silver-iron oxide particles as heterogeneous catalysts for the cross coupling of arenes and heterocycles
Advances in nanomaterials research have stoked interests in the design of dispersible catalysts for specific organic transformations, with higher reaction efficiency or lower burden in post-reaction waste processing. Multicomponent heterogeneous catalysts generally offer higher catalytic performance than single-component catalysts, with metal–substrate interactions (MSI) playing a key role in their performance. This thesis focuses on silver–iron-oxide particles as heterogeneous catalysts, starting with a literature survey (Chapter 1) followed by the synthesis and catalytic properties of two novel types of Ag–Fe3O4 particles that show strong potential for mediating C(sp2)–H arylation reactions (Chapters 2 and 3). Silver and especially iron oxide are much less expensive than other types of metals, and the magnetic properties of the Fe3O4 support transferability and reuse of the active catalytic species which enables us to reduce the ratio of catalyst to reactant. These features address multiple goals outlined by the principles of green chemistry. The arylation of heterocyclic compounds is frequently used in the preparation of organic dyes, polymers, and pharmaceutical intermediates, and is a useful benchmark reaction for comparing our cross-coupling catalyst with those from prior reports.
In Chapter 2, we describe the synthesis of colloidal silver–iron-oxide (SIO) and investigate its conversion into an efficient catalyst for C(sp2)–H arylation using novel modes of activation. This includes electrochemical activation using mild cathodic potentials, and photoactivation using a white light source. Both methods dramatically improve the efficacy of colloidal SIO as a catalyst for the cross coupling of diazonium salts with heteroaromatic rings at room temperature. High-resolution transmission electron microscopy analysis reveals that the SIO particles are primarily composed of colloidal Ag that are coated with nanosized islands of Fe3O4. The SIO catalysts are magnetically responsive and can be collected and reused multiple times, without requiring reactivation. The SIO is susceptible to acid degradation but can be preserved with neutralization by added base during reaction cycling.
In Chapter 3, we describe a second-generation catalyst in which Fe3O4 microspheres serves as the supporting substrate for Ag islands, with synthetic control over Ag size distribution. This material does not require any activation for cross-coupling catalysis, which can be attributed to better charge transfer between the Ag islands and Fe3O4 substrate. A comparison of Ag–Fe3O4 microspheres with different Ag/Fe ratios suggests that catalytic activity correlates with smaller particle sizes, where the strongest charge-transfer interactions are likely to occur. The role of MSI between Ag and Fe3O4 was further explored using X-ray absorption spectroscopy. The second-generation Ag–Fe3O4 catalysts are far more robust than the previous version and are better able to withstand acidic degradation, with less mass loss after multiple reaction cycles and no loss in catalytic function. Lastly, we have found that Ag–Fe3O4 microspheres can also be an efficient catalyst for the reduction of nitro groups into amines, and describe progress toward the one-pot conversion of nitroarenes into cross-coupling products.
- Doctor of Philosophy
- West Lafayette