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 pKa of trans-[W(NH)X(dppe)2]+ is sensitive to the nature of the trans ligand, and direct comparison of the pKa 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.
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