COMPUTATIONAL TRANSITION METAL CHEMISTRY
The interests of this group lie in the use of computational modelling to understand the reactivity of organometallic complexes. The main results of our calculations are molecular structures and energies. Such information is especially valuable when not easily determined experimentally.
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For example, in the lab the nature of the reactants and products is usually known, but determining the structures and relative energies of reactive intermediates and transition states is very difficult - yet it is often these species that determine the ease of a chemical reaction, or the degree of selectivity that a reaction may display. |
Computational chemistry also provides the opportunity to relate the reactivity of a system to the underlying electronic structure - and may suggest new ways to enhance reactivity.
1. Structure and Reactivity of Low-Valent Transition Metal Amides and Alkoxides
Metal-amides, M-NR2, and metal-alkoxides, M-OR, are isoelectronic to metal-alkyl species and are important intermediates in homogeneous catalytic cycles, such as aryl amination (1) and alkene hydroamination (2):
Such processes involve fundamental reaction steps similar to those of organometallic chemistry, such as alkene insertion or reductive elimination. The group's research aims to understand such key reaction steps and how they are altered by the presence of lone pairs on the N or O atoms.
Example: CO Insertion into a Pt-O Bond
The Pt-O bond in (dppe)Pt(Me)(OMe) displays enhanced reactivity with CO compared to the M-C bond also present in this species. Experimentally, CO insertion is only observed into the Pt-O bond. In addition, the mechanism is thought to involve a 5-coordinate intermediate:
Calculations on model species show that the transition state for CO insertion into the Pt-O bond is about 90 kJ/mol more stable than the alternative insertion into the Pt-C bond. Calculations also provide a rationalisation of this behaviour (below right). Insertion into the Pt-C bond involves sacrificing a Pt-CH3 bond in order to form a new C- C bond. For insertion into the Pt-O bond the hydroxyl ligand can orient itself such that one of its lone pairs is involved in C-O bond formation, enabling some stabilizing interaction with the metal to be maintained. The result is a lower activation energy which favours insertion of CO into the Pt-O bond.
2. Selectivity of Double Insertion Reactions of Group 10 Alkyne Complexes
(In collaboration with Dr. E. Wenger and Prof. M.A. Bennett, Australian National University)
Group 10 alkyne complexes can react with 2 moles of CO to produce metallacycle complexes. Intriguingly, a symmetric species is obtained for M = Ni, but an asymmetric one when M = Pt. The latter product implies a rare insertion of CO into a metal-acyl bond:
Similar double insertions with alkynes molecules produce arenes:
For unsymmetrical alkynes a number of isomers may be produced, depending on the regioselectivity of the alkyne insertion step. Research in this area aims to provide a deeper understanding of the factors that affect the selectivity of these types of reactions.
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