Mechanism of the base-catalysed substitution of halide for methoxide in the reactions of trans-[W(NH)X(Ph2PCH2CH2PPh2)2]+(X = Cl, Br, or I)

Abstract

The kinetics of the reaction between trans-[W(NH)X(dppe)₂]⁺[X = Cl, Br, or I; dppe = 1,2-bis(diphenylphosphino)ethane] and triethylamine or lithium methoxide in methanol to yield the complex trans-[W(NH)(OCH₃)(dppe)₂]⁺ have been investigated. It is proposed that the reactions with both bases proceed via a common mechanism. This involves initial deprotonation of trans-[W(NH)X(dppe)₂]⁺, to generate trans-[W(N)X(dppe)₂]. Rapid dissociation of the halide yields the relatively stable ion pair [W(N)(dppe)₂]⁺·X^–. Subsequent rate-limiting attack of methoxide on the ion pair gives trans-[W(N)(OCH₃)(dppe)₂] which rapidly abstracts a proton from a solvent molecule to yield the product trans-[W(NH)(OCH₃)(dppe)₂]⁺. The pK_a of trans-[W(NH)X(dppe)₂]⁺ is sensitive to the nature of the trans ligand, and direct comparison of the pK_a for trans-[M(NH)X(dppe)₂]⁺(M = Mo or W, X = F or Cl) shows that the tungsten complexes are about a 1 000-fold less acidic than their molybdenum counterparts.