Rhenium polyhydride complexes of boron-hydride ligands and their application in catalysis
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Date
27/11/2021Item status
Restricted AccessEmbargo end date
27/11/2022Author
Donnelly, Liam J.
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Abstract
The synthesis of transition metal complexes containing metal-boron bonds is of particular
interest due to the relevance of these complexes as intermediates in catalytic C–X (X = H or
halogen) borylation to prepare synthetically valuable organoboronate esters. The most widely
used catalysts are Ir complexes typically of the form [Ir(Bpin)3(L)2], where L is a neutral
monodentate or bidentate ligand, and invoke an Ir(III)/Ir(V) redox couple. In the past 30
years metal-oxos have been studied in the activation of X–H (X = B, Si, P) bonds for
applications in catalytic hydrofunctionalisation reactions. The mechanism of these reactions
can proceed by functionalisation and/or cleavage of the M=O bond to produce compounds
that are inaccessible by conventional methods. However, this reactivity has not been
significantly explored for anionic metal-oxo complexes (e.g. ReO4¯, RuO4¯ and WO4
2¯). This thesis presents the synthesis of a number of rhenium polyhydride complexes of boron-hydride
ligands from perrhenate (ReO4−), the structural characterisation of these complexes, their
application in C–H borylation catalysis and hydroboration, and the exploration of the
mechanisms of these reactions.
Chapter One introduces C–H functionalisation catalysis and the relevance of transition metal
polyhydride and transition metal dihydroborate/σ-borane/boryl complexes as catalysts, and
catalytically-relevant intermediates, in this chemistry. Previously reported methods for X–H
(X = B or Si) activation by high oxidation transition metal-oxo complexes are also discussed.
Chapter Two details the synthesis and characterisation of the reactive rhenium boron-polyhydride anion [K(DME)(18-c-6][ReH7(Bpin)3] by exhaustive deoxygenation of the commercially available ReO4−anion with pinacol borane (HBpin). This complex is shown to
be a reagent for the stoichiometric C–H borylation of carboarenes with high meta
regioselectivity. The reaction with other hydroboranes is also described, resulting in reductive
Bpin substitution to form new anionic dihydroborate complexes.
Chapter Three describes how [K(DME)(18-c-6][ReH7(Bpin)3], either isolated or prepared in
situ, is a catalyst for the 1,4-hydroboration of N-heteroaromatic substrates under simple
operating procedures. A full description of the optimisation of the reaction conditions and
substrate scope is presented. A mechanistic investigation of the hydroboration reactivity is
outlined and used to elucidate the catalytic cycle of this reaction. The use of other anionic
metal-oxo complexes for hydrofunctionalisation catalysis is also discussed.
Chapter Four details the synthesis and characterisation of phosphine-ligated Re boron-polyhydride complexes through the reductive substitution of HBpin from [K(DME)(18-c-6][ReH7(Bpin)3] with mono- and bidentate phosphine ligands. In particular, the synthesis of
σ-borane complex [K(18-c-6)][ReH4(η2-HBpin)(dppp)] is highlighted which, upon
protonation, forms the neutral boryl complex [ReH6(Bpin)(dppp)]. The catalytic activity of
[ReH6(Bpin)(dppp)] in the C–H borylation of heteroaromatics is described, and a full
substrate scope and mechanistic study is outlined.
Chapter Five presents a summary of the work presented in this thesis, and Chapter Six
outlines the experimental procedures and analytical data for all described compounds and
complexes.