Difluorenylsilanes, -germanes, and -stannanes Exhibiting an Unprecedented Parallel Arrangement of the Fluorene Units

New, yet unprecedented molecular structures with parallel fluorene units were found by X-ray diffraction in (9,9'-disubstituted difluorenyl)(dimethyl) derivatives of group 14 elements Me 2 E(CR'R 2 )(CR"R 2 ) (E = Si, R' = Me, R" = SiMe 3 , 6a; E = Ge, R' = Me, R" = SiMe 3 , 6b; and E = Sn, R' = R" = SiMe 3 , 5c; CR 2 = fluorene units). This is in sharp contrast with the structure of monosubstituted Me 2 Ge(CHR 2 )(CR 2 -SiMe 3 ) (3b) and disubstituted Me 2 E(CMeR 2 ) 2 (E = Ge, 4b; E = Sn, 4c) derivatives where the two fluorene units are almost perpendicular. Intramolecular C-H...π interactions are revealed by solution 1 H NMR spectroscopy and supported by the AM1 molecular orbital calculations. Intermolecular (aryl, alkyl)C-H...π interactions leading to supramolecular associations are shown by the crystal structures of 3b, 4b, 4c, 5c, and 6a. In 4b and 5c rather unusual CH...HC distances resembling the "dihydrogen" bonds are noticed. Synthesis, NMR spectra, and gas phase conformations predicted on the basis of AM1 semiempirical molecular orbital calculations are discussed for Me 2 E(CR"R 2 )(CR"R 2 ) (E = Si, R' = R" = SiMe 3 , 5a; E = Ge, R' = R" = SiMe 3 , 5b; E = Sn, R' = H, R" = Me, 2c; R' = H, R" = SiMe 3 , 3c; R' = Me, R" = SiMe 3 , 6c; CR 2 = fluorene units).