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)2·2H2O (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.

Investigation of the Spin Crossover Properties of Three Dinulear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type

“…As such, differences in molecular arrangement and non-covalent interactions leads to separate degrees of cooperativity between Fe(II) centres throughout the crystal lattice. The lesser extent to which counter ions provide intermolecular interactions between individual helicates may explain why the spin-crossover observed in this study is less abrupt in nature [36]. The asymmetric unit obtained by Hannon et al contained two waters, two acetonitriles and one diisopropyl ether molecule, whereas the crystal structure of 1 contained only two partially occupied acetonitrile molecules in the asymmetric unit.…”

“…In this structure, these "linked" Fe(II) centres were of the same spin-state (see above), suggesting a greater degree of cooperativity between Fe(II) centres in this structure. As has been shown previously, the intermolecular interactions, particularly hydrogen bonding, are a crucial influence on the cooperativity of the spin transition [36,38,39]. This cooperativity may be a factor influencing the more abrupt nature of the spin transition in 2.…”

Evolution of Spin-Crossover Transition in Hybrid Crystals Involving Cationic Iron Complexes [Fe(III)(3-OMesal2-trien)]+ and Anionic Gold Bis(dithiolene) Complexes Au(dmit)2 and Au(dddt)2

“…Although a significant loss of acetonitrile molecules was observed in the 145-200 °C range ( Figure S3), the desolvation was evidently taking place at ambient conditions. These small changes significantly affect the configuration of the ligand and therefore the spin transition [25,[31][32][33]. We believe, solvent vacancies in the seasoned crystals 2, as well as the defects, provoke stabilization of A comparison of the χT evolution in 1 and 2 with their structural characteristics gives rise to several essential notions:…”

Spin Cross-Over (SCO) Complex Based on Unsymmetrical Functionalized Triazacyclononane Ligand: Structural Characterization and Magnetic Properties

“…This large synthetic work led to several series of stable metal complexes, in which the functionalized macrocycle molecule acts as penta-, hexa-, hepta-, or octa-dentate ligand . Such metal complexes contribute to better understand the metal environment in some metalloenzymes and metalloproteins [63][64][65][66], better adapt chelators design to radiometals for radiopharmaceuticals [67,68], and also display catalytic properties [69] and, as reported in the last few years, original cooperative magnetic switching behaviors [41][42][43][50][51][52].…”

“…In this context, some of us have described two new systems of iron(II) monomeric complexes that are based on hexadentate functionalized tetraazamacrocycle ligands containing two 2-pyridylmethyl pendant arms, namely 1,7-bis(2′-pyridylmethyl)-1,4,7,10-tetraazacyclododecane (L1 in Scheme 1) and 1,8-bis(2′-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (L2 in Scheme 1) [41,42]: the first one, [Fe(L1)](BF4)2, containing the functionalized cyclen ligand (L1), remains in the high spin state (S = 2) in the whole temperature range, while the second one, [Fe(L2)](BF4)2.H2O, based on a slightly larger macrocycle (cyclam) presents a SCO transition with a transition temperature of ca. 150 K [41].…”