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 Me2E(CR'R-2)(CR"R-2) (E = Si, R' = Me, R" = SiMe3, 6a; E = Ge, R' = Me, R" = SiMe3, 6b; and E = Sn, R' = R" = SiMe3, 5c; CR2 = fluorene units). This is in sharp contrast with the structure of monosubstituted Me2Ge(CHR2)(CR2-SiMe3) (3b) and disubstituted Me2E(CMeR2)(2) (E = Ge, 4b; E = Sn, 4c) derivatives where the two fluorene units are almost perpendicular. Intramolecular C-H(...)pi interactions are revealed by solution H-1 NMR spectroscopy and supported by the AM1 molecular orbital calculations. Intermolecular (aryl, alkyl)C-H(...)pi interactions leading to supramolecular associations are shown by the crystal structures of 3b, 4b, 4c, 5c, and 6a. In 4b and 5c rather unusual (CHHC)-H-... 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 Me2E(CR'R-2)(CR"R-2) (E = Si, R' = R" = SiMe3, 5a; E = Ge, R' = R" = SiMe3, 5b; E = Sn, R' = H, R" = Me, 2c; R' = H, R" = SiMe3, 3c; R' = Me, R" = SiMe3, 6c; CR2 = fluorene units).