Spontaneous Resolution of a Racemic Nickel(II) Complex and Helicity Induction via Hydrogen Bonding: The Effect of Chiral Building Blocks on the Helicity of One-Dimensional Chains

Abstract

The reactions of a racemic four-coordinated nickel(II) complex [Ni(alpha-rac-L)](ClO4)2 (containing equal amount of SS and RR enantiomers) with l- and d-phenylalanine in acetonitrile/water gave two less-soluble six-coordinated enantiomers of {[Ni( f-SS-L)(l-Phe)](ClO4)}n (Delta-1) and {[Ni(f- RR-L)(d-Phe)](ClO4)}n (Lambda-1), respectively. Evaporation the remaining solutions gave two six-coordinated diastereomers of {[Ni 3(f- RR-L)3(l-Phe)2(H 2O)](ClO4)4}n (a-2) and {[Ni3(f- SS-L)3(d-Phe)2(H2O)](ClO4)4}n (b-2), respectively (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, Phe(-) = phenylalanine anion). The reaction of [Ni(alpha-rac-L)](ClO4)2 with dl-Phe(-) gave a conglomerate of c-1; in which, the SS and RR enantiomers preferentially coordinate to l- and d-Phe(-) respectively to give a racemic mixture of Delta-1 and Lambda-1, and the spontaneous resolution occurs during the reaction, in which each crystal crystallizes to become enantiopure. Removing Phe(-) from Delta-1 and Lambda-1 using perchloric acid gave two enantiomers of [Ni(alpha-SS-L)](ClO4)2 (S-3) and [Ni(alpha-RR-L)](ClO4)2 (R-3). Dissolving S-3 and R-3 in acetonitrile gave two six-coordinated enantiomers of [Ni( f-SS-L)(CH3CN)2](ClO4)2 (S-4) and [Ni( f- RR-L)(CH3CN)2](ClO4)2 (R-4), while dissolving [Ni(alpha-rac-L)](ClO4)2 in acetonitrile gave a racemic twining complex [Ni(f-rac-L)(CH3CN)2](ClO4)2 (rac-4). Delta-1 and Lambda-1 belong to supramolecular stereoisomers, which are constructed via hydrogen bond linking of [Ni( f-SS-L)(l-Phe)](+) and [Ni(f-RR-L)(d-Phe)](+) monomers to form 1D homochiral right-handed and left-handed helical chains, respectively. The reaction of S-3 with d-Phe(-) gave {[Ni(f-SS-L)(d-Phe)](ClO4)}n (5), which shows a motif of a 1D hydrogen bonded zigzag chain instead of a 1D helical chain. Compound a-2/ b-2 contains dimers of [{Ni(f-RR-L)}2(l-Phe)(H2O)](3+)/[{Ni( f- SS-L)}2(d-Phe)(H2O)](3+) and 1D zigzag chains of {[Ni(f-RR-L)(l-Phe)](+)}n /{[Ni(f-SS-L)(d-Phe)](+) n . The homochiral nature of Delta-1/Lambda-1, a-2/b-2, S-3/R-3, and S-4/R-4 are confirmed by the results of circular dichroism (CD) spectra measurements.

Separation of α‐ from β‐arylalanines by nickel nitrilotriacetate chromatography

“…Therefore, the choice of selecting the nickel nitrilotriacetate resin to separate α‐ from β‐arylalanines in the present investigation was based on the reported formation of a five‐membered ring complex between the α‐isomers and nickel (Fig. 3A) 11, compared with the hypothetically less stable, sterically encumbered six‐membered ring chelate between nickel and β‐isomers (Fig. 3B).…”

Synthesis and Crystal Structure of Two Succinato‐Bridged Macrocyclic Nickel(II) Complexes

“…General: The macrocyclic ligand (L) and its nickel(II) complexes were prepared according to the literature methods,23,24 and separated as the racemic form of rac ‐L. The nickel(II) complex [Ni( RR ‐L)](ClO 4 ) 2 /[Ni( SS ‐L)](ClO 4 ) 2 was prepared according to the previously reported methods 25. All of the other chemicals are commercially sourced and used without further purification.…”

Synthesis and crystal structures of terephthalato-bridged macrocyclic nickel(II) complexes with cis configurations

“…and [Ni(RR-L)(dPhe)] ? , respectively [23]. As a continuance of our research on the construction of chiral helical chains, and in order to explore the factors that influence the construction and chirality of helical chains, we have now employed [Ni(arac-L)] 2?…”