RESUMO
Two polymorphic modifications 1 and 3 of binuclear compound [{Fe(dpia)(NCS)(2)}(2)(bpe)] and pseudo-polymorphic modification [{Fe(dpia)(NCS)(2)}(2)(bpe)]·2CH(3)OH (2), where dpia = di-(2-picolyl)amine, bpe = 1,2-bis(4-pyridyl)ethene, were synthesized, and their structures, magnetic properties, and Mössbauer spectra were studied. Variable-temperature magnetic susceptibility measurements of three binuclear compounds show different types of magnetic behaviour. The complex 1 exhibits a gradual two-step spin crossover (SCO) suggesting the occurrence of the mixed [HS-LS] (HS: high spin, LS: low spin) pair at the plateau temperature (182 K), at which about 50% of the complexes undergoes a thermal spin conversion. The complex 2 displays an abrupt full one-step spin transition without hysteresis, centred at about 159 K. The complex 3 is paramagnetic over the temperature range 20-290 K. The single-crystal X-ray studies show that all three compounds are built up from the bpe-bridged binuclear molecules. The structure of 1 was solved for three spin isomers [HS-HS], [HS-LS], and [LS-LS] at three temperatures 300 K, 183 K, and 90 K. The crystal structures for 2 and 3 were determined for the [HS-HS] complexes at room temperature. The analysis of correlations between the structural characteristics and different types of magnetic behaviour for new 1-3 binuclear complexes, as well as for previously reported binuclear compounds, revealed that the SCO process (occurrence of full one-step, two-step, or partial (50%) SCO) is specified by the degree of distortion of the octahedral geometry of the [FeN(6)] core, caused by both packing and strain effects arising from terminal and/or bridging ligands. The comparison of the magnetic properties and the networks of intra- and inter-molecular interactions in the crystal lattice for the family of related SCO binuclear compounds suggests that the intermolecular interactions play a predominant role in the cooperativeness of the spin transition relative to the intramolecular interactions through the bridging ligand.
RESUMO
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.
