Poly(ferrocenylsilane)s from planar-chiral sila[1]ferrocenophanes: How to twist a zig-zag chain into a helix

Abstract

In addition to the known enantiopure, Me2Si-bridged [1]ferrocenophane, three new enantiopure sila[1]ferrocenophanes were prepared and characterized by NMR spectroscopy, mass spectrometry, and differential scanning calorimetry. Two of the new sila[1]ferrocenophanes were also characterized by single-crystal X-ray analysis. For all these species, which were either equipped with a Me2Si-, Ph2Si-, Et2Si-, or Hex2Si-bridging unit, the planar-chiral ferrocene moiety (Sp,Sp)-2,2′-di(isopropyl)ferrocene-1,1′-diyl had been used. Only the hexyl-substituted monomer resulted in high-molecular-weight polymers that readily dissolved in common organic solvents (Mw (PS) = 7.4 (batch 1) or 6.2 × 105 (batch 2) Da, with Ð = 2.7 or 2.6). With this study, we investigated if the chiral, C2 symmetric repeating units result in a chiral secondary structure of respective macromolecules. A combination of theoretical (DFT calculations) and experimental methods (1H NMR, UV/Vis, and CD spectroscopy) provided strong evidence for a P-helicity of the hexyl-substituted macromolecules. To the best of our knowledge, this is the first reported poly(ferrocenylsilane) with a chiral secondary structure.