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 H-2 by MnO4- and RuO4, performed by UV-vis spectroscopy, showed these reactions to be quite rapid at 25 degreesC (k(1) approximate to (3-6) x 10(-2) M-1 s(-1)). Rates measured for H-2 oxidation by MnO4- in aqueous solution (using KMnO4) and in chlorobenzene (using (Bu4NMnO4)-Bu-n) 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 H-2. Experiments were pet-formed to probe for a catalytic hydrogen/deuterium exchange between D-2 and H2O as possible evidence of dihydrogen sigma -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 H-2 was observed. On the basis of the available evidence, a concerted mechanism for the cleavage of H-2 by metal tetraoxo compounds is proposed. Theoretical models were developed for pertinent MnO4- + H-2 transition states using density functional theory in order to differentiate between concerted [2 + 2] and [3 + 2] scissions of H-2. The density functional theory calculations strongly favor the [3 + 2] mechanism and show that the H-2 cleavage shares some mechanistic features Z with related hydrocarbon oxidation reactions. The calculated activation energy for the [3 + 2] pathway (DeltaH(double dagger) = 15.4 kcal mol(-1)) is within 2 kcal mol(-1) of the experimental value.