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)(2)C3H3]), 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 Be-9 NMR shift expected for a three-coordinate Be center (delta 22.8 ppm). A single crystal X-ray structure reveals that the three allyl ligands are bound to beryllium in an arrangement with approximate C-3 symmetry (Be-C (avg) = 1.805(10) angstrom), with the potassium cation engaging in cation-pi 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'L-3] (M' = Zn, M = Li, Na, K; M' = Sn, M = K; L = thf). Density functional theory (DFT) calculations indicate that the observed C-3-symmetric framework of the isolated anion ([BeA'(3)](-)) is 20 kJ.mol(-1) higher in energy than a C-1 arrangement; the K+ counterion evidently plays a critical role in templating the final conformation.