**5. Conclusions**

154 Current Trends in X-Ray Crystallography

Indeed, complex **76** is being oxidized in aqueous solutions in two steps, first oxidation of the organic ligand to semiquinone (complex **75**) at pH 2.8 and then transfer of an electron from VIV to the semiquinone at pH 2.2 resulting in the formation of the hexanuclear complex **74** 

Although the bpmah2- complexes (two dinuclear oxo-bridged units) are structurally different from the respective compounds of bicah6- (one dinuclear unit), from the comparison of the reaction mechanism in figure 13 with this in figure 14, it is apparent that complex **76** is analogous to **id1**, **id3**, complex **75** to **id2**, **id4** and complex **74** to **29**, **27**. Complex **76** is stable in solution at pH 3.4, however, it is very rapidly oxidized to semiquinone radical at pH=2.8 from dioxygen. As we have predicted, the pyridine nitrogen donor atoms stabilize the oxidation state IV of vanadium. At pH 2.2 the oxophilicity of the metal increases, resulting in replacement of the chlorine with an oxo bridging group. This change of vanadium coordination environment is expected to reduce the VIV→VV redox potential and in combination with the stronger oxidative ability of semiquinone at lower pH (Baruah, et al., 2002; Drouza & Keramidas, 2008; Rath, et al., 1999) it results in electron transfer from VIV to semiquinone and the slow formation of the VIV/VV mixed valent

Hydroquinone is not a strong reducing agent and thus cannot directly reduce O2. It is certain that the interaction of the metal ion with O2 increases the one electron reduction potential of dioxygen to superoxide radical oxidizing hydroquinone to semiquinone. Superoxide is further reduced to peroxide. One of the most surprising features of the semiquinone complex **75** is its low oxophilicity that increases at lower pH, as shown by the formation of **74**. Examples in the literature show that VIV complexes with pyridine type donors are highly oxophilic, thus, it is probable that the bonding with the semiquinone radical softens the metal ion. This is important for further understanding the reactivity of vanadium at the oxidation reactions of organic substrates from the metal activated O2. However, full understanding of the properties of these radical complexes requires more *p*semiquinone complexes to be synthesized and crystallographically characterized. In detailed investigation of these mechanisms, the involvement of additional electrochemical, magnetic and spectroscopic techniques are required. However, crystallography is always the best way to start. It constitutes the first priority in the development of new bioinorganic

Fig. 14. Mechanism of the two step oxidation of **76** in aqueous solution

(figure 14).

hydroquinone complex **74**.

model compounds.

The difficulties in the synthesis of stable transition metal – hydroquinone/semiquinone/ quinone complexes have delayed the development of this chemistry relative to the phenol and catechol ones. The notion that *p*-dioxolene chemistry resembles that of phenols or *o*dioxolenes and thus the study of those molecules also covers hydroquinones is mistaken. *p*-Dioxolenes have different reactivity. They are more reactive than phenols and less reactive than *o*-dioxolenes. In addition, as it has been shown, it is a bridging ligand which provides functional polymeric materials with novel optical, redox and magnetic properties. The last ten years, the importance of these ligands in the synthesis of bioinorganic models, in the development of bioinspiring "green" catalysts and of functional materials has been recognized, resulting in an increase of the structurally characterized *p*-dioxolene transition metal complexes.

Here, we have reviewed the rich coordination chemistry of *p*-dioxolenes with transition metals found in their crystal structures, examined the structural data that can be applied for the calculation of ligand charge and understood the factors in the metal induced stabilization of *p*-semiquinone radicals. VIV is the only ion found up to now to stabilize the σ-bonded *p*-semiquinone radical. The stabilization is a result of the very strong bond between the metal and the oxygen of the dioxolene ligand. These binary metal-organic redox bioinorganic models have rich pH induced redox chemistry in aqueous solution as it has been proven from the detailed crystallographic study of the species produced at various pHs. Particular impetus for future research aimed at these molecules is provided by the established significance in O2 activation reactions.

The structural chemistry of *p*-dioxolenes transition metal complexes is to a large extent unexplored. However, the unique redox properties and structural diversity of these ligands in combination with the recent advances in novel syntheses for the stabilization of the complexes have attracted the scientific curiosity, and thus, a prosperous future for this chemistry is waiting to be seen.

### **6. Acknowledgment**

We thank the Research Promotion Foundation of Cyprus and the European Structural Funds for the financial support of this work with the proposals ΔΙΔΑΚΤΩΡ/ΔΙΣΕΚ/0308/49.

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**7** 

*Iran* 

**Structural Diversity on Copper(I)** 

*Department of Chemistry, Faculty of Science, Golestan University, Gorgan* 

Azomethines (known as Schiff bases), are perspective materials for wide spectrum of applications, particularly for anion sensor [1], antimicrobial agents [2-4] and nonlinear optical materials [5,6]. There has been considerable interest in some Schiff bases derived from salicylaldehyde because they show photochromism and thermochromism in the solid state [7]. The preparation of these compounds is simple and elegant. Since their discovery by Hugo Schiff in 1864 [8], they are prepared by condensing an active carbonyl compounds (ketone or aldehyde) with an amine, generally in refluxing alcohol [9-15]. Schiff bases are

In recent years, there has been a growing interest in the synthesis, characterization and crystal structures of copper(I) Schiff base complexes, not only because they have interesting properties and structural diversity [23-25] but also because they have found important application in catalysis for the coupling of phenylacetylene with halobenzene [26], preparation of supramolecular assemblies [27,28], the design of single and double-stranded architectures [29,30] and the grid complexes [31,32]. Then, Many efforts have been devoted to the design and synthesis of new Schiff base ligands that would be able to control the crystal structure of copper(I) complexes [33-40]. The purpose of this chapter is to present the

Schiff bases are functional groups that contain a carbon-nitrogen double bond (C=N) with the nitrogen atom connected to an aryl or alkyl group, but not hydrogen. They are of the general formula R1R2C=N-R3, where R3 is an aryl or alkyl group that makes the Schiff base a stable imine. Schiff base compounds can be synthesized from an amine and a carbonyl compound by nucleophilic addition, followed by a dehydration to generate an imine [9-15],

The basic symmetric bidentate Schiff base ligands (Scheme 1) have different R1 and R2 substituents [41-62]. Schiff bases based on aldehydes have hydrogen atom as one of the

**1. Introduction** 

**2. Schiff base ligands** 

**2.1 Bidentate schiff-bases** 

**2.1.1 Symmetric bidentate schiff bases** 

often used as ligands in inorganic chemistry [16-22].

current status of chemistry of copper(I) Schiff base complexes.

and are broadly classified as bidnetate and bis-bidentate Schiff bases.

**Schiff Base Complexes** 

Aliakbar Dehno Khalaji

