Density Functional Investigation on the Reaction Mechanisms of Oxidative Activation of Dihydrogen by Osmium Oxide Cation in Gas Phase

作者：Liu Qiong; Wang Pei; Zhang Ganbing

来源：Acta Chimica Sinica, 2012, 70(12): 1337-1346.

• density functional calculations
• multiple-state-reactivity
• dihydrogen activation by osmium oxide cation
• oxidative addition-reductive elimination mechanism
• [2+2] cycloaddition of dihydrogen across to Os=O
• H-abstraction-rebound mechanism
• oxene-insertion mechanism

Density functional calculations with UB3LYP functional and an extended ECP basis set are employed to calculate the geometries and energies for all possible reactants, intermediates, transition states and products on sextet, quartet and doublet surfaces in four pathways of addition (oxidative addition and [2+2] cycloaddition)-elimination, abstraction-rebound and oxene-insertion for investigating the mechanisms of oxidative activation of dihydrogen by osmium oxide cation. From the results calculated, the title reaction is spin-forbidden, which starts on the quarter surface and ends on sextet surface, the overall reaction is exothermic by 21.0 kJ.mol(-1). Oxene-insertion process is unfavorable thermodynamically due to more positive Gibbs free energy for the reactant complexes. The other three mechanisms proposed exhibit multiple-state-reactivity (MSR) or two-state-reactivity (TSR). Individually the surfaces in three spin states for the two addition-elimination pathways may cross over three times, while the sextet and quartet surfaces for abstraction-rebound may cross once, respectively. The abstraction-rebound mechanism starts on the H-abstraction process with uphill potential surfaces and high endothermicity, followed by a barrierless and highly exothermic rebound of H atom, thus it cannot take place at normal temperature. While the two addition-elimination processes have the same rate-determining step, where each barrier is about 156.9 kJ.mol(-1), which is a little higher than that for the usual reactions in liquid, however it is possible to take place due to coupling with the highly exothermic steps before. Furthermore, the concerted [2+2] cycloaddition step has a lower barrier of only 28.7 kJ.mol(-1), which is 113.7 kJ.mol(-1) lower than that for the step of the reductive elimination of the first hydride in oxidative addition-elimination process. Thus, [2+2] cycloaddition process is more favorable than oxidative addition process kinetically.

• 出版日期2012-6-28
• 单位湖北大学