&#963;-Bonded p-Dioxolene Transition Metal Complexes (2011)

Current Trends in X-Ray Crystallography 138 form chelate rings. In addition to the stabilization of the metal complexes, hydroquinones substitution offers a direct control of the redox properties of the metal ion and increases the number of new possible structural motives by changing the number and the type of the donor atoms of the chelating group. One of the problems that someone has to face working with "non-innocent" ligands, such as hydroquinones, is the determination of their formal charge in the complex. Sometimes, physicochemical properties of the complexes, such as strong magnetic coupling between the metal ion and the organic radical, may give misleading results regarding the oxidation states. It has been shown that X-ray crystallography can be used for the determination of the oxidation states of the non innocent ligands in the complexes. For example, the C-O hydroquinonate and the C-C bond lengths of the p-dioxolene ligands are strongly dependent on the formal charge of the ligands. In this chapter we demonstrate that the rich structural chemistry of hydroquinonate complexes is predicated on a) the ability of the metal ions to reversibly deprotonate the -OH groups, b) the remote and adjacent bridge ligating modes of hydroquinone and c) the reversible metal ion -hydroquinone electron transfer which results in stabilization of the psemiquinone oxidation state. The determination of the oxidation state of the p-dioxolene ligand based of C-O and intraring bond distances is also analyzed. The application of a statistical approach for the determination of the ligand formal charges is being discussed. In addition, a graphical method for the assignment of the oxidation states has been included in this chapter. Finally, the factors that promote the stabilization of the semiquinone radical versus the hydroquinone are discussed based on the structural data. Here, we will mainly focus on the V IV/V complexes with the 2,5-bisubstituted hydroquinone with iminodiacetic acid or bis(2-methylpyridyl)amine in o-position. These are the only universally structurally characterized p-semiquinone examples in the literature up to today and the structure of the hydroquinone complexes can be directly compared with that of the p-semiquinone analogues. These compounds are oxidized from the atmospheric oxygen to form stable semiquinone radicals, trapping intermediates of dioxygen reduction that have been identified by X-ray crystallography. This is an important development towards the better understanding of the catalytic reduction mechanisms of dioxygen from metal ions in biological systems as well as in the catalytic oxidation of organic substrates from metal complexes. It is clear that -bonded hydroquinone/p-semiquinone-metal complexes have many interesting properties that have only begun to be explored or exploited (vide infra). X-ray crystallography represents a basic and irreplaceable tool in this exploration. This chapter will provide a glimpse of the fascinating structural chemistry exhibited by hydroquinones/p-semiquinones metal complexes and the utilization of X-ray crystallography into the exploration of the chemistry and the development of hydroquinones/p-semiquinones based functional bioinorganic models.