Application of Transition Metal Phosphine Complexes in the Modeling of Catalytic Processes: Reactivity with Hydrosilanes and Other Industrially Relevant Substrates
Creators: Zuzek, Ashley
Publisher: name: Columbia University
Year: 2014
Subjects: Catalysis
Silane compounds
Transition metal complexes
Chemistry
Chemistry, Inorganic

description: The first two chapters of this thesis are devoted to exploring the reactivity of electron rich molybdenum and tungsten trimethylphosphine complexes with hydrosilanes. These complexes, Mo(PMe₃)₆ and W(PMe₃)₄(η²-CH₂PMe₂)H, have been shown to be highly reactive species that undergo a number of bond cleavage reactions. In the presence of the hydrosilanes PhxSiH₄-x (x = 0 - 4), Mo(PMe₃)₆ and W(PMe₃)₄(η²-CH₂PMe₂)H effect Si-H and Si-C bond cleavage, along with Si-Si bond formation; however, the products derived from these reactions are drastically different for Mo(PMe₃)₆ and W(PMe₃)₄(η²-CH₂PMe₂)H and are highly dependent on the substitution of the silane. Mo(PMe₃)₆ reacts with SiH₄, PhSiH₃, and Ph₂SiH₂ to afford novel silyl, hypervalent silyl, silane, and disilane complexes, as respectively illustrated by Mo(PMe₃)₄H2(SiH3)₂, Mo(PMe₃)₄H(k₂-H₂-H₂SiPh₂H), Mo(PMe₃)₃H₄(s-HSiHPh₂), and Mo(PMe₃)₃H₂(k₂-H₂-H₂Si₂Ph₄). Mo(PMe₃)₄H(k₂-H₂-H2Si2Ph₄) is the first example of a complex with a hypervalent [H₂SiPh₂HH] ligand, and Mo(PMe₃)₃H2(k₂-H₂-H₂Si₂Ph₄) represents the first structurally characterized disilane complex. In addition to being structurally unique, these complexes also possess interesting reactivity. For example, Mo(PMe₃)₄((SiH₃)₂H₂ undergoes isotope exchange with SiD₄, and NMR spectroscopic analysis of the SiHxD₄₋ₓ isotopologues released indicates that the reaction occurs via a sigma bond metathesis pathway. In contrast, W(PMe₃)₄(η²-CH₂PMe₂)H affords a range of products that includes metallacycle, disilyl, silane, and bridging silylene complexes. The disilyl compounds, W(PMe₃)₄H₃(SiH₂SiHPh₂) and W(PMe₃)₃H₄SiH₂Ph)(SiH₂SiHPh₂), exhibit the ability of W(PMe₃)₄(n₂-CH₂PMe₂)H to cause both redistribution and Si-Si bond formation. A mechanism involving silylene intermediates is proposed for the generation of these complexes, and this mechanism is supported computationally. Additional support for the presence of intermediates comes from the isolation of a unique complex with a bridging silylene ligand, "WSiW". The bridging silylene bonding motif is unprecedented. The reactivity of the simplest hydrosilane, SiH₄, was also examined with IrCl(CO)(PPh₃)₂ (i.e. Vaska's compound). Previous reports on this reaction have assigned the product as trans-IrH(SiH₃)(Cl)(CO)(PPh₃)₂, in which the hydride and silyl ligands are mutually trans. It is noteworthy, therefore, that we have now obtained a crystal structure of the product of this reaction in which the hydride and silyl ligands are cis, namely cis-IrH(SiH₃)(Cl)(CO)(PPh₃)₂. Calculated energies of the isomeric species also suggest that the product of this reaction was originally misassigned. These results, and the analogous reactions with germane (GeH₄), are described in Chapter 4. Chapter 4 also discusses some reactions of transition metal phosphine complexes, including Ru(PMe₃)₄H₂, Mo(PMe₃)₆, W(PMe₃)₄(η²-CH₂PMe₂)H, and Mo(PMe₃)₄(η²-CH₂PMe₂)H, with industrially relevant substrates. Ru(PMe₃)₄H₂ effects the water gas shift reaction of CO and H2O to form CO2 and H2. Furthermore, Ru(PMe₃)₄H₂ reacts with CO₂, CS₂, and H₂S to respectively form formate, thiocarbonate, and hydrosulfido complexes. The reactivity of Mo(PMe₃)₆ and W(PMe₃)₄(η²-CH₂PMe₂)H towards molecules relevant to the hydrodeoxygenation industry, including dihydrofuran and benzofuran, was studied. The products of these reactions exhibit hydrogenation of unsaturated bonds and C-O bond cleavage, both of which are essential to the hydrodeoxygenation process. Mo(PMe₃)₄(η²-CH₂PMe₂)H reacts with PhI to form an alkylidyne species, [Mo(PMe₃)₄(CPMe₂Ph)I]I, which was structurally characterized by X-ray diffraction. W(PMe₃)₄(η²-CH₂PMe₂)H forms a k2-adduct when treated with 2-seleno-2-methylbenzimidazole, namely W(PMe₃)₄(sebenzimᴹᵉ)H. Chapter 3 discusses the development of two new ruthenaboratrane complexes, [k⁴-B(mimᴮᵘᵗ)₃]Ru(CO)(PR₃) (R = Ph, Me). The structures of these complexes are described, and their d⁶ metal configuration is supported by both Fenske-Hall and Natural Bond Orbital calculations. Some reactivity of these complexes was also explored. For example, [k⁴-B(mimᴮᵘᵗ)₃]Ru(CO)(PMe₃) appears to add MeI across the Ru-B bond. Finally, as an extension of the work that we have done on tungsten trimethylphosphine complexes, the structure of W(PMe₃)₃H₆ in solution was investigated, and the results are presented in Chapter 5. T₁ measurements of the hydride ligands and deuterium isotope effect shifts both confirm that this complex exists as a classical hydride in solution, which is in accord with the classical hydride formulation in the solid state that was previously determined by X-ray diffraction.