Mechanism of Dihydrogen Cleavage by High-Valent Metal Oxo Compounds:  Experimental and Computational Studies

The oxidation of dihydrogen by metal tetraoxo compounds was investigated. Kinetic measurements of the oxidations of H₂ by MnO₄^- and RuO₄, performed by UV−vis spectroscopy, showed these reactions to be quite rapid at 25 °C (k₁ ≈ (3−6) × 10^-2 M^-1 s^-1). Rates measured for H₂ oxidation by MnO₄^- in aqueous solution (using KMnO₄) and in chlorobenzene (using ⁿBu₄NMnO₄) revealed only a minor solvent effect on the reaction rate. Substantial kinetic isotope effects [(k_H2/k_D2 = 3.8(2) (MnO₄^-, aq), 4.5(5) (MnO₄^-, C₆H₅Cl soln), and 1.8(6) (RuO₄, CCl₄ soln)] indicated that H−H bond cleavage is rate determining and that the mechanism of dihydrogen cleavage is likely similar in aqueous and organic solutions. Third-row transition-metal oxo compounds, such as OsO₄, ReO₄^-, and MeReO₃, were found to be completely unreactive toward H₂. Experiments were performed to probe for a catalytic hydrogen/deuterium exchange between D₂ and H₂O as possible evidence of dihydrogen σ-complex intermediates, but no H/D exchange was observed in the presence of various metal oxo compounds at various pH values. In addition, no inhibition of RuO₄-catalyzed hydrocarbon oxidation by H₂ was observed. On the basis of the available evidence, a concerted mechanism for the cleavage of H₂ by metal tetraoxo compounds is proposed. Theoretical models were developed for pertinent MnO₄^- + H₂ transition states using density functional theory in order to differentiate between concerted [2 + 2] and [3 + 2] scissions of H₂. The density functional theory calculations strongly favor the [3 + 2] mechanism and show that the H₂ cleavage shares some mechanistic features with related hydrocarbon oxidation reactions. The calculated activation energy for the [3 + 2] pathway (ΔH^‡ = 15.4 kcal mol^-1) is within 2 kcal mol^-1 of the experimental value.