Schiff bases spin-crossover

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. 

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

Abstract In this chapter, selected results obtained so far on Fe(III) spin crossover compounds are summarized and discussed. Fe(III) spin transition materials of ligands containing chalcogen donor atoms are considered with emphasis on those of M,f T-disubsti-tuted-dithiocarbamates, Ar,N-disubstituted-XY-carbamates (XY=SO, SSe, SeSe), X-xan-thates (X=0, S), monothio-/J-diketonates and X-semicarbazones (X=S, Se). In addition, attention is directed to Fe(III) spin crossover systems of multidentate Schiff base-type ligands. Examples of spin inter-conversion in Fe(III) compounds induced by light irradiation are given. 

Schiff base-type systems are the second most widespread class of ligands which have been used to obtain Fe(III) spin crossover materials. These ligands may be classified according to the number of donor atoms available for coordination to the Fe(III) ion. In Sects. 3.1 to 3.4 attention is drawn to tri-, tetra-, penta- and hexadentate Schiff base-type ligands, severally. Section 3.5 focuses on spin crossover in iron(III) induced by light irradiation, whereas Sect. 3.6 is devoted to recent developments in the field of materials science with the objective of incorporation of Fe(III) spin crossover materials in devices. 

Spin crossover in iron(III) has been generated using the N4-donating macro-cyclic ligands 1,4,8,11-tetraazacyclotetradecane (cyclam) and its tetramethy-lated derivative (tmcyclam), as well as with the Schiff base system H2amben (Fig. 13). 

The use of appropriate tetradentate N202-donating Schiff base ligands (Fig. 14) together with the incorporation of the N-donating nitrosyl anion has resulted in the formation of a unique series of five-coordinate Fe(III) spin crossover materials containing FeN302 chromophores. 

In the only instances where spin crossover has been observed for a system involving a pentadentate ligand this has been an N302-donating Schiff base and the sixth coordination site has been occupied by a nitrogen donor, giving rise to an FeN402 coordination core. Most examples involve salten (Fig. 17). 

The N402 ligand systems depicted in Fig. 18 have been shown to generate spin crossover in Fe(III). Two families of Schiff base ligands have been obtained from the 1 2 condensation of triethylenetetramine with derivatives of... 

Spin-crossover iron(III)-Schiff base complexes 252... 

Dithiocarbamates and Schiff base complexes provide many good examples of spin crossovers and low-spin—high-spin equilibria. 

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