SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF THE SANDWICHLIKE COMPOUND [(12-CROWN-4)LI(ETA-5-C5H5)] AND THE ETA-0-SUBSTITUTED-CYCLOPENTADIENIDE SALT [LI(12-CROWN-4)2][1,2,4-(ME3SI)3C5H2]

The addition of 12-crown-4, in stoichiometric amounts or in excess, to a solution of LiC5H5 in THF affords the complex [(12-crown-4)Li(eta-5-C5H5)] (1). X-ray structural data demonstrate that it possesses a unique sandwichlike structure in which the Li+ ion is coordinated on one side by four oxygens from a 12-crown-4 ligand and on the other side by the cyclopentadienide anion, [C5H5]-, in an eta-5 fashion. In contrast, the addition of 12-crown-4 to THF solutions of Li(1,2,4-(Me3Si)3C5H2) gives the ion pair [Li(12-crown-4)2]+[1,2,4-(Me3Si)3C5H2]- (2), which has a free (eta-0) substituted-cyclopentadienide ion as part of its lattice. The latter complex represents a very rare example of the structure of an eta-0 delocalized carbanion, and the structure of 1 is the first of a new class of sandwich structures. Crystal data at 130 K: 1, C13H21LiO4, a = 7.729 (5) angstrom, b = 11.268 (7) angstrom, c = 15.618 (8) angstrom, orthorhombic, P2(1)2(1)2(1), Z = 4; 2, C30H61LiO8Si3, a = 11.924 (4) angstrom, b = 12.565 (4) angstrom, c = 14.530 (3) angstrom, alpha = 92.65 (2)-degrees, beta = 90.03 (2)-degrees, gamma = 120.28 (2)-degrees, triclinic, P1BAR, Z = 2.