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Theoretical Investigation of the Rh(TFA)3(CO)2 Catalyst for the Oxidative Carbonylation of Toluene to P-Toluic Acid

Andrew Behn1, Alexis T. Bell1, and Martin Head-Gordon2. (1) Chemical Engineering, University of California, Berkeley, 201 Gilman Hall, University of California, Berkeley, CA 94720, (2) Department of Chemistry, University of California, Berkeley, 419 Latimer Hall, Berkeley, CA 94720

Ligand chemistry plays a central role in the activity of homogeneous transition metal catalysts by strongly influencing the electronic environment of the active center. It has been shown experimentally that slightly changing the ligand structure of a catalyst can have a drastic effect on activity and selectivity. To better understand the role that ligand structure plays in the oxidative carbonylation of toluene to p-toluic acid, density functional theory was used to investigate the resting state of the Rh(TFA)3(CO)2 catalyst. Three distinct and possibly catalytically active isomers of Rh(TFA)3(CO)2 were identified. Several transition states were found for the insertion of CO into Rh(TFA)3CO, confirming that all three isomers are thermodynamically accessible at room temperature. By performing energy decomposition analysis (EDA) calculations on the catalyst resting state, the lowest energy isomer was found to possess an unprecedented association between the carbon of a carbonyl ligand, and the oxygen atom of a unidentate trifluoroacetate ligand. The inter-atomic distance between the associated atoms (1.53 ) is comparable to a canonical C-O sigma bond. Further, the EDA calculations show a high degree of charge transfer between the carbonyl and trifluoroacetate upon formation of the associated complex. A vibrational analysis was performed, and predicts that the ligand association of the lowest energy isomer will produce a spectroscopic signal (IR and Raman active) which is absent in the other isomers. The role of isomer composition on the activation of the C-H bond in toluene, the rate-limiting step in the oxidative carbonylation of toluene, will be discussed in detail.