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(μ-I)(thf) n ] 2, with η 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) Å; the Sr−I and Sr−I‘ distances, at 3.278(3) and 3.355(3) Å, 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°) 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 alkali-metal-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(SiMe 3 ) 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 CaI 2 is an important driving force for this system.