Chiral Recognition and Partial Resolution of 1-Phenylethylamine through Noncovalent Interactions Using Binuclear Ni(II) Complex as Host

Abstract

Chiral recognition through noncovalent interaction occurs in many biological systems. Structural characterization of chiral host–guest complexes can enhance the understanding of recognition mechanism. but such examples are relatively few. Chiral recognition and resolution of 1-phenylethylamine was studied where a binuclear Ni(II) complex acts as chiral host. Structural characterization showed that (R)-1-phenylethylamine forms a triangular assembly of three host–guests adducts through both H-bond and CH···π interactions. In contrast, use of (S)-1-phenylethylamine leads to formation of ∼11 Å wide one-dimensional channels by the binuclear Ni(II) anionic complex where the chiral guest cation has been organized inside the channels in a zigzag fashion. Use of racemic amine yielded crystals having identical channel structure where S-amine and R-amine are inside the channels in a 55:45 ratio. HPLC results of the isolated amines from bulk of the crystals show that the S:R isomer ratio is 65:35, supporting the structural observation. HPLC on the isolated amine from amorphous precipitate of host–guest adduct showed minor (4% ee) chiral enhancement, indicating that this enhancement is mostly due to noncovalent interactions in the crystalline state. Resolution of enantiomers through crystallization is a result of solution thermodynamics, nucleation, and other factors pertaining to the crystalline state. Results presented attempted to show the extent of this interdependence within a chiral metallo-organic host and an amine.