Alkaline-Earth Metal Alkylaluminate Chemistry Revisited

Heterobimetallic peralkylated calcium complexes [Ca(AlR4)2]n (R = Me, Et) were obtained from {Ca[N(SiMe3)2]2}2 and an excess AlR3 in almost quantitative yield. A similar silylamide elimination protocol was previously applied for the synthesis of the corresponding ytterbium and samarium complexes. The solid-state structure of polymeric [Ca(AlEt4)2]n revealed agostically linked right-handed helices featuring a pitch of ∼14 Å and a repeat length of ∼27 Å with the latter composed of nine alkyl-bridged [Ca(AlEt4)3]− and [Ca(AlEt4)]+ building units. Reaction of [Ca(AlR4)2]n (R = Me, Et) with equimolar amounts of donor molecules gave X-ray structurally authenticated hexacoordinate monomeric adducts Ca(AlMe4)2Phen (Phen = 1,10-phenanthroline) and Ca(AlEt4)2(thf)2. Treatment of [Ca(AlEt4)2]n, [Yb(AlEt4)2]n, and Mg(AlMe4)2 with excess thf resulted in the formation of solvent-separated ion pairs of composition [M(II)(thf)6][AlR4]2. Heating barely soluble [Ca(AlMe4)2]n in a pressure tube produced vinyloxide-bridged complex [{Ca(μ-OCHCH2)(thf)4}2][AlMe4]2 via thf degradation. Furthermore, treatment of MgMe2 with thf yielded quantitatively the donor adduct [Mg(Me)(μ-Me)(thf)]2.