronically-tunable metal clusters using calixarene ligands is applied to Ir4-based catalysts.  The first calixarene-bound metal polyhedra are synthesized, which consist of an Ir4 metal core.  Using FTIR spectroscopy of bound CO ligands in these clusters, single-crystal X-ray diffraction, ethylene hydrogenation catalysis, and solid-state NMR spectroscopy before and after catalysis, we demonstrate the calixarene phosphine ligand in these clusters to impart extra electron density to the cluster as well as act as a steric barrier that prevents metal core aggregation during ligand exchange processes accompanying catalysis.