Uncovering Multimetallic Cooperativity and Reactivity in Molecular Triiron Nanoclusters

Abstract

The cooperation of multiple metal centers is critical for difficult multielectron chemical transformations, especially in industrial heterogeneous catalysis. Each metal center participates in catalysis, whether directly or indirectly, but deciphering the specific role of each metal center is often challenging due to the nondiscrete nature of heterogeneous interfaces coupled with the numerous amounts of physiochemical processes occurring within the system. Surface characterization methods can identify active sites and capture surface reorganization but provide limited atomistic insights into the electronic structure and coordinative geometry of the metal components. To address this gap of knowledge, we utilize atomically precise nanoclusters to serve as molecular models for complex catalytic systems, enabling us to gain fundamental insights into how multiple metal centers respond to electronic and coordinative changes. In Chapter 1, we introduce the concept of catalysis and discuss how it has shaped the world we inhabit. We then explore the complicated interfaces and processes of heterogeneous catalysts. Finally, we introduce a molecular method of investigating of heterogeneous catalysts which involves designing, manipulating, and characterizing atomically precise nanoclusters. In Chapter 2, we probe the loci of oxidation of a series tri-oxidized triiron clusters. Despite the coordination of halides to the Fe centers, the assignment of redox states is ambiguous due to the redox non-innocent nature of the Co6Se8 core. The electronic structure of the oxidized clusters was investigated with a variety of spectroscopic methods including X-ray diffractometry, 57Fe Mössbauer spectroscopy, and electronic absorption spectroscopy. We find that the oxidized clusters are mixed valent, where charges are delocalized across the Fe edges and Co6Se8 support, enabling inter-site electronic coupling. In Chapter 3, we report the functionalization of the Fe edge sites with azides by performing anion exchange with the tri-oxidized triiron cluster. Subsequently, the reactivity of the installed azides towards photolysis is investigated. In addition, we investigate the electronic consequences of installing thiocyanates onto the Fe edges, which includes the formation of mono- and di-oxidized clusters. In Chapter 4, we introduce the synthesis of organometallic complexes that model the edge sites of the nanopropeller clusters. We explore the reactivity of these complexes towards ligand binding and oxygen atom transfer reagents. Through this study, we were able to further deconvolute the role the Co6Se8 core plays on the reactivity and electronic structure of the metal edge sites.