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Synthesis and ligand reactivity of group 6 organometallic nitrosyl complexe

University of British Columbia

Reaction of Cp*WR₂(NO) (R = CH₂SiMe₃ , CH₂Ph) complexes with HBF₄ yields the nitrosyl-borane adduct species Cp*WR₂ (NOBF₃) via degradation of the tetrafluoroborate counterion. The similar materials, Cp*WR₂(NOB(C₆F₅)₃), can be prepared by the addition of perfluorophenylborane to the metallonitrosyl complex. The reaction of a protic acid containing a very weakly coordinating anion ([H(OEt₂)₂][B(3,5-(CF₃)C₆H₃)₄]) with the tungsten nitrosyl species yields the first complexes containing a terminal hydroxylimido ligand, [Cp*WR₂ (NOH)]⁺. These complexes have been characterised by single crystal X-ray crystallography, and their solution behaviour has been studied by NMR spectroscopy. The dimethyl complexes, Cp*MoMe₂ (NO), have proven to be remarkably elusive synthetic targets in our laboratories. Thus, treatment of [Cp*MoCl(NO)(μ-Cl)]₂ with magnesium (Me₂Mg-dioxane, MeMgCl) or aluminum (Me₃Al) based methylating reagents leads only to metathesis of a single chloro ligand and the formation of the known compound [Cp*MoMe(NO)(μ-Cl)]₂. Similar treatment of the dichloro precursor with MeLi in ethereal solvents generates an equimolar mixture of the mono(methyl) complex and the trimethyl 'ate' complex, Cp*MoMe₃(NO-Li(OEt₂)[sub n]). In toluene at low temperatures, MeLi can be utilised to effect complete metathesis of the chloro ligands of [Cp*MoCl(NO)(μ-Cl)]₂ and generate the target dimethyl complex, Cp*MoMe₂(NO), in high yield. In solution this compound is predominantly a monomeric species similar to its more sterically encumbered congeners, whereas in the solid state it adopts a dimeric or oligomeric structure containing isonitrosyl bridges as indicated by IR and solid-state NMR spectroscopies. The reaction of the dimethyl complex with a range of Lewis bases, L, to form the 18e adducts Cp*MoMe₂L(NO), has established it to be the most electrophilic complex of its family. Most interestingly, the dimethyl complex is thermally unstable in solution and undergoes an (R-)[M](-NO) to (0=)[M](=NR) isomerisation to its oxo imido form, Cp*Mo(NMe)(O)Me, which is isolable from the final reaction mixture as the μ-oxo bridged adduct Cp*Mo(NO)Me₂(μ-O)Cp*Mo(NMe)Me. The rate of this isomerisation is significantly faster for the tungsten dimethyl complex; hence Cp*WMe₂(NO) is not isolable free of a supporting donor interaction. Under mild thermolysis conditions the tungsten complexes CpW(o-tolyl)₂(NO) undergo an aryl group isomerisation to form the mixed o-tolyl p-tolyl and o-tolyl m-tolyl complexes. The overall in-plane rotation of the tolyl group occurs via an η²-toluene η²-toluyne intermediate.

Thesis/Dissertation

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2009-09-30

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http://hdl.handle.net/2429/13416

Chemistry

University of British Columbia

UBCV

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