Spin-crossover phenomenon structure

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

Spin-crossover phenomenon is one of the hot topics in the assembled complexes [5-20]. In the study of spin-crossover complexes, intermolecular interaction (cooperativity) is important. High-spin and low-spin states can coexist at the same temperature (bistability), depending on the cooperativity. The bistability will be stabilized by the cooperativity. In this situation, two states of bistability are interconverted by outer stimuli, bringing a function of molecular switch to the sample. In order to achieve the bistability, assembly of complexes becomes important. In this context, it is important to investigate the assembled complexes, especially from the viewpoint of spin-crossover phenomenon, that is, whether the presence of a guest molecule has a great influence on the construction of assembled structures and the spin state of the metal ions [21,22]. 

The spin-crossover phenomenon has gained renewed importance since the discovery of the LIESST for [Fe(ptz)s] (Bp4)2 (ptz = I -propyltetrazole) [ 10,11 ]. In this section, the LIESST and reverse-LIESST for [Fe(ptz)6](Bp4)2 are described. [Pe(ptz)fi](Bp4)2 has a highly symmetric rhombohedral structure (R3 Z= 3) [19]. The Pe(ll) ion, which is coordinated by six N4 atoms belonging to I -propyltetrazole (Scheme 9.1), shows the thermally induced spin transition from the LS state... 

The spin crossover (SCO) phenomenon was studied extensively over the last 50 years (26). Compounds that exhibit a transition between the LS and HS states upon changes in temperature, pressure, or irradiation with light are attractive candidates for the development of magnetic sensors and memory devices (27). Most of the SCO compounds documented in the literature are Fe(II) complexes. An examination of the coordination environment of Fe(II) ions in many of these structures, includingthe very first Fe(II) SCO complex, [Fe (phen)2(NCS)2] (272), suggests that spin transition also can be observed in compounds where CN acts as a bridge between metal ions and is N bound to the Fe(II) center. 

Multi-temperature X-ray diffraction data for a series of spin-crossover complexes differing in cooperativity indicates that the molecule and crystal volume variations upon spin conversion are similar in all the cases irrespective of the cooperative nature of the spin conversion [47]. So, a systematic structural analysis of specifically designed spin-crossover compounds should be of utmost importance to establish correlations between intermolecular interactions and cooperativity. The comparative structural study of [Fe(phen)2(NCS)2] and [Fe(btz)2(NCS)2] where btz = 2,2 -bi-4,5-dihydrothiazine (Figure 10) represents the sole example so far reported oriented in this direction [48,49]. It illustrates the dependence of the nature of the phenomenon on the efficiency of the intermolecular contacts in transmiting the intramolecular reorganization upon spin conversion. 

Here, one has to be careful because spin isomerism behaviour in solution may well be different to that in the solid state. In the solid state the individual magnetic ions couple weakly together but often sufficiently strongly for the phenomenon to have a cooperative aspect and to show hysteresis the stronger the cooperativity the more abrupt the transition. The spin-crossover may be induced not only thermally but also by application of pressure, for small structural changes accompany the spin change. 

The major part of the chapter will be devoted to the phenomenon of thermally induced spin crossover in iron complex compounds. This research topic has recently gained increasing interest by chemists and physicists for the promising potential of technical applications as devices. It wiU be demonstrated that Mossbauer spectroscopy, together with magnetic measurements, is particularly suited to follow the electronic structure dynamics of such materials under various conditions. 

---