Ligand-directed assembly of trinuclear and one-dimensional heterotrimetallic CuIILnIIIFeIII complexes: unusual antiferromagnetic CuIIFeIII coupling via cyano bridges

Abstract

New heterotrimetallic complexes [Cu(valpn)(H₂O)Ln(H₂O)₃Fe(CN)₆]·xH₂O (Ln = Gd, x = 3 for complex 1; Ln = Tb, x = 3 for complex 2; Ln = Dy, x = 1.75 for complex 3; H₂valpn = N,N′-bis(3-methoxysalicylidene)-1,3-diaminopropane), {[Cu(Me₂valpn)]₂Ln(H₂O)Fe(CN)₆·xH₂O·yCH₃CN}_n (Ln = Gd, x = 5, y = 2.5 for complex 4; Ln = Tb, x = 4.5, y = 1.5 for complex 5; Ln = Dy, x = 5.5, y = 2.5 for complex 6; Ln = Y, x = 6, y = 4 for complex 7; H₂Me₂valpn = N,N′-bis(3-methoxysalicylidene)-2,2-dimethyl-1,3-diaminopropane) have been synthesised by a reaction of Cu(valpn) (or Cu(Me₂valpn)), Ln³⁺ and [Fe(CN)₆]³⁻ in a molar ratio of 2 : 1 : 1. Complexes 1–3 have a CuLnFe trinuclear structure, consisting of phenoxo-bridged Cu–Ln and cyano-bridged Ln–Fe moieties. Complexes 4–7 contain phenoxo-bridged Cu₂Ln units that are connected by [Fe(CN)₆]³⁻ to form a one-dimensional (1D) zig-zag structure. In the trinuclear complexes 1–3, the [Fe(CN)₆]³⁻ anion coordinates to Ln³⁺, whereas it coordinates to Cu²⁺ in complexes 4–7. Magnetic studies demonstrate that there exists a ferromagnetic interaction between Cu(II) and Gd(III) ions via the phenoxo bridges, while the cyano-bridges transit antiferromagnetic coupling between low-spin Fe(III) and Cu(II) or Gd(III). The fitting to the magnetic susceptibilities of complexes 1 and 4 gave the parameters of J_CuGd = 2.392 and 2.945 cm⁻¹, and J_FeGd = −0.341 cm⁻¹ and J_FeCu = −0.378 cm⁻¹. The ac magnetic measurements reveal that complex 6 shows slow magnetization relaxation with an energy barrier of 12.3 K and a relaxation time of τ₀ = 1.65 × 10⁻⁶ s, which is typical of a single-chain magnet.