Structural features of transition metal complexes having ligands with different electronic properties and mechanistic aspect of C-H bond activation and functionalization by transition metal complexes
by Lam Wai Han
xix, 156 leaves : ill. ; 30 cm
Theoretical studies on the structural features of transition-metal complexes having ligands with different electronic properties and mechanism for stoichiometric and catalytic transition-metal reactions are reported in this thesis. Having a clear picture of the nature of transition-metal-ligand bonding is important in understanding structures, properties and reactivity of transition metal complexes. Knowledge of mechanistic aspect is very useful in designing better and effective catalysts....[ Read more ]
Theoretical studies on the structural features of transition-metal complexes having ligands with different electronic properties and mechanism for stoichiometric and catalytic transition-metal reactions are reported in this thesis. Having a clear picture of the nature of transition-metal-ligand bonding is important in understanding structures, properties and reactivity of transition metal complexes. Knowledge of mechanistic aspect is very useful in designing better and effective catalysts.
The development of Dewar-Chatt-Duncanson model has a great impact on coordination/organometallic chemistry in terms of understanding the structure and bonding in metal complexes containing π-accepting ligands. The majority of π-acceptor ligands can be categorized into two types, double-face and single-face π-accepting ligands. Metal complexes containing both single-face and double-face π-accepting ligands show unique structural preferences. The structural consequence for these complexes will be discussed with the aid of density functional theory (DFT) calculations. Examples include η1-alkenyl, η2-silane, η2-alkene and boryl octahedral complexes.
The X-ray diffraction shows that the relative orientations of three amido ligands in the titanium and zirconium amido complexes containing a hydrotris(pyrazolyl)borate (Tp) or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) ligand TpM(NMe2)3 and Tp*M(NMe2)3 (M = Ti, Zr) are quite different. Theoretical calculations at the B3PW91 level of the density functional theory on model complexes TpTi(NMe2)3 and TpZr(NMe2)3 were performed to rationalize the observed orientations and the rotational behavior of amido ligands in these metal complexes.
Bonding analysis has been performed for titanocene borane σ-complexes Cp2Ti(η2-HBcat)2 and Cp2Ti(PMe3)(η2-HBcat) to investigate the unusual metal η2-HBcat bonding interactions with the aid of density functional theory calculations at the level of B3PW91. The significantly short B...B contact in the former complex is explained with a three-center-two-electron bond involving the B-Ti-B triangle. In the Cp2Ti(PMe3)(η2-HBcat) complex, a related bonding feature involving a metal d orbital and an sp3 hybridized fragment orbital derived from the HBcat ligand is found to be responsible for the metal-boron interaction.
The H/D exchange processes between CH4 and deuterated organic solvents - benzene-d6 and diethyl ether-d10 catalyzed by the ruthenium complex TpRu(PPh3)(CH3CN)H (Tp = hydrotris(pyrazolyl)borate) have been investigated by density functional theory calculations at the B3LYP level. Theoretical study on the reaction mechanism suggests that σ-complexes TpRu(PPh3)(η2-H-R)H are active species in the exchange processes. During the exchange processes, the reversible transformations of TpRu(PPh3)(η2-H-R)H to TpRu(PPh3)(η2-H2)R are the crucial steps. The barriers for the transformations are in the range of 10 - 13.4 kcal/mol. Interestingly, the transition states for the transformations correspond to the seven-coordinate TpRu(PPh3)(R)(H)(H), which are species derived from the oxidative addition of H-R to the metal center. The exchange processes involve transformations of the (η2-H-R) species to the (η2-H2) species followed by the H-H rotation in the latter. The rotation barriers are calculated to be in the range of 2 - 4 kcal/mol. The exchange process having aromatic R group is found to be most favorable due to the strong Ru-C(sp2) bonding which stabilizes the (η2-H2) species and lowers the transformation barrier.
Theoretical calculations on the metathesis process, Tp(PH3)MR(η2-H-CH3) ---> Tp(PH3)M(CH3)(η2-H-R) (M= Fe, Ru, and Os; R= H and CH3), have been systematically carried out to study their detailed reaction mechanisms. Other than the one-step mechanism via a four-center transition state and the two-step mechanism via the oxidative additional/reductive alive elimination pathway, a new one-step mechanism with an oxidatively-added transition state has been found. Based on the intrinsic reaction coordinate calculations, we found that the trajectories of the transferring hydrogen in the studied metathesis processes studied are similar to each other regardless of the nature of reaction mechanisms.
Reaction mechanisms of the methane and benzene functionalizations (borylation) by CpFe(CO)(BO2C2H2) and CpW(CO)2(BO2C2H2 have been investigated with the aid of B3LYP density functional theory calculations. The significant barrier difference between the functionalizations of methane and benzene by the Fe complex and the small barrier difference between the functionalizations by the W complex from our calculations are in good agreement with the experimental observation in a series of photochemical reactions of the transition-metal boryl complexes with alkanes and arenes. Cp*W(CO)3)Bcat' [Bcat' = B-1,2-O2C6H2-3,5-Me2] has comparable reactivity toward both alkanes and arenes while the iron boryl complexes complexes Cp’Fe(CO)2Bcat (Cp’ = Cp and Cp*; Bcat = B-1,2-O2C6H4) are very reactive toward the aromatic C-H bonds of arenes and show unreactive toward the alkane C-H bonds. The distinct barriers between the functionalizations of methane and benzene by the Fe complex can be explained by the significant stabilization interaction between the ‘‘empty” boron p orbital of the boryl group and the π orbitals of the benzene ring in the oxidatively added transition state for the iron-benzene system. For the purpose of comparison, a mechanistic study on the functionalizations of methane and benzene by model complex CpRu(CO)(BO2C2H2) has also been done.