Iron spin-crossover systems

Spin-crossover phase transition of a manganese(IU) complex [Mn(taa)] was studied by variable-temperature laser Raman spectroscopy and it was found that the vibrational contribution in the transition entropy is not dominant in contrast to the cases of ordinary iron spin-crossover systems. The discovery of a dynamic disorder in the HS phase by means of dielectric measurements provided an alternative entropy source to explain the thermally induced spin-crossover transition. This dynamic disorder was attributed to the reorienting distortion dipoles accompanying the E e Jahn-Teller effect in HS manganese(III) ions. 

In a limited number of systems which stay in the HS state down to low temperatures but sufficiently close to the crossover point, it is possible to populate the LS state as metastable state by irradiation in the near infrared. " Cooperative effects can result in a true light-induced bistability for such systems, which can persist to quite high temperatures. " In exchange-coupled binuclear iron spin-crossover systems, which are in the LS state at low temperatures, it is also possible to induce the HS state by irradiation. Although the individual iron centers are... 

Toftlund H, McGarvey JJ (2004) Iron(II) Spin Crossover Systems with Multidentate Ligands.233 151-166 Toftlund H, see Brady C (2004) 235 1-22... 

Iron(II) Spin Crossover Systems with Multidentate Ligands... 

A more subtle chemical influence is the variation of the anion associated with a cationic spin crossover system, or of the nature and degree of solvation of salts or neutral species. These variations can result in the displacement of the transition temperature, even to the extent that SCO is no longer observed, or may also cause a fundamental change in the nature of the transition, for example from abrupt to gradual. The influence of the anion was first noted for salts of [Co(trpy)2]2+ [142] and later for iron(II) in salts of [Fe(paptH)2]2+ [143] and of [Fe(pic)3]2+ [127]. For the [Fe(pic)3]2+ salts the degree of completion and steepness of the ST curve increases in the order io-dide[Pg.41]

Real (1999) Bistability in iron(II) spin crossover systems a supramolecu-lar function [243]. 

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],... 

Over the past few decades, a large variety of ligand systems have been tested with the aim of obtaining novel iron(II) spin crossover systems which could possibly be utilised in electronic devices [1]. In most cases an Fe(II)N6 chro-mophore is required in order to generate the spin crossover phenomenon [2]. A large majority of the ligands used are represented by heterocyclic systems, in which the lone electron pair on the nitrogen atom coordinates to the Fe(II) ion. 

It is expected that ligands providing a weaker ligand field than tpa will be obtained if one or more of the 2-pyridylmethyl arms are replaced by 2-pyr-idylethyl arms. However, the first ligand of such a series of expanded tripo-dal ligands (7) still forms an iron(II) spin crossover system. However, if more than one of the chelate rings are six-membered, only high-spin complexes are formed [14]. 

Iron(III) Spin Crossover Systems of Multidentate Schiff Base-Type Ligands 285... 

Real, Jose Antonio, Bistability in Iron (II) Spin-Crossover Systems A Supramolecular Function, 5, 53. 

The first examples exhibiting a temperature dependent conversion between different spin states have been found by Cambi and his school4) in the early thirties while studying trisdithiocarbamates of iron(III) with various N-substituted ligands. By now many more spin crossover systems have been discovered, particularly in the complex chemistry of iron(II), iron(III), and cobalt(II) a few examples of nickel(II) and manganese(II) compounds have been added recently. 

The present review deals with spin crossover in iron(II) complexes only. It was intended to cover the relevant literature rigorously up to early 1980. The author wishes to apologize in case one or the other important publication slipped his attention. It is planned that a similar account on spin crossover systems of other 3 d ions follows later in this series. 

Aj(Oh) transition, whereas compounds I and III behave as normal paramagnetic complexes174). Spin state lifetime measurements on these and other iron(II) spin crossover systems have been carried out by Sutin et al.17S) and Beattie et al.176) the results are given in Chap. 9. 

Some of the polycrystalline spin crossover systems of iron(II) described above retain their spin equilibrium property upon dissolution in appropriate solvents. The Evans NMR method of measuring the change of the paramagnetic shift with temperature is the most common technique to study the magnetic behaviour of such systems. The spin transition characteristics has been observed to depend on various chemical modi-... 

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