SYNTHESIS AND CHARACTERIZATION OF LEWIS BASE-FREE, SIGMA-BONDED LITHIUM ARYLS - A STRUCTURAL MODEL FOR UNSOLVATED PHENYLLITHIUM IN THE SOLID-STATE

The synthesis and characterization of four Lewis base-free, sigma-bond ed lithium aryls are reported. This work was undertaken in order to pr ovide a model for the solid-state structure of phenyllithium, which is currently unknown. Nondonor hydrocarbon solubility of the four lithiu m aryls (LiC6H3-3,5-t-Bu(2))(6) (1), (LiC6H4-4-t-Bu)(n) (2), (LiC6H4-4 -n-Bu)(n) (3), and {LiC6H4-4-SiMe(2)(t-Bu)}(n) (4) was achieved by the incorporation of meta- or para-substituents on the aryl rings. This p ermitted C-13 NMR spectroscopy and crystal growth using their solution s. 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 C-13 NMR data for the ipso-carbon atoms s uggest an association number of at least four or, more probably, six i n solution. These data are in agreement with the X-ray crystal structu re of 1, which is hexameric, with a distorted octahedral (trigonal ant iprismatic) array of lithium ions. Six of the eight Lis faces are capp ed 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, sigma-bonded Lithium aryl. C rystal data with Cu K alpha (lambda = 1.541 78 Angstrom) radiation for 1 at 130 K: 1, C84H126Li6, M = 1177.49, a = 13.516(2) Angstrom, b = 1 5.124(3) Angstrom, c = 20.958(3) Angstrom, alpha = 84.084(13)degrees, beta = 86.249(11) Angstrom, gamma = 68.675(13)degrees, V = 3967.7(11) Angstrom(3), Z = 2, space group <P(1)over bar>, R(1) = 0.085 for 7983 (I > 2 sigma(I)) data.