Control of Helicity in a Metal Complex

Significance

Schiff base 1 adopts an M-helical conformation in the solid state and in solution upon metallation with nickel(II). The remote di­hydrobenzofuran stereocenters direct the formation of this structure, which is also stabilized by a series of hydrogen bonds. In particular, the introduction of an ester substituent was crucial for obtaining the M-helical arrangement: complex 2, which lacks this group, exists as a mixture of helical conformers as a result of facile rotation about the amide carbon-nitrogen bonds. The ester group biases the conformational preference of this amide bond through steric effects and the formation of a three-centered hydrogen bond.