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 (LiC 6 H 3 -3,5- t -Bu 2 ) 6 ( 1 ), (LiC 6 H 4 -4- t -Bu) n ( 2 ), (LiC 6 H 4 -4- n -Bu) n ( 3 ), and {LiC 6 H 4 -4-SiMe 2 ( t -Bu)} n ( 4 ) was achieved by the incorporation of meta - or para -substituents on the aryl rings. This permitted 13 C 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 13 C 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 Li 3 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, C 84 H 126 Li 6, 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) Å 3, Z = 2, space group P 1̄, R 1 = 0.085 for 7983 ( I > 2σ( I )) data.