Development of tungsten nitrosyl alkylidene complexes for activation of hydrocarbons

• Main Author: Tran, Elizabeth
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/13226

series of tungsten alkyls of stoichiometry Cp'W(NO)(R)(R') [Cp' = Cp, Cp*; R = CH₂CMe₃ , CHȈ2CMe₂Ph, CH₂SiMe₃ ; R' = CH₂CMe₃ , CH₂CMe₂Et, CH₂CMe₂Ph, CH₂SiMe₃, CH₃, Ph, C l , CH₂Ph, OCH₃, NME₂ ] is shown by NMR and, in some cases, IR spectroscopies to adopt an a-agostic structure when secondary interactions such as lone pair or 7i-electron donation are not possible. This was confirmed by X-ray diffraction studies of several of these compounds and by a neutron diffraction study of Cp*W(NO)( CH₂CMe₃)₂ . In each, the agostic interaction is best described as a closed, 3-center, 2-electron bond and serves to give the electron-deficient tungsten center an 18-electron configuration. The complexes in which R and R' are neopentyl-like display fluxional behavior in solution, exhibiting averaged NMR spectra as a result of a competition between R and R' for formation of an a-agostic bond with the metal. This competition is manifested in terms of unusual NMR chemical shifts and coupling constants for the α-CH2 groups. These data establish that the strength of the C-H—W interaction is dependent on the nature of C p ' , R, and R ' . For complexes with R = R ' , it varies in the order Cp* > Cp and R = R ' = CH₂CMe₃ ~ CH₂CMe₂Ph > CH₂SiMe₃ . For complexes with R ± R ' , two trends are observed: First, among all the alkyl ligands studied, the relative a-agostic donor ability is CH₂CMe₃ > CH₂CMe₂Et > CH₂CMe₂Ph > CH₂SiMe₃ » CH₃ (CH₂Ph is nonagostic). Second, when R = CH₂CMe₃ and R ' = CH₂CMe₃ , CH₂CMe₂ E t , CH₂CMe ₂Ph, CH₂SiMe₃ , CH₃, Ph, Cl , or CH₂Ph, the α-agostic donor ability of R decreases as R' is changed from Ph to CH₃ to Cl to CH₂SiMe₃ to CH₂CMe₂Ph to CH₂CMe₂Et to CH₂CMe₃ to CH₂Ph. The neopentyl complexes, Cp'W(NO)(CH₂CMe₃)₂ [Cp' = Cp, Cp*] and Cp*W(NO)(CH₂CMe₃)(CH₂Ph), undergo facile a-abstraction of neopentane at 60-70 °C to give transiently the 16-electron alkylidene complexes, [Cp'W(NO)(=CHCMe₃ ) ] and [Cp*W(NO)(=CHPh)], respectively. Both [Cp'W(NO)(=CHCMe₃ ) ] and [Cp*W(NO)(=CHPh)] can be trapped with tertiary phosphines in THF solvent to yield the corresponding phosphine adducts as the anti rotamer. In the case of [Cp*W(NO)(=CHCMe₃)], the trapping can also be effected in neat pyridine or Me2NEt. Cp*W(NO)(=CHCMe₃)(PMe₃ ) undergoes associative addition reactions with Me₂NH, MeOH, and PhC0₂H to give Cp*W(NO)(CH₂CMe₃)(ER) [ER = NMe2, OMe, 02CPh] and free PMe3. The amido complex is also formed smoothly when [Cp*W(NO)(= CHCMe₃)j is generated in THF in the presence of Me₂NH. When [Cp*W(NO)(= CHCMe₃)] is generated in alkene solvents, it undergoes [2+2] cycloaddition and/or C - H activation reactions depending on the alkene structure. Cycloaddition is observed for acyclic alkenes which lack allylic C - H bonds and for cyclic alkenes with strained C=C bonds. In the case where the acyclic alkenes contain accessible allylic hydrogens, complicated C - H activation product mixtures are formed. Alkanes and arenes also react readily with [Cp*W(NO)(=CHCMe3)] at 70 °C by C -H activation. Reactions with alkanes proceed with moderate steric selectivity and produce mixed bis(alkyl), metallacyclobutane, alkene, or allyl complexes depending on the alkane structure. Cp*W(NO)( CH₂CMe₃)(CH₂SiMe₃) is produced in high yield in the reaction with Me₄Si . The major product of the reaction with 1,1,2,2-tetramethylcyclopropane is a metallacycle formed by intermolecular C - H activation followed by y-cyclometalation. Neohexane and cyclohexane, which are capable of P-elimination, react to give alkene complexes which can be trapped by PMe₃ or Me₂NH. In contrast, alkanes such as pentane and methylcyclohexane react to give Cp*W(NO)(73-allyl)(H) complexes in both the presence and absence of PMe₃. Reactions with arenes are similarly sensitive to steric effects and involve competitive aryl vs benzyl C - H activation. The benzyl-activated products are unstable at 70 °C, decomposing to give benzylidene complexes that also readily cleave aryl and benzyl C - H bonds. Mechanistic studies, including isotopic labeling, isotope effect, and kinetic studies, strongly implicate that the thermal decompositions of Cp'W(NO)( CH₂CMe₃)₂ proceed by a reversible, rate-determining a-abstraction process to form the neopentane complex, [Cp'W(NO)(= CHCMe₃)(772-H- CH₂CMe₃)], in which the metal can coordinate interchangeably to the neopentane a- and y - C - H bonds. Concurrently, the coordinated neopentane dissociates irreversibly from the metal center to yield [Cp'W(NO)(= CHCMe₃) ] , which can rapidly form adducts with dative ligands or coordinate another hydrocarbon and undergo addition reactions. The thermal decompositions of the mixed bis(alkyl) complexes, Cp*W(NO)( CH₂CMe₃)(R), where R = CH₂CMe₂ E t , CH₂CMe₂Ph, CH₂SiMe₃ , CH₃ , and Ph, are also reported. These studies show that the pathway by which these compounds decompose is governed primarily by the strengths of the M - C bond broken and formed, and that the presence of α-agostic interactions in the ground state has little, i f any, effect on the mechanism of decomposition.