On the structure of bis(9-fluorenyl)dimethylsilane: a sterically crowded molecule with relatively low barriers of rotations around the Si−C(fluorenyl) bonds X-ray diffraction analysis and AM1 molecular orbital calculations

Abstract

The X-ray structure of bis(9-fluorenyl)dimethylsilane, the first silane compound containing two voluminous fluorenyl groups, has been determined. The crystal system is orthorhombic with Aba2 space group. The unit cell ( a = 18.672, b = 36.089, c = 12.802 Å) contains two sets of four molecules, each one of one set being symmetrically related with the others of the same set. The conformations around the IIC(fluorenyl)SiC(fluorenyl) bonds may be grossly described as in-in, in-out and out-out. The AM1 calculations have shown that the barrier between the in-out and in-in conformations is relatively low while that between the in-out and out-out is higher; thus, probably, the first two are the most populated in solution. These results provide a better understanding of some unusual chemical shifts previously observed in 1H-NMR for Me 2Si(CHR 2) 2 and Me 2Si(CHR 2)(CR′R 2).