Does an Axial Propeller Shape on a Dirhodium(III,III) Core Affect Equatorial Ligand Chirality?

The dirhodium(III,III) tetrakis(1-aza-2-cyclooctanoate) framework, which exists solely in a propeller conformation, has been prepared with nonidentical apical aryl ligands. The presence of these nonidentical ligands on the propeller conformation was expected to define this structural unit as a chiral element and the resulting aryl(1)aryl(2)dirhodium(III,III) tetrakis(1-aza-2-cyclooctanoate) as a chiral compound. Dirhodium(III,III) tetrakis(1-aza-2-cyclooctanoate) compounds having phenyl and p-tolyl, 1-(3-phenylphenyl) and 2-methoxy-6-naphthyl, and p-(N-p-toluenesulfonylpyrolinamidophenyl) apical ligands were prepared and subjected to chromatographic, NMR, and X-ray crystallographic analysis to ascertain if the propeller core provided a chiral element that could define two enantiomers when dirhodium(III,III) tetrakis(1-aza-2-cyclooctanoate) was bound to nonidentical apical aryl ligands. X-ray analysis suggests chirality in those compounds for which structures could be obtained, but chromatographic analyses were not able to separate enantiomers. Studies of dirhodium(III,III) tetrakis(1-aza-2-cyclooctanoate) compounds possessing different apical ligands by NMR spectroscopy using binuclear chiral lanthanide–silver shift reagents were also performed, but conclusive evidence for chirality in these dirhodium complexes was not obtained.