A temperature-dependent order-disorder and crystallographic phase transition in a 0D Feii spin crossover compound and its non-spin crossover Coiiisomorph

Abstract

The new dipyridylamino/triazine ligand DDE (N²,N²,N⁴,N⁴-tetraethyl-N⁶,N⁶-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine) has been incorporated into the mononuclear Fe^IISCO compounds cis-[Fe^II(NCSe)₂(DDE)₂] (1), cis-[Fe^II(NCBH₃)₂(DDE)₂] (2), and cis-[Fe^II(NCS)₂(DDE)₂] (3). Magnetic susceptibility measurements reveal that each of 1, 2 and 3 undergoes a complete, continuous spin transition with a T_½ of ∼260 K, ∼300 K and ∼205 K, respectively. An analogue and isomorph of 1, cis-[Co^II(NCSe)₂(DDE)₂] (4), remains high spin down to low temperatures. Variable temperature single crystal data reveal that 1 and 4 undergo a crystallographic phase transition (from orthorhombic Pbcn at high temperatures to monoclinic P2/c at low temperatures) accompanied by an order-disorder transition of ethyl moieties of the DDE ligand. In the Pbcn phase, the structures of 1 and 4 contain one crystallographically unique M^II centre, while in the P2/c phase, 1 and 4 contain two crystallographically unique M^II centres. Variable temperature powder X-ray diffraction experiments reveal that the crystallographic phase transition occurs at ∼250 K for 1. The occurrence of the concomitant order-disorder and crystallographic phase transitions undergone by 1 and 4 is not directly apparent in their magnetic susceptibility measurements, and this is likely due to the local environment of the M^II centres remaining largely undisturbed as the transitions occur. The compound 2 is isostructural to 1 and 4 at low temperatures.