Spin-crossover studies on a series of 1D chain and dinuclear iron(II) triazine-dipyridylamine compounds

Four isostructural one-dimensional polymeric iron(II) spin crossover materials containing the ligand 2-chloro-4,6-bis(di-pyrid-2-ylamino)-1,3,5- triazine (cddt) have been synthesised and magnetically and structurally characterised. The polymeric material Fe(NCS)₂(cddt)·n(guest) (1), self assembles into three crystallographically distinct phases; the compounds 1b and 1c, [Fe(NCS)₂(cddt)]·2(CH₃OH), are polymorphs, however, 1a, [Fe(NCS)₂(cddt)]·1/2(CHCl ₃)·(H₂O) contains a different solvent system. Each phase shows very different magnetic behaviour; phases 1a and 1b remain high spin over all temperatures, whilst phase 1c undergoes a "half" spin crossover between 225 and 125 K. The structural consequences of the spin transition in 1c have been followed using variable temperature synchrotron powder X-ray diffraction techniques. Variation of the chalcogenide ligand in the complex [Fe(NCSe)₂(cddt)]·2(CH₃OH), 2, leads to a full one-step spin crossover with a T_1/2 of 200 K. Structural analysis of 1c below the transition temperature reveals alternating, crystallographically distinct, HS and LS iron(II) centres (HS: high spin, LS: low spin) along the chain, and structural analysis of 2 below the transition temperature reveals all LS iron(II) centres. Also reported are the structural characterisations of two dinuclear materials, containing the ligand 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine (tdt), of the formula [Fe-(tdt)(X)₂]₂·(ClO₄)·(solvent) (X = H₂O, Cl^- for 3a, 3b) which are analogous to those recently reported by Gamez et al. In addition, the structural characterisations are described for dinuclear materials, containing the ligand cddt, of the formula [Fe(cddt)(Cl)(X)]₂·(Y)₂·n(solvent) (X: H₂O or NCCH₃, Y: ClO₄^-, BF ₄^- 4a, 4b and 4d) and for a cobalt(II) analogue, [Co(cddt)(Cl)(H₂O)]₂·(BF₄) ₂·n(solvent) (4c).