title: Ligand strain and the nature of spin crossover in binuclear complexes: two-step spin crossover in a 4,4'-bipyridine-bridged iron(II) complex [{Fe(dpia)(NCS)(2)}(2)(4,4'-bpy)] (dpia = di(2-picolyl)amine; 4,4'-bpy = 4,4'-bipyridine).
abstract: The synthesis and detailed characterization of the new spin crossover (SCO) binuclear complex [{Fe(dpia)(NCS)(2)}(2)(4,4'-bpy)] (1; dpia=di(2-picolyl)amine, 4,4'-bpy = 4,4'-bipyridine) are reported. Variable-temperature magnetic susceptibility measurements show a relatively cooperative two-step spin transition suggesting the occurrence of three spin-state isomers: [HS-HS], [HS-LS], and [LS-LS] (HS: high spin, LS: low spin). A short plateau at 204 K separates the two steps and conforms with about 50% of the complexes having undergone a thermal spin conversion. Routine Mössbauer spectroscopy without applying a magnetic field clearly separates four iron(II) one-center spin states in three [HS-HS], [HS-LS], and [LS-LS] pairs and unambiguously confirms that the spin transition at the plateau temperature goes through the intermediate [HS-LS] state. The single-crystal X-ray structure was solved for three spin isomers at 293, 208, and 120 K. The structural study at the plateau temperature was unable to resolve the HS and LS sites in the [HS-LS] pair and only an average Fe-N bond length was obtained, which suggests that there is an intermediate [HS-LS] phase. The structural analysis at three temperatures revealed a dense three-dimensional network of both intra- and intermolecular interactions. The relative energies of the three spin-state isomers were evaluated by quantum-chemical DFT calculations. Comparison of compound 1 with previously known analogues, as well as the overall analysis of structural data for numerous binuclear complexes, allowed a conclusion to be reached on the crucial role of ligand strain effects in the SCO behavior of binuclear complexes. The suggested intramolecular mechanism explains the different types of SCO observed in binuclear complexes: one-step, two-step, and partial (50%) transitions.
authors.affiliation: Laboratoire de Chimie (UMR CNRS and ENS-Lyon no. 5182), Ecole Normale Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon cedex 07, France.
authors.forename: Alexander Yu
authors.initials: AY
authors.lastname: Verat
authors.forename: Nawel
authors.initials: N
authors.lastname: Ould-Moussa
authors.forename: Erwann
authors.initials: E
authors.lastname: Jeanneau
authors.forename: Boris
authors.initials: B
authors.lastname: Le Guennic
authors.forename: Azzedine
authors.initials: A
authors.lastname: Bousseksou
authors.forename: Serguei A
authors.initials: SA
authors.lastname: Borshch
authors.forename: Galina S
authors.initials: GS
authors.lastname: Matouzenko