DFT Study of Solvent Coordination Effects on Titanium-Based Epoxidation Catalysts. Part One: Formation of the Titanium Hydroperoxo Intermediate

Abstract

Density functional theory has been used to study the effects of solvent coordination on the structure and formation of the titanium hydroperoxo species believed to be the active intermediate in oxidation reactions with Ti(IV)−H2O2 catalytic systems. Titanium hydroperoxo intermediates possessing a variety of coordination environments have been modeled with unconstrained single coordination sphere clusters using a B3LYP/ECP methodology. Titanium hydroperoxo intermediates bearing one or two solvent ligands may assume several degenerate structures with the hydroperoxo moiety coordinated to titanium in a monodentate or bidentate manner. Hydrogen bonding of the hydroperoxo moiety with a protic solvent ligand via a monodentate five-membered ring structure does not confer any significant additional stabilization to the titanium hydroperoxo intermediate. The presence of a single solvent ligand on the titanium center lowers the Gibbs free energy of activation for the formation of titanium hydroperoxo intermediates by 4−8.5 kcal/mol. Hydrogen-bonded protic molecules can facilitate proton transfer from a hydrogen peroxide ligand on titanium and thereby lower the Gibbs activation barrier for intermediate formation by 5−6 kcal/mol. NBO analysis provides an enriched understanding of how solvent ligands alter structural and electronic properties and reduce activation barriers. Consideration of thermodynamic and kinetic results indicates that the most abundant titanium hydroperoxo intermediates formed under liquid-phase reaction conditions will most likely possess a single solvent ligand on titanium and possibly a hydrogen-bonded protic solvent molecule.