Constructions of 1D helical chains with left-handed/right-handed helicity: a correlation between the helicity of 1D chains and the chirality of building blocks

Abstract

The reactions of [Ni(α-SS-L)](ClO₄)₂/[Ni(α-RR-L)](ClO₄)₂ with dca⁻ gave two enantiomers of [Ni(SS-L)](dca)₂ (S-1) and [Ni(RR-L)](dca)₂ (R-1) in acetonitrile/water, and two supramolecular stereoisomers of {[Ni(SS-L)]₂(dca)₃(ClO₄)·DMF}_n (Δ-2) and {[Ni(RR-L)]₂(dca)₃(ClO₄)·DMF}_n (Λ-2) in acetonitrile/DMF, respectively (L = 5,5,7,12,12, 14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, dca = dicyanamide, N(CN)₂⁻). While the reaction of racemic [Ni(α-rac-L)](ClO₄)₂ (containing an equal amount of SS and RR enantiomers) with dca⁻ in acetonitrile/water or acetonitrile/DMF produced a trimer of [Ni(rac-L)(dca)]₃·(dca)·(ClO₄)₂·MeCN (3) rather than S-1/R-1 or Δ-2/Λ-2. X-ray single-crystal analysis reveals that S-1/R-1 and Δ-2/Λ-2 crystallize in a chiral space groupP2₁, while 3 crystallizes in a centrosymmetric space groupC2/c. In S-1/R-1, the [Ni(SS-L)](dca)₂/[Ni(RR-L)](dca)₂ monomers are linked through intermolecular hydrogen bonds to form a two-dimensional sheet. While the intermolecular hydrogen bonding linking of {[Ni(SS-L)]₂(dca)₃}⁺/{[Ni(RR-L)]₂(dca)₃}⁺ leads to the formation of a 1D right-handed helical chain of Δ-2 and 1D left-handed helical chain of Λ-2, respectively, demonstrating there is a correlation between the helicity of 1D chains and the chirality of building blocks. Complex 3 forms a triangular structure of {[Ni(rac-L)(dca)]₃}³⁺, in which the macrocyclic ligand L adopts unsymmetrical RR/RR/SS or RR/SS/SS configurations. The homochiral nature of S-1/R-1 and Δ-2/Λ-2 are confirmed by the results of solid circular dichroism (CD) spectra measurements. The magnetic properties of chiral Δ-2/Λ-2 and achiral 3 were investigated.