Theoretical Studies on the Mechanisms of Chemical Reactions

               

Principal Investigator

Brian Yates

School of Chemistry, University of Tasmania

Co-Investigators

Jeremy Rackham

David Graham

Nadja Saendig

School of Chemistry, University of Tasmania

Projects

g29 - VPP, PC

This project is concerned with applying ab initio
molecular orbital theory to study the mechanisms
of reaction of a number of palladium catalysts. These catalysts are used experimentally (and in some cases, industrially) to manufacture novel polymers with specific properties including photodegradability. Some of the most exciting catalysts developed recently are those for the co-polymerisation of carbon monoxide and ethylene. Some of these have been synthesised at the University of Tasmania.

This computational project at the ANU has enabled us to determine the mechanisms for some of these catalytic processes, and to determine some of the electronic and steric effects controlling the rate-determining steps. What is the effect of different phosphine and chelating ligands? What precise steps lead to the selective formation of particular products? Theory is being used to address these questions and, in collaboration with experimentalists in Hobart, to determine new directions for synthesis.

This project was supported by both small and large ARC grants.

   
         
               

     
               

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

We have previously studied the organometallic system shown on the left:

with X = CH2, C=O and C=CH2.

               
- Appendix B

 
               

       

During 1998, our work on the VPP allowed us to extended this series to include the system with X = C=NH which was predicted to have interesting electronic properties due to conjugative effects. Preliminary results suggest that the rate determining step has an activation energy similar to that of X=C=O and this novel compound may be a useful target to approach synthetically.

Such calculations represent a model of the true experimental system shown at the right in the diagram above. Therefore also during 1998 we began calculations on the real ligand system. The study of these large systems would not have been possible without access to the VPP. We will continue to study these systems and apply the latest QM/MM and QM/QM techniques for taking account of the experimental ligand structures.

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 and PC. 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

R.H. Hertwig, W. Koch, B.F. Yates Economical Treatments of Relativistic Effects and Electron Correlation in WH6, J. Comp. Chem., 19, 1998, 1604-1611.

Green, M.J.; Britovsek, G.J.P., Cavell, K.J.; Gerhards, F., Yates, B.F., Frankcombe, K.; Skelton, B.W.; White, A.H., Cationic Methylpalladium(II) Complexes Containing Bidentate N-O Ligands as Catalysts for the Copolymerisation of CO and Ethylene - Identification and Isolation of Intermediates from the Stepwise Insertion Reactions, and Subsequent Detailed Mechanistic Interpretation, J. Chem. Soc. Dalton Trans., 1998, 1137-1144.

Heard, G.L., Yates, B.F., Rearrangement Reaction Mechanisms in Heterocyclic Chemistry, invited article in the Encyclopedia of Computational Chemistry, Wiley, 1998.

Uddin, J.; Dapprich, S., Frenking, G., Yates, B.F., Theoretical Studies on the Nature of the Metal-Alkene Bond in Platinum Complexes of Strained Olefins, Organometallics, 1999, in press.

       
Appendix B -