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

A new series of cyanido-bridged {FeIIILnIII} heterobimetallic chains of general formula {[(NC)FeIII{HB(pz)3}(μ-CN)2LnIII(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 [FeIIILn2III] 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 (Cs symmetry) and a spherical tricapped trigonal prism (D3h 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 SA = 1/2 and SB = 7/2] whereas the coexistence of the spin–orbit coupling of the lanthanide(III) [TbIII (2), DyIII (3), HoIII (4)] and low-spin iron(III) ions together with the ligand field effects masked the visualization of the possible magnetic interactions in 2–4.