Formation of the alkoxyalkyl complexes [M{CHMe(OMe)}(CO)2(η-C5H4R)](M = Mo, R = H; M = W, R = H or Me) and their transformations by loss of MeOH into η3-propenoyl or vinyl complexes. Crystal structures of [Mo{CHMe(PPh3)}(CO)2(PPh3)(η-C5H5)]BF4, [Mo(η3-CH2CHCO)(CO)(PPh3)(η-C5H5)] and [W(σ-CHCH2)(CO)2(PPh3)(η-C5H4Me)]

Abstract

Addition of MeSO₃F or [Me₃O]BF₄ followed by PPh₃ to the aldehyde complex [Mo(MeCHO)(CO)₂(η-C₅H₅)]^– results in formation of the methoxyalkyl complex trans-[Mo{CHMe(OMe)}(CO)₂(PPh₃)(η-C₅H₅)] in a reaction that proceeds via the unstable [Mo{η²-CHMe(OMe)}(CO)₂(η-C₅H₅)]. A by-product of this reaction is the ylide trans-[Mo{CHMe(PPh₃)}(CO)₂(PPh₃)(η-C₅H₅)]X (X = BF₄^– or SO₃F^–), a species (X = BF₄) better prepared by treatment of trans-[Mo{CHMe(OMe)}(CO)₂(PPh₃)(η-C₅H₅)] with HBF₄ in the presence of PPh₃. The crystalline ylide trans-[Mo{CHMe(PPh₃)}(CO)₂(PPh₃)(η-C₅H₅)]BF₄ is monoclinic, with a= 11.378(34), b= 18.903(35), c= 20.17(8)Å, β= 115.28(31)° and for the 3918 independent reflections with I/σ(I) > 3.0, R= 0.0510. In solution, trans-[Mo{CHMe(OMe)}(CO)₂(PPh₃)(η-C₅H₅)] either degrades to [MoH(CO)₂(PPh₃)(η-C₅H₅)] or loses MeOH to form the η³-propenoyl [Mo(η³-CH₂CHCO)(CO)(PPh₃)(η-C₅H₅)]. This molecule is monoclinic, with a= 17.231(50), b= 8.189(14), c= 16.727(36)Å, β= 106.79(22)° and for the 1654 independent reflections for which |F|/σ(|F|) > 4.0, R= 0.100. The proposed mechanism for its formation is based on isotopic tracking experiments and includes a vinyl to carbonyl migration. Treatment of [WMe(CO)₃(η-C₅H₄R)](R = H or Me) with PPh₃ in MeCN gives the acyl [W(σ-COMe)(CO)₂(PPh₃)(η-C₅H₄R)]. These react with [Me₃O][BF₄] to form the cationic carbenes [W{CMe(OMe)}(CO)₂(PPh₃)(η-C₅H₄R)]BF₄, which in turn react with LiBHEt₃ to give the alkoxyalkyls trans-[W{CHMe(OMe)}(CO)₂(PPh₃)(η-C₅H₄R)]. The alkoxyalkyls trans-[W{CHMe(OMe)}(CO)₂(PPh₃)(η-C₅H₄R)] react with traces of acid to form the σ-vinyls trans-[W(σ-CHCH₂)(CO)₂(PPh₃)(η-C₅H₄R)], the methylcyclopentadienyl species of which is crystalline, monoclinic, with a= 36.952(40), b= 10.971(13), c= 25.148(23)Å and β= 103.60(8)°. For the 2542 independent reflections for which |F|/σ(|F|) > 5.0, R converged to 0.0750.