Issue 16, 1997

Tertiary phosphine ligand-exchange reactions involving the M[quadruple bond, length as m-dash]>M quadruply bonded complexes M2Cl4L4, where L = PMe3, PEt3, PBun3 or PMe2Ph

Abstract

The reactions between M 2 Cl 4 L 4 complexes and an excess of L or L′ (PMe 3 , PEt 3 , PBu n 3 , PMe 2 Ph or PMePh 2 ) have been studied in [ 2 H 8 ]toluene by 31 P-{ 1 H} NMR spectroscopy. The substitutions proceed in a stepwise manner wherein L′ displaces L, except for L′ = Me 2 PCH 2 CH 2 PMe 2 (dmpe) which yields Mo 2 Cl 4 (dmpe-P) 4 . No tertiary phosphine in this series is capable of displacing PMe 3 from a M 2 Cl 4 (PMe 3 ) 4 complex but by spin magnetization transfer the degenerate exchange involving Mo 2 Cl 4 (PMe 3 ) 4 and PMe 3 (added in excess) can be detected. The complexes Mo 2 Cl 4 (PMe 3 ) 4 and Mo 2 Cl 4 (PEt 3 ) 4 in benzene showed no PMe 3 for PEt 3 scrambling at +50 °C over several days despite the fact that Mo 2 Cl 4 (PMe 3 ) 4 -  n (PEt 3 ) n , where n = 2 or 3, are kinetically inert to ligand redistribution. In the presence of [ 2 H 5 ]pyridine Mo 2 Cl 4 (PMe 3 ) 4 and Mo 2 Cl 4 (PEt 3 ) 4 underwent tertiary phosphine scrambling at 25 °C and in neat [ 2 H 5 ]pyridine Mo 2 Cl 4 (PMe 3 ) 4 revealed the formation of an equilibrium concentration of Mo 2 Cl 4 (PMe 3 ) 3 (py) (py = pyridine) and free PMe 3 . Under similar conditions Mo 2 Cl 4 (PEt 3 ) 4 yielded an equilibrium mixture of Mo 2 Cl 4 (PEt 3 ) 3 (py) and Mo 2 Cl 4 (PEt 3 ) 2 (py) 2 and free PEt 3 . From kinetics the ΔH values are positive in the range +24 to +34 kcal mol -1 and the ΔS values range from +12 to +28 cal K -1 mol -1 . Collectively the data reported are consistent with an interchange dissociative mechanism, I d , wherein M–P bond breaking contributes significantly to the rate-determining step with related values of ΔH being larger for M = W than M = Mo. The rate dependence on the entering ligand is clearly evident from temperature-dependent studies and leads to varying ΔS values. The I d mechanism is proposed to involve pre-equilibria between M 2 Cl 4 L 4 and the entering L′ in an axial site followed by rate-determining M–L displacement. In neat [ 2 H 5 ]pyridine this may be viewed as a solvent-assisted displacement. The present results are discussed in terms of earlier studies from which researchers inferred a simple dissociative process, D, involving M–PR 3 bond rupture as the first and rate-determining step.

Article information

Article type
Paper

J. Chem. Soc., Dalton Trans., 1997, 2735-2744

Tertiary phosphine ligand-exchange reactions involving the M[quadruple bond, length as m-dash]>M quadruply bonded complexes M2Cl4L4, where L = PMe3, PEt3, PBun3 or PMe2Ph

M. H. Chisholm and J. M. McInnes, J. Chem. Soc., Dalton Trans., 1997, 2735 DOI: 10.1039/A701222E

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