Theoretical Studies On The Mechanism Of Cooperative Activation Of Chemical Bonds By Trinuclear Metal Polyalkylide And Polyhydride Complexes

In recent years, an increasing number of chemists pay attention to "multimetallic cooperative effect" during the design of catalysts. Although experimentalists have made significant advances in this field, the knowledge of mechanism on multimetallic cooperative effect has remained limited. This is mainly due to the difficulty in isolating and detecting intermediates experimentally. Computational chemistry can effectively explore the mechanism of chemical reactions, which would further help design new multinuclear metal catalytic systems. Herein, for better understanding of multimetallic cooperative effect, a series of reactions of chemical bond activation by trinuclear metal polyalkylide and polyhydride complexes have been investigated by density functional theory (DFT) approach.The theoretical study on methane elimination reaction in a trinuclear thulium complex shows that the reaction occurs through rearrangement of Tm-CH3 connections and subsequent C-H bond activation. The multimetal-cooperated C-H bond activation via ?-bond metathesis (?-BM) mechanism is reported here for the first time, which enriched the chemistry of ?-BM. The studies on the E-H (E= N, P) bond activation of PhEH2 by a trinuclear yttrium complex suggest that the first E-H bond activation is monometal-mediated ?-BM mechanism and the second E-H bond activation is multimetal-cooperated a-BM mechanism, being analogous to the C-H bond activation aforementioned.As for the activation of unsaturated bond by trinuclear metal alkylide complex, the reaction of trinuclear scandium methylidene complex with ketone (CH2-/O2- exchange) reveals that it occurs via three stages, viz., nucleophilic addition, multimetal-cooperated intramolecular isomerization, and olefination elimination. Such a result is different from the case of conventional CH2-transfer mechanism, which is generally considered as a two-step reaction, viz., the initial addition and subsequent elimination. This study sheds light on the intermetallic cooperation for methylidene (CH2) transfer. Additionally, the DFT study on CS2 activation by a trinuclear yttrium phosphinidene cluster suggests that this reaction goes through a ?3-S bridged intermediate by S2-/PPh2- exchange, analogously to CH22-/O2-exchange process mentioned above. This mechanism also works for other reactions of the trinuclear yttrium phosphinidene cluster with substrates containing C=X (X= O, S) bond. During the reaction, the change in coordination manner of the leaving groups follow the order of ?3??2??1 while the forming groups (or atoms) follow a reverse trend ?1??2??3.The detailed activation mechanisms of three kinds of unsaturated substrates (dinitrogen, benzene, and pyridine) by a trinuclear titanium polyhydride complex were also investigated by DFT calculations. The results suggest that all the reactions are initiated by the approach of a substrate to the Ti atoms with release of one molecule of H2 to form partially reductive intermediate. As for H2 elimination process, one of bridged H ligand firstly changes to the terminal-H fasion and then could bind with an adjacent H ligand to form a molecule of H2. During the reactions, the multimetallic cooperation, flexible hydride ligands, and redox-active metal centers are three important factors for achieving chemical transformation of these inert molecules. These results could not only deepen the understanding of the related industrial processes, such as Haber-Bosch process, petroleum catalytic reforming, and oil hydrodenitrogenation, but also provided theoretical knowledge for developing new multinuclear polyhydride complexes.