New cyanido-bridged iron() 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 1&cmb.macr; 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. The newly reported cyanido-bridged {Fe II M II } coordination polymers, with rare ladder-like topologies, show gradual HS-to-LS SCO on cooling due to the flexible N6 surrounding of Fe II metal ions from one pincer and three CN - ligands (M = Pd, Pt).