Spin crossover systems

The driving force for the spin crossover is the change in the Gibbs energy the enthalpy-unfavourable high-spin state becomes entropy-favoured above Tc. An opposite transition from the high-spin to a low-spin state with rising temperature has not been reported so far. 

---

Linert W, Grunert MC, Koudriavtsev AB (2004) Isokenetic and Isoequilibrium Relationships in Spin Crossover Systems. 235 105-136 Liu S, Edwards DS (2002) Fundamentals of Receptor-Based Diagnostic Metalloradio-pharmaceuticals. 222 259-278 Liu Y, see Arico F (2005) 249 in press Liz-Marzan L, see Mulvaney P (2003) 226 225-246 Llamas-Lorente P,see Alajan n M (2005) 250 77-106... 

Murray KS, Kepert CJ (2004) Cooperativity in Spin Crossover Systems Memory, Magnetism and Microporosity. 233 195-228... 

Smith DK, Diederich F (2000) Supramolecular Dendrimer Chemistry - A Journey Through the Branched Architecture. 210 183-227 Sorai M (2004) Heat Capacity Studies of Spin Crossover Systems. 235 153-170 Sour A, see Boillot M-L (2004) 234 261-276 Spiegel A, see Easier B (2005) 243 1-42... 

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... 

Winkler H, Chumakov AI, Trautwein AX (2004) Nuclear Resonant Forward and Nuclear Inelastic Scattering Using Synchrotron Radiation for Spin Crossover Systems. 235 105-136... 

Brady C, McGarvey JJ, McCusker JK, Toftlund H, Hendrickson DN (2004) Time-Resolved Relaxation Studies of Spin Crossover Systems in Solution. 235 1-22 Brand SC, see Haley MM (1999) 201 81-129 Bravic G,see Guionneau P (2004) 234 97-128... 

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

Isokinetic and Isoequilibrium Relationships in Spin Crossover Systems... 

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]. 

Linert and Kudryavtsev (1999) Isokinetic and isoequilibrium relationships in spin crossover systems [244]. 

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. 

In the following section, attention is directed towards these linear polynuclear Fe(II) spin crossover systems, whereas subsequent sections focus on mononuclear Fe(II) spin transition compounds containing chelating 1,2,4-triazole derivatives. 

Table 1 Thermodynamic parameters for some Fe(II) spin crossover systems in solution...

The combination of pyridine and aliphatic nitrogen donors creates an intermediate ligand field which has yielded a long list of spin crossover systems [2]. Recently, Kahn et al. obtained two new tetradentate ligands in which two 2-pyridylmethyl groups replace two hydrogen atoms on the same amine function in 1,2-diaminoethane and 1,3-diaminopropane (4, 5) [5-6]. 

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]. 

Even though many ligands of this type have been prepared in recent years, none of their Fe(II) complexes is a spin-crossover system. 

---