Magnetic Bistability in Macrocycle‐Based Fe^II Spin‐Crossover Complexes: Counter Ion and Solvent Effects

Abstract

Two new iron(II) complexes of formula [Fe(L2)](tcm) 2 · 2H 2 O ( 1 ) and [Fe(L2)][Ni(CN) 4 ] · H 2 O ( 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)](BF 4 ) 2 · H 2 O ( 3 ). The crystal structures of the three compounds show discrete iron(II) complexes in which the Fe II ions adopt distorted FeN 6 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 ( T 1/2 down = 136 K; T 1/2 up = 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 Fe II 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.