Threefold Cation−π Bonding in Trimethylsilylated Allyl Complexes

Abstract

Reaction of zinc triflate with the lithium, sodium, and potassium salts of the bis(1,3-trimethylsilyl)allyl anion, M[A′] (A′ ) [1,3-(SiMe 3 ) 2 C 3 H 3 ]), produces the triallylzincates Li[ZnA′ 3 ] (1), Na[ZnA′ 3 ] (2), and K[ZnA′ 3 ] (3) rather than the inititially expected neutral ZnA′ 2 . The molecules are fluxional in solution, and the chemical shifts of the molecules in C 6 D 6 are different for the three molecules, indicating that the cations remain associated with the triallylzinc anion. Single-crystal X-ray structures of 2 and 3 reveal that the three allyl ligands are bound to the zinc in an arrangement with approximate C 3 symmetry, with the alkali-metal cations situated between the double bonds of the allyl ligands. The distances are consistent with noncovalent cation-π interactions. The structure of the lithium derivative 1 is similar to those of 2 and 3, but the asymmetry in the metal-carbon distances suggests that some Li-C σ-bonding is involved. Density functional theory calculations were performed on [M(C 6 H 6 )] + and [M(C 2 H 4 ) n ] + (M ) Li, Na, K; n ) 1-3) cations with the PBE1PBE functional and basis sets of triple-ζ quality. The binding enthalpies (∆H°) of the three metals to two ethylene molecules or to benzene are approximately the same, despite the latter's greater number of π-electrons. The binding energy of three ethylene molecules to the metal cations exceeds that of benzene by 30-50%, underscoring the importance of geometric factors to cation-π interactions.

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)

“…In the solid state, 1 exhibits approximate C3-symmetry, with σ-bound A' ligands and a potassium cation engaging in cation-π interactions with the three double bonds of the allyls. It is isostructural with the previously reported M[ZnA '3] (M = Li, Na, K) and K(thf)[SnA '3] complexes [15,16]. The beryllium center is in a nearly planar trigonal environment (sum of C-Be-C´ angles = 357.7°) (Figure 2).…”

“…The formation of the zinc species K[ZnA' 3 ] is also similar to that of 1 in that it is formed from the reaction of 2 equiv. of K[A'] and ZnCl 2 , i.e., in a non-stoichiometric reaction [15]. However, both it and K(thf) [SnA' 3 ] are synthesized in THF, so it is clear that the driving force for -ate formation compared to that for the neutral (Zn,Sn)A' 2 species is greater than that for 1.…”

“…As detailed below, an unsolvated neutral complex was not isolated via this route, and the beryllate anion that was produced instead has structural parallels with previously described -ate complexes of Zn [15] and Sn [16]. In all of these species, the alkali metal counterion, usually K + but sometimes Na + and Li + , appears to play a critical role in the assembly of the symmetric complexes.…”

Aromatische Ringe in chemischer und biologischer Erkennung: Energien und Strukturen

“…Zusätzliche Belege für die Bindung von Alkalimetallkationen an Doppelbindungen stammen von trimethylsilylierten Allylkomplexen. [263] In Kristallstrukturen sind Li + , Na + und K + zwischen drei Allylbindungen mit Kation-p-Bindungsabständen eingelagert (bei Li + ist auch eine s-Bindung involviert). Rechnungen sagen eine Abnahme der Wechselwirkung in der Reihe Li + > Na + > K + vorher.…”

Alkali Metal Complexes of Silyl‐Substituted ansa‐(Tris)allyl Ligands: Metal‐, Co‐Ligand‐ and Substituent‐Dependent Stereochemistry

“…[36] The bimetallic sodium tris(allyl)zincate [Zn{(1,3-Me 3 Si) 2 C 3 H 3 }Na] is also related to 4, and is a contact ion-pair in which three allyl ligands form σ-bonds to zinc and the localized allyl double bonds coordinate to the sodium cation by means of η 2 -, or cation-π, type interactions. [10] In order to explore the impact of varying the substituent on the central silicon atom in ansa-tris(allyl) ligands on the structures of their corresponding alkali-metal complexes,…”