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Polymorphism in Spin Transition Systems. Crystal Structure, Magnetic Properties, and Mössbauer Spectroscopy of Three
Galina S. Matouzenko1, Azzedine Bousseksou, Sylvain Lecocq
1Stéréochimie et Interactions moléculaires (UMR CNRS and ENS-Lyon No. 117; chaire de l'Institut universitaire de France), École normale supérieure de Lyon, 46, allée d'Italie, 69364 Lyon cedex 07, France, Laboratoire de chimie de coordination (UPR CNRS No. 8241), 205, route de Narbonne, 31077 Toulouse cedex, France, and School of Chemistry, Queen's University of Belfast, Belfast, BT9 5AG, Northern Ireland, Laboratoire de reconnaissance et organisation moléculaire (ESA Q5078), Université Claude Bernard-Lyon 1, 69622 Villeurbanne cedex, France, and Laboratoire des sciences moléculaires, Institut de chimie de la matière condensée de Bordeaux (UPR CNRS No. 9048), 33608 Pessac, France.
Abstract:
Three polymorphic modifications A-C of [Fe(II)(DPPA)(NCS)(2)], where DPPA = (3-aminopropyl)bis(2-pyridylmethyl)amine is a new tetradentate ligand, have been synthesized, and their structures, magnetic properties, and Mössbauer spectra have been investigated. For polymorph A, variable-temperature magnetic susceptibility measurements as well as Mössbauer spectroscopy have revealed the occurrence of a rather gradual HS if LS transition without hysteresis, centered at about 176 K. The same methods have shown that polymorph B is paramagnetic over the temperature range 4.5-295 K, whereas polymorph C exhibits a very abrupt S = 2 if S = 0 transition with a hysteresis. The hysteresis width is 8 K, the transitions being centered at T(c) downward arrow = 112 K for decreasing and T(c) upward arrow = 120 K for increasing temperatures. The crystal structures of the three polymorphs have been solved by X-ray diffraction at 298 K. Polymorph A is triclinic, space group P&onemacr; with Z = 2, a = 8.710(2) Å, b = 15.645(2) Å, c = 7.985(1) Å, alpha = 101.57(1) degrees, beta = 112.59(2) degrees, and gamma = 82.68(2) degrees. Polymorph B is monoclinic, space group P2(1)/c with Z = 4, a = 8.936(2) Å, b = 16.855(4) Å, c = 13.645(3) Å, and beta = 97.78(2) degrees. Polymorph C is orthorhombic, space group Pbca with Z = 8, a = 8.449(2) Å, b = 14.239(2) Å, and c = 33.463(5) Å. In the three polymorphs, the asymmetric units are almost identical and consist of one chiral complex molecule with the same configuration and conformation. The distorted [FeN(6)] octahedron is formed by four nitrogen atoms belonging to DPPA and two provided by the cis thiocyanate groups. The two pyridine rings of DPPA are in fac positions. The main differences between the structures of the three polymorphs are found in their crystal packing. The stabilization of the high-spin ground state of polymorph B is tentatively explained by the presence of two centers of steric strain in the crystal lattice resulting in the elongation of the Fe-N(aromatic) distance. The observed hysteresis in polymorph C seems to be due to the existence of an array of intermolecular contacts in the crystal lattice making the spin transition more cooperative than in polymorph A.