Ligand Strategies based on Electronically Flexible Carbon Donors in Magnesium and Bismuth Chemistry
Obi, Akachukwu, Chemistry - Graduate School of Arts and Sciences, University of Virginia
Gilliard, Robert, University of Virginia
Main-group (s- and p-block) organometallic reagents based on environmentally benign and biocompatible metals are increasingly being adopted in chemical synthesis for bond activation events competitive with the more versatile transition metal complexes. Central to these advances is the correct choice of ancillary ligand for stabilizing well-defined species with unusual bonding scenarios (e.g., low coordinate, low valent or highly electrophilic complexes), in order to mimic the synergistic effect of d-orbitals for canonical organometallic reactivities (i.e., oxidative addition, reductive elimination, insertion and elimination reactions). In addition to their practical application for “greener” catalysis, inquests into unusual bonding in main group species typically enable enigmatic reactivities with fundamental and pedagogical significance. To this end, it is widely accepted that bulky, polydentate, anionic ligands based on electronegative elements (e.g., N, O) are critical for kinetic stabilization, especially for electropositive metals such as magnesium and bismuth, which tend to form metastable bonds with neutral donors. This dissertation investigates the stabilization of organo-magnesium and -bismuth complexes primarily through sterically unencumbered, electronically-flexible donor ligands such as carbenes and carbones.
Chapter One addresses ligand stabilization strategies in group 2 chemistry through the lens of periodic trends, which suggests that a one-size-fits-all is not appropriate. As an alternative, the electronic diversity achievable using carbenes and carbones is presented. In the p-block, these carbon-based donors are widely adopted, except for the heaviest element, bismuth. Therefore, new strategies for stabilizing organobismuth complexes using carbones were highlighted (and further elaborated in Chapter Seven). Chapter Two discusses the stabilization of redox-flexible magnesium complexes using redox non-innocent diimines, with unprecedented multiple bonding at magnesium due to electromeric carbene-diimine interactions. It was also discovered that multiple carbenes at the same magnesium center discouraged deleterious redox disproportionation. The same strategy enabled the stabilization of highly electrophilic, organomagnesium cations benefitting from bis- or tris-carbene stabilization in Chapter Three. The electronic influence of carbenes for stabilizing small molecule building blocks at magnesium was further demonstrated in the isolation of thermally-stable magnesium phosphaethynolate (O–C≡P) complexes in Chapter Four.
Chapter Five introduces the first example of reversible migratory insertion chemistry at normal valent s-block species. This process is mediated by N-heterocyclic carbenes (NHCs), which shuttle unsaturated aminoboranes (Me2N=BH2) within the coordination sphere of magnesium amidoboranes. Notably, the utilization of sterically unencumbered NHCs was critical to the thermodynamic stability of these species. Analogous calcium amides are described in Chapter Six, and no dynamic migratory insertion processes were observed. Therefore, the NHC-assisted aminoborane migration at magnesium was attributed to comparative Lewis acidities of base-free magnesium amides and Me2N=BH2.
Finally, Chapter Seven details the isolation of remarkably air-stable carbone-bismuth halides, which benefit from geometrically-constrained, persistent carbone coordination. Their unprecedented trans carbone-Bi-halide ligation facilitated rapid dehydrosilylation redox catalysis, and the transient Bi-H intermediate was captured using B(C6F5)3 as the first isolable bismuth hydridoborate.
PHD (Doctor of Philosophy)
magnesium, bismuth, calcium, carbene, amine borane