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 n BuNa/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 n BuLi/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 ‐O t Bu)] 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 n BuLi 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 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)