Two-Step Spin Crossover in a Mononuclear Compound [Fe(DPEA)(bim)](ClO4)2·0.5 H2O[DPEA = (2-Aminoethyl)bis(2-pyridylmethyl)amine, bim = 2,2-Bisimidazole] − Crystal Structure, Magnetic Properties, Mössbauer Spectroscopy, and Photomagnetic Effects

Abstract

The synthesis and characterization of the new spin crossover mononuclear complex [FeII(DPEA)(bim)](ClO4)2·0.5 H2O, where DPEA = (2-aminoethyl)bis(2-pyridylmethyl)amine and bim = 2,2-bisimidazole, are reported. Variable-temperature magnetic susceptibility measurements (77−295 K) reveal the occurrence of a two-step spin transition. Two steps on the magnetic curve are separated by an inflection point at 200 K, corresponding to about 50% of the complexes that have undergone a thermal spin transition. The first step is centered at 171 K and the second one at 218 K. Mössbauer spectroscopy and X-ray analysis show that the profile of the magnetic curve is a consequence of the presence of two inequivalent iron(II) molecules in the crystal lattice. 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 group P21/c (Z = 4). The main differences between high-spin and low-spin isomers are found in the geometry of the [FeN6] core, the shorter Fe−N distances are seen at lower temperatures. Two inequivalent HS molecules at lattice sites 1 and 2 successively undergo a thermal spin transition. The analysis of the mean Fe−N distances for functionally different nitrogen donor atoms show that the two steps of the spin transition can be assigned to the inequivalent lattice sites. The gradual character of the spin transition at both lattice sites is accounted for in terms of intermolecular H-bonding via the perchlorate ions. At 10 K the light-induced excited spin state trapping (LIESST) effect is observed within the SQUID magnetometer cavity. Two critical temperatures Tc(LIESST) were recorded (36 K and 21 K) and are attributed to the lattice sites 1 and 2.