A homologous series of base-free organo(alkaline-earth) metallocenes:: Synthesis and molecular structures of [1,2,4-(SiMe3)3C5H2]2(Ca, Sr, Ba)

Bis[1,2,4-tris(trimethylsilyl)cyclopentadienyl] complexes [1,2,4-(SiMe3)(3)C5H2](2)Ae [(Cp-3Si)(2)Ae; Ae = Ca (1), Sr (2), Ba (3)] are isolated from the 2:1 reaction of K[Cp-3Si] and AeI(2) in diethyl ether. Under the same reaction conditions used for the heavier metallocenes, the corresponding (Cp-3Siz)(2)Be compound was not formed, although calculations suggest that the beryllocene would be sterically feasible. Compound 1 crystallizes as a bent monomer from hexanes (ring centroid-Ca-ring centroid = 166.7degrees) with two eta(5)-Cp-3Si ligands on the calcium atom [Ca-C(av) = 2.63(2) Angstrom]. Metallocene 2, the first crystallographically characterized example of an unsolvated strontocene, is also isolated as a bent monomer (159.4degrees) from hexanes with two eta(5)-Cp-3Si ligands on the strontium atom [Sr-C(av) = 2.819(2) Angstrom]. Compound 3 crystallizes on sublimation as a coordination dimer in which each barium atom is flanked by two eta(5)-Cp-3Si rings [Ba-C(av) = 3.01(2) Angstrom] and one methyl group from a neighboring Cp' ring; the intermolecular Ba...C(methyl)' distance is 3.275(6) Angstrom. The amount of bending observed in 1-3 and other Group-2 metallocenes varies with the metals, the steric bulk of the cyclopentadienyl ligands, and crystal packing effects, and is not easily predictable. (C) Wiley-VCH Verlag GmbH Co.