Symmetric Assembly of a Sterically Encumbered Allyl Complex: Mechanochemical and Solution Synthesis of the Tris(allyl)beryllate, K[BeA′3] (A′ = 1,3-(SiMe3)2C3H3)

The ball milling of beryllium chloride with two equivalents of the potassium salt of bis(1,3-trimethylsilyl)allyl anion, K[A′] (A′ = [1,3-(SiMe3)2C3H3]), produces the tris(allyl)beryllate K[BeA’3] (1) rather than the expected neutral BeA’2. The same product is obtained from reaction in hexanes; in contrast, although a similar reaction conducted in Et2O was previously shown to produce the solvated species BeA’2(OEt2), it can produce 1 if the reaction time is extended (16 h). The tris(allyl)beryllate is fluxional in solution, and displays the strongly downfield 9Be NMR shift expected for a three-coordinate Be center (δ22.8 ppm). A single crystal X-ray structure reveals that the three allyl ligands are bound to beryllium in an arrangement with approximate C3 symmetry (Be-C (avg) = 1.805(10) Å), with the potassium cation engaging in cation-π interactions with the double bonds of the allyl ligands. Similar structures have previously been found in complexes of zinc and tin, i.e., M[M′A′3L] (M′ = Zn, M = Li, Na, K; M′ = Sn, M = K; L = thf). Density functional theory (DFT) calculations indicate that the observed C3-symmetric framework of the isolated anion ([BeA′3]−) is 20 kJ•mol−1 higher in energy than a C1 arrangement; the K+ counterion evidently plays a critical role in templating the final conformation.