POLYMORPHISM IN SPIN TRANSITION-SYSTEMS - CRYSTAL-STRUCTURE, MAGNETIC-PROPERTIES, AND MOSSBAUER-SPECTROSCOPY OF 3 POLYMORPHIC MODIFICATIONSOF [FE(DPPA)(NCS)(2)][DPPA = (3-AMINOPROPYL)BIS(2-PYRIDYLMETHYL)AMINE]

Three polymorphic modifications A-C of [Fe-II(DPPA)(NCS)(2)], where DP PA = (3-aminopropyl)bis(2-pyridylmethyl)amine is a new tetradentate li gand, have been synthesized, and their structures, magnetic properties , and Mossbauer spectra have been investigated. For polymorph A, varia ble-temperature magnetic susceptibility measurements as well as Mossba uer spectroscopy have revealed the occurrence of a rather gradual HS d ouble left right arrow LS transition without hysteresis, centered at a bout 176 K. The same methods have shown that polymorph B is paramagnet ic over the temperature range 4.5-295 K, whereas polymorph C exhibits a very abrupt S = 2 double left right arrow S = 0 transition with a hy steresis. The hysteresis width is 8 K, the transitions being centered at T-c down arrow = 112 K for decreasing and T-c up arrow = 120 K for increasing temperatures. The crystal structures of the three polymorph s have been solved by X-ray diffraction at 298 K. Polymorph A is tricl inic, space group <p(1)over bar> with Z = 2, a = 8.710(2) Angstrom, b = 15.645(2) Angstrom, c = 7.985(1) Angstrom, 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) Angstrom, b = 16.855(4) Angstrom, c = 13.645(3) Angstrom, and beta = 97.78(2)degree s. Polymorph C is orthorhombic, space group Pbca with Z = 8, a = 8.449 (2) Angstrom, b = 14.239(2) Angstrom, and c = 33.463(5) Angstrom. In t he three polymorphs, the asymmetric units are almost identical and con sist of one chiral complex molecule with the same configuration and co nformation. The distorted [FeN6] octahedron is formed by four nitrogen atoms belonging to DPPA and two provided by the cis thiocyanate group s. The two pyridine rings of DPPA are infne positions. The main differ ences between the structures of the three polymorphs are found in thei r 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 t he Fe-N(aromatic) distance. The observed hysteresis in polymorph C see ms to be due to the existence of an array of intermolecular contacts i n the crystal lattice making the spin transition more cooperative than in polymorph A.