Synthesis and Crystallographic Study of Indenyl and Isopropylated Indenyl Complexes of Calcium, Strontium, and Barium

Reaction of 2 equiv of potassium indenide (KInd) with AeI₂ (Ae = Ca, Sr, Ba) in THF produces the bis(indenyl) complexes (Ind)₂Ae(thf)<i>_n</i> (<i>n</i> = 1 for Sr, Ba; <i>n</i> = 2 for Ca) in high yields. The use of potassium 1,3-diisopropylindenide (KInd²ⁱ) with the same diiodides produces the analogous complexes (Ind²ⁱ)₂Ae(thf)<i>_n</i> (<i>n</i> = 1 for Ca; <i>n</i> = 2 for Sr and Ba). All the complexes are air-sensitive solids. Single-crystal X-ray diffraction studies were completed for (Ind)₂Ca(thf)₂, (Ind²ⁱ)₂Ca(thf), (Ind²ⁱ)₂Ba(thf)₂, and (Ind)₂Sr(thf); these represent the first structurally characterized indenyl complexes of the heavier alkaline-earth metals. The calcium and barium compounds are monomeric, with bent metallocene geometries and average Ae−C distances of 2.73(3), 2.69(2), and 3.03(4) Å for (Ind)₂Ca(thf)₂, (Ind²ⁱ)₂Ca(thf), and (Ind²ⁱ)₂Ba(thf)₂, respectively. The strontium complex is an infinite coordination polymer, [(Ind)₂Sr(thf)]_∞, with both terminal (Sr−C = 2.94 Å (av)) and bridging (Sr−C = 3.07 Å (av)) indenyl ligands. Although the individual Sr−C bond distances differ as much as 0.25 Å, there is no clear indication of slippage toward an η³-configuration. The presence of the two isopropyl substituents on the indenyl ligands makes them effectively as bulky as triisopropylcyclopentadienyl rings.