Two Novel Iron(II) Materials Based on Dianionic N₄O₂ Schiff Bases:  Structural Properties and Spin-Crossover Characteristics in the Series [Fe(3-X,5-NO₂-sal-N(1,4,7,10)] (X = H, 3-MeO, 3-EtO)

Two spin-crossover (SCO) complexes [Fe^II(3-MeO,5-NO₂-sal-N(1,4,7,10)] (1) and [Fe^II(3-EtO,5-NO₂-sal-N(1,4,7,10)] (2) have been prepared and studied (3-MeO,5-NO₂-sal-N(1,4,7,10) and 3-EtO,5-NO₂-sal-N(1,4,7,10) are deprotonated 2-[12-(hydroxy-3-methoxy-5-nitrophenyl)-2,5,8,11-tetraazadodeca-1,11-dien-1-yl]-2-methoxy-4-nitrophenol and 2-[12-(hydroxy-3-ethoxy-5-nitrophenyl)-2,5,8,11-tetraazadodeca-1,11-dien-1-yl]-2-ethoxy-4-nitrophenol, respectively). The X-ray diffraction analysis of complex 1 (C₂₂H₂₆N₆O₈Fe) evidenced the same Pbnb orthorhombic system at 160 K (high-spin (HS) state) and 100 K (low-spin (LS) state). At 160 K, a = 8.4810(9) Å, b = 14.7704(14) Å, c = 18.769(2) Å, V = 2351.2(4) ų, and Z = 4. At 100 K, a = 8.5317(8) Å, b = 14.4674(15) Å, c = 18.814(2) Å, and V = 2322.2(4) ų. Complex 2 (C₂₈H₃₈N₆O₉Fe) crystallizes in the P1̄ triclinic system. At 160 K (HS state), a = 10.265(4) Å, b = 10.861(4) Å, c = 14.181(5) Å, α = 84.18(4)°, β = 70.53(5)°, γ = 88.95(5)°, V = 1482.6(10) ų, and Z = 2. The iron(II) coordination sphere is distorted octahedral in 1 and 2 with a cis-α arrangement of the N₄O₂ donor set of the hexadentate ligand. The molecules are connected into 1D infinite chains through hydrogen contacts involving the secondary amine functions and O_nitro atoms of the ligands in adjacent molecules. Investigation of their magnetic properties and Mössbauer spectra has revealed very different SCO behaviors:  complex 1 exhibits a cooperative SCO without residual LS or HS fraction; complex 2 shows a LS ↔ HS SCO involving ∼5% of the Fe^II ions in the 30−150 K range. The phenomenological cooperative interaction parameter J = 138 K evaluated from the area of the hysteresis loop indicates a cooperative effect weaker in 1 than in [Fe^II(5NO₂-sal-N(1,4,7,10))]. The theoretical approach to the SCO in 2 indicates a HS ground state and a LS first excited level 53 K above:  the thermal dependence of the system occurs through population of vibrational states. Comparison of the structural and electronic properties of the ferrous SCO materials with parent N₄O₂ ligands shows that the properties of SCO are closely related to intermolecular interactions and crystal packing.