Computational studies have been widely applied in understanding the nature of bonding in transition-metal complexes and mechanistic study on reactions of transition-metal complexes. With the aid of quantum chemical calculation, we are able to study the complicated electronic structures of these complexes and provide insights into the nature of metal-ligand bonding. In term of mechanistic aspect, the proposed possible reaction pathways based on experimental findings can be elucidated by determining the structures and energies of reactants, intermediates, transition states, and products.
Theoretical studies on the structure, bonding and reactivity of some transition metal complexes and mechanistic picture on catalytic and stoichiometric reactions are reported in this thesis. Knowledge of mechanistic aspect is very useful in designing better and effective catalysts.
To obtain a better understanding of the interesting chemistry that treatment of [OsCI(H
2)(PPh
3) {Ph
2P(CH
2)
2CH=CH(CH
2)
2PPh
2}]OTf with H
2 produced the (hydrido)dihydrogen complex [OsH(H
2) {Ph
2P(CH
2)
2CH=CH(CH
2)
2PPh
2}]OTf rather than the hydrogenated products, we calculated the structures of complexes involved in the above process. It has been found that the hydrogenation of [OsCI(H
2)(PPh
3) {Ph
2P(CH
2)
2CH=CH(CH
2)
2PPh
2}]OTf was thermodynamically fessible but kinetically unfavorable.
The reactions of H
2 with NBD in the model complexes [CpRu(H
2)(NBD)]
+ were found to proceed through a stepwise mechanism. Theoretical calculations support the proposal that [CpRu(H
2)(NBD)]
+ is the active species in the observed reaction. The energy profile shows that the reaction of H
2 with NBD is energetically very favorable. The [2+2] hydrogenation of NBD ligand is kinetically unfavorable.
The hydration reaction of CH
3CN can be catalyzed by some Ru-complexes. Experiments show that (η
5-C
9H
7)Ru(dppm)H can catalyze the hydration reaction, but (η
5-C
9H
7)Ru(dppm)CI cannot. In addition, a remarkable rate acceleration in the [(η
5-C
9H
6CH
2CH
2NMe
2)Ru(dppm)H]-catalyzed nitrile hydration reactions was observed. In order to understand these phenomena, theoretical calculations based on the B3LYP density functional theory have been carried out to examine the structural and energetic aspects related to the possible reaction pathways. The calculations have indicated that in the catalytic process the dissociation of one arm in the bidentate ligand from the metal center followed by the coordination of CH
3CN is the initial event. It was learned that the presence of Ru-H⋅⋅⋅H-OH dihydrogen-bonding interaction in the transition state lowers the reaction barrier in the (η
5-C
9H
7)Ru(dppm)H system, but not in the chloro system. Further theoretical study shows that the energy barriers in the [(η
5-C
9H
6CH
2CH
2NMe
2)Ru(dppm)H]-catalyzed hydration of acetonitrile are lower than those of the corresponding reaction catalyzed by (η
5-C
9H
7)Ru(dppm)H. This work demonstrates that with a suitably designed structure, we can make use of hydrogen bonding interactions (conventional and unconventional) to promote catalysis.
Density functional theory calculations at the B3LYP level have been performed to study the reaction mechanism of the Ru-catalyzed cycloaddition of 1,5-cyclooctadiene (COD) with alkynes. Our calculations point towards the proposed mechanism that the cycloaddition reaction occurs via an intermediate formed by the active species [CpRu(COD)]
+. The first C-C coupling step was found to be the rate-determining step. The active species [CpRU(COD)]
+, which is crucial for the catalytic reaction, can be obtained through the ionization of CI
- from [CpRu(COD)Cl] under the polar solvent of MeOH. The extra π⊥ bond of alkynes in comparison to alkenes has been found to play a key role in stabilizing the relevant reaction intermediate as well as lowering the reaction barriers. In the olefin case, the absence of the extra π⊥ bond leads to the instability of the corresponding intermediate as well as the high reaction barriers.
In addition, theoretical calculations have been carried out to examine the structural and energetic aspects of β-hydrogen elimination in several metallocycle complexes of ruthenium and platinum. Factors affecting barriers of the elimination reactions have been examined. It was found that favorable structural arrangements, in which the transferring β-hydrogen is in close proximity to the metal center, for β-hydrogen elimination exist in certain ring conformations of metallocycle complexes. However, favorable electronic requirements, which allow the transferring β-hydrogen to have effective orbital overlap with the hydride-receiving unoccupied orbital from the metal center, cannot always be achieved. Calculations show that β-hydrogen elimination of several five- and six-membered-ring, 16-electron ruthenium complexes occurs easily. The corresponding reactions of platinum complexes were found to be difficult.
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