Synthesis and crystallographic study of indenyl and isopropylated indenyl complexes of calcium, strontium, and barium

Reaction of 2 equiv of potassium indenide (KInd) with AeI(2) (Ae = Ca, Sr, Ba) in THF produces the bis(indenyl) complexes (Ind)(2)Ae(thf)(n) (n = 1 for Sr, Ba; n = 2 for Ca) in high yields. The use of potassium 1,3-diisopropylindenide (KInd(2i)) with the same diiodides produces the analogous complexes (Ind(2i))(2)Ae(thf)(n) (n = 1 for Ca; n = 2 for Sr and Ba). All the complexes are air-sensitive solids. Single-crystal X-ray diffraction studies were completed for (Ind)(2)Ca(thf)(2), (Ind(2i))(2)Ca(thf), (Ind(2i))(2)Ba(thf)(2), and (Ind)(2)Sr(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) Angstrom for (Ind)(2)Ca(thf)(2), (Ind(2i))(2)Ca(thf), and (Ind(2i))(2)Ba(thf)(2), respectively. The strontium complex is an infinite coordination polymer, [(Ind)(2)Sr(thf)]infinity, with both terminal (Sr-C = 2.94 Angstrom (av)) and bridging (Sr-C = 3.07 Angstrom (av)) indenyl ligands. Although the individual Sr-C bond distances differ as much as 0.25 Angstrom, there is no clear indication of slippage toward an eta(3)-configuration. The presence of the two isopropyl substituents on the indenyl ligands makes them effectively as bulky as triisopropylcyclopentadienyl rings.