Chemistry of highly reactive group 5 and 6 transition metal compounds

• Main Author: Sattler, Aaron
• Language: English
• Published: 2012
• Subjects: Chemistry
• Online Access: https://doi.org/10.7916/D8N58THB

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(!2– 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(!2– CH2PMe2)H and haloarenes, namely the formation of (i) the alkylidene complex, [W(PMe3)4(!2–CHPMe2)H]X (X = Br or I) and (ii) the phosphoniocarbyne complex, W(PMe3)3Cl2(CPMe2Ph). Additionally, treatment of [W(PMe3)4(!2–CHPMe2)H]X with LiAlD4, allows for the isolation of the isotopomer W(PMe3)4(!2–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(!2–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(!2–CH2PMe2)H, giving the chelating bisisocyanide complex, ["2-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(!2–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, ["3- ArTol’2]Ta(PMe3)2MeCl (Tol’ = C6H3Me). Reduction of ["3-ArTol’2]Ta(PMe3)2Cl2 with KC8 in benzene gives the arene complex ["3-ArTol’2]Ta(PMe3)2(!6-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 (["3-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 ["2-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 ["2-ArTol,Tol’]Ta(NMe2)2Np and neopentane, of which the former isomerizes to produce ["2-Ar*Tol,Tol’]Ta(NMe2)2Np (* indicates the new connectivity of the aryl ligand).