Highly Reactive Beryllium and Bismuth Complexes: Structure Analysis, Chemical Bonding, and Redox Chemistry

The field of catalysis is dominated by transition metals. In industry, these metals are often used as catalysts for the transformation of simple small molecules into value-added commodities. Many of these catalytic systems feature expensive and geologically scarce precious metals such as platinum, palladium, rhodium, and iridium. Therefore, there is a growing synthetic effort to develop the chemistry of geologically abundant and inexpensive main-group elements. Such well-defined processes are difficult to many main-group elements, thus mimicking the reactivity of transition metals is an enormous synthetic challenge. Access to d-orbitals allows for multiple coordination sites and variable redox states for transition metals, while main-group elements are limited to s- and p-orbitals. Therefore, it is important to study fundamental reaction pathways for main-group elements to discover chemistry that both mimics and diverges those known for transition metals. Herein, beryllium and bismuth chemistry is studied through the scope of neutral carbene and carbone ligands. This research uses a neutral carbon-based ligand (i.e., carbenes and carbones) scaffolding as an approach to develop new bonding modes, oxidation states, and reactivity for main-group elements.
Beryllium, the lightest member of the alklaine earth series, is one of the least explored elements on the periodic table. This is, in part, due to the presumed toxicity of its complexes. Nevertheless, studying beryllium is important for developing a better understanding of periodic trends and synthetic strategies for the alkaline earth elements. Chapter 2 discusses the isolation and study of the first examples of carbodicarbene beryllium complexes. The isolation of these compounds led to the first example of a beryllium mediated C(sp3)–H activation event, which is promoted by either a base or a one-electron reducing agent. A new class of five-membered beryllacycles was also developed, and the coordination chemistry of carbenes led to the first example of a beryllacycle ring-expansion reaction mechanism. A series of carbene-beryllium complexes were also developed and is discussed in chapter 3. These include carbene-supported aryl- and alk-oxo beryllium complexes and doubly reduced carbene-beryllium(α-diamide) molecules.
The chemistry of bismuth has exhibited a remarkable transition-metal-like reactivity profile in recent years. For example, bismuth is capable of both Bi(III/V) and Bi(I/III) redox cycles. Chemists are also interested in bismuth cations for their applications in Lewis acid catalysis. In chapter 4, a new class of bismuth cations (bismaalkene cations) were developed by the complexation of carbodicarbene, followed by halide abstractions. These molecules were studied using X-ray crystallography, NMR spectroscopy, and DFT to investigate the bonding. In chapter 5, the first reactions of organometallic bismuth complexes with sodium 2-phosphaethynolate, the P-analogue of cyanate, were studied. In this study, carbene transfer and thermal reduction of the bismuth center (BiIII to BiII) was observed. The same reactivity profile was observed for analogous antimony complexes.