Carbon-hydrogen activation by high and low valent transition metal complexes

The aim of this research was to identify the factors that control the selectivity and activity of methane activation by transition metal complexes. Such research is relevant to the identification of industrial catalysts for the conversion of methane (the main component of natural gas) to methanol, a more easily transportable liquid.; Methane adducts play an important part in methane activation. The bonding forces in methane adducts were analyzed using the reduced variational space - self-consistent field (RVS-SCF) technique. The main contributions to the methane binding energy, ΔE_add, were found to be the Coulomb-exchange energy (CEX), charge transfer from methane to the metal complex (CT_2→1 ), and polarization of methane (POL₂). The latter two components were exothermic while the former was endothermic. Furthermore, the CEX and POL₂ components nearly cancel each other, leaving trends in ΔE _add as a function of metal and ligands to be primarily determined by CT_2→1.; Isotope effects are a valuable mechanistic tool in organometallic chemistry. However, their interpretation from experimental data is not straightforward. Hence, equilibrium and kinetic isotope effects were calculated and compared with experimental isotope effects. It was found that isotope effects can be calculated with great accuracy, and that the main contributions to the equilibrium isotope effects (EIEs) are from low energy and “core” modes. The results suggest that computations can be an effective tool in aiding the experimental analysis of isotope effects for organometallic catalysis.; Most previous computational work has only focused on methane activation. However, since methane is only one component of natural gas, selective activation is important. Extensive research was aimed at probing the role of R (hydrocarbon) & R (ligand substituent) on the reaction coordinate for Ti(OR′ )₂(=NR′) + RH → adduct → transition state → (OR′)₂Ti(N(H)R ′)(R). Titanium compounds with R = H, Me, Et, Vy, cPr, Ph, Cy, Bz, and cubyl were studied using quantum (R′ = H, SiH ₃, SiMe₃) and classical (R′ = Si tBu₃) techniques. Through computations, the response of the potential energy surface for C-H activation was seen to vary in terms of the geometries, energetics, and electronic structure of key stationary points. Several significant results were revealed, primarily that weakly-bound alkane adducts play a pivotal role in controlling the selectivity of C-H bond activation by transition metal complexes.; It is also of interest to research how the previous results of this doctoral work apply to other families of C-H activating complexes, in particular those involving later, lower-valent transition metals that activate through oxidative addition. Rhenium was used to compare cyclopentadienyl (Cp) and tris(1-pyrazolyl)borate (Tp) complexes for their C-H activating ability. This research indicates a much greater difference in the kinetic and thermodynamics of C-H activation for Cp and Tp complexes than has been appreciated in the experimental literature.