Tuning the Spin States of Two Apical Iron(II) Ions in the Trigonal‐Bipyramidal [{Fe^II(μ‐bpt)₃}₂Fe^II₃(μ₃‐O)]²⁺ Cations Through the Choice of Anions

Abstract

When located in octahedral environment, the iron(II) ion with a d 6 electronic configuration may adopt two different stable electronic states, namely, a diamagnetic low-spin (LS) state and a paramagnetic high-spin (HS) state, which both give rise to different magnetic, optical and electronic properties. So tuning the spin state of the iron(II) ion is significant and contributes to the development of amazing materials that can be used as molecular switches, sensors and display devices.[1] As is well established, the spin state depends on relative strength of spin paring energy (P) and splitting energy (D 0 ). If the former is much stronger than the latter, the HS state will be stabilised, and if D 0 is stronger then the LS state will be the ground state. If P and D 0 are comparable a spin crossover (SCO) may occur between the HS and LS state by external perturbations such as temperature, pressure or light irradiation. So the main task is to make a judicious choice of ligand, which can impose a proper ligand-field strength.[2] However, in reality, the spin state of the iron(II) ion is quite sensitive to even more subtle changes such as solvent molecules, [3] polymorphism [4] and counterions. [5] As one of the best representatives of switchable molecules, SCO materials have attracted considerable interest in the chemistry and materials fields. Current work mainly focuses on the enhancement of cooperativity, which results in an abrupt transition and a wider thermal hysteresis.[6] Another promising research area, but with few examples, [7] is the combination of magnetic-exchange and spin-transition (ST) phenomena in the same molecule or polymeric network, which could eventually afford new switching materials with considerable amplification of the response signal.[1d]Much more work should be done to expand our knowledge of spin electronics.To induce a ST, the most common method is by varying the temperature. However, light-and pressure-induced SCOs play an increasingly important role owing to their potential applications as optical and pressure switches, for example. Moreover, the latter two methods are not restricted to thermal SCO compounds: LIESST (light-induced excited spinstate trapping) may also be observed in LS compounds, whereas HS compounds may experience STs with the application of external hydrostatic pressure.Although there are many examples that exhibit thermal STs, rare spin-crossover clusters of iron(II) have been found to exhibit a mixed-spin structure and synergy between ST and magnetic interaction. Fortunately, by introducing counterions, we have successfully tuned the spin states of two apical iron(II) ions in the pentanuclear [{Fe II

Tuning the Spin States of Two Apical Iron(II) Ions in a Pentanuclear Iron(II) Cluster Helicate through the Choice of Anions

“…Similar results were reported in [44]. [35], they can be omitted for simplification. Moreover, the Fe1 ion remains in an LS state in the whole temperature region.…”

“…Since the intramolecular magnetic coupling between the SCO-active Fe2 ion and central [Fe II 3(μ3-O)] 4+ core and intermolecular magnetic interactions between anions and cations are very weak [35], they can be omitted for simplification. Moreover, the Fe1 ion remains in an LS state in the whole temperature region.…”

“…The spin state of the two iron(II) ions in the axial direction was successfully realized by regulating different counter anions: one apical [ [35]. The main reason might be a variation of the supermolecular structure coordination environment of the {Fe 5 } cation cluster caused by anion replacement.…”

“…The disordered water was removed by SQUEEZE [38] to achieve reasonable refinements, which were determined by thermogravimetric analysis ( Figure S1) and elemental analysis [36]. The geometry of the pentanuclear cation is similar to that of our previous work [35,36] in which two terminal triple-stranded [Fe II (bpt) 3 (Table 2). Theoretically, the τ values for perfect trigonal bipyramid and square pyramid are 1 and 0, respectively [39,40].…”

“…To make this SCO behavior apparent, the χMT contributions of [Fe II 3(μ3-O)] 4+ core and dinuclear anion can be subtracted by the following two steps. First, the experimental χMT value of [Fe II 3(μ3-O)] 4+ core can be obtained from a similar cluster [{Fe II (μ-bpt)3} 2 Fe II 3(μ3-O)](NCS)2·10H2O, in which two apical Fe II ions are in the LS state [35]. Second, after the above subtraction, the magnetic susceptibility plot between 35 K and 100 K (Fe1 and Fe2 are in the LS state) can be fitted by a simple dinuclear S = 2 model [43].…”