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

Abstract

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 Fe^II compounds of type [Fe^II(NCX)₂(L)₂]·Solvent (where NCX = NCS⁻, NCSe⁻ or N(CN)₂⁻). The polymorphic compounds α-trans-[Fe^II(NCS)₂(DBB)₂] (1) and β-trans-[Fe^II(NCS)₂(DBB)₂] (2) display, respectively, a relatively abrupt, complete, one-step spin transition with T_½ ∼ 170 K, and a more gradual, complete, one-step spin transition with T_½ ∼ 300 K. Gradual, one-step spin transitions are observed for trans-[Fe^II(N(CN)₂)₂(DBB)₂]·2CH₃CH₂OH (3) and trans-[Fe^II(NCSe)₂(DCCl)₂]·2CH₃OH (6) with T_½ ∼ 280 K for both, while the one-step spin transition observed for a desolvated sample of trans-[Fe^II(NCSe)₂(DDB)₂]·2CH₃OH (4) is relatively abrupt, showing hysteresis with T_½↑ = 285 K and T_½↓ = 275 K. The compounds cis-[Fe^II(NCS)₂(DDB)₂] (5) and trans-[Fe^II(NCS)₂(DDT)₂]·4CH₂Cl₂ (7) remain high spin, while structural data on trans-[Fe^II(NCSe)₂(DDT)₂]·4CH₂Cl₂ (8) suggests a spin transition at low temperatures. It is likely that distortion of the Fe^IIN₆ octahedron, intermolecular interactions and molecular conformation are crucial in deciding both the T_½ 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.