Spin transition in [Fe(DPEA)(NCS)(2)], a compound with the new tetradentate ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (DPEA): Crystal structure, magnetic properties, and Mossbauer spectroscopy

The new spin transition compound [Fe-II(DPEA)(NCS)(2)], where DPEA [(2-aminoethyl)bis(2-pyridylmethyl)amine] is a new tetradentate ligand, has been synthesized, and its structure, magnetic properties, and Mossbauer spectra have been investigated. The crystal structure has been determined by X-ray diffraction at 298 K. The compound crystallizes in the monoclinic system, space group is P2(1)/c, with Z = 4, a = 9.358(1) Angstrom, b = 11.812(2) Angstrom, c = 17.135(2) Angstrom, and beta = 94.5(4)degrees. The distorted [FeN6] octahedron is formed from four nitrogen atoms belonging to DPEA and two provided by the cis thiocyanate groups. The two pyridine rings of DPEA are in mer positions. Each molecule is linked to its neighbors by hydrogen-bonding interactions as well as by numerous van der Waals contacts supposed to be responsible for the cooperativity of the system. Variable-temperature magnetic susceptibility measurements (20-290 K) have evidenced a relatively abrupt S = 2 reversible arrow S = 0 transition centered at T-1/2 = 138 K. The thermal variation: of the high spin state fraction observed by Mossbauer spectroscopy is in agreement with that obtained from magnetic susceptibility measurements. The fitting of Mossbauer and magnetic data with the Ising-like model allowed us to determine the energy gap between the high-spin and low-spin states (Delta(eff) = 835 K) and to estimate the variation of the thermodynamic parameters upon spin transition. The calculated variations of enthalpy (Delta H = 6.76 kJ mol(-1)) and entropy (Delta S = 49 J mol(-1) K-1) associated with the spin transition are in agreement with those previously observed for iron(II) spin-crossover compounds. The spin conversion is found to be close to a first-order phenomenon.