Water Exchange Reactions and Hydrolysis of Hydrated Titanium(III) Ions. A Density Functional Theory Study

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Abstract

The structure and water exchange mechanism of hexahydrated Ti(III), its hydrolysis, and the water exchange mechanism of analogous hydroxo−aqua complexes have been studied using density functional theory (DFT) calculations. Isolated metal−aqua and metal−hydroxo clusters corresponding to the gas-phase (T = 0 K) were used to approximate the model reactions. The structure of [Ti(H2O)6]3+ was found to have Ci symmetry and Ti−O bond lengths of 2.094 Å. The water exchange reaction of this complex follows an (almost) limiting A mechanism with an energy of activation of 15.8 kcal mol-1. The hydrolysis of hexahydrated Ti(III) was modeled by an in vacuo proton-transfer process between water molecules of the first and second coordination spheres of [Ti(H2O)6]3+·H2O. This process was found to be activationless and leads to the unusually stable dication:cation pair [Ti(H2O)5(OH)]2+·H3O+, which is lower in energy than the reactant by 4.5 kcal mol-1. Only a weak structural influence, indicated by a slight increase in the mean value of the Ti−O bond lengths of water molecules in the first coordination sphere, is observed when the hydroxo ligand is formed. The water exchange reactions of the corresponding hydroxo−aqua complexes [Ti(H2O)5(OH)]2+ and [Ti(H2O)5(OH)]2+·H2O, respectively, were found to proceed via limiting D mechanisms. The energies of activation for the exchange of the water molecule in the trans-position to the hydroxo ligand were calculated to be only 9.8 and 7.2 kcal mol-1, respectively. This, however, implies that the apparently weak influence of coordinated hydroxide still results in a significant reduction in the energy barrier for the water exchange reaction and also leads to a complete changeover in the preferred exchange pathway from A to D.

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