摘要：

Cyclooctatetraene complexes of the type M(C8H8)2R (M = Nb, Ta; R = C6H5, CH3) and Ta(C8H8)(CH3)3 have been prepared employing [K+]2[C8H8]2- as the reducing agent and source of C8H8. In the former the η4 and η8 rings interconvert on the NMR time scale with ΔG≠337 = 15.7-17.5 kcal mol-1 depending on M and R. Except where R = C6H5 both form adducts with some donor ligands such as (CH3)2PCH2CH2P(CH3) 2. When R = C6H5, forcing conditions give complexes containing the 2,3,4,5,6-n-endo-8-phenylbicyclo[5.1.0]octadienyl ligand, e.g., Nb(η4-C8H8)[η5-C 8H8(C6H5)][(CH3) 2AsC6H4As(CH3)2], the structure of which was determined by x-ray methods. The cyclopropyl ring is exo to the metal and the phenyl substituent endo, the first instance where the geometry of this bicyclic ligand has been established. The η4-C8H8 and diars geometries are normal. Similar complexes are obtained when electrophiles (E = H+, D+, and C(C6H5)3+) attack [M(η4-C8H8)(η3-C 8H8)2]- in the presence of (CH3)2PCH2CH2P(CH3) 2. The facility of the η8-C8H8 → (e.g.) η4-C8H8 process suggests that complexes containing η8-C8H8 are potentially valuable catalysts (or catalyst precursors) for reactions taking place at the metal center.

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