Polymeric spin-crossover systems

The aim that has motivated the present contribution is to place the spin-crossover phenomenon in the context of supramolecular chemistry. There are two reasons for doing this. Firstly, cooperativity depends on both the intermolecular and intramolecular interactions occurring in discrete mononuclear and polynuclear as well as in extended nD (n = l,2, 3) polymeric spin-crossover systems. Thus, understanding cooperativity requires knowing the factors that govern the aggregation... 

A further step consists of extending the connectivity between iron(II) metal ions by polymerizing of a molecular fragment, that could undergo spin conversion, to achieve D (n = 1-3) spin-crossover systems. 

Among all Fe(II) spin crossover compounds known to date, the extensively studied polymeric [Fe(4-R-l,2,4-triazole)3](anion)2 systems (R=amino, alkyl, hydroxyalkyl) appear to have the greatest potential for technological applications, for example in molecular electronics [1, 24, 25] or as temperature sensors [24, 26]. This arises because of their near-ideal spin crossover characteristics pronounced thermochromism, transition temperatures near room temperature, and large thermal hysteresis [1, 24, 27]. 

Solid [Fe(salacen)(l-methyl-imidazole)2]C104 displays a relatively complete, gradual spin crossover [180]. Measurements of both 57Fe Mossbauer spectra and magnetism indicate that the transition observed in the one-dimensional polymeric system [Fe(salacen)(l,l -tetramethylenediimidazole)] C104 is also gradual but incomplete at both 290 K (/ze/j=5.37 B.M.) and 4.2 K (Heff=337 B.M.) [180],... 

Despite these differences, the similarities predominate and virtually all the features noted for spin crossover in iron(II) are also found for iron(III). Because of the great emphasis on the cooperative aspects of the spin crossover phenomenon, iron(II) systems have tended to dominate more recent research. However, there are very striking examples among the iron(III) systems which are of strong relevance to these aspects and there is certainly scope for future work in this area. This is evident in much of the very recent work where it can be seen that specific strategies to increase the cooperativity have been successful and have led, for example, to solid iron(III) systems which display the LIESST effect [137, 138]. The generation of polymeric species as a means of increasing cooperativity, an approach which has been widely adopted for iron(II), has received relatively little attention for iro-n(III) and this is an area which can be expected to be exploited further. 

Concerning the understanding of the spin-crossover mechanism in polymeric systems, the results are still limited in number but give support to the idea that... 

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