Gradual spin crossover behaviour in a linear trinuclear FeII complex

Abstract

The synthesis, structures and magnetic properties of a new trinuclear spin crossover complex, [Fe II 3 (npt) 6 (EtOH) 4 (H 2 O) 2 ](ptol) 6 $4EtOH (1), and of its Co II (2 and 3) and Ni II (4) analogues, are reported here. The complexes were synthesized by reacting a 1,2,4-triazole-based ligand, 4-(4 0 -nitrophenyl)-1,2,4-triazole (npt), with the p-tolylsulfonate (ptol) metal salts in methanol or ethanol. Structural analyses revealed that all complexes are iomorphous and consist of a linear trinuclear core where metal centres are bridged by triazole groups. For 1, dc susceptibility measurements exhibit gradual spin transition with T 1/2 ¼ 148 K which corresponds to HS / LS crossover for the triazole bridged central Fe II ion. This spin transition was confirmed by Single-Crystal X-Ray Diffraction data of 1 at 100 K and 181 K, where the low temperature measurement revealed a decrease in volume for the central Fe II ion, which is in agreement with a HS / LS transition.

Structure and Magnetic Properties of the Spin Crossover Linear Trinuclear Complex [Fe3(furtrz)6(ptol)2(MeOH)4]·4(ptol)·4(MeOH) (furtrz: furanylidene-4H-1,2,4-triazol-4-amine ptol: p-tolylsulfonate)

“…The overall gradual SCO behavior of this material suggests weak cooperativity between molecules in the lattice. While structural analysis revealed a dense network of intramolecular interactions, the molecular species are well separated due to the bulky aromatic substituents on the furtrz ligands and the presence of many aromatic anions, meaning the communication between SCO units would be weak [16]. Furthermore, the relatively low spin transition temperature of this material is consistent with other polymeric species which contain bulky aromatic 1,2,4-triazole ligands, thought to contribute a lower overall ligand field strength [16,21].…”

“…Several trinuclear SCO materials have been reported, composed of the linear arrangement of iron(II) sites bridged by 1,2,4-triazole-type ligands of various 4-functionalisation [15][16][17][18]. These materials are particularly useful as a tool for modelling the SCO behavior of one-dimensional 1,2,4-triazole ligand chain materials as the central iron(II) sites are of the same [FeN 6 ] coordination environment [19,20].…”

Abrupt Spin Crossover Behavior in a Linear N1,N2-Triazole Bridged Trinuclear Fe(II) Complex

“…A further study can be followed by synthesis of similar compounds with different degree of aromatic substitutions, either from the triazole ligands or from the counter anions. Furthermore, the relatively low transition temperature (120 K) for this compound is consistent with other similar discrete molecular species containing bulky aromatic 1,2,4-triazole ligands [19,[24][25][26].…”

“…The multiple intra-and inter-molecular H-bonding interactions found in the crystal lattice explain the strong cooperativity between the active Fe(II) centers. Interestingly, even though studies have shown that ligands with bulky aromatic substituents and aromatic anions might have a "dilute or separation effect" on the overall cooperativity of the SCO compounds [19,26]. The abrupt transition observed here stands for an example with the supramolecular interactions overcome the "aromatic dilute effect".…”

“…Numerous intra-and inter-molecular interactions involving the sulfonate oxygen atoms of the free p-tolylsulfonate anions lead to the final crystal packing. These supramolecular interactions explains the unusual sharp transition found in the rather bulky π-rich (conjugated) ligand system [19,26]. A comprehensive list of all the interactions is given in Table S5.…”