ISOLATION AND STRUCTURAL CHARACTERIZATION OF UNSOLVATED LITHIUM ARYLS

The synthesis and characterization of three examples of unsolvated lit hium aryls are described. The compounds {Li(Trip)}4 (1, Trip = 2,4,6-i -Pr3C6H2), (Li(2,6-Mes2C6H3)}2 (2, Mes = 2,4,6-Me3C6H2), {Li(n-Bu)}2(L iMes)2 (3), and {Li(n-Bu)12(LiMes*)2.Mes*Br (3.Mes*Br, Mes* = 2,4,6-t -Bu3C6H2) were synthesized by a metal-halogen exchange reaction betwee n n-butyllithium and the appropriate aryl bromide (I and 3) or iodide (2). The X-ray crystal structures of 1 and 3 show that they are tetram etallic lithium species involving almost planar arrays of four lithium atoms. The bonding involves both sigma- and pi-interactions to the ar yl ligands. In contrast, the structure of 2 is dimeric with the two ar yl groups linked by two mu2-bridging lithiums which are primarily sigm a-bonded to ipso carbons. Weaker interactions were also observed betwe en the ortho mesityl substituents and the lithiums. The structure of 3 -MesBr shows that the halide starting material Mes*Br can be included in the isolated crystals. This is a consequence of the fact that the metal-halogen exchange reaction used in their synthesis is sluggish. T he main reasons for this probably are the use of hydrocarbon rather th an ether solvent and the large size of the aryl ligands. The compounds were characterized by H-1, C-13, and Li-7 NMR spectroscopy and X-ray crystallography and represent the first examples of structurally chara cterized simple, nonsolvated, sigma-bonded lithium aryls. Crystal data with Mo Kalpha (lambda = 0.71069 angstrom) at 130 K: 1, a = b = 13.09 8(2) angstrom, c = 16.791(8) angstrom, V = 2881(2) angstrom3, Z = 2, s pace group I4BAR, 597 (I > 2sigma(1)) data, R = 0.089; 2, a = 30.190(7 ) angstrom, b = 13.423(2) angstrom, c = 19.048(4) angstrom, V = 7719(3 ) angstrom3, Z = 8 (two independent molecules), space group Pnna, 3419 (I > 2sigma(I)) data, R = 0.097; 3, a = 22.721(11) angstrom, b = 10.0 72(5) angstrom, c = 19.327(8) angstrom, V = 4423(3) angstrom3, Z = 4, space group Pna2(1), 1811 (I > 2.5sigma(I)) data, R = 0.082; 3-MesBr, a = 11.385(2) angstrom, b = 15.261(3) angstrom, c = 17.458(5) angstro m, beta = 91.16(2)degrees, V = 3032.4(12) angstrom3, Z = 2, space grou p C2/m, 1821 (I > 3sigma(I)) data, R = 0.104.