Solvent-resistant structures of base-free lithium and potassium allyl complexes, M[(SiMe3)nC3H5-n] (M = Li, n=3; M = K, n=2)

The structures of unsolvated {Li[1,1',3-(SiMe3)(3)C3H2]}(2) ([LiA ''](2)) and {K[1,3-( SiMe3)(2)C3H3]}(infinity) ([KA'](infinity)) are reported. The base-free complex LiA '' is a dinuclear species in the solid state in which the two Li atoms are bridged by the allyls in a mu(2)-eta(1), eta(2) fashion. The Li-C sigma-bonded distance is 2.232(7) angstrom; the Li-C eta(2)-interaction occurs at distances of 2.230(7) angstrom and 2.241(7) angstrom. Density functional theory calculations on [Li(C3H5)](2) and {Li[1,1',3-(SiH3)(3)C3H2]}(2) indicate that inclusion of the silyl groups is critical to reproducing the distortions observed in [LiA '']. The structure of the solvated Li(C3H5)(thf)(2) complex is calculated to be stable as a dimer, consistent with previous measurements on allyllithium in THF solution. Like related DME and THF solvates, [KA '](infinity) is a coordination polymer with potassium ions linked by bridging pi-allyl ligands; in [KA'](infinity) the polymer takes the form of helical chains with a broad range of K-C distances (2.87-3.15 angstrom).