The X-ray structures of the complexes clearly show the predicted conformation; i.e., in all cases the substituent was oriented "anti" and the overall twist of the complex was determined by the absolute configuration od the chiral aplha-carbon. Only one conformer (anti) was observed in solid phase, for complexes derived from either racemic (e.g. [Zn((R,S)-a-MeTPA)Cl]ClO4, [Zn((R,S)-a-PhTPA)Cl]ClO4, or [Zn((R,S)-a-MeBQPA)Cl] ClO4) or enantiomerically pure ligands (e.g. [Cu(R-a-MeBQPA)(CH3CN)](ClO4)2). In general, the geometries predicted by computational methods correlates well with the solid phase results
While the solid phase data is encouraging, direct evidence of a similar behavior in solution is desired. The CD spectra are consistent with the presence of a propeller twist in solution. Two things are important to note from these spectra:
The modeling of these dynamically controlled asymmetric complexes was successful, as judged by the physical studies of their complexes. Studies of possible application of these complexes in asymmetric reactions are underway.
We are grateful to the National Institutes of Health (GM 49170) for support of this research, and Dr. Nina Berova for fruitful discussions on the interpretation of the CD spectra.