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Theoretical design and development of catalyst systems

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Frankcombe, Katrina Ellen (1997) Theoretical design and development of catalyst systems. PhD thesis, University of Tasmania.

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Abstract

Wave function based ab initio and nonlocal density functional methods have
been employed to impart a unique insight into the competing mechanisms operating in
the carbonylation of alkyl-palladium(H) complexes of bidentate ligands. The
theoretical methods employed were established by two benchmark studies.
Geometries of physically meaningful accuracy were obtained using second-order
Moller-Plesset (MP2) methods or nonlocal density functional theory (DFT) with a
small-core pseudopotential on palladium and double-c basis sets with polarisation
functions on the ligands. Reliable reaction energies required a higher level of
correlation (such as CCSD(T)) together with large basis sets incorporating diffuse
functions and polarisation functions. For every system investigated, the lowest energy pathway proceeded via novel
five-coordinate intermediates and transition structures. This is in contrast to the
generally assumed four-coordinate pathways and has important ramifications in the
context of rationalising experimentally observed CO/olefin copolymensation activities.
The competing carbonylation mechanisms for the model neutral and cationic
palladium(fl) systems Pd(N-0)(CH3)(PH3) + CO - Pd(N-0)(COCH3)(PH3)
(N-O = NHCHCOO (1), NHCHCHO (21)) have been investigated in detail.
Despite marked differences in the lowest energy mechanisms, variation in the overall
energetics for the neutral and cationic systems was found to be less than 15 kJ/mol.
Furthermore, it has been unequivocally demonstrated that differences in CO/ethylene
copolymerisation activity of neutral and cationic palladium(II) complexes of bidentate
N-O ligand can be attributed to the activation energy of ethylene insertion (a variation
of 55.4 kJ/mol). This is the first theoretical investigation to reproduce the distinct
CO/ethylene copolymerisation rates of cationic and neutral complexes, thereby
allowing clear elucidation of this important phenomenon.
Ligand influences have been assessed by modelling the carbonylation reaction
for a range of complexes: Pd(X-Y)(CH3)(L) (X-Y = NHCHCHO, L = PF3 (41), P(CH3)3 (51); X-Y = NHCHCHNH, L = PH3 (61), CH3 (71)). With the exception
of the neutral diimine complex (71) variations in the activation energy for the methyl
migration step are surprisingly small. Values ranged from 39.5 kJ/mol for 41 to
55.6 kJ/mol for 1. Conversely, it was shown that differences in carbonylation
reactivity typically arise in the ligand substitution and isomerisation steps. The high
migration barrier associated with 71 (67.5 kJ/mol) is due to a reduction in cy-donation
by the migrating methyl group and an increase in metal-carbonyl it back-donation.
The study provides the most definitive theoretical evidence to date for the
participation of five-coordinate species in the migratory insertion reactions of
alkyl-palladium(H) complexes. In particular, a novel transition structure has been
identified which accounts for the isomerisation of square-pyramidal d 8complexes.
The integral role proposed for five-coordinate species in the carbonyl and olefin
migratory insertion reactions has led to the formulation of novel rationale to account
for several ambiguous experimental trends.
In summary, all of the theoretical results presented exhibit outstanding
agreement with all available experimental structural and kinetic data. This study clearly
demonstrates that computational chemistry is now mature and can make a significant
contribution at the leading edge of catalyst design and development.

Item Type: Thesis (PhD)
Copyright Holders: The Author
Copyright Information:

Copyright 1997 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
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Additional Information:

Examines the competing mechanisms operating in the carbonylation of alkyl-palladium(II) complexes of bidentate ligands. Thesis (Ph.D.)--University of Tasmania, 1997. Includes bibliographical references. Examines the competing mechanisms operating in the carbonylation of alkyl-palladium(II) complexes of bidentate ligands

Date Deposited: 09 Dec 2014 00:02
Last Modified: 16 Aug 2016 06:10
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