Cyano-Bridged Homochiral Heterometallic Helical Complexes: Synthesis, Structures, Magnetic and Dielectric Properties

Abstract

The reaction of the chiral macrocyclic compound [Ni(SS-L)](ClO4)2 with Na2[Fe(CN)5NO] in acetonitrile/water gave {[Ni(SS-L)][cis-Fe(CN)5NO]}n (Δ-2) and {[Ni(SS-L)][trans-Fe(CN)5NO]}n (Λ-3), and the reaction of chiral [Ni(RR-L)](ClO4)2 with Na2[Fe(CN)5NO] gave the corresponding {[Ni(RR-L)][cis-Fe(CN)5NO]}n (Λ-2) and {[Ni(RR-L)][trans-Fe(CN)5NO]}n (Δ-3), while the reaction of racemic [Ni(rac-L)](ClO4)2 with Na2[Fe(CN)5NO] generated five supramolecular isomers of one achiral {[Ni(rac-L)][cis-Fe(CN)5NO]}2 (meso-1) and four homochiral Δ-2/Λ-2 and Δ-3/Λ-3 (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). In meso-1, a pair of enantiomers of [Ni(SS-L)]2+ and [Ni(RR-L)]2+ are bridged by two [Fe(CN)5NO]2− through two cis cyano groups to form a [2 + 2] type of discrete molecular square. In Δ-2/Λ-2, the homochiral [Ni(SS-L)]2+/[Ni(RR-L)]2+ cations are alternately bridged by [Fe(CN)5NO]2− anions through two cis cyano groups to produce one-dimensional (1D) homochiral right-handed and left-handed helical chains of Δ-2 and Λ-2, respectively. In Δ-3/Λ-3, however, the homochiral [Ni(RR-L)]2+/[Ni(SS-L)]2+ cations are alternately bridged by [Fe(CN)5NO]2− anions through two trans cyano groups to produce 1D homochiral right-handed and left-handed helical chains in Δ-3 and Λ-3, respectively. Their thermal stability, CD spectra, magnetic and dielectric properties were also investigated.