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| General Overview |
Work in our group focuses on synthetic chemistry, with the primary goal being to make, purify, and study new molecules and materials. We have extensive experience in the synthesis and study of new and unusual molecular inorganic and organometallic compounds of the d- and p- block, and lanthanide elements. The emphasis is on preparing compounds that exhibit novel reactivity and/or catalytic behavior. In addition to the dry-box and Schlenk techniques used to prepare and manipulate compounds, we exploit a variety of characterization methods, including multinuclear NMR, X-ray crystallography, EPR and cyclic voltammetry. Characterization techniques in our materials work include electron microscopy, TGA/DTA, and powder diffraction.
Our work is funded by grants from the NSF, DOE, and AFOSR.
Research is concentrated in the following main areas that span organometallic and coordination chemistry, catalysis, and the design of new materials: |
| Ligand Design in Organometallic Chemistry & Catalysis |
| A major focus of work in this area involves the design and synthesis of new ligands to support and promote novel chemistry at metal centers. Nitrogen donors are heavily employed, although recent work in the group has now expanded this chemistry to encompass electon-rich phosphines. We are especially interested in tetradentate monoanionic (TDMA) ligand sets, which are of interest for the stabilization of low-valent metal fragments. |
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| Through the variation of the anionic donor, we can tune the sterics and electronics of the metal center and modulate its reactivity, both catalytic and stoichiometric. Target molecules include Group 4 alkylidenes and imidos, as well as metal-alkyl and metal-amido species with a focus on catalytic coupling reactions, such as hydroamination. We have a long-standing collaboration in the area of Group 5 chemistry with Professor R.G. Bergman. |
| Hexatantalum Imido clusters |
| Where do transition metal catalysts go to die? That question has been of great importance in industry as well as in pure research, and the answer is sometimes very interesting. During our investigations of the imidotantalum-catalyzed hydroamination of alkynes by anilines, a bright red decomposition product was observed which we found to be a highly symmetrical octahedral hexatantalum cluster complex bearing fourteen arylimido ligands. |
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| The cluster contains an oxygen atom in the center of the tantalum cage, the identity of which we proved by isotopic labeling studies and mass spectrometry. An oxide source is necessary for cluster formation and thus a small quantity of water or tantalum oxide must be added to the reaction mixture if useful synthetic yields are to be attained. While the species were first isolated using (benzyl)3Ta=N-t-Bu as a tantalum source, commercially available synthons such as Ta(NMe2)5 are also effective. A variety of substituted anilines can be employed, including p-tolNH2, p-t-BuC6H4NH2, p-MeOC6H4NH2, and p-BrC6H4NH2. These compounds are extremely air- and moisture-sensitive, but persist in solution for at least two weeks at 200 oC under an atmosphere of nitrogen. Computational results indicate that the cluster LUMO is completely metal-centered and non-bonding, and thus an anionic, reduced species should be attainable. Indeed, a one-electron reduction with decamethylcobaltocene affords a stable salt. Reactivity studies with this new class of metal-imido complex are underway, in collaboration with Professor R.G. Bergman. |
| Actinide Chemistry |
| Since early 2006 we have also developed an active interest in actinide chemisty, focusing on interesting uranium (+3, +4, +5, +6 oxidation states) and thorium (+4 oxidation state) complexes, particularly with sterically demanding ligands and ligands utilizing soft-donor atoms. The complexes we synthesize can then be used as a base for exploring interesting reactivity with small molecules, ligand substitution chemistry and redox chemistry. |
| Materials Chemistry |
Our current emphasis is on the modification of inorganic surfaces to create hybrid materials with control over chemical and physical properties. This work benefits from a strong collaboration with the research group of Professor Peidong Yang.
Two examples which have been explored to date are surface initiated polymerization from oxide materials, which modify the hydrophobicity of the oxide surfaces |
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| Also new thiol linkages for silver nanoparticles which can be used for Surface Enhanced Raman Spectroscopy (SERS). Our current goal is to develop a sensor for Arsenic in ground water by changing the surface of silver colloids like those seen below. |
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This can be achieved through either organic modification with thiols (examples seen below), or through inorganic routes which will deposit oxide materials. |
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| Group 13 Chemistry |
In 2005 we began a new jointly-funded (AFOSR) project with Professor J.R. Long that aims to develop the chemistry of main-group clusters. Owing to the instability of aluminum in the +1 and +2 formal oxidation states, compounds bearing reduced aluminum centers are rare in comparison with the plethora of organoaluminum(III) compounds. We seek to expand the base of synthetic knowledge in this field by targeting new complexes incorporating monoanionic tetradentate ligands capable of stabilizing monomeric aluminum centers, and non-chelating bulky cyclopentadienyl-based ligands that will allow aggregation and formation of metal-metal bonds. |
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| 2,4-Di-tert-butyl-6-bis(2-pyridyl-2-ethyl)aminomethylphenol (H(BPPA)) was chosen as a precursor to monomeric Al(I) compounds because it is a chelating monoanionic ligand with hard donor atoms and had previously been shown to bind transition metals and lanthanides. We are extending this chemistry to tren-like ligands with hard anionic nitrogen and soft phosphine donor atoms. |
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