pH Values Dependent Helicity of 1D Homochiral Helical Chains

The reaction of a racemic four-coordinated nickel(II) complex [Ni(α- rac -L)](ClO 4 ) 2 with l -Leu − in acetonitrile/water under basic conditions gave two six-coordinated diastereomers of {[Ni( SS -L)( l -Leu)](ClO 4 )} n (Δ- 1 ) and {[Ni( RR -L)( l -Leu)](ClO 4 )} n ( a - 2 ). While the reaction of [Ni(α- rac -L)](ClO 4 ) 2 with d -Leu − in acetonitrile/water under basic condition generated two six-coordinated diastereomers of {[Ni( RR -L)( d -Leu)](ClO 4 )} n (Λ- 1 ) and {[Ni( SS -L)( d -Leu)](ClO 4 )} 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)](ClO 4 ) 2 with l / d -leucine in acetonitrile/water under acidic condition gave the left-handed helical chains of {[Ni( SS -L)( l -HLeu)](ClO 4 ) 2 } n (Λ- 3 ) and right-handed helical chains of {[Ni( RR -L)( d -HLeu)](ClO 4 ) 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.