Effects of Ligand Composition on the Oxidative Carbonylation of Toluene to Toluic Acid Catalyzed by Rh(III) Complexes

Experimental and theoretical studies were conducted to investigate the influence of anionic ligands (e.g., CF(3)COO(-), CH(3)SO(3)(-)) on the catalytic activity and selectivity of Rh(III) in the oxidative carbonylation of toluene to toluic acid. The catalyst activity was found to pass through a maximum as the pK(a) of the conjugate Brønsted acid decreases from 4.63 to -2.00, with the maximum activity occurring at pK(a) = 0.35, corresponding to CClF(2)COOH. The theoretical analysis showed that the strength of toluene coordination increases with decreasing basicity of the anion (i.e., decreasing pK(a) of the corresponding acid). In contrast, the activation barrier for C-H activation increases with decreasing ligand pK(a). The experimentally observed effect of anion composition on catalyst activity can be explained using calculations of the apparent rate coefficient for toluene activation based on density functional theory and transition state theory. The ratio of p- to m-toluic acid formed increases with decreasing acid pK(a) and passes through a maximum for pK(a) = 0, corresponding to CF(3)COOH. The effect of anion composition on the isomer selectivity is attributed to changes in the charge density on the Rh(III) cation, which in turn affect the distribution of charge on the para and meta carbon atoms of the aromatic ring.