Cyanido-bridged {FeIIILnIII} heterobimetallic chains assembled through the [FeIII{HB(pz)3}(CN)3]− complex as metalloligand: synthesis, crystal structure and magnetic properties

Abstract

A new series of cyanido-bridged {Fe^IIILn^III} heterobimetallic chains of general formula {[(NC)Fe^III{HB(pz)₃}(μ-CN)₂Ln^III(bpdo)(NO₃)₂(H₂O)]·CH₃CN}_n [HB(pz)₃⁻ = hydrotris(pyrazol-1-yl)borate, bpdo = 2,2′-bipyridine-N,N′-dioxide and Ln = Gd (1), Tb (2), Dy (3) and Ho (4)], were obtained by using the low-spin [Fe{HB(pz)₃}(CN)₃]⁻ complex as a metalloligand towards the preformed [Ln(bdpo)(NO₃)₂(H₂O)]⁺ species. Single-crystal X-ray diffraction shows that 1–4 are isostructural compounds that crystallize in the monoclinic P2₁/c space group. Their crystal structure consists of neutral 1D coordination polymers where the [Fe{HB(pz)₃}(CN)₃]⁻ fragment adopts a bis-monodentate coordination mode, through two cis-cyanido ligands, towards two [Ln(bdpo)(NO₃)₂(H₂O)]⁺ units. The two cis-oriented cyanide bridges determine the formation of angular cyanido-bridged [Fe^IIILn₂^III] trinuclear moieties that are alternatively perpendicular to each other, describing a unique zig-zag chain. Each crystallographic independent iron(III) ion in 1–4 is six-coordinate in a distorted octahedral surrounding, the coordination sphere being built by one tridentate HB(pz)₃⁻ ligand and three cyanide groups, disposed in a fac-arrangement. The lanthanide(III) ions from 1–4 are nine-coordinate in an environment close to a distorted muffin-like geometry (C_s symmetry) and a spherical tricapped trigonal prism (D_3h symmetry). The iron⋯lanthanide intermetallic distances through the cyanide ligands are 5.36/5.50 (1), 5.49/5.35 (2), 5.34/5.48 (3) and 5.33/5.47 Å (4). Two-dimensional supramolecular networks are built from the heterobimetallic d/f chains through weak slipped-off π–π and C–H⋯π stacking interactions. Solid-state direct-current (dc) magnetic susceptibility measurements in the temperature range 1.9–300 K for 1–4 revealed the occurrence of weak antiferromagnetic interactions in 1 [J = −0.298 cm⁻¹ (α = 0), the spin Hamiltonian being defined as with S_A = 1/2 and S_B = 7/2] whereas the coexistence of the spin–orbit coupling of the lanthanide(III) [Tb^III (2), Dy^III (3), Ho^III (4)] and low-spin iron(III) ions together with the ligand field effects masked the visualization of the possible magnetic interactions in 2–4.