Spin crossover effect

Abstract This review deals with spin crossover effects in small polynuclear clusters, particularly dinuclear species, and in extended network molecular materials, some of which have interpenetrated network structures. Fe(II)Fe(II) species are the main focus but Co(II)Co(II) compounds are included. The sections on dinuclear compounds include short background reviews on (i) synergism of SCO and spin-spin magnetic exchange (ii) cooperativity (memory effects) in polynuclear compounds, and (iii) the design of dinu-... 

DFT calculations have shown that in C-H hydroxylation of cyclohexane, the nonhaem oxidant (N4Py)FeIV=02+ is more reactive than P450 Cpd I and is predicted to involve multi-state reactivity with a strong solvent effect and a temperature-dependent stereoselectivity reflecting spin crossover effects.58... 

As well as for Fe complexes spin-crossover effects are observed for Fe complexes Co° complexes (3d ) and, to a lesser extent, for Cr complexes (3d ) and for Mn complexes (3d ). Spin-crossover materials could find applications for the fabrication of rewritable optical, thermal or pressure memories at a nanometre scale. 

Ksenofontov V, Caspar AB, Giitlich P (2004) Pressure Effect Studies on Spin Crossover and Valence Tautomeric Systems. 235 23-64 Ksenofontov V, see Real JA (2004) 233 167-193... 

The authors of this book consider it appropriate to include in this section two contributions from their own laboratories, one on Mossbauer spectroscopy of spin crossover (SCO) phenomena in iron(II) compounds and the other on applications to biological systems. Both chapters will demonstrate the effectiveness of Mossbauer spectroscopy in these particular fields. 

Pressure Effect Studies on Spin Crossover and Valence Tautomeric Systems... 

Substitution of one ligand by another can generate, or alter, spin crossover characteristics. The systems studied early provide the classic illustration of this effect. Thus [Fe(py)4(NCS)2] is high spin at room temperature and does not undergo a thermal spin transition. Substitution of two of the pyridine molecules by a phenanthroline molecule gives [Fe (phen)(py)2 (NCS)2] which does undergo a thermal transition [99, 141], as does the species in which the remaining two pyridines are substituted [Fe(phen)2 (NCS)2]. As would be expected, T1/2 for the former complex (106 K) is lower... 

The same effect is observed for the substituted pyridyl-pyrazole and -imidazole systems. While 2-(pyrazol-l-yl)pyridine 24 gives a low spin iron(II) complex a continuous spin transition is observed centred just above room temperature in solid salts of [Fe (31)3]2+ and just below in solution [39]. Spin crossover occurs in the [Fe N6]2+ derivative of 2-(pyridin-2-yl)benzimidazole 32 (Dq(Ni2+)=1050 cm"1) but not in that of the 6-methyl-pyridyl system 33 (Dq(Ni2+)=1000 cm"1). Although the transition in salts of [Fe 323]2+ is strongly influenced by the nature of the anion and the extent of hydration, suggesting an influence of hydrogen-bonding, in all instances it is continuous [40]. 

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 formation of diimine systems by Schiff -base-type condensation of suitable aldehydes and primary amines has been widely applied. Those reported are mostly strong field systems and their relevance to the spin crossover field is generally in systems of the kind [Fe(diimine)2(NCS)2]. The effect of the incorporation of substituents likely to hinder coordination has been studied. Robinson and Busch noted a fundamental difference at room temperature in the electronic properties of the [Fe N6]2+ derivatives of 2-pyridi-nalmethylhydrazone and 2-pyridinal-dimethylhydrazone, those of the former being low spin and those of the latter high spin [49]. The temperature-dependence of the magnetism of the latter complex was not reported but may well be of interest. However, spin crossover [Fe(diimine)3]2+ systems have been characterised for systems where the incorporation of appropriate substituents has reduced the ligand field. 

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]. 

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],... 

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 mononuclear hexakis(l-alkyl-tetrazole)iron(II) compounds with various anions have been extensively studied. It appears that the spin crossover characteristics of compounds with different alkyl substituents attached to N-1 of the tetrazole heavily depend on the crystal structure features. The transitions may be abrupt or rather gradual, complete or only involving a fraction of the Fe(II) ions, and the Tm values lie in the range 63-204 K [2c, 2f, 2g, 74-81]. Interest in these systems has focused on their suitability for detailed studies of the LIESST effect (A. Hauser, this volume). 

Most interestingly, [Fe(btzp)3](Cl04)2 is the first one-dimensional Fe(II) spin crossover compound, which shows the LIESST effect, detected in this instance by 57Fe Mossbauer spectroscopy (Fig. 19). 

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