Guest-Induced Enantioselective Self-Assembly of a Pd_6 Homochiral Octahedral Cage with a C_3-Symmetric Pyridyl Donor

摘要

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 C_3-symmetric tripyridyl donor L·HNO_3 with cis-[(tmeda)Pd(NO_3)_2] (M) [tmeda = N,N,N',N'-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.