Spin crossover process

Abstract This review reports on the study of the interplay between magnetic coupling and spin transition in 2,2 -bipyrimidine (bpym)-bridged iron(II) dinuclear compounds. The coexistence of both phenomena has been observed in [Fe(bpym)(NCS)2]2(bpym), [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) (bpym = 2,2 -bipyrimidine, bt = 2,2 -bithiazoline) by the action of external physical perturbations such as heat, pressure or electromagnetic radiation. The competition between magnetic exchange and spin crossover has been studied in [Fe(bpym)(NCS)2]2(bpym) at 0.63 GPa. LIESST experiments carried out on [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) at 4.2 K have shown that it is possible to generate dinuclear molecules with different spin states in this class of compounds. A special feature of the spin crossover process in the dinuclear compounds studied so far is the plateau in the spin transition curve. Up to now, it has not been possible to explore with a microscopic physical method the nature of the species... 

As mentioned above, the particular characteristics of the spin crossover process in dinuclear compounds is the appearance of a plateau in the spin transition curve. From the analysis of the results of the pressure experiments, it is inferred that the plateau results from successive ST in the two metal centres, leading first to the formation of relatively stable [HS-LS] pairs and then, above a critical pressure, to the formation of [LS-LS] pairs on further lowering of the temperature. The intermolecular interactions between [HS-LS] pairs leads to domains that contribute to the stability of the... 

The special feature of the spin crossover process in all bpym-bridged dinuclear compounds studied so far is the occurrence of a plateau in the spin transition curve. A reasonable assumption to account for this observation is that a thermal spin transition takes place successively in the two metal centres. However, it cannot be excluded that spin transition takes place simultaneously in the dinuclear units leading directly from [HS—HS] pairs to [LS-LS] pairs with decreasing temperature. Therefore, two possible conversion pathways for [HS—HS] pairs with decreasing temperature may be proposed [HS—HS]<->[HS—LS]<->[LS—LS] or [HS-HS] [LS-LS]. The differentiation of the existence of the [LS—LS], [HS—LS], and [HS—HS] spin pairs is not trivial and has recently been solved experimentally by utilisation of magnetisation versus magnetic field measurements as a macroscopic tool [9], and by Mossbauer spectroscopy in an applied magnetic field as a microscopic tool [11]. 

Figure 14 Schematic representation of solvation/spin-crossover process. The short horizontal bars in the unsolvated triplet surface denote vibrational levels. 

The magnitude of pressure effects in electronic spectra can vary over a very wide range, depending critically on both the chromophore and its environment. The most dramatic effects occur when there is a pressure-dependent phase-change whereby the nature of a chromophore may be completely changed. We present an example of pressure dependence of spin-crossover processes in an example in Section 4.1. 

The spin-crossover process can be induced thermally or optically. The physical properties of the high-spin and low-spin states are very different. In particular, the change in volume between the two states is 25. Consequently, a... 

Ikeue et have characterized temperature-dependent spin crossover processes in a series of saddle-shaped (S4) ferric porphyrins, using for this purpose proton chemical shifts of the axial ligand resonances. Curie plots showed dra-... 

For the [Fe(bpp)2]2+ system, spin transition behaviour is also observed in acetone solution. For the three salts examined, the tetrafluoroborate, iodide and hexafluorophosphate, the behaviour is virtually independent of the associated anion, unlike the situation in solid samples, and in this instance the molecular process occurs essentially independently of cooperative effects [86]. Analysis of the systems in terms of a simple low spin high spin thermal equilibrium gives AH=20 1 kj mol-1 and AS=80 4 J K-1 mol-1 for the forward process, values typical for iron(II) spin crossover systems and similar to those obtained for solid [Fe 592][BF4]2 (AH=24 kj mol-1 and AS=100 J K-1 mol-1) from differential scannning calorimetry measurements [94],... 

The progress achieved in the detailed understanding of photophysical and photochemical processes that may be induced by light-irradiation in particular spin crossover systems has driven research efforts towards the development of materials that may be used for various technological applications. Only relatively recently, reports have appeared exploring this field for Fe(III) spin crossover materials. 

The entire crossover process is found to be completed in the samarium materials at room temperature over a pressure span of close to 50 kB. This is again much as for the high-spin -> low-spin transition of Fe2+ in the dilute pyrites solid solution MnS2 Fe (47). 

By the same technique of resonance and pulsed ultrasonic perturbation Beattie et al.176) have also studied the spin crossover dynamics in aqueous solutions of [Fe(paptH)2]Cl2- Relaxation times of ca. 40 ns were measured at 25 °C, again independent of concentration and thus presumably arising from a unimolecular spin isomerization process. Some of the thermodynamic parameters of particular interest are here AH0 = 3.91 kcal mol-1, AS° = 14.8 eu, AH = 7.63 kcal mol-1, AH = 3.72 kcal mol-1,, , = 10-2S. 

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