| In this work, we studied the mechanism of hydrogenation of ketones and dinitrogen activation and hydrogenation catalyzed by transition metal complexes using DFT method.H2activation processes in hydrogenation of ketones catalyzed by late transition metal-ligand bifunctional catalysts have been studied using the DFT method. For systems A (RuH2-diphosephine/diamine complex) and B (Ru-η5-Cp*-1,2-diamine complex), the dihydrogen activation process in neutral and basic conditions (path1) consisted of two steps:H2coordination and H-H cleavage. However, dihydrogen activations catalyzed by complexes C-F (Ru-η6-arene and Rh/Ir-cyclopentadiene complexes) along path1consist of only H-H cleavage due to the absence of H2coordination. Thus, systems C-F have higher energy barriers (ΔG>27kcal/mol) for dihydrogen activation than systems A and B. However, for systems C-F under acidic conditions, dihydrogen activation (path2) consists of the two steps involving H2coordination; thus the dihydrogen activation barriers decrease greatly, resulting in an easy splitting of H2. These results agree well with experiments. In the conversion from transfer hydrogenation to H2hydrogenation for C-F, the protonation of16e complex MNC-F changes the N2-M’-Y3(Y=N or O) delocalized Ï€-bond into a M’-Y3localized Ï€-bond. Therefore, the16e complexes, which can provide a vacant site for H2coordination, tend to perform H2hydrogenation.In order to elucidate:(i) physico-chemical properties of the ((η5-C5Me5)[Taâ…£](i-Pr)C(Me)N(i-Pr)}2(μ-η1:η1-N2),I,[Taâ…£]2(μ-η1:η1-N2),and ((η5-C5Me5)[Taâ…£](i-Pr)C(Me)N(i-Pr)}2(μ-N)2, â…¡,[Taâ…£]2(μ-N)2,complexes;(â…±) the mechanism of the â… â†'â…¡ isomerization; and (iii) reactivity of these complexes toward H2molecule, we launched density functional (B3LYP) studies of the corresponding model systems1,2and3where C5Me5and (i-Pr)C(Me)N(i-Pr) were replaced by C5H5and HC(NCH3)2, respectively. These calculations shown that the lower-lying electronic states of1,[TaTV]2(μ-η1:η1-N2), are almost degenerate open-shell singlet and triplet states with two unpaired electrons located on the Ta centers. This finding is in reasonable agreement with experiments showing easy accessibility of paramagnetic and diamagnetic states of I. The ground electronic state of the bis(μ-nitrido) complex2,[TaTV]2(μ-N)2, is a closed-shell singlet state in agreement with experimentally reported diamagnetic feature of â…¡. The1-to-2rearrangement is a multistep process and occurs with a maximum of28.7kcal/mol free energy barrier required for the (μ-η1:η1-N2)â†'(μ-η2:η2-N2) transformation and is highly exothermic. Reaction of1with H2to form the1,4-addition product3proceeds with a maximum of24.2kcal/mol free energy barrier associated by the (μ-η1:η1-N2)â†'(μ-η2:η2-N2) isomerization. Overall reaction1+H2â†'3is exothermic by20.0kcal/mol. Thus, addition of H2to1is kinetically and thermodynamically feasible and proceeds via the rate-determining (μ-η1:η1-N2)â†'(μ-η2:η2-N2) isomerization step. The thermodynamically most favorable isomer [TaTV]2(μ-N)2, i.e. bis(μ-nitrido) complex2, does not react with H2because of large barrier (49.5kcal/mol) and high endothermicity. This conclusion is also in an excellent agreement with the experiment... |
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