Mono(cyclopentadienyl) complexes of calcium, strontium, and barium, {[C5(SiMe3)3H2](Ca,Sr,Ba)I(thf)n}x.: Influence of alkali-metal cations on ligand exchange reactions

Reaction of the alkali-metal cyclopentadienide MCp' with an alkaline-earth dihalide (AeX(2)) in ethers is usually an efficient route to group 2 cyclopentadienyl complexes Cp'(n)AeX(2-n). This is not true for every Cp'/Ae combination, however, and the precipitation of an insoluble MX halide may not always be an adequate driving force for ligand exchange. As examples of this, [1,2,4-tris(trimethylsilyl)cyclopentadienyl]metal iodides (Cp-3Si)AeI(thf)(n) (Ae = Ca, n = 1; Ae = Sr, Ba, n = 2) are isolated from the 1:1 reaction of K[Cp-3Si] and AeI(2) in THF, but the yields range from very good (79%) when Ae = Ca to poor (26%) when Ae = Ba. In the case of Ae = Sr, Ba, substantial amounts of K[Cp-3Si] are recoverable at the end of the reaction. No redistribution of (Cp-3Si)AeI(thf)(n) into (Cp-3Si)(2)Ae and AeI(2)(thf)(n) is observed in either THF or aromatic solvents at room temperature. Both (Cp-3Si)CaI(thf) and (Cp-3Si)SrI(thf)(2) crystallize from THF/toluene as iodide-bridged dimers, [(Cp-3Si)Ae(mu-I)(thf)(n)](2), with eta(5)-[Cp-3Si](-) ligands and one or two terminal thf's on each metal atom. The Ca-I and Ca-I' distances are nearly equal at 3.066(4) and 3.102(4) Angstrom; the Sr-I and Sr-I' distances, at 3.278(3) and 3.355(3) Angstrom, are only slightly asymmetric. Unlike the two lighter homologues, the organobarium complex (Cp-3Si)BaI(thf)(2) crystallizes from THF/toluene as a coordination polymer containing both linear and near-linear (177.8 degrees) Ba-I-Ba links in a zigzag motif; this is an unprecedented arrangement for bridging iodide ligands. Treatment of (Cp-3Si)CaI(thf) with various alkalimetal-containing reagents yields primarily the group 1 cyclopentadienides (M[Cp-3Si]): e.g., the reaction of (Cp-3Si)CaI(thf) with LiI forms Li[Cp-3Si], the use of Na[N(SiMe3)(2)] generates Na[Cp-3Si], and the reaction with KCp* produces K[Cp-3Si]. Models of the reaction of Cp'CaI with LiI were studied with density functional theory calculations, which indicate that the formation of THF-solvated CaI2 is an important driving force for this system.