First Dicyanamide-Bridged Spin-Crossover Coordination Polymer: Synthesis, Structural, Magnetic, and Spectroscopic Studies

• Published in: Chemistry : a European journal 2008-01, Vol.14 (2), p.697-705
• Main Authors: Genre, Caroline, Jeanneau, Erwann, Bousseksou, Azzedine, Luneau, Dominique, Borshch, Serguei A., Matouzenko, Galina S.
• Format: Article
• Language: English
• Subjects: Aminoquinolines - chemistry; chain structures; Chemical Sciences; Crystallography, X-Ray; Cyanamide - chemistry; Ferrous Compounds - chemical synthesis; Ferrous Compounds - chemistry; Hydrogen Bonding; iron; Iron - chemistry; Ligands; magnetic properties; Magnetics; Models, Molecular; Molecular Structure; or physical chemistry; polymers; Polymers - chemical synthesis; Polymers - chemistry; Quantum Theory; Spectroscopy, Mossbauer - methods; Spectrum Analysis, Raman - methods; spin crossover; Temperature; Theoretical and
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200700998

We report here on the synthesis and characterisation of a first iron(II) spin‐crossover coordination polymer with the dca spacer ligand, having the formula [Fe(aqin)2(dca)]ClO4⋅MeOH (aqin=8‐aminoquinoline, dca=dicyanamide), which displays a two‐step complete spin transition. Variable‐temperature magnetic susceptibility measurements and Mössbauer spectroscopy have revealed that the two relatively gradual steps are centred at 215 and 186 K and are separated by an inflection point at about 201 K, at which 50 % of the complex molecules undergo a spin transition. The two steps are related to the existence of two crystallographically inequivalent metal sites, as confirmed by the structural and Mössbauer studies. The crystal structure was resolved at 293 K (HS form) and 130 K (LS form). Both spin‐state structures belong to the triclinic P$\bar 1$ space group (Z=2). The complex assumes a linear chain structure, in which the active iron(II) sites are linked to each other by anionic dicyanamide ligands acting as chemical bridges. The Fe–Fe distances through the dca ligand are 8.119(1) and 7.835(1) Å in the high‐spin and low‐spin structures, respectively. The polymeric chains extend along a (1, 0, −1) axis and are packed in sheets, between which the perchlorate anions and methanol molecules are inserted. The complex molecules are linked together by π‐stacking interactions and H‐bonding between the H‐donor aqin ligands and the perchlorate ions. These structural features provide a basis for cooperative interactions in the crystal lattice. Analysis of the two‐step spin‐crossover character in this compound suggests that covalent interactions through the spacer ligand do not provide the main mechanism of cooperativity. Novel iron spin‐crossover polymer: Structural and physical studies suggest that non‐covalent intermolecular interactions, rather than interactions through bridging ligands, are mainly responsible for the cooperativity in some spin‐crossover polymers (see figure) described herein.