Mechanism of alkylation of alkyldiazenido-complexes of molybdenum(II) and tungsten(II): influence of metal and co-ligands on the nucleophilicity of a diazenido-group

Abstract

The alkylation reaction (1) proceeds via an S_N2 process. The nucleophilicity of the diazenido-group is sensitive to the metal, M = Mo or W, and the diphosphine ligands, R′₂PCH₂CH₂PR′₂, where trans-[MBr(N¹N²Et)(R′₂PCH₂CH₂PR′₂)₂]+ Mel [graphic omitted] trans-[MBr(N¹N²MeEt)(R′₂PCH₂CH₂PR′₂)₂]⁺+ I^–(1) R′=p-X′C₆H₄ and X′= CF₃, Cl, H, Me, or MeO. The second-order rate constants, k₂, for the molybdenum and tungsten analogues are related by the linear-free-energy expression (2); the Ink₂(W)= 1.12 Ink₂(Mo)+ 1.81 (2) tungsten ethyldiazenido-complexes react about an order of magnitude faster than do their molybdenum counterparts. Substituting CF₃ for MeO groups on the aryl rings of the diphosphine ligands decreases k₂ by about three orders of magnitude for W, somewhat less for Mo. Correlation of k₂ and E^0′(M^III/M^II) with the Hammett constant σ shows that the substituent influence on k₂ is predominantly inductive; there is no evidence of through-conjugation from X′ to the reaction centre, N². The cis-PWN¹ unit transmits the influence of X′ to the reaction centre more efficiently than does the corresponding Mo unit.