New cyanido-bridged iron(ii) spin crossover coordination polymers with an unusual ladder-like topology: an alternative to Hofmann clathrates

Abstract

Two new cyanido-bridged {Fe^IIM^II} double chains were obtained by reacting cyanido anions [M(CN)₄]²⁻ with complex cations [Fe^II(tptz)]²⁺ (preformed in situ by mixing a hydrated tetrafluoroborate salt of iron(II) and a tptz ligand, tptz = 2,4,6-tri(2-pyridyl)-1,3,5-triazine) having the general formula [Fe^II(tptz)M^II(CN)₄]·2H₂O·CH₃CN, where M = Pd (1) or Pt (2). Additionally, two molecular complexes formulated as [Fe^II(tptz)₂][M^II(CN)₄]·4.25H₂O, where M = Pd (3) or Pt (4), were subsequently obtained from the same reaction, as secondary products. Single crystal X-ray analysis revealed that 1 and 2 are isostructural and crystallize in the P-1 triclinic space group. Their structure consists of a double-chain with a ladder-like topology, in which cyanido-based [M(CN)₄]²⁻ metalloligands coordinate, through three CN⁻ ligands and three [Fe^II(tptz)]²⁺ complex cations. Compounds 3 and 4 are also isostructural and crystallize in the P triclinic space group, and the X-ray structural data show the formation of [Fe^II(tptz)₂]²⁺ and [M^II(CN)₄]²⁻ ionic units interconnected through H-bonds and π⋯π stacking supramolecular interactions. The static DC magnetic measurements recorded in the temperature range of 2–300 K showed that 1 and 2 exhibit incomplete spin transition on cooling, which is also confirmed by single crystal XRD analysis and Mössbauer spectroscopy. Compounds 3 and 4 are diamagnetic, most likely due to the encapsulation of Fe(II) in a tight pocket formed by two tptz ligands that preserve the low-spin state in the temperature range of 2–400 K.