Synthesis of Optically Pure Helicene Metallocenes

Abstract

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 1 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.

Phosphorescence Resulting from Interaction between Two Non‐equivalent Metals on a Helical π‐Conjugated Surface

“…Complex 3 exhibited phosphorescence both in the solution and solid states, which is in sharp contrastt ot he non-emissive nature of monometallic complexes 1 and 2 ( Figure 1c). [13] The quantum yield of the emission of complex 3 was by far the highest among those of the previously reported arene-or Cp-complexes with late-transition metals.D ensity functional theory (DFT) calculations have suggested charge transfer from the Cp-Ru moiety to the arene-Ru part via a p-conjugated system.…”

“…Herein, we report the synthesis of η 5 ,η 6 ‐[7]helicene bis‐ruthenium complex 3 (Figure d). Complex 3 exhibited phosphorescence both in the solution and solid states, which is in sharp contrast to the non‐emissive nature of monometallic complexes 1 and 2 (Figure c) . The quantum yield of the emission of complex 3 was by far the highest among those of the previously reported arene‐ or Cp‐complexes with late‐transition metals.…”

“…In UV‐vis absorption and circularly dichroism (CD) spectra, complex 3 showed peaks at a longer wavelength range than complex 1 (Figure ). In the absorption spectrum, complex 1 showed a weak broad peak with an absorption shoulder at 430 nm, which is attributed to MLCT transition . Bimetallic complex 3 exhibited a similar broad absorption peak, with its shoulder above 500 nm.…”

“…In the absorption spectrum,c omplex 1 showed aw eak broad peak with an absorption shoulder at 430 nm, which is attributedt oM LCT transition. [13] Bimetallic complex 3 exhibited as imilar broad absorptionp eak, with its shoulder above 500 nm. Also, in the CD spectra, complex 3 showedr ed-shifted peaks compared to complex 1;( P)-3/(M)-3 showedastrongd ichroic peak at 360 nm and aw eak peak at 460 nm, while (P)-1/(M)-1 exhibited as trong peak at 334 nm and aw eak peak around4 40 nm.…”

“…We have previously reported the synthesis of (h 5 -[7]helicene)Cp*Ru (1)a nd (h 5 -[7]helicene) 2 Ru (2), in which the [7]helicene ligand possessed aC pm oiety in the middle of the skeleton ( Figure 1c). [13] In additiont ot his Cp part, one of the 6membered rings in [7]helicene possibly served as as econd coordination site to afford an [(h 5 ,h 6 -[7]helicene)(Cp*Ru) 2 ] + (3). Since benzene and Cp rings have different valences, each metal on these positions will have non-equivalent electron density.…”