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Heterocyclic carbene complexes : reaction chemistry and catalytic applications

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McGuinness, David S.(David Shane) (2001) Heterocyclic carbene complexes : reaction chemistry and catalytic applications. PhD thesis, University of Tasmania.

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Abstract

This thesis describes the synthesis of a broad range of heterocyclic carbene complexes
of transition metals, their fundamental reaction chemistry and their use as catalysts in
a number of reactions. Mechanistic studies have been performed in order to gain
insight into likely catalytic cycles and the decomposition processes of the complexes.
Pd complexes of donor-functionalised heterocyclic carbene ligands have been
synthesised through a route involving carbene transfer from Aglcarbene precursors. A
variety of complexes containing 0 and N donor-functionalised carbenes have been
prepared and tested as catalysts for Heck-type and polymerisation reactions. The
complexes are highly active for Heck and Suzuki couplings, but do not catalyse chain
growth reactions such as ethylene polymerisation or CO-ethylene copolymerisation.
Zerovalent carbene complexes of Pd of formula Pd(tmiy)2(alkene) (tmiy = 1,3,4,5-
tetramethylimidazolin-2-ylidene, alkene = maleic anhydride, tetracyanoethylene) have
been synthesised. Spectroscopic studies on the complexes provide strong evidence of
the almost purely donor nature of the carbene ligand. Reaction with a number of
substrates, including aryl halides, leads to Pd11 complexes through oxidative addition.
The zerovalent Ni complex Ni(tmiy)2 has also been produced in situ and found to
undergo oxidative addition with organohalides and dihalides. Halide abstraction from
Pd(tmiy)2(4-nitrophenyl)I in the presence of n-butylacrylate leads to migratory
insertion of the olefin and elimination of the Heck coupling product. Under
stoichiometric reaction conditions hydrocarbyl-imidazolium ions are also produced as
major byproducts, indicating that carbene elimination as the hydrocarbyl-imidazolium
salt is a probable route to catalyst deactivation. Additionally, the reaction of CO with
[PdMe(tmiy)(µ-C1)]2 has been studied, and the migratory insertion of CO into the PdMe
bond, followed by rapid decomposition by elimination of an acyl-imidazolium
cation is reported. This observation provides an explanation for the lack of activity of
the Pd carbene complexes in chain growth reactions.
Elimination of the imidazolium ion from hydrocarbyl Pd and Ni carbene complexes
was considered a potentially serious route to catalyst deactivation. In order to elucidate
the mechanism of the reaction a number of new methyl Pdri complexes of
the form [PdMe(tmiy)L2]BF4 (L = cyclooctadiene, methyldiphenylphosphine,
triphenylphosphite, triphenylphosphine) were prepared and their decomposition
kinetics studied. The complexes decompose via elimination of
pentamethylimidazolium and generation of Pd° following first order kinetics. The
kinetic investigations combined with density functional studies show that the
complexes decompose via a mechanism of concerted reductive elimination. An
understanding of the mechanism of the reaction led to the prediction of methods by
which it can be impeded.
Density functional studies of Ni, Pd and Pt complexes show that the energetics of the
reaction can be changed such that oxidative addition of imidazolium to M ° is
energetically favourable. This is demonstrated experimentally by the synthesis of
hydrido Pt carbene complexes, via oxidative addition of azolium salts to Pt °. The
results suggest that conducting reactions in ionic liquids of imidazolium salts will be a
promising method of stabilising M-carbene complexes against decomposition.
Consistent with this proposal, Ni complexes of the form NiI2(carbene)2 have been
synthesised (and crystal structures obtained) and shown to be very active and stable
catalysts for olefin dimerisation when immobilised in ionic liquids. A variety of carbenes
and imidazolium salts with two flanking donor groups were
prepared and their coordination behaviour investigated. Coordination of a dipicolyl
carbene ligand to Pd was found to increase the stability of the complex (to reductive
elimination) relative to other cationic methyl Pd carbene complexes. The ligands were
tested in conjunction with the metals Ti, V, Cr and Pd as catalysts for ethylene
polymerisation, and several combinations were found to give moderately active
catalysts.

Item Type: Thesis (PhD)
Keywords: Carbenes (Methylene compounds), Heterocyclic compounds
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:41
Last Modified: 26 Sep 2017 01:35
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