Four new dipyridylamino-substituted s-triazine ligands DBB (N2,N2,N4,N4-tetrabenzyl-N6,N6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DDB (N2,N2,N4,N4-tetrabutyl-N6,N6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DCCl (6-chloro-N2,N2-dicyclohexyl-N4,N4-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine) and DDT (N2,N2,N4,N4-tetraphenyl-N6,N6-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), have been incorporated into eight new, 0D FeII compounds of type [FeII(NCX)2(L)2]·Solvent (where NCX = NCS−, NCSe− or N(CN)2−). The polymorphic compounds α-trans-[FeII(NCS)2(DBB)2] (1) and β-trans-[FeII(NCS)2(DBB)2] (2) display, respectively, a relatively abrupt, complete, one-step spin transition with T½ ∼ 170 K, and a more gradual, complete, one-step spin transition with T½ ∼ 300 K. Gradual, one-step spin transitions are observed for trans-[FeII(N(CN)2)2(DBB)2]·2CH3CH2OH (3) and trans-[FeII(NCSe)2(DCCl)2]·2CH3OH (6) with T½ ∼ 280 K for both, while the one-step spin transition observed for a desolvated sample of trans-[FeII(NCSe)2(DDB)2]·2CH3OH (4) is relatively abrupt, showing hysteresis with T½↑ = 285 K and T½↓ = 275 K. The compounds cis-[FeII(NCS)2(DDB)2] (5) and trans-[FeII(NCS)2(DDT)2]·4CH2Cl2 (7) remain high spin, while structural data on trans-[FeII(NCSe)2(DDT)2]·4CH2Cl2 (8) suggests a spin transition at low temperatures. It is likely that distortion of the FeIIN6 octahedron, intermolecular interactions and molecular conformation are crucial in deciding both the T½ 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.
