A theoretical study on the nature of organometallic carbene bonding in Titanium Schrock carbene complexes

Abstract

Although various organometallic carbene complexes have found use in industry or research, they still lack some fundamental footing in theory. These complexes have found significant use in catalysis. This is especially true for Schrock carbene complexes in olefination reactions. A few such titanium-based olefination catalyst examples are the Tebbe reagent, Petasis reagent and the Ziegler-Natta catalyst. The nature of organometallic carbene bonding is still not well defined theoretically. Schrock carbenes are expected to have a covalent bonding nature, but multiconfigurational studies have shown this to not be the best description of the bonding. Furthermore, expected Schrock carbenes have been shown to be Fischer carbenes due to the electronic structure. This work investigates the nature of the carbene bond in titanium Schrock carbene complexes by utilising DFT and further application of MO, NBO, QTAIM and FALDI methods. This allows for a modernised description of the nature of this bond as well as the identification of an important long-range ligand-ligand interaction that has not been reported on previously. The research aims to define the nature of titanium Schrock carbene bonding on a theoretical basis by the use of integrated cross-sections on the electron and orbital densities to determine the σ- and π-character of the interaction. These cross-sections provided the means to determine the major components of the bonding interaction. This is further investigated by defining FALDI fragment-based delocalisation indices which revealed the presence of long-range ligand-ligand interactions. The FALDI fragment approach also provided the means to quantify the inter-fragment delocalisation along with intra-fragment localisation and delocalisation which would prove useful for further investigation into the characteristics or various chemical interactions. The fragment-based description should prove to be more intuitive to the chemist than diatomic interactions between atoms where a chemical bond or interaction is not classically expected. This study was followed by a decomposition of the molecular orbitals into localised and delocalised components from atomic contributions which provides a novel approach to determining the bond order in compounds. This provided a quantitative means to describe which atoms contribute to the formation of each molecular orbital as well as providing a measure of the degree to which these atoms are contributing localised as well as delocalised electrons to the molecular orbital.