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

Two new cyanido-bridged {Fe II M 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 ]·2H 2 O·CH 3 CN, where M = Pd (1) or Pt (2). Additionally, two molecular complexes formulated as [Fe II (tptz) 2 ][M II (CN) 4 ]·4.25H 2 O, 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 1̄ 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.