1,1'-DIMESITYLURANOCENE AND ITS UNUSUAL DYNAMIC NMR PHENOMENA

1,1'-Dimesityluranocene gives a H-1 NMR spectrum showing the mesityl g roups in a locked position as expected with exo- and endo-ortho-methyl groups. The [8]annulene ring protons give four resonances at room tem perature in a 2:2:2:1 ratio, but at low temperatures three of these re sonances coalesce and reappear as six resonances at still lower temper atures. These results are interpreted in terms of two lowest energy en antiomeric structures in which the two mesityl groups are oriented clo se to each other, presumably held by van der Waals attraction. Separat ion with interconversion of the enantiomers involves a barrier of abou t 10 kcal mol(-1) but is rapid on the NMR time-scale at room temperatu re.