PREPARATION AND REACTIONS OF [MOH(ETA(3)-C4H7)(PH2PCH2CH2-PPH2-KAPPA(2)P)(PH2PCH2CH2PPH2-KAPPA-P)] - THE INTRAMOLECULAR INTERCONVERSION BETWEEN ETA(3)-2-METHYLALLYL AND ETA(4)-TRIMETHYLENEMETHANE LIGANDS

The complex [MoH(eta3-C4H7)(dppe-kappa2P)(dppe-kappaP)] (dppe = Ph2PCH2CH2PPh2) has been prepared. Variable-temperature H-1, C-13 and P-31 NMR spectroscopy demonstrates that this 16-electron, eta3-2-methylallyl species (the dominant format 25-degrees-C) rapidly interconverts, intramolecularly, to the 18-electron, eta4-trimethylenemethane species, [MoH2(eta4-C4H6) (dppe-kappa2P) (dppe-kappaP)] (the dominant form at -80-degrees-C). Protonation of (MoH(eta3-C4H7) (dppe-kappa2P) (dppe-kappaP)] with HCI at 25-degrees-C gives stoichiometric yields of [MoH2Cl2(dppe-kappa2P)2] and ButCH2C(Me)CH2. Stopped-flow spectrophotometric studies of the protonation reaction indicate that the alkene formation involves rapid protonation of the molybdenum to give (MoH2 (eta3-C4H7)(dppe-kappa2P) (dppe-kappaP)]+, which undergoes rate-limiting intramolecular hydrogen migration to form [MoH(eta2-Me2C=CH2)(dppe-kappa2P)2]+. Subsequent formation of ButCH2C(Me)=CH2 occurs rapidly and is not amenable to kinetic analysis. However, the dimerisation is too fast to be catalysed by the excess of HCI alone, and a mechanism involving formation of ButCH2C(Me)=CH2 at the molybdenum centre is proposed.