Ligand Strain and tine 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)

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'-bipylidine) 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-HS]. 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 Mossbauer spectroscopy without. applying, a magnetic field clearly separates lour iron(II) one-center spin state, in three [HS-HS]. [HS-HS] 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. 205, and 120 K The structural study at the plateau temperature was unable to resolve the HS kind I-S sites in the [HS-LS] pair and only an average Fe-N bond length wits 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 (is 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. .