Solvent Drives Switching between Λ and Δ Metal Center Stereochemistry of M8L6 Cubic Cages.

Abstract

An enantiopure ligand with four bidentate metal-binding sites and four (S)-carbon stereocenters self-assembles with octahedral ZnII or CoII to produce O-symmetric M8L6 coordination cages. The Λ- or Δ-handedness of the metal centers forming the corners of these cages is determined by the solvent environment: the same (S)-ligand produces one diastereomer, (S)24-Λ8-M8L6, in acetonitrile but another with opposite metal-center handedness, (S)24-Δ8-M8L6, in nitromethane. Van 't Hoff analysis revealed the Δ stereochemical configuration to be entropically favored but enthalpically disfavored, consistent with a loosening of the coordination sphere and an increase in conformational freedom following Λ-to-Δ transition. The binding of 4,4'-dipyridyl naphthalenediimide and tetrapyridyl Zn-porphyrin guests did not interfere with the solvent-driven stereoselectivity of self-assembly, suggesting applications where either a Λ- or Δ-handed framework may enable chiral separations or catalysis.