Crossover behaviour

For five-membered heterocycles other than thiazole, (such as pyrazole [27], imidazole [28], and triazole [29]) the effect of replacement of just one pyridine moiety in 1 is greater and the [Fe N6]2+ derivatives in these instances show crossover behaviour. The [Fe N6]2+ derivative of 2-(pyridin-2-yl)imidazole 19 (Dq(Ni2+) 1150 cm-1 [22]) was shown relatively early on to be a crossover system [28]. In solid salts and in solution the transition is continuous and centred above room temperature. The dynamics for the 5T2— Ai relaxation for this system have been investigated by a number of techniques [30-32] and Beattie and McMahon have shown that in solution there is not only a spin equilibrium but also a ligand dissociation process, very reasonably ascribed to the high spin form of the tris complex [32]. 

Replacement of both pyridine rings of bipyridine by imidazole has a much greater effect than replacement by thiazole and the [Fe N6]2+ derivative of 2,2 -biimidazole 34 (Dq(Ni2+)=1080 cm-1) is purely high spin [41]. The spin-crossover behaviour of tris(2,2/-bi-imidazoline)iron(II) salts seems somewhat unexpected in light of this and the relatively low a-donor power as indicated by the small Dq(Ni2+) value (1030 cm-1) for 36. 

The incorporation of two terminal pyrazole or triazole rings into the ter-pyridine framework leads to a diversity of spin crossover behaviour not seen, for example, in the bis(thiazolyl) systems discussed above. It is likely that the presence of a non-coordinating >NH group and its involvement in hydrogen bonding gives rise to the striking effects. For a series of salts of [Fe(bpp)2]2+ (bpp is 2,6-bis(pyrazol-3- yl)pyridine 58) a marked dependence of the spin state on the anion and the extent of hydration has been observed [85-88]. 

The tris(pyrazolyl)borate and tris(pyrazolyl)methane systems represent an important class of tridentates which lead to spin crossover behaviour in iron(II) but they belong to a totally different structural category and are... 

In contrast to the phenanthroline-based systems, the similar incorporation of an azo-methine linkage into the 6-position of 2,2/-bipyridine is less effective in producing spin crossover behaviour because the higher fields produced stabilise the singlet state for iron(II). The Dq(Ni2+) values for the... 

The 1,2,4-triazole system has been found to be particularly suited towards generating spin crossover behaviour in Fe(II)N6 derivatives of the simple molecule and in bidentate and tridentate systems containing at least one... 

Structure determinations of several Fe(II) compounds of 2-triazolyl-l,10-phenanthroline and its methyl-substituted derivatives proved that in addition to the two nitrogen donor atoms of the 1,10-phenanthroline entity, the N-4 of the 1,2,4-triazole ring participates in coordination, even when a methyl substituent occupies the position adjacent to this donor atom [15, 16]. All compounds obtained exhibit Fe(II) spin crossover behaviour, its extent depending on the nature of the anionic groups and the solvent content. 

These structural features observed for [Fe(abpt)2(TCNQ)2] involving pronounced and extended n-n stacking interactions lead to a duality with respect to its gradual spin crossover behaviour. It has generally been accepted that extended n-n interactions may lead to the occurrence of thermal hysteresis in mononuclear Fe(II) spin crossover compounds [62-65]. Clearly, the requirements responsible for cooperative Fe(II) spin crossover behaviour are not easy to define, since obviously [Fe(abpt)2(TCNQ)2] represents an exception to this rule in spite of the pronounced TCNQ n-n stacking interactions, the Fe(II) spin crossover displays at best weak cooperativity. 

Fe(III) compounds exhibiting spin crossover behaviour, its extent depending on the nature of the substituents R1 and R2 indicated in Fig. 4. 

Iron(III) compounds of 3-OEt-salAPA have been widely studied [127, 128, 139-145]. Both the anion and the incorporated solvent molecule influence the spin crossover behaviour of the complex salts. Thus T1/2 for [Fe(3-OEt-salAPA)2]ClC>4 is 295 K, whereas that for the dichloromethane solvate is 152 K [140]. The transition in [Fe(3-0Et-salAPA)2]C104 and [Fe(3-OEt-salA-PA)2]BPh4 is more gradual and occurs at a somewhat higher temperature than that for the benzene solvate [Fe(3-0Et-salAPA)2]C104-C6H6 [141]. 

Dinuclear Fe(III) compounds were also obtained using substituted salten derivatives together with 4,4 -bipyridine as bridging ligand [200]. The 3-OMe-salten tetraphenylborate compound seems to show the onset of spin crossover at about 270 K, the 5-OMe-salten material is probably a purely high spin compound, whereas the 5-Cl-salten derivative exhibits gradual spin crossover behaviour. 

Nakano et al. have demonstrated that Fe(III) spin crossover complexes adsorbed on the surface of silicon dioxide retain their spin crossover behaviour [220]. EPR and 57Fe Mossbauer spectral data indicated that the spin transitions observed are similar to those of the neat solid materials used, i.e. [Fe(acpa)2]PF6, [Fe(acpa)2]BPh4 (Hacpa=N-(l-acetyl-2-propylidene)(2-pyri-dylmethyl)amine) and [Fe(bzpa)2]PF6 (Hbzpa=(l-benzoylpropen-2-yl)(2-pyridylmethyl)amine). 

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