Synthesis, Structure, and Magnetic Properties of Tetranuclear Cubane-like and Chain-like Iron(II) Complexes Based on the N₄O Pentadentate Dinucleating Ligand 1,5-Bis[(2-pyridylmethyl)amino]pentan-3-ol

The tetranuclear complexes [Fe₄(pypentO)(pym)₃(Oac)(NCS)₃]·1.5EtOH (1), [Fe₄(pypentO)(pym)(Oac)₂(NCS)₂(MeO)₂(H₂O)]·H₂O (2), [Fe₂(pypentO)(NCO)₃]₂ (3), and [Fe₂(pypentO)(N₃)₃]₂ (4) have been prepared, and their structure and magnetic properties have been studied (pypentOH = 1,5-bis[(2-pyridylmethyl)amino]pentan-3-ol, pymH = 2-pyridylmethanol). The X-ray diffraction analysis of 1 (C₄₃H₅₃N₁₀O_7.5S₃Fe₄, monoclinic, P2₁/n, a = 11.6153(17) Å, b = 34.391(17) Å, c = 14.2150(18) Å, β = 110.88(5)°, V = 5305(3) ų, Z = 4) and 2 (C₃₁H₄₅N₇O₁₀S₂Fe₄, monoclinic, C2/c, a = 19.9165(17) Å, b = 21.1001(12) Å, c = 21.2617(19) Å, β = 104.441(10)°, V = 8652.7(12) ų, Z = 8) showed a Fe₄O₄ cubane-like arrangement of four iron(II) atoms, four μ₃-O bridging ligands, one (1) or two (2) syn−syn bridging acetates. The X-ray diffraction analysis of 3 (C₄₀H₄₆N₁₄O₈Fe₄, monoclinic, P2₁/c, a = 11.7633(18) Å, b = 18.234(3) Å, c = 10.4792(16) Å, β = 99.359(18)°, V = 2217.7(6) ų, Z = 2) and 4 (C₃₄H₄₆N₂₆O₂Fe₄, monoclinic, P2₁/c, V = 4412.4(10) ų, a = 23.534(3) Å, b = 18.046(2) Å, c = 10.4865(16) Å, β = 97.80(2)°, Z = 4) showed a zigzag bis-dinuclear arrangement of four iron(II) cations, two μ₂-O bridging pypentO ligands, four μ₂-N-cyanato bridging ligands (3) or four end-on azido bridging ligands (4):  they are the first examples of cyanato and azido bridged discrete polynuclear ferrous compounds, respectively. The Mössbauer spectra of 1 are consistent with four different high-spin iron(II) sites in the Fe₄O₄ cubane-type structure. The Mössbauer spectra of 3 are consistent with two high-spin iron(II) sites (N₅O and N₄O). Below 190 K, the Mössbauer spectra of 4 are consistent with one N₅O and two N₄O high-spin iron(II) sites. The temperature dependence of the magnetic susceptibility was fitted with J₁ ∼ 0 cm^-1, J₂ = −1.3 cm^-1, J₃ = 4.6 cm^-1, D = 6.4 cm^-1, and g = 2.21 for 1; J₁ = 2.6 cm^-1, J₂ = 2.5 cm^-1, J₃ = − 5.6 cm^-1, D = 4.5 cm^-1, and g = 2.09 for 2; J₁ = 1.5 cm^-1, J₂ = 0.2 cm^-1, D = − 5.6 cm^-1, D‘ = 4.5 cm^-1, and g = 2.14 for 3; and J₁ = − 2.6 cm^-1, J₂ = 0.8 cm^-1, D= 6.3 cm^-1, D‘ = 1.6 cm^-1, and g = 2.18 for 4. The differences in sign among the J₁, J₂, and J₃ super-exchange interactions indicate that the faces including only μ₃-OR bridges exhibit ferromagnetic interactions. The nature of the ground state in 1−3 is confirmed by simulation of the magnetization curves at 2 and 5 K. In the bis-dinuclear iron(II) compounds 3 and 4, the J₂ interaction resulting from the bridging of two Fe₂(pypentO)X₃ units through two pseudo-halide anions is ferromagnetic in 3 (X = μ₂-N-cyanato) and may be either ferro- or antiferromagnetic in 4 (X = end-on azido). The J₁ interaction through the central O_alkoxo and pseudo-halide bridges inside the dinuclear units is ferromagnetic in 3 (X = μ₂-N-cyanato) and antiferromagnetic in 4 (X = end-on azido). In agreement with the symmetry of the two Fe^II sites in complexes 3 and 4, D (pentacoordinated sites) is larger than D‘ (octahedral sites).