η2 Complexes of cyclic polyolefins: crystal structure of [Mn(CO)2(η2-C8H8)(η-C5H5)]

Abstract

Treatment of [Mn(CO)₂(thf)(η-C₅H₅)](thf = tetrahydrofuran) with cyclo-octatetraene produces [Mn(CO)₂(η²-C₈H₈)(η-C₅H₅)], whose structure was determined by X-ray diffraction. Crystals are monoclinic, space group P2₁/c, with four molecules in a unit cell of dimensions a= 10.813(7), b= 6.606(4), c= 17.164(9)Å, β= 94.68(5)°. The structure was solved by conventional techniques and refined by least squares to R 0.034 for 2 787 independent diffracted intensities. The molecule contains an η²-cyclo-octatetraene ligand which possesses mirror symmetry and present the convex face of a tub conformation towards the cyclopentadienyl ring. Nuclear magnetic resonance spectra show that the complex is non-fluxional below the temperature (ca. 75 °C) at which it decomposes to the dimanganese µ-cyclo-octatetraene complex [{Mn(CO)₂(η-C₅H₅)}₂(µ-C₈H₈)]. The η²-cyclo-octatetraene is readily displaced by phosphine ligands. Analogous η²-olefin complexes of manganese are obtained from cycloheptatriene and cyclo-octa-1,3,6-triene.