Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium

Abstract

Rare-earth metal alkyl and hydrides are highly reactive species that can act as a metal precatalyst or reactive intermediate in a variety of homogeneous catalytic reactions such as hydroelementation and polymerization of olefins. Rare-earth metal hydrides play an important role for researchers to study sigma bond metathesis reactions and develop better homogeneous catalysts. However, these complexes tend to oligomerize in order to stabilize the highly polarizable, soft hydride at the Lewis acidic metal center. This thesis presents the synthesis various scandium complexes supported by the tetrapodal pentadentate B2Pz4Py ligand with the aim to stabilize these reactive intermediates and to isolate their monomeric species. The single site of reactivity at the apical position allows for controlled reactivity and a better understanding of these transformations. The scandium alkyl complexes were shown to be highly robust and do not undergo sigma bond metathesis to afford the hydride complex. Alternatively, the scandium hydride was synthesized from the reaction between a ligated scandium chloro complex and NaHBEt3 and was found to exist as a dimeric species. These complexes reacted with small molecules like H2O, N2O and CO2, and the mechanism for CO2 insertion was studied by DFT calculations. Cationic scandium complexes were also generated by alkyl or hydride abstraction with B(C6F5)3 and employed as a catalyst for hydrosilylation of CO2. Efforts to synthesize an anionic scandium terminal oxo complex showed that the complex is highly basic and immediately scavenges any acidic protons to form the corresponding scandium hydroxo complex which condenses to form the more thermodynamically stable scandium µ-O dimer. The analogous B2Pz4Py titanium complexes were also synthesized which showed some differences in the coordination chemistry. The dimeric titanium hydride complex is further stabilized by π stacking interactions which made it unreactive to N2O and CO2. Although the titanium µ-O dimer is once again a common by-product in the presence of adventitious moisture, it can be further oxidized by O2 to form other peroxo and oxo products.