Preparation and reactions of [MoH(η3-C4H7)(Ph2PCH2CH2PPh2-κ2P)(Ph2PCH2CH2PPh2-κP)]: the intramolecular interconversion between η3-2-methylallyl and η4-trimethylenemethane ligands

Abstract

The complex [MoH(η³-C₄H₇)(dppe-κ³P)(dppe-κP)](dppe = Ph₂PCH₂CH₂PPh₂) has been prepared. Variable-temperature ¹H, ¹³C and ³¹P NMR spectroscopy demonstrates that this 16-electron, η³-2-methylallyl species (the dominant form at 25 °C) rapidly interconverts, intramolecularly, to the 18-electron, η⁴-trimethylenemethane species, [MoH₂(η⁴-C₄H₆)(dppe-κ²P)(dppe-κP)](the dominant form at –80 °C). Protonation of [MoH(η³-C₄H₇)(dppe-κ₂P)(dppe-κP)] with HCl at 25 °C gives stoichiometric yields of [MoH₂,Cl₂(dppe-κ₂P)₂] and Bu^tCH₂C(Me)CH₂. Stopped-flow spectrophotometric studies of the protonation reaction indicate that the alkene formation involves rapid protonation of the molybdenum to give [MoH₂(η³-C₄H₇)(dppe-κ₂P)(dppe-κP)]⁺, which undergoes rate-limiting intramolecular hydrogen migration to form [MoH(η²-Me₂CCH₂)(dppe-κ²P)₂]⁺. Subsequent formation of Bu^tCH₂C(Me)CH₂ occurs rapidly and is not amenable to kinetic analysis. However, the dimerisation is too fast to be catalysed by the excess of HCl alone, and a mechanism involving formation of Bu^tCH₂C(Me)CH₂ at the molybdenum centre is proposed.