2014 Theses Doctoral

Application of Transition Metal Phosphine Complexes in the Modeling of Catalytic Processes: Reactivity with Hydrosilanes and Other Industrially Relevant Substrates

Zuzek, Ashley

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.