Diimines Schiff base

The cylindrical helical binuclear complex [Fe2L3]4+, where L is the bis-diimine Schiff base 1, exists as two enantiomers, whose interactions with DNA differ markedly. The more strongly interacting enantiomer targets the major groove (cf. ruthenium(II) complexes, Section II.E.l below) and induces dramatic tightening of the DNA coil (118). 

The synthesis of a variety of dibenzodiaza-crowns from salicylaldehyde or its derivatives has been reported by Lindoy and coworkers (Adam et al., 1981a, 1981b Armstrong and Lindoy, 1975 Armstrong et al., 1977 Baldwin et al., 1987 Grimsley et al., 1977). The macrocycles were obtained via the corresponding diimines (Schiff bases), which were obtained by condensations of the appropriate dialdehydes and diamines as shown (method U-14). It was... 

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. 

Activation volumes for aquation of Schiff base complexes [Fe(C5H4NCH=NHR)3] + (R = Me, Et, Pr , Bu ) in 0.1 M aqueous HCl are between - -11 cm mol and - -14cm mol v and thus within the range established earlier " for (substituted) tris-l,10-phenanthroline-iron(II) complexes. These positive values are consistent with dissociative activation, as are those for dissociation of [Fe(5Brphen)3] + and of [Fe(5N02phen)3] " " in the presence of edta. AF and values for aquation of [Fe(5Brphen)3] have the subject of isochoric analysis. " Medium effects on activation volumes have been reviewed for iron-diimine complexes in binary aqueous solvent mixtures. 

The iron(II) complex of the Schiff base-diimine (121) is mentioned briefly in a review more concerned with Cu" "-diimine complexes leading to helicates and catenanes—but which is also concerned with moving from tetrahedral Cu+ to octahedral metal centers as templates. 

The highest flexibility for a variation of the functional group and the chains X and Y (i.e. the size of the rim of the lamp shade) will be realiad when the synthesis of 3 is convergent and modular (Scheme 1). Amide bonds can easily be formed in macrocyclizations [13], therefore macrocyclic diamines 7 and diacyl dichlorides 8 had to be prepared. For the synthesis of macrocyclic diamines 7, also a large number of reactions are known. However, in this case a reduction of a macrocyclic diamide could not be achieved [11]. Therefore, another route was used the formation of macrocyclic diimines 6 (bis-Schiff bases) followed by NaBH4 reduction to the macrocyclic diamines 7. This approach has the advantage that for the construction of macrocyclic diimines 6, the metal ion template effect [14] may be exploited. 

A number of observations show the profound effect of chelation on the stability of Schiff bases and similar unsaturated nitrogen-containing ligands. Eichhorn and coworkers found that the Schiff base of bis(thiaphenal)ethylene-diimine is hydrolyzed in the presence of Cu+2, with the formation of the ethylene-diamine complex (Equation 18) (26, 28). 

Schiff bases having two nitrogen atoms as donors may be derived either from condensation of dialdehydes and diketones with two molecules of an amine, or from reaction of diamines with aldehydes or ketones. In Section 20.1.2.1, it has been pointed out that coordination through the N atom may occur only under particular circumstances. However, in the case of diimines the formation of chelate rings stabilizes the metal-nitrogen bond. Thus, they can form both mono-41 and bis-chelate42 complexes. 

A large number of polydentate Schiff base ligands have been reported, and of particular interest are the iron(II) complexes of ligands such as (42) (R = H, Ph) because they are low-spin, even though they contain only two diimine units.578 The rate of acid aquation of [Fe(42)]2+ is slow, but this depends on acid concentration. Reaction with hydroxide ion is second order, as is that with cyanide ion. The product of the last reaction is not [Fe(CN)6]4-, but [Fe(CN)4(42)]2 that has only one diimine bound to the metal. Related ligands have been prepared and much of the early work has been reviewed.54615"5464... 

Condensation of an aldehyde or ketone with a primary amine yields a Schiff base, a class of compounds that have been widely studied as chelating ligands. iV,iV -Diimine ligands, resulting from condensation with pyridine-2-aldehyde or 2-pyridyl ketones, are discussed in Section 8.2.2, and N,0-ligands are discussed here. The condensation products of... 

Reactions of Schiff bases coordinated with metal ions have been examined from the standpoint of stability and reactivity. Eichhorn (23, 25) examined the complex formed between bis-(thiaphenal)ethylene-diimine and copper(II). It was found that, when complexed to copper(II), the ligand becomes unstable and hydrolysis occurs this leads to the aldehyde and the copper-ethylenediamine complex. 

Also other active metal complexes (i.e., several Co SchifF base [232] and Mn diimine complexes [233]) have been supported in this way. The high dispersion of the complexes in the cages of the molecular sieves allows to study the redox properties of mononuclear complexes that are imstable in solution [234]. The increased stability of the obtained materials, the easier handling of heterogeneous catalysts and the high yields achieved make these supramolecular systems a very promising candidate for further catalyst development in fine chemical synthesis. 

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