Exploring N-heterocyclic carbene ligands in gold redox chemistry

Abstract

Exploring N-heterocyclic carbene ligands in gold redox chemistry investigates the ability
of a range of ligands to facilitate gold redox processes in the absence of exogenous
oxidants. Focusing on an N-heterocyclic carbene ligand framework containing a hemi-
labile side arm, this work explores the effect of the organic component of the side arm
on oxidative processes performed by gold complexes.
An introduction to gold redox chemistry and the challenges it presents is found in
Chapter 1, along with an overview of carbenes and N-heterocyclic carbenes, reviewing
their value as ligands in inorganic chemistry.
The 4-step organic synthesis starting from 2,6-dibromopyridine described in Chapter 2
yielded a range of 5 imidazolium chloride salts which were characterised using NMR
spectroscopy, mass spectrometry and X-ray crystallography. Imidazolium ions were
deprotonated to form free N-heterocyclic carbenes, which were characterised using
their distinctive 13C-{1H} NMR spectrum; these free carbenes were unstable,
decomposing over a period of ca. 90 minutes.
N-heterocyclic carbene complexes of coinage metals Cu, Ag and Au were synthesised
and characterised using NMR spectroscopy, mass spectrometry and X-ray
crystallography as reported in Chapter 3. Complexes displayed linear structures
consistent with +1 oxidation state coinage metal compounds, along with typical lengths
for the carbene-metal bond.
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Chapter 4 describes how N-heterocyclic carbene gold(I) chloride complexes were
tested in oxidative addition reactions with 4-fluoroiodobenzene, with gold(III)
complexes isolated and characterised by NMR spectroscopy, mass spectrometry and X-
ray crystallography, displaying the expected square planar geometries about the gold
centres. Oxidative additions using iodomethane were attempted but were unsuccessful
or inconclusive. Catalytic C-H arylation reaction conditions were screened for each
complex, with coupling occurring at varying conversion rates for all complexes. Highest
conversion rates were observed for ligands containing the least bulky organic
component of the side arm.

Date of Award	1 Oct 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Chris A Russell (Supervisor)