New types of organometallic oxo and related complexes containing molybdenum and tungsten
Creators: name: Phillips, Everett C.
Publisher: name: University of British Columbia
Year: 2010

description: Treatment of solutions of the 16-electron dialkyl compounds Cp'M(NO)R₂ (Cp’ = Cp (η⁵-C₅H₅) or Cp* (η⁵-C₅Me₅); M = Mo or W; R = CH₂SiMe₃, CH₂CMe₃, CH₂CMe₂Ph, or Me) with either dioxygen or aqueous hydrogen peroxide in Et₂O at ambient temperatures and pressures produces novel 16-electron dioxo alkyl complexes Cp'M(O)₂R which can be isolated in moderate yields (45 - 60 %). The results of labelling studies with ¹⁸O₂ are consistent with the first steps of the reactions involving O₂ proceeding via the coordination of the diatomic molecule to the metal centers in the organometallic reactants. The Cp'Mo(NO)(CH₂SiMe₃)₂ complexes react with adventitious O₂ or H₂O to form [Cp'Mo(NO)(CH₂SiMe₃)]₂ -(μ -0) complexes, the Cp*-analogue of which has been structurally characterized by X-ray crystallography. The unique members in the class of cyclopentadienylnitrosyl dialkyl complexes that exhibit limited reactivity with dioxygen are the nitrosyl bis(benzyl) complexes Cp'M(NO)(CH₂Ph')₂ (M = Mo or W; Ph’ = C₆H₅ or C₆H₂-2,4,6-Me₃). Investigations into the molecular structure of these complexes has revealed that they are formally coordinatively saturated, 18-electron complexes possessing the unusual Cp'M(NO)( η²-CH₂Ph')(η¹-CH₂Ph') structure both in the solid-state and in low temperature solutions. Variable temperature ¹H and ¹³C NMR spectroscopic studies of these complexes in CD₂Cl₂ solutions have established that the bis(benzyl) complexes undergo a facile fluxional process, the two benzyl ligands interchanging their modes of attachment to the molybdenum or tungsten metal center. The fluxional process is more facile for the tungsten bis(benzyl) complexes than the molybdenum species (ΔG‡ being between 8.4 -12.0 kcal/mol). Interestingly, only the tungsten species of this group of bis(benzyl) complexes Cp'M(NO)(CH₂Ph')₂ react with dioxygen to afford the corresponding Cp'W(O)₂(CH₂Ph) compounds. Reactivity studies have revealed that the preferred sites of reactivity in the cyclopentadienyl dioxo alkyl complexes Cp'M(O)₂R are their M = O linkages. Thus, reactions of the dioxo alkyl complexes with 30% H₂O₂(aq) in Et₂O solution converts them cleanly to novel organometallic peroxo alkyl complexes Cp'M(O)( η²- O₂)R, several of which have been isolated and characterized. These complexes are monomelic 16-electron entities having a piano-stool molecular structure, a fact which has been confirmed by a single crystal X-ray crystallographic analysis of CpW(O)(η²- O₂)(CH₂SiMe₃). Reaction of Cp*W(O)(η²- O₂)(CH₂SiMe₃) with tetracyanoethylene (TCNE) results in the formation of a 2:1 charge-transfer complex [Cp*W(O)(η²- O₂)(CH₂SiMe₃)]₂-μ-[(CN)₂C=C(CN)₂] whose solid-state molecular structure has been determined by X-ray analysis. In solution however, the charge-transfer species between Cp*W(O)(η²- O₂)(CH₂SiMe₃) and TCNE exhibits Benesi-Hildebrand behavior and is, therefore, formulated as a 1:1 adduct. Treatment of representative Cp'W(O)₂R compounds with HCl (or PCI₅ or Me₃SiCl) in Et₂O produces the corresponding Cp'W(O)Cl₂R compounds in high yields. These dichloro oxo complexes are very useful synthetic precursors in their own right, undergoing metathesis with alkyllithium or alkyl Grignard reagents to produce the oxo trialkyl compounds Cp'W(O)R[sub n]R'[sub 3-n] in which R and R' represent selected alkyl groups. Sterically crowded members of this class of compounds are not isolable under ambient conditions since they spontaneously convert to the corresponding oxo alkylidene species Cp'W(O)( = alkylidene)R, presumably via intramolecular α-H abstraction. The isolable oxo trialkyl complexes can be induced to undergo the same conversions simply by gentle warming of their solutions. Treatment of representative Cp'M(O)₂R complexes with p-tolyl isocyanate in toluene solution at reflux results in their conversion to the corresponding Cp*M(=NC₆H₄-p-Me)[-N(C₆H₄-p-Me)C(O)N(C₆H₄-p-Me)]R compounds. These imido urea complexes are thought to form as a result of trapping of the bis(imido) complexes Cp*M(=NC₆H₄-p-Me)₂R, analogues to the dioxo alkyl complexes Cp*M(O)₂R, with an equivalent of the p-tolyl isocyanate reagent.