Theoretical Studies on the Mechanisms of Chemical Reactions

               

Principal Investigator

Brian Yates

Department of Chemistry

University of Tasmania

Powerful computers and the most modern algorithms are being used to help design better catalytic systems for the synthesis of industrially important products such as polyketone polymers. This new and versatile class of polymers has economic and environmental advantages over the present more familiar polyethylenes. The VPP300 is being used to improve the organometallic catalysts in a systematic manner and to provide a theoretical basis and understanding for many of the experimental observations.

In our department, experimental and theoretical work has focussed on using Pd(II) complexes containing pyridine carboxylate ligands (NC5H4COO­) to explore the fundamental mechanistic steps. With the use of the VPP300 we have been able to carry out calculations on model complexes approaching the complexity of the experimental systems. These calculations would not have been possible without access to these high performance computing facilities.

As a result of this ongoing research we will

· obtain detailed descriptions of possible reaction mechanisms for the copolymerisation of CO and ethene using prototype neutral and cationic catalysts

· determine the effect of different phosphine ligands (PF3, PMe3, and PH3 vs PPh3)

· obtain information for different chelating ligands (NO, NN and PP donors)

· provide a theoretical basis for the observation that a trans N­donor activates CO and a trans P­donor activates an alkyl group for migratory insertion

· determine precise changes in activation energies as a function of electronic and steric effects

· through a combination of theory and experiment determine principles for optimising catalyst performance and hence assist in the design of more efficient catalysts, and

· determine a rationale that can be applied to similar ligands and metals.

   

Co-Investigators

     

George Heard

Katrina Frankcombe

Trent Wale

Department of Chemistry

University of Tasmania

     

Projects

g29 - VPP

     
         
               

     
               

- Appendix B


 
               

     

What are the results to date and the future of the work?

Following a calibrative study on the fragment systems PdPH3 and PdCO, we have carried out extensive computer modelling on the mechanism of the palladium catalysed carbonylation reaction. At the MP2 level of theory for a prototype system, we found that the lowest energy pathway involved the following steps:

We have calculated the potential energy surfaces for both cationic and neutral complexes and found that there is a difference of over 30 kJ/mol in the key migration reaction (3 ­> 4):

 

Lowest energy pathways for carbonylation of the neutral (dashed) and cationic (filled) systems

The lower activation energy for the migration step in the cationic complex is due to an increased s-donation from the phosphine to the metal, accompanied by a decrease in the metal-carbonyl back-donation. We were also able to determine a unique isomerisation step in the reaction which provides the first ever rationalisation for the experimental observation of only one product isomer. These computational results are being used to optimise the conditions and selectivity of the reaction in collaboration with the experimental research group of A/Prof Kingsley Cavell in our Department.

What computational techniques are used?

We have used quantum chemistry techniques (mainly conventional ab initio SCF and MP2 methods together with density functional methods) as implemented in the Gaussian 94 program. This package has been well-vectorised and makes good use of the VPP. We have employed

     

     

     

large basis sets together with relativistic effective core potentials to enable geometry optimisations and wavefunction analyses to be carried out reliably.

Publications

Frankcombe, K.E., Cavell, K.J., Knott, R.B., Yates, B.F., Competing Mechanisms in the Carbonylation of Palladium (II) Complexes Containing Bidentate Ligands: Theoretical Insights, Chem. Commun., 1996, 781-2.

Heard, G.L., Yates, B.F., A Hybrid Supermolecule-Polarisable Continuum Approach to Solvation: Application to the Mechanism of the Stevens Rearrangement, J. Comp. Chem., 17, 1996, 1444-1452.

Heard, G.L., Yates, B.F., Competing Rearrangements of Ammonium Ylides: A Quantum Theoretical Study, J. Org. Chem., 61, 1996, 7276-7284.

Frankcombe, K.E., Cavell, K.J., Knott, R.B., Yates, B.F., Ligand Substitution: An Assessment of the Reliability of ab initio Calculations, J. Phys. Chem., 100, 1996, 18363-18370.

     

- Appendix B