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; 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 β 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 λmaxis proposed.