2-STEP SPIN CONVERSION OF [FE-II(5-NO2-SAL-N(1,4,7,10))] - 292, 153, AND 103 K X-RAY CRYSTAL AND MOLECULAR-STRUCTURE AND INFRARED, MAGNETIC, MOSSBAUER, CALORIMETRIC, AND THEORETICAL-STUDIES

A detailed study of the two-step spin-state conversion in [Fe-II(S-NO2-sal-N(1,4,7,10))] is presented with special emphasis on the structural changes evidenced by a multitemperature X-ray crystal and molecular structure study (292, 153, 103 K). The ligand (5-NO2-sal-N(1,4,7, 10)) results from the Schiff base condensation of 5-nitrosalicylaldehyde with tetraazadecane in a 2:1 ratio. The title molecule crystallizes in the monoclinic system, space group P2/c, with Z = 2 and a = 10.153(1) Angstrom, b = 8.490(3) Angstrom, c = 13.173(2) Angstrom, and beta = 109.93(2)degrees at 292 K. At 153 K, the space group. of the monoclinic system is P2 with Z = 2 and a = 9.952(1) Angstrom, b = 8.537(2) Angstrom, c = 13.070(1) Angstrom, and p 109.47(1)degrees, and at 103 K, the crystalline system is triclinic, space group P1, with Z = 2 and a 9.839(3) Angstrom, b e 8.336(4) Angstrom, c = 13.066(6) Angstrom, alpha = 90.06(3)degrees, beta = 107.84(3)degrees and gamma = 90.01(2)degrees. The structures were solved by the heavy-atom method and refined to conventional agreement indices R = 0.052 (292 K), R = 0.055 (153 K), and R = 0.092 (103 K). The structures consist of [Fe-II(5-NO2-sal-N(1,4,7,10))] complex molecules linked into infinite chains through two sets of N-H...O(nitro) hydrogen bonds, the central iron atom of each molecule being coordinated to the six donor atoms of the dianion of the N4O2 ligand. Two S = 2 <----> S = 0 spin-state conversions involving each similar to 50% of the molecules occur in the same temperature range as the two structural phase transitions which clearly allow one to distinguish two equally distributed sets of molecules in this material. The X-ray molecular structure determinations, corroborated by the low-temperature IR studies, also allow one to illustrate the role of intermolecular interactions in the cooperativeness of the spin-state conversion mechanism through modifications of the hydrogen bond network at each step of the spin-state conversion. The two discontinuities evidenced by the thermal dependence of the Massbauer parameters, Delta E(Q)(HS) and Delta E(Q)(IS), and area ratio, Delta(HS)/Delta(tot), and magnetic susceptibility and differential scanning calorimetry allow one to confirm that the two steps of the spin-state conversion are associated with the structural phase transitions evidenced by the X-ray molecular structure determinations. The theoretical approach based on the Ising-like model modified to take into account the nonequivalence of the two sublattices, and its application to [Fe-II(S-NO2-sal-N(1,4,7,10))], allows one to correctly account for the above-mentioned experimental observations, confirming that the origin of this unique experimental behavior must be related to the structural phase transitions.