Date on Master's Thesis/Doctoral Dissertation

5-2020

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Nantz, Michael

Committee Co-Chair (if applicable)

Zamborini, Francis

Committee Member

Zhang, Xiang

Committee Member

Ramezanipour, Farshid

Committee Member

Sumanasekera, Gamini

Author's Keywords

aminooxy; oximation; MPCs; solid-support; catalysis

Abstract

Aminooxy (-ONH2) groups are most commonly known for their chemoselective reaction with carbonyl compounds (aldehydes/ketones) under mild reaction conditions. Aminooxy-based click chemistry is a versatile means of ligation as evidenced by broad application in material science, biology, biochemistry, analytical chemistry, and nanoscience. Our work exploits the facile reaction of aminooxy groups presented on the surface of gold (Au) or palladium (Pd) monolayer protected clusters (MPCs) with various aldehydes via oximation reactions, which form the robust oxime ether adducts. The functionalization of hexanethiolate-stabilized Au MPCs with a newly developed trifunctional amine-containing aminooxy alkanethiol ligand by thiol place-exchange affords aminooxylated mixed monolayer protected clusters (MMPCs). The Au MMPCs-bound aminooxy groups react readily with various aldehydes in aqueous or non-polar organic solvents through oximation chemistry to form diversely functionalized Au MMPCs containing redox, aromatic, and fluorescent groups as well as chemical receptors that bind acetone. In addition, ligand and pH modification allows tuning of the solubility properties of the aminooxy functionalized Au MMPCs. Moreover, aminooxy-functionalized Au and Pd MMPCs readily bind to commercially available aldehyde-functionalized polystyrene and agarose gel spherical microbeads, which serves as a unique method for the preparation of solid-supported metal clusters. The goal is to use these materials as catalysts with both high turnover frequency (TOF) and long-term recyclability compared to unsupported homogeneous catalysts of the same clusters. Studies of Au and Pd MPCs as homogeneous catalysts reveal that although the MPCs are quite stable, the thiolate capping agent strongly inhibits the catalyst activity. Partial thiolate ligand removal by straightforward in-situ addition of iodine significantly enhances the catalytic activity while maintaining good stability under optimized conditions. An increased rate constant for NaBH4 reduction of 4-nitrophenol to 4-aminophenol demonstrates the effect of partial ligand removal on the catalytic activity of glutathione (SG)-capped Au MPCs serving as the catalyst. Similarly, an increased TOF for the hydrogenation/isomerization of allyl alcohol shows the benefit of iodine-activated partial ligand removal on the activity of SG-capped Pd MPCs. In both reactions, as the amount of I2 increases, the catalytic activity increases (due to partial ligand dissociation) while the catalyst recyclabilitydecreases (due to MPC aggregation). Low equivalents of I2 relative to SG ligands on the MPCs are optimal when considering both reaction rate and catalyst recyclability. The results on SG Au and SG Pd MPCs as homogeneous catalysts combined with successful loading of aldehyde-functionalized microbeads with aminooxy-functionalized Au and Pd MPCs demonstrates the feasibility of preparing unique MPC-loaded porous microbeads activated with iodine for use as stable, efficient heterogeneous catalysts. The combination of aminooxylation chemistry on metal MPCs and iodine activation provides a general approach to new catalyst design. The strategy also shows promise for creating nanomaterials useful for surface-enhanced Raman and chemiresistive selective sensing of carbonyl-containing volatile organic compounds (VOCs) of medical and environmental importance.

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