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]
- 1 January 1995
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 15,p. 2479-2487
- https://doi.org/10.1039/dt9950002479
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.Keywords
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