Chapter 25
Charge Distribution in Vanadium p-(Hydro/Semi)Quinonate Complexes
Downloaded by UNIV LAVAL on May 2, 2016 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch025
Chryssoula Drouza and Anastasios D. Keramidas* Department of Chemistry, University of Cyrus, Nicosia, 1678, Cyprus *Corresponding author: email:
[email protected] The known crystal structures of co-ordination compounds containing p-dioxolene ligands in the form of hydroquinone, semiquinone or quinone have been examined. A simple method is proposed to correlate the oxidation state of these ligands with the structural distortion based on crystallographic data. The results fit well with the literature oxidation-state assignments for ligands ligated either to one or to two bridged through the ligand metal ions including the vanadium(IV/V) (hydro/semi)quinonate complexes.
o- and p- dioxolenes, catechols (Cat) and hydroquinones (Hq), and the oxidation products, o- and p- semiquinones (Sq) and quinones (Q), are important compounds in chemical and biochemical reactions such as organic electron and hydrogen transfer reactions or as strong ligators for the transfer of metal ions in biological systems (1-3). For example, electron transfer reactions between transition metal centres and p-quinone cofactors are vital for all life, occurring in key biological processes as diverse as the oxidative maintenance of biological amine levels (4), tissue (collagen and elastin) formation (5,6), photosynthesis (7,8) and aerobic (mitochondrial) respiration (9,10). The interaction of o- and p- dioxolenes with vanadium presents additional interest, because a) it serves as a model of the possible biological function of tunichromes, compounds found in tunicate blood cells (morula cells), and b) helps in understanding the mechanism of the redox reactions of vanadium(V) in biological systems (//), such as the reduction of vanadium(V), present in sea 352
© 2007 American Chemical Society
Kustin et al.; Vanadium: The Versatile Metal ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
Downloaded by UNIV LAVAL on May 2, 2016 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch025
353 water, to vanadium(III) in the blood cells of tunicates (12, 13). In addition, the semiquinonate complexes of vanadium are important intermediates in biochemical and chemical redox reactions, such as the oxidative C - H activation of aliphatic and aromatic hydrocarbons (14,15). Dioxolenes have orbitals that can be close in energy to the transition-metal d orbitals generating the opportunity for considerable covalency between the redox-active metal centre and co-ordinated redox active ligand (16,17). These metal complexes which consist of two redox-active centres, are characterized by the existence of two electronic forms (valence tautomers) with different charge distribution, and consequently, different optical, electric and magnetic properties (Figure 1). These species might interconvert to each other by a reversible intramolecular electron transfer involving the metal ion and the redox active ligand and can been used for the preparation of new molecular electronic devices (16-18). In addition /?-dioxolenes can link two metal centres together serving as redox active bridging ligands. Redox active bridges create the possibility of modulating the degree of electronic coupling between the various molecular components since the electronic energy of both bridge and its adjoining units will depend implicitly on the redox state of the bridge (19). In marked contrast to the extensive structural chemical studies for chelate stabilized
