Design of 3d-4f molecular squares through the [Fe{(HB(pz)₃)}(CN)₃]^- metalloligand

A new series of {FeIII2LnIII2} heterobimetallic squares of general formula [FeIII{HB(pz)3}(CN)(μ-CN)2Ln(pyim)x(NO3)2(H2O)y]2·zH2O [Ln = La (1), Gd (2), Tb (3) and Dy (4); {HB(pz)3}- = hydrotris(pyrazolyl)borate and pyim = 2-(1H-imidazol-2-yl)pyridine; x = 2, y = 0 (1), x = y = 1 (2-4) and z = 10 (1), 6 (2), 2.76 (3), 4 (4)] were synthesized by reacting the low-spin [FeIII{HB(pz)3}(CN)3]- complex anion with the preformed [LnIII(pyim)x(NO3)2(H2O)y]+ complex cation [formed in situ by mixing the lanthanide(iii) salt and the pyim ligand]. Single-crystal X-ray diffraction shows that 1-4 crystallize in the P1[combining macron] triclinic space group, 2-4 being isomorphous. In all cases, the structure comprises neutral cyanido-bridged {FeIII2LnIII2} molecular squares in which two [FeIII{HB(pz)3}(CN)3]- units act as bis-monodentate ligands towards two [LnIII(pyim)x(NO3)2(H2O)y]+ moieties through two of the three cyanide groups. The iron(iii) and lanthanide(iii) cations regularly alternate in the corners of the square and the edges are defined by the cyanide bridges. Each iron(iii) ion in 1-4 is six-coordinate in a distorted octahedral surrounding with a tridentate tris(pyrazolyl)borate ligand and three cyanide groups in an approximately C3v symmetry. The lanthanum(iii) ion in 1 is ten-coordinate in a rare distorted sphenocorona environment, while the lanthanide(iii) ions in 2-4 are all nine-coordinate with a muffin-like geometry. The values of the FeIIILnIII distances across the cyanide bridges are 5.579(1) and 5.687(1) Å (1), 5.4695(20) and 5.4781(29) Å (2), 5.4550(12) and 5.4464(14) Å (3), 5.4463(10) and 5.4478(12) Å (4). The tetranuclear units in 1-4 are further interconnected through hydrogen bonds, π-π stacking and very weak C-Hπ type interactions leading to supramolecular three-dimensional networks with different topologies. Solid-state direct-current magnetic susceptibility analyses in the temperature range 1.9-300 K reveal the occurrence of weak intra- and intermolecular antiferromagnetic interactions in 1 [JFeFe' = -2.32(3) cm-1, gFe = 2.198(4), θ = -0.650(7) K and χTIP = 266(5) × 10-6 cm3 mol-1, the Hamiltonian being defined as H = -JFeFe'SFeSFe' + gβH(SFe + SFe')] and 2 [JFeFe' = -2.06(3) cm-1, JFeGd = -0.210(4) cm-1, gFe = 2.23(1), gGd = 2.0 (fixed), θ = -0.450(5) K and χTIP = 280(5) × 10-6 cm3 mol-1]. The coexistence of the spin-orbit coupling of the low-spin iron(iii) and lanthanide(iii) ions [TbIII (3) and DyIII (4)], together with the ligand field effects, masked the visualization of the possible magnetic interactions in 3 and 4. The small antiferromagnetic couplings in 1 and 2 are in line with previously reported weak antiferromagnetic interactions for these couples of ions through single cyanide bridges.