Synthesis, crystal structures and magnetic properties of mer-cyanideiron(iii)-based 1D heterobimetallic cyanide-bridged chiral coordination polymers

Abstract

Two pairs of cyanide-bridged Fe(III)–Mn(III)/Cu(II) chiral enantiomer coordination polymers {[Mn(S,S/R,R-Salcy)(CH₃OH)₂]{[Mn(S,S/R,R-Salcy)][Fe(bbp)(CN)₃]}}_2n (1,2) (bbp = bis(2-benzimidazolyl)pyridine dianion) and {[Cu(S,S/R,R-Chxn)₂]₂[Fe₂(tbbp)(CN)₆]}_n (3,4) (tbbp = tetra(3-benzimidazolyl)-4,4′-bipyridine tetraanion) have been successfully prepared by employing mer-tricyanometallate [PPh₄]₂[Fe(bbp)(CN)₃] or the newly bimetallic mer-cyanideiron(III) precursor K₄[Fe₂(tbbp)(CN)₆] as building blocks and with chiral manganese(III)/copper(II) compounds as assemble segments. The four complexes have been characterized by elemental analysis, IR spectroscopy, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra. Single X-ray diffraction reveals that complexes 1 and 2 possess a single anionic chain structure consisting of the asymmetric chiral {[Mn(S,S/R,R-Salcy)][Fe(bbp)(CN)₃]}₂²⁻ unit with free [Mn(S,S/R,R-Salcy)]⁺ as balanced cations. The cyanide-bridged Fe(III)–Cu(II) complexes 3 and 4 can be structurally characterized as neutral ladder-like double chains composed of the alternating cyanide-bridged Fe–Cu units. Our investigation of magnetic susceptibilities reveals the antiferromagnetic coupling between the cyanide-bridged Fe(III) and Mn(III)/Cu(II) ions for complexes 1–4. These results have been further confirmed by theoretical simulation through numerical matrix diagonalization techniques using a Fortran program or a uniform chain model, leading to the coupling constants J = −7.36 cm⁻¹, D = −1.52 cm⁻¹ (1) and J = −4.35 cm⁻¹ (3), respectively.