Theoretical Study of Oxidation of Cyclohexane Diol to Adipic Anhydride by [RuIV(O)(tpa)(H2O)]2+ Complex (tpa = Tris(2-pyridylmethyl)amine)

The catalytic conversion of 1,2-cyclohexanediol to adipic anhydride by (RuO)-O-IV(tpa) (tpa tris(2-pyridylmethyl)amine) is discussed using density functional theory calculations. The whole reaction is divided into three steps: (1) formation of a-hydroxy cyclohexanone by dehydrogenation of cyclohexanediol, (2) formation of 1,2-cyclohexanedione by dehydrogenation of alpha-hydroxy cyclohexanone, and (3) formation of adipic anhydride by oxygenation of cyclohexanedione. In each step the two-electron oxidation is performed by (RuO)-O-IV(tpa) active species, which is reduced to bis-aqua Ru-II(tpa) complex. The Ru-II complex is reactivated using Ce(IV) and water as an oxygen source. There are two different pathways of the first two steps of the conversion depending on whether the direct H-atom abstraction occurs on a C H bond or on its adjacent oxygen O-H. In the first step, the C-H (O-H) bond dissociation occurs in TS1 (TS2-1) with an activation barrier of 21.4 (21.6) kcal/mol, which is followed by abstraction of another hydrogen pathways. The second process also bifurcates into two reaction pathways. TS3 (TS4 1) is leading to dissociation of the C-H (O-H) bond, and the activation barrier of TS3 (TS4-1) is 20.2 (20.7) kcal/mol. In the third step, oxo ligand attack on the carbonyl carbon and hydrogen migration from the water ligand occur via TS5 with an activation barrier of 17.4 kcal/mol leading to a stable tetrahedral intermediate in a triplet state. However, the slightly higher energy singlet state of this tetrahedral intermediate is unstable; therefore, a spin crossover spontaneously transforms the tetrahedral intermediate into a dione complex by a hydrogen rebound and a C C bond cleavage. Kinetic isotope effects (k(H)/k(D)) for the electronic processes of the C H bond dissociations calculated to be 4.9-7.4 at 300 K are in good agreement with experiment values of 2.8-9.0.