Conformational properties of boron-bridged dimethylethylenediamino bis(boratabenzene) zirconium(IV) and iron(II) complexes

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 dilithium salt of N,N'-bis(1-boratabenzene)-N,N'-dimethylethylene-1,2 (14). The reaction of 14 with ZrCl4 afforded the boron-bridged dimethylethylenediamino bis(boratabenzene) zirconium dichloride complex 8, while reaction of 14 with 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 barrier to interconversion of the enantiomeric conformers of 8 was determined by 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 cyclopentadienyl complexes 10 and 11.