Synthesis of Optically Pure Helicene Metallocenes.

Two types of ruthenocenes and a ferrocene coordinated by rac-9H-cyclopenta[1,2-c:4,3-c']diphenanthrenyl anion(s), a [7]helicene with a cyclopentadienyl moiety at the center of its skeleton, were successfully synthesized: mono-helicene ruthenocene 1 and its iron analogue 1Fe with one [7]helicene ligand bound to the central metal, and bis-helicene ruthenocene 2 with two [7]helicenes. Starting from a racemic mixture of the ligand precursor, rac-2 and meso-2 were obtained in a 7:3 ratio. Since the [7]helicene has a high racemization barrier, enantiomers of the complexes were isolated in their pure forms; they showed large optical rotations and intense circular dichroism (CD) responses.