Two-step spin crossover in a mononuclear compound [Fe(DPEA)(bim)](ClO4)(2)center dot 0.5 H2O[DPEA = (2-aminoethyl)-bis(2-pyridylmethyl)amine, bim=2,2-bisimidazole] - Crystal structure, magnetic properties, Mossbauer spectroscopy, and photomagnetic effects

The synthesis and characterization of the new spin crossover mononuclear co mplex [Fe-II(DPEA)(bim)](ClO4)2 .0.5H(2)O, where DPEA = (2-aminoethyl)bis(2 -pyridylmethyl)amine and bim. = 2,2-bisimidazole, are reported. Variable-te mperature magnetic susceptibility measurements (77-295 K) reveal the occurr ence of a two-step spin transition. Two steps on the magnetic curve are sep arated by an inflection point at 200 K, corresponding to about 50% of the c omplexes that have undergone a thermal spin transition. The first step is c entered at 171 K and the second one at 218 K. Mossbauer spectroscopy and X- ray analysis show that the profile of the magnetic curve is a consequence o f the presence of two inequivalent iron(II) molecules in the crystal lattic e. The crystal structure was resolved at 293 K (high-spin form) and at 123 K (low-spin form). Both spin-state isomers belong to the monoclinic space g roup P2(1)/c (Z = 4). The main differences between high-spin and low-spin i somers are found in the geometry of the [FeN6] core, the shorter Fe-N dista nces are seen at lower temperatures. Two inequivalent HS molecules at latti ce sites I and 2 successively undergo a thermal spin transition. The analys is of the mean Fe-N distances for functionally different nitrogen donor ato ms show that the two steps of the spin transition can be assigned to the in equivalent lattice sites. The gradual character of the spin transition at b oth lattice sites is accounted for in terms of intermolecular H-bonding via the perchlorate ions, At 10 K the light-induced excited spin state trappin g (LIESST) effect is observed within the SQUID magnetometer cavity, Two cri tical temperatures T-c(LIESST) were recorded (36 K and 21 K) and are attrib uted to the lattice sites I and 2.