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 (<b>1</b>), [Fe₄(pypentO)(pym)(Oac)₂(NCS)₂(MeO)₂(H₂O)]·H₂O (<b>2</b>), [Fe₂(pypentO)(NCO)₃]₂ (<b>3</b>), and [Fe₂(pypentO)(N₃)₃]₂ (<b>4</b>) 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 <b>1</b> (C₄₃H₅₃N₁₀O_7.5S₃Fe₄, monoclinic, <i>P</i>2₁/<i>n</i>, <i>a</i> = 11.6153(17) Å, <i>b</i> = 34.391(17) Å, <i>c</i> = 14.2150(18) Å, β = 110.88(5)°, <i>V</i> = 5305(3) ų, <i>Z</i> = 4) and <b>2</b> (C₃₁H₄₅N₇O₁₀S₂Fe₄, monoclinic, <i>C</i>2/<i>c</i>, <i>a</i> = 19.9165(17) Å, <i>b</i> = 21.1001(12) Å, <i>c</i> = 21.2617(19) Å, β = 104.441(10)°, <i>V</i> = 8652.7(12) ų, <i>Z</i> = 8) showed a Fe₄O₄ cubane-like arrangement of four iron(II) atoms, four μ₃-O bridging ligands, one (<b>1</b>) or two (<b>2</b>) syn<i>−</i>syn bridging acetates. The X-ray diffraction analysis of <b>3</b> (C₄₀H₄₆N₁₄O₈Fe₄, monoclinic, <i>P</i>2₁/<i>c</i>, <i>a</i> = 11.7633(18) Å, <i>b</i> = 18.234(3) Å, <i>c</i> = 10.4792(16) Å, β = 99.359(18)°, <i>V</i> = 2217.7(6) ų, <i>Z</i> = 2) and <b>4</b> (C₃₄H₄₆N₂₆O₂Fe₄, monoclinic, <i>P</i>2₁/<i>c</i>, <i>V</i> = 4412.4(10) ų, <i>a</i> = 23.534(3) Å, <i>b</i> = 18.046(2) Å, <i>c</i> = 10.4865(16) Å, β = 97.80(2)°, <i>Z</i> = 4) showed a zigzag bis-dinuclear arrangement of four iron(II) cations, two μ₂-O bridging pypentO ligands, four μ₂-N-cyanato bridging ligands (<b>3</b>) or four end-on azido bridging ligands (<b>4</b>):  they are the first examples of cyanato and azido bridged discrete polynuclear ferrous compounds, respectively. The Mössbauer spectra of <b>1</b> are consistent with four different high-spin iron(II) sites in the Fe₄O₄ cubane-type structure. The Mössbauer spectra of <b>3</b> are consistent with two high-spin iron(II) sites (N₅O and N₄O). Below 190 K, the Mössbauer spectra of <b>4</b> are consistent with one N₅O and two N₄O high-spin iron(II) sites. The temperature dependence of the magnetic susceptibility was fitted with <i>J</i>₁ ∼ 0 cm^-1, <i>J</i>₂ = −1.3 cm^-1, <i>J</i>₃ = 4.6 cm^-1, <i>D</i> = 6.4 cm^-1, and <i>g</i> = 2.21 for <b>1</b>; <i>J</i>₁ = 2.6 cm^-1, <i>J</i>₂ = 2.5 cm^-1, <i>J</i>₃ = − 5.6 cm^-1, <i>D</i> = 4.5 cm^-1, and <i>g</i> = 2.09 for <b>2</b>; <i>J</i>₁ = 1.5 cm^-1, <i>J</i>₂ = 0.2 cm^-1, <i>D</i> = − 5.6 cm^-1, <i>D</i>‘ = 4.5 cm^-1, and <i>g</i> = 2.14 for <b>3</b>; and <i>J</i>₁ = − 2.6 cm^-1, <i>J</i>₂ = 0.8 cm^-1, <i>D</i><i></i>= 6.3 cm^-1, <i>D</i>‘ = 1.6 cm^-1, and <i>g</i> = 2.18 for <b>4</b>. The differences in sign among the <i>J</i>₁, <i>J</i>₂, and <i>J</i>₃ super-exchange interactions indicate that the faces including only μ₃-OR bridges exhibit ferromagnetic interactions. The nature of the ground state in <b>1</b>−<b>3</b> is confirmed by simulation of the magnetization curves at 2 and 5 K. In the bis-dinuclear iron(II) compounds <b>3</b> and <b>4</b>, the <i>J</i>₂ interaction resulting from the bridging of two Fe₂(pypentO)X₃ units through two pseudo-halide anions is ferromagnetic in <b>3</b> (X = μ₂-N-cyanato) and may be either ferro- or antiferromagnetic in <b>4</b> (X = end-on azido). The <i>J</i>₁ interaction through the central O_alkoxo and pseudo-halide bridges inside the dinuclear units is ferromagnetic in <b>3</b> (X = μ₂-N-cyanato) and antiferromagnetic in <b>4</b> (X = end-on azido). In agreement with the symmetry of the two Fe^II sites in complexes <b>3</b> and <b>4</b>, <i>D</i> (pentacoordinated sites) is larger than <i>D</i>‘ (octahedral sites).