Title: Chemistry of highly reactive group 5 and 6 transition metal compounds
Description: 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, W(PMe₃)₄(η²-CH₂PMe₂)H is the first tungsten complex that is capable of desulfurization of thiophene, benzothiophene and dibenzothiophene.

Chapter 5 describes the reactivity of trimethylphosphine complexes of tungsten and molybdenum with furans, in order to model aspects of hydrodeoxygenation. Most notably, Mo(PMe₃)₄H₄ is capable of cleaving the C–C, C–O, and C–H bonds of furan, thereby producing propene and carbon monoxide. Chapter 6 describes the synthesis of the first [CCC] X3-donor pincer ligand for a transition metal. Specifically, addition of PMe₃ to [Arᵀᵒˡ²]TaMe₃Cl ([Arᵀᵒˡ²] = 2,6-di-ptolylphenyl) induces elimination of methane and formation of the pincer complex, [κ³- Arᵀᵒˡ’²]Ta(PMe₃)₂MeCl (Tol’ = C6H₃Me). Reduction of [κ³-Arᵀᵒˡ’²]Ta(PMe₃)₂Cl2 with KC8 in benzene gives the arene complex [κ³- Arᵀᵒˡ’²]Ta(PMe₃)2(η⁶-C₆H₆), which is the first structurally characterized benzene complex of tantalum. Deuterium labeling employing Ta(PMe₃)₂(CD₃)₃Cl2 demonstrates that the pincer ligand is generated by a pair of Ar–H/Ta–Me sigma-bond metathesis transformations, rather than by a mechanism that involves #–H abstraction by a tantalum methyl ligand.

The [CCC] pincer ligand ([κ³- Arᵀᵒˡ’²]) was also synthesized on niobium. Chapter 7 describes the synthesis of a variety of other terphenyl complexes of tantalum, namely [Arᵀᵒˡ²]Ta(NMe₂)₃X (X = Me, Et, Prn, Bun, Np, BH₄, and [Arᵀᵒˡ²]), all of which cyclometalate under varying conditions to give [κ²-Arᵀᵒˡ,ᵀᵒˡ’]Ta(NMe₂)₃. The dialkyl complexes, [Arᵀᵒˡ²]Ta(NMe₂)₂R₂ (R = Me, Et, Prn, Bun, and Np) have also been synthesized from the dichloride complex, [Arᵀᵒˡ²]Ta(NMe₂)₂Cl2. The bis-neopentyl complex, [Arᵀᵒˡ²]Ta(NMe₂)₂Np₂, is not stable in solution at room temperature and converts to [κ²-Arᵀᵒˡ,ᵀᵒˡ’]Ta(NMe₂)₂Np and neopentane, of which the former isomerizes to produce [κ²-Ar*ᵀᵒˡ,ᵀᵒˡ’]Ta(NMe₂)₂Np (* indicates the new connectivity of the aryl ligand).