Synthesis and Characterization of Lewis Base-Free, σ-Bonded Lithium Aryls:  A Structural Model for Unsolvated Phenyllithium in the Solid State

The synthesis and characterization of four Lewis base-free, σ-bonded lithium aryls are reported. This work was undertaken in order to provide a model for the solid-state structure of phenyllithium, which is currently unknown. Nondonor hydrocarbon solubility of the four lithium aryls (LiC₆H₃-3,5-t-Bu₂)₆ (1), (LiC₆H₄-4-t-Bu)_n (2), (LiC₆H₄-4-n-Bu)_n (3), and {LiC₆H₄-4-SiMe₂(t-Bu)}_n (4) was achieved by the incorporation of meta- or para-substituents on the aryl rings. This permitted ¹³C NMR spectroscopy and crystal growth using their solutions. It is proposed that the absence of bulky ortho-substituents allows association of the lithium aryls to occur in a manner similar to that of phenyllithium itself. The ¹³C NMR data for the ipso-carbon atoms suggest an association number of at least four or, more probably, six in solution. These data are in agreement with the X-ray crystal structure of 1, which is hexameric, with a distorted octahedral (trigonal antiprismatic) array of lithium ions. Six of the eight Li₃ faces are capped by an aryl group that interacts primarily through the C(ipso) atom. Weaker Li−C(ortho) interactions are also apparent. This structure is the first of this type for an unsolvated, σ-bonded lithium aryl. Crystal data with Cu Kα (λ = 1.541 78 Å) radiation for 1 at 130 K:  1, C₈₄H₁₂₆Li₆, M = 1177.49, a = 13.516(2) Å, b = 15.124(3) Å, c = 20.958(3) Å, α = 84.084(13)°, β = 86.249(11) Å, γ = 68.675(13)°, V = 3967.7(11) ų, Z = 2, space group P1̄, R₁ = 0.085 for 7983 (I > 2σ(I)) data.