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

AbstractThe structures of alkali metal complexes of silyl‐substituted ansa‐tris(allyl) ligands [RSi(C3H3SiMe3)3]3– (R = Me, L1; or Ph, L2) are discussed. Triple deprotonation of L1H3 by nBuNa/tmeda affords [L1{Na(tmeda)}3] (4) in which the sodium cations are complexed by ηn‐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 L2H3 with nBuLi/tmeda results in the formation of [L2{Li(tmeda)}3] (5), the structure of which features silyl substituents with [exo,exo]2[endo,exo] stereochemistries. The trisodium complex [L2Na{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 L1 resulted in the formation of the bimetallic potassium/lithium complex [L2{K(OEt2)2}2KLi(μ4‐OtBu)]2 (7), in which the lithium tert‐butoxide by‐product is incorporated into a hexa(allylpotassium) macrocycle. Triple deprotonation of L1H3 with nBuLi and the terdentate Lewis base pmdeta results in [L1Li(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 andstereochemical preference of the ansa‐tris(allyl) ligands in 4 and 5 is provided by detailed density functional theory calculations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)