Coordination complexes of bivalent ansa-ytterbocenes: synthesis, structure and comparison with related unbridged ytterbocenes and ansa-ferrocenes

The dimethylsilyl bridged bis(cyclopentadienide) ligands, {[2,4-(Me3C)2C5H2]2SiMe2}2- and {[(Me3Si)2C5H2]2SiMe2}2-, have been prepared and used for the preparation of ansa-metallocene derivatives of iron and bivalent ytterbium metallocene adducts. The structures of two ansa-ferrocenes, as well as that of the free tetraene [2,4-(Me3C)2C5H3]2SiMe2, have been determined by X-ray crystallography, and the lability of the Me3Si group leads to a different ring substitution pattern in the structure of ansa-{[3,4-(Me3Si)2C5H2]2SiMe2}Fe compared with that in ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Fe. The structures of ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(OEt2) and an analogous ansa-ytterbocene isocyanide complex are also reported, and compared with those of non-bridged ytterbocene diethyl ether complexes. Short distances from the ytterbium atom to the diethyl ether beta carbon atoms are observed in several structures, consistent with the tendency of ytterbium to maximise its coordination number. Spectroscopic properties for the ansa¬-ytterbocene etherates and the isocyanide complex are reported and compared with the corresponding adducts of non-bridged ytterbocenes. The reduced Cp(centroid) - metal - Cp(centroid) angle enforced by the ansa-bridge does not lead to significant changes in the optical spectra compared with unbridged metallocene diethyl ether and tetrahydrofuran adducts, but significant changes are observed in the isocyanide adducts; a model rationalising the red and blue shifts of lambda¬max is proposed