Towards modular design of chiroptically switchable molecules based on formation and cleavage of metal-ligand coordination bonds

Article abstract of DOI:10.1055/s-0029-1217090

The results presented in this paper demonstrate that the proposed design of C₂-symmetric, pentadentate achiral or chiral ligands 8a-d and 15 allows to generate, upon coordination with Ni(II) and Pd(II), the corresponding diastereomeric complexes possessing three new elements of chirality: stereogenic axis, center, and helix. Of particular importance is that due to the specific steric characteristics of the designed ligands the formation of the corresponding diastereomeric products is highly stereoselective allowing preparation of only two out of four possible stereochemical combinations. For instance, each diastereoisomeric product (R_a,P_h ,S_c)-9a and (R_a,M_h,R_c)-12a can be selectively prepared and characterized in solid state, simply by the choice of the chelating metal [Ni(II) or Pd(II)]. Furthermore, introduction of stereochemical information into the ligands design with application of a simple chiral Amine Module allows for complete transfer of the corresponding stereochemistry to the newly generated axial, helical, and central chirality. For example, starting with chiral ligand (R)-15, out of eight possible products, only a single product of (R_c,R_a,P_h ,S_c) absolute configuration was obtained in the solid state. Taking into account the modular nature of this design, one may agree that modification of the three major phenone, acid, and amine modules, or application of different metals, will allow for virtually unlimited structural and functional flexibility in fine-tuning the diastereomeric relationships of this type of complexes making them more selective and controllable by an external stimulus.