Spin-crossover iron(II) complexes [Fe(Medpq)(py)2(NCS)2] and [Fe(Medpq)(py)2(NCSe)2]:: syntheses, characterization and magnetic properties

Two new spin-crossover complexes, [Fe(Medpq)(py)(2)(NCS)(2)] center dot py center dot 0.5H(2)O (1) and [Fe(Medpq)(py)(2)(NCSe)2] center dot py (2) (Medpq = 2-methyldipyrido[3,2-f:2',3'-h]-quinoxaline, py = pyridine), have been synthesized. The crystal structures were determined at both room temperature (298 K) and low temperature (110 K). Complexes 1 and 2 crystallize in the orthorhombic space group Pbca and monoclinic space group P2(1)/n, respectively. In both complexes, the distorted [FeN6] octahedron is formed by six nitrogen atoms from Medpq, the trans pyridine molecules and the cis NCX- groups. The thermal spin transition is accompanied by the shortening of the mean Fe-N distances by 0.194 angstrom for 2. The mononuclear [Fe(Medpq)(py)(2)(NCS)(2)] and [Fe(Medpq)(py)(2)(NCSe)(2)] neutral species interact each other via pi-stacking, resulting in a one-dimensional extended structure for both 1 and 2. There exist C-H center dot center dot center dot X (X = S, Se) hydrogen bonds for both complexes. Variable-temperature magnetic susceptibility measurements and Mossbauer spectroscopy reveal the occurrence of a gradual spin transition. The transitions are centered at T-1/2 = 120 K for 1 and T-1/2 = 180 K for 2, respectively. (C) 2007 Elsevier B.V. All rights reserved.