Guest-Induced Enantioselective Self-Assembly of a Pd₆ Homochiral Octahedral Cage with a <i>C</i>₃-Symmetric Pyridyl Donor

Abstract

Self-assembly of an achiral acceptor of square-planar Pd(II) or Pt(II) ion with a symmetric donor generally yields achiral architecture or a racemic mixture of chiral assemblies. Selective formation of an enantiopure assembly in such processes is very challenging. We report here a new approach of converting a dynamic mixture of multiple homochiral assemblies to an enantiopure architecture through the interaction of a chiral guest molecule. One-pot reaction of a cationic <i>C</i>₃-symmetric tripyridyl donor <b>L·</b>HNO₃ with <i>cis</i>-[(tmeda)Pd(NO₃)₂] (<b>M</b>) [tmeda = <i>N</i>,<i>N</i>,<i>N</i>',<i>N</i>'-tetramethylethane-1,2-diamine] yielded a complex mixture of stereoisomers of a chiral octahedral cage. Surprisingly, the presence of R-BINOL as a chiral guest in the above self-assembly induced selective formation of a single enantiopure octahedral cage. S-BINOL induced formation of the other enantiomer of the cage selectively. While selective recognition of an enantiomeric guest from a racemic mixture by a chiral host is well-known, present observation of "reverse chiral recognition" where the guest molecule determines the handedness of the host leading to the formation of an enantiopure cage is noteworthy.