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) A, b = 14.7704(14) A, c = 18.769(2) A, V = 2351.2(4) A3, and Z = 4. At 100 K, a = 8.5317(8) A, b = 14.4674(15) A, c = 18.814(2) A, and V = 2322.2(4) A3. Complex 2 (C28H38N6O9Fe) crystallizes in the P1 triclinic system. At 160 K (HS state), a = 10.265(4) A, b = 10.861(4) A, c = 14.181(5) A, alpha = 84.18(4) degrees, beta = 70.53(5) degrees, gamma = 88.95(5) degrees, V = 1482.6(10) A3, and Z = 2. The iron(II) coordination sphere is distorted octahedral in 1 and 2 with a cis-alpha 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 Mossbauer 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 approximately 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.