Magnetostructural relationships in polymorphic ethylmalonate-containing copper(ii) coordination polymers

Abstract

Three ethylmalonate-containing copper(II) chiral complexes of the formula {[Cu(H₂O)₄][Cu(Etmal)₂(H₂O)]}_n (1), [Cu(Etmal)(H₂O)]_n (2) and {[Cu(Etmal)(H₂O)]·H₂O}_n (3) (H₂Etmal = ethylmalonic acid) were obtained by reacting copper(II) nitrate trihydrate, ethylmalonic acid and sodium carbonate in water. Compound 1 is a chiral zigzag chain with regular alternation of [Cu(Etmal)₂(H₂O)]²⁻ and [Cu(H₂O)₄]²⁺ units, the former acting as bis-monodentate ligands toward the latter ones through the two carboxylate groups. The chirality of 1 is a result of the [Cu(Etmal)₂(H₂O)]²⁻ fragment whose five-coordinate copper(II) surrounding exhibits the Δ or Λ conformation within the Δ or Λ-crystals. The structure of 2 is made up of corrugated square grids of aquacopper(II) units connected through ethylmalonate-carboxylate bridges, with the chirality arising from the relative orientation of the stacking layers. 3 is a chiral three-dimensional compound which consists of ethylmalonate-carboxylate bridged aquacopper(II) units with pores of 9.5 Å diameter where the water molecules of crystallization are hosted. Cryomagnetic measurements of 1–3 show the occurrence of weak intrachain antiferromagnetic coupling (1), ferromagnetic coupled layers (2) and ferromagnetic ordering (3), the ethylmalonate-carboxylate bridge in the anti–syn conformation providing the exchange pathway in this series of compounds. Alternating current magnetic measurements at very low temperatures reveal the presence of a frequency-independent out-of-phase signal at 3.0 K for 3, a feature which is the signature of the occurrence of long-range ferromagnetic ordering in this compound (T_cca. 3.0 K).