Allylation of cobalt-co-ordinated cyclo-octatetraene by the [Fe{P(OMe)3}(NO)2(η3-allyl)]+ cation: ring closure to 1,2,3,3a-tetrahydropentalenyl derivatives and the X-ray structure of [Co(η5-C8H8R)(η-C5Me5)][PF6][R = CH2C(Me)CH2]

Abstract

The reaction of [Fe{P(OMe)₃}(NO)₂(η³-allyl)][PF₆][1; allyl = CH₂CHCH₂, CH₂CHCHMe, or CH₂C(Me)CH₂] with [Co(η⁴-cot)(η-C₅R′₅)](2; R′= H or Me, cot = cyclo-octatetraene) results in C–C coupling and the formation of 1,2,3,3a-tetrahydropentalenyl derivatives [Co(η⁵-C₈H₈R)(η-C₅R′₅)][PF₆][3; R = CH₂CHCH₂, CH₂CHCHMe, or CH₂C(Me)CH₂; R′= H or Me]. An X-ray diffraction study on [Co(η⁵-C₈H₈R)(η-C₅Me₅)][PF₆][R = CH₂C(Me)CH₂] confirms the structure assignment with the cobalt atom co-ordinated to an η⁵-1,2,3,3a-tetrahydropentalenyl ligand having the 2-methylallyl substituent exo to the metal. The co-ordination of the η⁵-1,2,3,3a-tetrahydropentalenyl ligand is slightly asymmetric with the substituted cárbons furthest from the cobalt. The observed structure shows that complexes (3) may also be regarded as substituted cobaltocenium salts, and as such undergo reversible one-electron reduction to the corresponding cobaltocenes at a platinum bead electrode in CH₂Cl₂.