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-(SiMe ₃)₃C₅H₂]₂Ae [(Cp ^3Si)₂Ae; Ae = Ca (1), Sr (2), Ba (3)] are isolated from the 2:1 reaction of K[Cp^3Si] and AeI₂ in diethyl ether. Under the same reaction conditions used for the heavier metallocenes the corresponding (Cp^3Si)₂Be 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 = 166.7°) with two η⁵-Cp^3Si ligands on the calcium atom [Ca-C(av) = 2.63(2) Å]. Metallocene 2, the first crystallographically characterized example of an unsolvated strontocene, is also isolated as a bent monomer (159.4°) from hexanes with two η ⁵-Cp^3Si ligands on the strontium atom [Sr-C(av) = 2.819(2) Å]. Compound 3 crystallizes on sublimation as a coordination dimer in which each barium atom is flanked by two η⁵-Cp^3Si rings [Ba-C(av) = 3.01(2) Å] and one methyl group from a neighboring Cp′ ring; the intermolecular Ba⋯C(methyl)′ distance is 3.275(6) Å. 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. (