DFT Mechanistic Investigation of a Large Molecularcatalytic System

DFT Mechanistic Investigation of a Large MolecularCatalytic System.

Marrigje Marianne Conradie

Department of Chemistry, University of the Free State, Bloemfontein 9300, South Africa;
Department of Chemistry and Centre for Theoretical and Computational Chemistry, University of Tromsø, Tromsø N-9037, Norway.

Abstract

Rhodium complex compounds are of the most prevalent industrial homogeneous catalysts for organic raw material processing. A classic example is the Monsanto process, where methanol is converted into acetic acid with the aid of a rhodium catalyst and methyl iodide. The oxidative addition of methyl iodide to square planar rhodium complexes has important implications in catalysis. One of the factors involved in the increase and decrease of the activity of metal centre, is the ligand attached to the metal. The ligand plays a significant role in intra- and intermolecular interactions that can be observed by studying the reaction mechanism. In order to achieve a better understanding of the ways of purposeful alteration of the reactivity of catalytic systems, a detail study of the reaction mechanism are thus of great importance.

The Monstanto process has been studied thoroughly, both experimentally and theoretically. The same reaction promoted from [Rh(L,L'-BID)(CO)(PPh3)] complexes (where L,L'-BID is a monoanionic bidentate ligand with donor atoms L and L') has mostly been studied experimentally. On the basis of extensive experimental kinetic studies of these type of reactions, a general reaction scheme has been proposed for the addition of methyl iodide to [Rh(L,L'-BID)(CO)(PPh3)] complexes [1]. Depending on the nature of the L,L'-BID ligand, the reaction is experimentally found to terminate at different stages in this scheme: [Rh(L,L'-BID)(CO)(PPh3)] + CH3I  [Rh(L,L'-BID)(CH3)(CO)(PPh3)(I)]-alkyl1 [Rh(L,L'-BID)(COCH3)(PPh3)(I)]-acyl1  [Rh(L,L'-BID)(CH3)(CO)(PPh3)(I)]-alkyl2  [Rh(L,L'-BID)(COCH3)(PPh3)(I)]-acyl2.

The reaction between [Rh(CH3COCRCOCH3)(CO)(PPh3)] (where R is a bulky ligand (OCOPhO(CH2)9CH3)) and methyl iodide, however, does not follow this general reaction scheme. The following reaction was observed [2]: [Rh(CH3COCRCOCH3)(CO)(PPh3)] + CH3I [Rh(CH3COCRCOCH3)(CH3)(CO)(PPh3)(I)]-alkyl1  [Rh(CH3COCRCOCH3)(COCH3)(PPh3)(I)]-acyl1  [Rh(CH3COCRCOCH3)(COCH3)(PPh3)(I)]-acyl3  [Rh(CH3COCRCOCH3)(CH3)(CO)(PPh3)(I)]-alkyl2. It was observed that both acyl1 and acyl3 formed before formation of alkyl2. A DFT mechanistic study was conducted to shed light on this deviation, which could not be explained with experimental techniques.

Figure 1: DFT structure for the transition state of the reaction [Rh(CH3COCRCOCH3)(CO)(PPh3)] + CH3I.

References

[1] Conradie, J., Lamprecht, G.J., Roodt,A. and Swarts,J.C., Polyhedron 2007 (26) 5075-5087.

[2] Molefi, N.F., Twigge, L., Buitendach, B.E. and Conradie, M.M., Unpublished.