ISOLATION AND STRUCTURAL CHARACTERIZATION OF UNSOLVATED LITHIUM ARYLS

The synthesis and characterization of three examples of unsolvated lithium 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(LiMes*)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 between 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 tetrametallic lithium species involving almost planar arrays of four lithium atoms. The bonding involves both sigma- and pi-interactions to the aryl ligands. In contrast, the structure of 2 is dimeric with the two aryl groups linked by two mu2-bridging lithiums which are primarily sigma-bonded to ipso carbons. Weaker interactions were also observed between the ortho mesityl substituents and the lithiums. The structure of 3-Mes*Br 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. The main reasons for this probably are the use of hydrocarbon rather than 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 characterized simple, nonsolvated, sigma-bonded lithium aryls. Crystal data with Mo Kalpha (lambda = 0.71069 angstrom) at 130 K: 1, a = b = 13.098(2) angstrom, c = 16.791(8) angstrom, V = 2881(2) angstrom3, Z = 2, space 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 = 10.072(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-Mes*Br, a = 11.385(2) angstrom, b = 15.261(3) angstrom, c = 17.458(5) angstrom, beta = 91.16(2)degrees, V = 3032.4(12) angstrom3, Z = 2, space group C2/m, 1821 (I > 3sigma(I)) data, R = 0.104.