(529f) Theoretical Investigation of the Rh(TFA)3(CO)2 Catalyst for the Oxidative Carbonylation of Toluene to P-Toluic Acid

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 Å) 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.