Density Functional Theory (DFT) calculations have been widely applied to understand mechanisms of various chemical reactions. In this thesis, theoretical studies employing DFT calculations on recently discovered stoichiometric and catalytic reactions are reported. Insights into developing new reactions, selecting appropriate substrates, and designing more efficient catalytic systems are given.

Part I: The first example of complete cleavage of isonitrile carbon-nitrogen triple bond without the mediation of a transition metal has been investigated. The mechanism of the reaction is found to be passing through a succession of 1,2-migrations that reduce the carbon-nitrogen π-bonds. Breaking of the strong carbon-nitrogen σ-bond is attributed to the presence of nucleophilic nitrogen atom. Th...[ Read more ]

Density Functional Theory (DFT) calculations have been widely applied to understand mechanisms of various chemical reactions. In this thesis, theoretical studies employing DFT calculations on recently discovered stoichiometric and catalytic reactions are reported. Insights into developing new reactions, selecting appropriate substrates, and designing more efficient catalytic systems are given.

Part I: The first example of complete cleavage of isonitrile carbon-nitrogen triple bond without the mediation of a transition metal has been investigated. The mechanism of the reaction is found to be passing through a succession of 1,2-migrations that reduce the carbon-nitrogen π-bonds. Breaking of the strong carbon-nitrogen σ-bond is attributed to the presence of nucleophilic nitrogen atom. The influences of excess amount of tert-butyl isonitrile in resulting different products are discussed. For the analogous reaction using carbon monoxide, the reason for unsuccessful complete carbon-oxygen triple bond cleavage has been related to the poorer nucleophilicity of the oxygen atom.

Part II: The reaction mechanisms for the formation of the first rhenabenzene complex Re[–C(Np)=(C(CO₂Et)C(OEt)=CHC(OEt)=}(CO)₄ (Np = naphthalene-1-yl) and an unexpected vinyl-substituted rhenacyclobutadiene complex Re{–C(Np)=C(C(OEt)=CH(CO₂Et))C(OEt)=}(CO)₄ have been studied. The results suggest that the reactions are initiated by the direct nucleophilic attack of the ethoxyethyne on a carbene carbon of the rhenacyclobutadiene complex Re{–C(Np)=(C(CO₂Et)C(OEt)=}(CO)₄. Energies of the lowest unoccupied molecular orbitals (LUMOs) and the charges on the carbene carbons of the rhenacyclobutadiene complexes are important in determining the reactivity and the regioselectivity of the reactions. Structural analysis and aromatic stabilization energy calculations on model metallabenzenes show that rhenabenzene complexes are aromatic in nature.

Part III: DFT calculations have also been employed to study an unforeseen formation of the η⁵-oxocyclohexadieneyl complex Re{η⁵-C₆O(Np)(CO₂Et)(OEt)(Ph)(NEt₂)}(CO)₃ from a reaction of the rhenacyclobutadiene complex Re{–C(Np)=(C(CO₂Et)C(OEt)=}(CO)₄ with (diethylamino)phenylacetylene. A competitive formation of the η⁵-cyclopentadienyl complex Re{η⁵-C₅(Np)(CO₂Et)(OEt)(Ph)(NEt₂)}(CO)₃ via the rhenabenzene intermediate C(Np)=(C(CO₂Et)C(OEt)=C(Ph)C(NEt₂)=}(CO)₄ has also been investigated. The mechanistic studies unveil that the respective formations of the two complexes are led from a free carbene in a zwitterion intermediate, derived from a nucleophilic initiation step, competitively attack to a carbonyl ligand and the metal center. Further calculations suggest that alkynes with an amine substituent are potential reactants for analogous reactions. The kinetic stability of rhenabenzenes is found to depend on the steric and electronic properties of substituents on the β-carbon positions of rhenabenzenes.

Part IV: The performance of lately developed ruthenium catalysts in hydrogenation of esters have been studied via DFT calculations. The calculation results show that the rate-determining transition state of a complete catalytic cycle is related to the complexation of dihydrogen molecule to the ruthenium center for catalyst regeneration. The results also indicate that deprotonation of amine in the ligand by base significantly lowers down the overall barrier of the reaction. Absence of electron-withdrawing carbonyl ligand and presence of an electron-donating pyridine moiety in the multidentate ligand are the keys for the high-performance of the ruthenium catalysts.