Protonation of trans-[Mo(η2-MeCCH)2(Ph2PCH2CH2PPh2)2]: mechanism of formation of trans-[MoX(CHCHMe)(Ph2PCH2CH2PPh2)2](X = Cl or Br) or trans-[MoF(CCH2Me)(Ph2PCH2CH2PPh2)2]

Abstract

The reaction between trans-[Mo(η²-MeCCH)₂(Ph₂PCH₂CH₂PPh₂)₂] and an excess of anhydrous HX (X = Cl or Br) in tetrahydrofuran gives trans-[MoX(CHCHMe)(Ph₂PCH₂CH₂PPh₂)₂] and the evolution of 1 mol equivalent of MeCCH. Mechanistic studies indicated that initial protonation of trans-[Mo(η²-MeCCH)₂(Ph₂PCH₂CH₂PPh₂)₂] occurs at a propyne ligand to form the vinyl species, trans-[Mo(CHCHMe)(η²-MeCCH)(Ph₂PCH₂CH₂PPh₂)₂]⁺, and at low concentrations of acid rate-limiting dissociation of the other trans-propyne, followed by attack of halide ion at the molybdenum, produces trans-[MoX(CHCHMe)(Ph₂PCH₂CH₂PPh₂)₂]. At high concentrations of acid further rapid protonation of the vinyl ligand occurs to give trans-[Mo(CHCH₂Me)(η²-MeCCH)(Ph₂PCH₂CH₂PPh₂)₂]²⁺. This second protonation further labilises the trans-propyne which is lost in the rate-limiting step and subsequent attack of halide gives trans-[MoX(CHCH₂Me)(Ph₂PCH₂CH₂PPh₂)₂]⁺. Dissociation of a proton gives the product, trans-[MoX(CHCHMe)(Ph₂PCH₂CH₂PPh₂)₂]. The reaction between trans-[Mo(η²-MeCCH)₂(Ph₂PCH₂CH₂PPh₂)₂] and HBF₄·OEt₂ gives a mixture of trans-[MoF(CHCHMe)(Ph₂PCH₂CH₂PPh₂)₂] and the alkylidyne complex: trans-[MoF(CCH₂Me)(Ph₂PCH₂CH₂PPh₂)₂]. The alkylidyne complex has been isolated pure by fractional crystallisation. The factors which discriminate between the formation of the vinyl and alkylidyne species are discussed.