Spin-Crossover Iron(II) Coordination Polymer with Zigzag Chain Structure

Abstract

We report on the synthesis and characterization of a first iron(II) spin-crossover coordination polymer of formula [Fe(pyim) 2 (bpy)](ClO 4 ) 2 ·2C 2 H 5 OH, where pyim = 2-(2-pyridyl)imidazole and bpy = 4,4‘-bipyridine, with zigzag chain structure. The chains are linked by π stacking into a two-dimensional network. Variable-temperature magnetic susceptibility measurements have revealed the occurrence of the HS ↔ LS transition without hysteresis. The spin conversion temperature T 1/2 (temperature for which the HS fraction is equal to 0.5) is found equal to 205 K. The crystal structure was resolved at 293 K (high-spin form) and at 173 K (low-spin form). Both spin-state structures belong to the monoclinic space group C 2/ c ( Z = 4). The complex assumes one-dimensional zigzag chain structure, in which the iron(II) active sites are linked to each other by a chemical bridge as the rigid rodlike 4,4‘-bipyridine molecule. The Fe−Fe separation via the bpy ligand equals 11.523(2) and 11.201(2) Å in the high-spin and low-spin structures, respectively. The polymeric chains extended along the c axis are stacked in the b direction forming two-dimensional sheets of bound molecules. The sheets are linked together by several intermolecular H-bonds in the a direction. The ensemble of structural features of the complex, potentially leading to the cooperativity, provides a basis for strong interactions. However, they are not strong enough in comparison with the HS−LS energy gap and result in a relatively gradual spin transition of the complex. The structural features and magnetic properties of the complex have been discussed and compared with those for earlier reported iron(II) spin-crossover coordination polymers.