COMPETITIVE LIGAND C-H BOND ACTIVATIONS - RING-METALATED AND PHOSPHINE-METALATED FORMS OF (C5ME5)TA(PME3)2

The reduction of (eta-C5H5)TaCl4 with sodium sand in neat trimethylphosphine gives a mixture of products from which (eta-C5H5)2Ta(H)PMe3 (1) may be isolated in low yield and (eta-C5H5)Ta(PMe3)2-(H)(eta-2-CH2PMe2) (2) may be identified by NMR spectroscopy. Treatment of (eta-C5Me5)TaCl4 under similar conditions affords (eta-C5Me5)Ta(PMe3)(H)2(eta-2-CHPMe2) (3) which has been characterized spectroscopically and by an X-ray diffraction study. When a more conventional sodium amalgam/tetrahydrofuran reducing medium is used, (eta-C5Me5)TaCl4 reacts with excess trimethylphosphine to give (eta-7-C5Me3(CH2)2)Ta(H)2(PMe3)2 (4) in which two ring methyl C-H bonds have been cleaved. 4 has been characterized spectroscopically and by an X-ray diffraction study. Complexes 3 and 4 do not undergo interconversion readily under thermal conditions, and dynamic NMR studies on 3 provide no evidence for reversible hydrogen migrations. Investigations into use of other reducing media show that the ratio of the isomers obtained is highly dependent upon reducing agent and solvent.