Polymorphism and spin crossover in mononuclear FeII species containing new dipyridylamino-substituted s-triazine ligands

Four new dipyridylamino-substituted s-triazine ligands DBB (N-2,N-2,N-4,N-4-tetrabenzyl-N-6,N-6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DDB (N-2,N-2,N-4,N-4-tetrabutyl-N-6,N-6-di(pyridin-2-yl)-1,3,5- triazine-2,4,6-triamine), DCCl (6-chloro-N-2,N-2-dicyclohexyl-N-4,N-4-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine) and DDT (N-2,N-2,N-4,N-4-tetraphenyl-N-6,N-6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), have been incorporated into eight new, 0D Fe-II compounds of type [Fe-II(NCX)(2)(L)(2)]center dot Solvent (where NCX = NCS-, NCSe- or N(CN)(2)(-)). The polymorphic compounds alpha-trans-[Fe-II(NCS)(2)(DBB)(2)] (1) and beta-trans-[Fe-II(NCS)(2)(DBB)(2)] (2) display, respectively, a relatively abrupt, complete, one-step spin transition with T-1/2 similar to 170 K, and a more gradual, complete, one-step spin transition with T-1/2 similar to 300 K. Gradual, one-step spin transitions are observed for trans-[Fe-II(N(CN)(2))(2)(DBB)(2)]center dot 2CH(3)CH(2)OH (3) and trans-[Fe-II(NCSe)(2)(DCCl)(2)]center dot 2CH(3)OH (6) with T-1/2 similar to 280 K for both, while the one-step spin transition observed for a desolvated sample of trans-[Fe-II(NCSe)(2)(DDB)(2)]center dot 2CH(3)OH (4) is relatively abrupt, showing hysteresis with T-1/2 up arrow = 285 K and T-1/2 down arrow = 275 K. The compounds cis-[Fe-II(NCS)(2)(DDB)(2)] (5) and trans-[Fe-II(NCS)(2)(DDT)(2)]center dot 4CH(2)Cl(2) (7) remain high spin, while structural data on trans-[Fe-II(NCSe)(2)(DDT)(2)]center dot 4CH(2)Cl(2) (8) suggests a spin transition at low temperatures. It is likely that distortion of the (FeN6)-N-II octahedron, intermolecular interactions and molecular conformation are crucial in deciding both the T-1/2 and abruptness of the spin transition for these species, although the nature of their influence varies. Variable temperature powder X-ray diffraction measurements on the polymorphs 1 and 2 reveal anisotropy in the unit cell parameters as the spin transition occurs.