Theoretical study of oxidation of cyclohexane diol to adipic anhydride by [Ru(IV)(O)(tpa)(H2O)]2+ complex (tpa ═ tris(2-pyridylmethyl)amine).

The catalytic conversion of 1,2-cyclohexanediol to adipic anhydride by Ru(IV)O(tpa) (tpa ═ tris(2-pyridylmethyl)amine) is discussed using density functional theory calculations. The whole reaction is divided into three steps: (1) formation of α-hydroxy cyclohexanone by dehydrogenation of cyclohexanediol, (2) formation of 1,2-cyclohexanedione by dehydrogenation of α-hydroxy cyclohexanone, and (3) formation of adipic anhydride by oxygenation of cyclohexanedione. In each step the two-electron oxidation is performed by Ru(IV)O(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 with the spin transition in both 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.