The reaction of 1-chloro-1-boracyclohexadiene (12) with N,N'-dimethyl-N,N'-
bis(trimethylsilyl)ethylene-1,2-diamine followed by LDA afforded the dilith
ium salt of N,N'-bis(1-boratabenzene)-N,N'-dimethylethylene-1,2 (14). The r
eaction of 14 with ZrCl4 afforded the boron-bridged dimethylethylenediamino
bis(boratabenzene) zirconium dichloride complex 8, while reaction of 14 wi
th FeCl2 afforded the corresponding dimethylethylenediamino-bridged iron(II
) complex 9. The structures of 8 and 9 were determined by X-ray diffraction
. In each case the boratabenzene units are bridged by an approximately C-2-
symmetric dimethylethylenediamino bridge which makes 8 and 9 chiral. A barr
ier to interconversion of the enantiomeric conformers of 8 was determined b
y 2D H-1 NOESY as 21.2 +/- 0.2 kcal/mol, while the corresponding barrier of
9 was determined from variable-temperature C-13 NMR spectroscopy to be 16.
9 +/- 0.5 kcal/mol. Both barriers involve rotation about the exocyclic B-N
pi -bonds. Comparison has been made with the corresponding bridged cyclopen
tadienyl complexes 10 and 11.