pH Values Dependent Helicity of 1D Homochiral Helical Chains

The reaction of a racemic four-coordinated nickel(II) complex [Ni(α-rac-L)](ClO4)2 with l-Leu− in acetonitrile/water under basic conditions gave two six-coordinated diastereomers of {[Ni(SS-L)(l-Leu)](ClO4)}n (Δ-1) and {[Ni(RR-L)(l-Leu)](ClO4)}n (a-2). While the reaction of [Ni(α-rac-L)](ClO4)2 with d-Leu− in acetonitrile/water under basic condition generated two six-coordinated diastereomers of {[Ni(RR-L)(d-Leu)](ClO4)}n (Λ-1) and {[Ni(SS-L)(d-Leu)](ClO4)}n (b-2) (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, Leu− = leucine anion). Δ-1 and Λ-1 belong to supramolecular stereoisomers in which the [Ni(SS-L)(l-Leu)]+ and [Ni(RR-L)(d-Leu)]+ monomers are linked through the intermolecular hydrogen bonding interactions to form 1D homochiral right- and left-handed helical chains, respectively, while the connections of [{Ni(RR-L)}(l-Leu)]+/[{Ni(SS-L)}(d-Leu)]+ in a-2/b-2 through similar intermolecular hydrogen bonding interactions give 1D zigzag chains instead of 1D helical chains. In contrast, the reactions of [Ni(α-rac-L)](ClO4)2 with l/d-leucine in acetonitrile/water under acidic condition gave the left-handed helical chains of {[Ni(SS-L)(l-HLeu)](ClO4)2}n (Λ-3) and right-handed helical chains of {[Ni(RR-L)(d-HLeu)](ClO4)2}n (Δ-3), respectively, demonstrating the helicity of 1D helical chains is pH dependent The homochiral nature of Δ-1/Λ-1 and a-2/b-2 are confirmed by the results of CD spectra measurements.