Thermally-Induced Spin Crossover and LIESST Effect in the Neutral [FeII(Mebik)2(NCX)2] Complexes: Variable-Temperature Structural, Magnetic, and Optical Studies (X = S, Se; Mebik = bis(1-methylimidazol-2-yl)ketone).

Siddhartha De,Hasnaa El Said,Gopalan Rajaraman,Subrata Tewary,Alexandrine Flambard,Lise-Marie Chamoreau,Yanling Li,Marie-Laure Boillot,Rodrigue Lescouëzec

doi:10.3389/fchem.2018.00326

Abstract

Two new iron(II) neutral complexes of bis(1-methylimidazol-2-yl)ketone (Mebik) with molecular formula [FeII(Mebik)2(NCS)2] (1) and [FeII(Mebik)2(NCSe)2] (2) have been synthesized and characterized by magnetic measurements, single-crystal X-ray diffraction, and solid state UV-vis spectroscopy. The temperature dependent magnetic susceptibility measurements of crystalline samples of both compound show the occurrence of a gradual spin transition centered at T1/2 = 260 K and 326 K, respectively. The crystal structures of both compounds were determined at different temperatures, below and above the transition, in order to detect the structural changes associated with the spin transition. The main structural modifications, when passing from the low-spin to the high-spin form, consist of an important lengthening of the Fe-N(Mebik) and Fe-N (C-S/Se) distances (by ca. 0.20 and 0.18 Å, respectively) and a noticeable variation of the N-Fe-N angles, leading to a more distorted [Fe-N6] octahedron. The spin-transition phenomenon also affects the optical properties, with significant decrease of the intensity of the Metal-to-Ligand charge transfer band upon increasing the temperature. Finally, both complexes exhibit a light-induced excited spin-state trapping under laser light irradiation at low temperature. DFT calculations were also carried out on these complexes in order to rationalize the theoretically predicted magnetic and optical behavior with those of the experimental one. The results clearly highlights the dramatic alteration of the magneto-structural behavior of the tris-chelate spin-crossover complex upon substituting one Mebik with NCS and NCSe ligands.

Highlights

The rational design of switchable molecular materials featuring stimuli-responsiveness is attracting strong research efforts because of the potential use as molecular sensors, switches, actuators, or memories in future molecular devices (Sauvage and Amendola, 2001; Feringa, 2011; Shepherd et al., 2013; Ferrando-Soria et al, 2017)
The current report presents a straightforward synthesis of two new air-stable Fe(II) spin-crossover complexes based on β-diimine ligands, namely [Fe(Mebik)2(NCS)2], 1, and
The magnetic studies reveal the occurrence of weakly cooperative spin transition

Summary

Introduction

The rational design of switchable molecular materials featuring stimuli-responsiveness is attracting strong research efforts because of the potential use as molecular sensors, switches, actuators, or memories in future molecular devices (Sauvage and Amendola, 2001; Feringa, 2011; Shepherd et al., 2013; Ferrando-Soria et al, 2017). Complexes represent an emblematic class of switchable molecular systems. In these complexes, the application of thermal variation, pressure, light-irradiation, electric, or magnetic fields can induce an electronic change between a low-spin (LS) and a high-spin (HS) state, which leads to important changes in both optical and magnetic properties (Halcrow, 2013). The Fe(II) SCO complexes are of particular interest as the electronic reorganization involves a transition between a diamagnetic low-spin state (S = 0, t62g) and a paramagnetic high-spin state (S = 2, t42ge2g). The spin-state change is accompanied by a structural reorganization, in particular with a significant lengthening of the Fe-N(ligand) distances The important structural reorganization have been shown to favor the Light-Induced Spin-State Trapping (LIESST) and photomagnetic effects are often observed in Fe(II)

Methods

Results

Conclusion