Correlating Structure and Reactivity of First-Row Late Transition Metal Carbenes in a Weak-Field Environment

Abstract

Leveraging metal-ligand multiple bonds (MLMBs) for the functionalization of inert C–H bonds is an attractive strategy for selectively and efficiently converting simple molecules and feedstocks to more complex architectures. Indeed, nature employs an open-shell iron oxo MLMB unit featuring an attenuated Fe–O bond to accomplish selective C–H hydroxylation under mild conditions. Understanding the structural features (geometric and electronic) that influence the reactivity of MLMB complexes is key to expanding their chemistry to other functionalities. To this end, this thesis focuses on the synthesis, isolation, and characterization of first-row late transition metal carbene complexes with the objective of correlating their structure and reactivity. Chapter 1 summarizes the utility of transition metal carbenes in catalysis and how variations in their electronic structure give rise to distinct reactivity including olefin metathesis, cyclopropanation, and C–H bond insertion. Although efficient, many of these systems rely on 4d and 5d metals, illuminating an opportunity to investigate 3d transition metal carbenes and their structure-function relationships. In Chapter 2, we demonstrate the synthesis and isolation of a series of copper benzylidenes (EmL)Cu(CHAr) (Em = 1,1,7,7-tetraethyl-1,2,3,5,6,7-hexahydro-3,3,5,5-tetramethyl-s-indacene). Despite bearing a Schrock-type motif, these copper complexes are best described as Fischer-type carbenes. The use of a weak-field ligand and minimal stabilizing substituents on the carbene renders the copper benzylidene sufficiently electrophilic to achieve carbene insertion into C–H bonds. Expansion of this chemistry to cobalt is explored in Chapter 3 to interrogate whether late 3d metal carbenes with higher metal–carbene covalency (Schrock-type alkylidenes or carbene radicals) are accessible. Characterization of (EmL)Co(CHAr) and (EmL)Co[CH(SiMe3)] reveal high-spin CoII centers antiferromagnetically coupled to a carbene radical, consistent with their cyclopropanation reactivity towards electron-deficient olefins. Considering the utility of the sterically encumbered (EmL)H ligand for kinetic stabilization of reactive transition metal complexes, we employed this ligand platform to isolate the smallest carbene, a methylidene (CH2), as described in Chapter 4. A cobalt methylidene (EmL)Co(k2-CH2) can be accessed using a phosphorus ylide as the methylidene source, and efforts to understand related ylide chemistry are detailed. Chapter 5 presents the spectroscopic characterization of a putative copper hydride, which can be trapped in the presence of excess CO2 to generate formate species (EmL)Cu(k2-O2CH). Further studies investigating the reactivity of the cobalt methylidene and copper hydride are ongoing in the Betley laboratory.