Solvent-resistant structures of base-free lithium and potassium allyl complexes, M[(SiMe₃)_nC₃H_5-n] (M = Li, n = 3; M = K, n = 2)

Abstract

The structures of unsolvated {Li[1,1′,3-(SiMe₃)₃C₃H₂]}₂ ([LiA″]₂) and {K[1,3-(SiMe₃)₂C₃H₃]}_∞ ([KA′]_∞) 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 μ₂-η¹, η² fashion. The Li[sbnd]C σ-bonded distance is 2.232(7) Å; the Li[sbnd]C η²-interaction occurs at distances of 2.230(7) Å and 2.241(6) Å. Density functional theory calculations on [Li(C₃H₅)]₂ and {Li[1,1′,3-(SiH₃)₃C₃H₂]}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′]_∞ is a coordination polymer with potassium ions linked by bridging π-allyl ligands; in [KA′]_∞ the polymer takes the form of helical chains with a broad range of K[sbnd]C distances (2.87–3.15 Å).

Keywords:

• lithium
• potassium
• allyl
• crystal structure
• bulky ligands
• trimethylsilyl group

Acknowledgment

Acknowledgment is made to the National Science Foundation for support.