Magnetic Bistability in Macrocycle-Based FeII Spin-Crossover Complexes: Counter Ion and Solvent Effects

• Published in: European journal of inorganic chemistry Vol. 2016; no. 34; pp. 5305 - 5314
• Main Authors: Milin, Eric, Benaicha, Bouabdellah, El Hajj, Fatima, Patinec, Véronique, Triki, Smail, Marchivie, Mathieu, Gómez-García, Carlos J., Pillet, Sébastien
• Format: Journal Article
• Language: English
• Published: Weinheim Blackwell Publishing Ltd 01-12-2016; Wiley Subscription Services, Inc; Wiley-VCH Verlag
• Subjects: Chemical Sciences; Iron; Macrocyclic ligands; Magnetic properties; Material chemistry; N ligands; Spin crossover; Macrocyclic ligands · N ligands · Iron · Spin crossover · Magnetic properties
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201600660

Two new iron(II) complexes of formula [Fe(L2)](tcm)2·2H2O (1) and [Fe(L2)][Ni(CN)4]·H2O (2) {L2 = 1,8‐bis(2′‐pyridylmethyl)‐1,4,8,11‐tetraazacyclotetradecane; tcm– = [C(CN)3]– = tricyanomethanide anion} have been synthesized and characterized by X‐ray diffraction and magnetic measurements and compared with the previously described compound of the same series [Fe(L2)](BF4)2·H2O (3). The crystal structures of the three compounds show discrete iron(II) complexes in which the FeII ions adopt distorted FeN6 octahedral geometries. A hydrogen‐bonding network involving the water molecules and the different counter ions, tcm– in 1 and [Ni(CN)4]2– in 2, leads to chains for both compounds whereas compound 3 reveals no significant intermolecular contacts. For 1, the magnetic measurements show an abrupt and incomplete high‐spin→low‐spin (HS→LS) spin transition (ST) with a hysteresis of 9 K (T1/2down = 136 K; T1/2up = 145 K) that is also optically observed. Compound 2 presents slightly weaker intermolecular contacts and shows an abrupt ST at 85 K without hysteretic behaviour. Finally, compound 3, in which the FeII complex is well isolated, exhibits a gradual spin transition centred at 150 K. Detailed X‐ray diffraction studies performed at various temperatures (293–120 K) show strong modifications of the iron coordination sphere in 1 and 3, in agreement with the presence of a ST in this temperature range in both complexes.
A new series of FeII spin‐crossover complexes based on an N‐functionalized macrocycle ligand have been prepared and fully characterized. One exhibits magnetic bistability with a 9 K wide hysteresis loop (136–145 K) and the other two show either an abrupt (T1/2 = 83 K) or gradual (T1/2 = 150 K) transition without hysteresis. The different behaviour can be explained by intermolecular interactions.