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)

Abstract

Abstract: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-(SiMe 3 ) 2 C 3 H 3 ]), 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 Et 2 O was previously shown to produce the solvated species BeA' 2 (OEt 2 ), 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 9 Be 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 C 3 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 3 L] (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.

An η3‐Bound Allyl Ligand on Magnesium in a Mechanochemically Generated Mg/K Allyl Complex

“…Isolated complexes involving the A′ ligand and a monovalent/ divalent metal combination have to date been of the form [M I M II A′ 3 ], where three allyl ligands are bound to the M II metal in a C 3 ‐symmetric tris ‐(η 1 ) fashion and sequester an [M I ] + cation in the ansa ‐tris(η 2 ‐olefin) pocket through a cation–π interaction (for example, Figure ); such interaction is calculated to contribute as much as 24 kcal mol −1 to the stability of the compound when M I =K . The range of M II ions that have been incorporated into structurally characterized compounds includes Zn (for which combinations with M I =Li, Na, and K are known), Be, and Sn . It seemed unusual that this was not the form of the metalate adopted by the Mg derivative, especially as the ionic radii of Mg 2+ and Zn 2+ , for example, are similar (0.57 Å and 0.60 Å, respectively, for CN=4) .…”

“…However, for the Group 2 metals, only a limited range of bonding patterns has been documented. To date, beryllium compounds have been found with σ‐bound (η 1 ) (Figure a) and μ 2 ‐η 1 ,η 2 allyls (Figure h) . In contrast, with very rare exceptions, η 3 ‐conformations are uniformly the norm with complexes of the heavier alkaline‐earth metals (Ca‐Ba) .…”

An η3‐Bound Allyl Ligand on Magnesium in a Mechanochemically Generated Mg/K Allyl Complex

“…, where three allyl ligands are bound to the M II metal in a C 3 -symmetric tris-(h 1 )fashion and sequester an [M I ] + cation in the ansa-tris(h 2 -olefin) pocket through ac ation-p interaction (for example,F igure 6);s uch interaction is calculated to contribute as much as 24 kcal mol À1 to the stability of the compound when M I = K. [29] Ther ange of M II ions that have been incorporated into structurally characterized compounds includes Zn (for which combinations with M I = Li, Na, and Ka re known), Be, [8] and Sn. [21,30] It seemed unusual that this was not the form of the metalate adopted by the Mg derivative,e specially as the ionic radii of Mg 2+ and Zn 2+ ,f or example,a re similar (0.57 and 0.60 ,r espectively,f or CN = 4).…”

“…To date,b eryllium compounds have been found with s-bound (h 1 )( Figure 1a) [7] and m 2 -h 1 ,h 2 allyls (Figure 1h). [8] In contrast, with very rare exceptions, h 3conformations are uniformly the norm with complexes of the heavier alkaline-earth metals (Ca-Ba). [9] Magnesium represents ab orderline case;e xplicit attempts to prepare magnesium species with h 3 -bound allyls (Figure 1b)h ave not been successful, [10] and at one time,b ased on the absence of any structural evidence to the contrary, s-bonding was thought to be the preferred bonding mode for allyl ligands in magnesium compounds.…”

Halide metathesis in overdrive: mechanochemical synthesis of a heterometallic group 1 allyl complex

“…As the generation of MX normally provides a substantial portion of the energy for the exchange, M should be more electropositive than M', in order to maximize hard–soft acid–base interactions [2]. The reaction will proceed without solvent, and mechanochemical activation, which promotes reactions through grinding or milling with no, or minimal, use of solvents, has been used in conjunction with halide metathesis to form organometallic compounds of the transition metals [3–7] and both s- [8–9] and p-block [10–11] main group elements.…”