Synthetic Investigations of Molecular s-block Organometallic Chemistry Supported by Singlet Carbenes

Singlet N-heterocyclic carbenes and cyclic(alkyl)(amino) carbenes have transformed many aspects of modern organic and organometallic chemistry. These ligands have been used to support and stabilize a wide array of highly reactive molecular fragments. Carbenes have proven especially useful in stabilizing electron-rich or hydridic main group organometallic compounds that would be otherwise inaccessible. In the s-block, the isolation and study of such compounds is a highly sought-after goal at the forefront of the field. As such, much effort has been placed on developing appropriate ligand platforms to allow for reliable synthetic chemistry with s-block elements under mild conditions. Success toward this goal has been achieved when a strongly coordinating, anionic supporting ligand was used, as will be discussed in Chapter 1. These ligands afford a high degree of control over the many decomposition pathways known for alkaline earth compounds; however, this stability comes at the price of somewhat restricting the chemistry that can occur at the metal center. By contrast, carbene-supported systems allow for a high degree of coordinative flexibility at the metal center, but using singlet carbenes to support alkali and alkaline earth chemistry has proved difficult. A number of synthetic challenges remain in this area, but solving these difficulties with well-designed synthetic protocols is critical to unlocking new reaction motifs for alkaline earth systems.
The doctoral research presented herein covers a multi-year effort to expand the known coordination chemistry of carbene-supported alkaline earth fragments. In work presented in chapter 2, the reduction chemistry of carbene–magnesium complexes was studied in detail. This effort resulted in a series of magnesium–diimine compounds that can be reduced in a stepwise fashion to yield highly sought-after electron-rich alkaline earth compounds with unusual electronic structures. Moreover, these reduced compounds were compared with their beryllium analogs, yielding a rare opportunity to comparatively study the first two alkaline earth elements in a combined synthetic and computational investigation.
Described in chapter 3, a reliable and high-yielding synthetic method was discovered for obtaining hydrocarbon-soluble, low-nuclearity, carbene-supported magnesium hydride complexes. Despite their high synthetic utility, it is notoriously challenging to synthesize alkaline earth hydride compounds which can be studied in solution and maintain discrete stable molecular structures without ligand dissociation or exchange. This is especially true when using neutral donor ligands such as carbenes, so there is substantial value associated with reliable synthetic procedures like those presented in chapter 3.
Chapter 4 details the investigation of direct reduction chemistry of carbene–CO2 adducts, known as carbene-2-carboxylates, resulting in both metal-free and alkali-mediated reduction of C═O bonds of carbon dioxide. In electrochemical systems, a CAAC-2-carboxylate can accept on electron to form a radical anion which itself can undergo further reaction with free CO2. The result of this transformation is the reductive disproportionation of two units of CO2 to CO and CO32-. When alkali metal reductants were used in place of electrochemical reduction, one- and two-electron reduced CAAC–CO2 clusters incorporating alkali cations were obtained. Notably, these reduced compounds were soluble, crystalline, and thermally stable thanks to the presence of the carbene fragment. These results represent a substantial addition to the known chemistry of these popular synthetic reagents. Prior to this study, no reports existed focused on solely on the electron-acceptor capabilities of carbene-2-carboxylates, despite their common use in systems focused on CO2 reduction.