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

The structures of alkali metal complexes of silyl-substituted ansa-tris(allyl) ligands [RSi(C 3 H 3 SiMe 3 ) 3 ] 3- (R = Me, L 1 ; or Ph, L 2 ) are discussed. Triple deprotonation of L 1 H 3 by nBuNa/tmeda affords [L 1 (Na(tmeda)} 3 ] (4) in which the sodium cations are complexed by η n -allyl ligands and the silyl substituents adopt [exo,exo][endo,exo] 2 stereochemistries in one crystallographically disordered form and [endo,exo] 3 in another. Triple deprotonation of L 2 H 3 with nBuLi/tmeda results in the formation of [L 2 {Li(tmeda)} 3 ] (5), the structure of which features silyl substituents with [exo,exo] 2 [endo,exo] stereochemistries. The trisodium complex [L 2 Na{Na-(tmeda)} 2 ] 2 (6) consists of a hexa(allylsodium) macrocycle that aggregates as a result of cation-π interactions between the phenyl substituents and the sodium cations. An attempt to prepare the tripotassium complex of L 1 resulted in the formation of the bimetallic potassium/lithium complex [L 2 (K(OEt 2 ) 2 } 2 KLi(μ 4 -OtBu)] 2 (7), in which the lithium tert-butoxide by-product is incorporated into a hexa(allylpotassium) macrocycle. Triple deprotonation of L 1 H 3 with nBuLi and the terdentate Lewis base pmdeta results in [L 1 Li-(pmdeta)} 3 ] (8), in which the three allyl groups do not μ-bridge between lithium cations, resulting in an [exo,exo] 3 stereochemistry of the silyl substituents. NMR spectroscopic studies reveal complicated solution-phase behaviour for 4, 6 and 7, whereas the solid-state structures of 5 and 8 are preserved in solution. Further insight into the structures and stereochemical preference of the ansa-tris(allyl) ligands in 4 and 5 is provided by detailed density functional theory calculations.