Chiral Recognition of Racemic Proline based on Chiral Macrocyclic Nickel(II) Complex: Synthesis and Crystal Structures

The reactions of dl ‐proline with chiral macrocyclic Ni II complex [Ni( SS ‐ L )](ClO 4 ) 2 in acetonitrile/water gave a six‐coordinate enantiomer formulated as [Ni( SS ‐ L )( l ‐Pro)](ClO 4 ) 2 · H 2 O ( 1 ). Another enantiomer of [Ni( RR ‐L)( d ‐Pro)](ClO 4 ) 2 · H 2 O ( 2 ) was obtained when [Ni( RR‐ L)](ClO 4 ) 2 was used (L = 5,5,7,12,12,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane, Pro = proline). Single‐crystal X‐ray diffraction analyses of complexes 1 and 2 revealed that the Ni II atom has a distorted octahedral coordination arrangement, being coordinated by four nitrogen atoms of L in a folded configuration, plus two carboxylate oxygen atoms of proline in mutually cis positions. Complexes 1 and 2 are supramolecular stereoisomers, which are constructed with hydrogen bonding linking of [Ni( SS ‐L)( l ‐Pro)] 2+ and [Ni( RR ‐L)( d ‐Pro)] 2+ monomers to form one‐dimensional zigzag chains. The homochiral natures of complexes 1 and 2 were confirmed by solid CD spectroscopy.