Chemistry of highly reactive group 5 and 6 transition metal compounds

Trimethylphosphine complexes of tungsten and molybdenum have been used to model the coordination chemistry and reactivity that may be observed on the surface of an industrial hydrotreating catalyst. Most notably, it was observed that W(PMe₃)₄(η²-CH₂PMe₂))H is capable of (i) the unprecedented cleavage of an aromatic carbon–carbon bond, and (ii) desulfurizing thiophene, benzothiophene, and dibenzothiophene. In addition to the group 6 chemistry, the first [CCC] X₃-donor pincer ligand for a transition metal was synthesized by two consecutive cyclometalations of a terphenyl complex of the group 5 metal tantalum. Chapter 1 describes two new transformations that occur between W(PMe₃)₄(η²-CH₂PMe₂))H and haloarenes, namely the formation of (i) the alkylidene complex, [W(PMe₃)₄(η²-CH₂PMe₂)H]X (X = Br or I) and (ii) the phosphoniocarbyne complex, W(PMe₃)₃Cl₂(CPMe₂Ph). Additionally, treatment of [W(PMe₃)₄(η²-CH₂PMe₂)H]X with LiAlD₄, allows for the isolation of the isotopomer W(PMe₃)₄(η²–CHDPMe₂)H, thereby providing a means to measure the rate constant for the formation of the 16-electron species [W(PMe₃)₅] from W(PMe₃)₄(η²-CH₂PMe₂)H. Chapter 2 describes the reactivity of trimethylphosphine complexes of molybdenum with phenazine and related N-heterocycles, in order to model aspects of hydrodenitrogenation. Several new coordination modes of phenazine to molybdenum were observed. Studies also indicate that oxidative addition of H2 is promoted by (i) incorporation of nitrogen substituents into the central ring and (ii) ring fusion. Furthermore, ring fusion promotes hydrogenation of the heterocyclic ligand. Chapter 3 describes the novel aromatic carbon–carbon bond cleavage and dehydrogenation of quinoxaline by W(PMe₃)₄(η²-CH₂PMe₂)H, giving the chelating bisisocyanide complex, [κ²-C₂-C6H₄(NC)₂]W(PMe₃)₄. Chapter 4 describes the reactivity of trimethylphosphine complexes of tungsten and molybdenum with thiophenes, in order to model aspects of hydrodesulfurization. Mo(PMe₃)₄H₄ desulfurizes thiophene and benzothiophene. Moreover, ...