**6. References**

134 Current Trends in X-Ray Crystallography

QIXZAY03 *Inorg.Chem.* 43 3742 2004 SABCII *Inorg.Chem.* 43 4106 2004 SABCII01 *Acta Crystallogr.,Sect.E* 61 o986 2005

KEHBIJ *Z.Naturforsch.,B:Chem.Sci.* 61 528 2006 BIBBOE *Polyhedron* 23 1629 2004 BIBCEV *Polyhedron* 23 1629 2004 QIXZAY *J.Organomet.Chem.* 623 74 2001 QIXZAY03 *Inorg.Chem.* 43 3742 2004 GETHOC *J.Organomet.Chem.* 349 95 1988 BIBCAR *Polyhedron* 23 1629 2004 BIBBUK *Polyhedron* 23 1629 2004 DIBTEP02 *Inorg.Chem.* 24 1814 1985 DIBTEP10 *J.Organomet.Chem.* 178 423 1979 FPHTEL *J.Chem.Soc.,Dalton Trans.* 2306 1980 OPNTED10 *J.Chem.Soc.,Dalton Trans.* 251 1982 DPHTEB01 *Acta Crystallogr.,Sect.E* 60 o2511 2004 CIFLEI *J.Chem.Soc.,Dalton Trans.* 869 1984 CIFLEI01 *J.Chem.Soc.,Dalton Trans.* 869 1984 YASVAP *Z.Anorg.Allg.Chem.* 619 1269 1993 YASVET *Z.Anorg.Allg.Chem.* 619 1269 1993 IDEXOE *Inorg.Chem.* 40 5169 2001 IDEXIY *Inorg.Chem.* 40 5169 2001 TUWPAC *Dalton Trans.* 973 2003 TUWPIK *Dalton Trans.* 973 2003 Table 9. Short-form references to individual CSD entries. In blue, structures measured at low

In the majority of structures having tellurium and halogens, intermolecular Te···X contacts

Despite the great chemical diversity found in these compounds, the majority of them can be grouped considering only a few supramolecular patterns, *i.e.*, dimer, simple chain and chain

Isostructurality is frequent in series of compounds with the same molecular structure but different halogen and, moreover, in some non isostructural compounds the supramolecular

No relations have been found between supramolecular arrangements of polymorphs

although due to the reduced number of cases can not ruled them out.

**Halogen series**

temperature

**4. Conclusion** 

of 4-membered rings.

arrangement is retained.

are observed.


**6** 

σ**-Bonded** *p***-Dioxolene** 

*1University of Cyprus* 

*Cyprus* 

*2Cyprus University of Technology* 

**Transition Metal Complexes** 

**O**

**O**

**H+, e-**

**H+, e-**

Anastasios D. Keramidas1, Chryssoula Drouza2 and Marios Stylianou1

Hydroquinones(HQ) are molecules of great importance in chemistry and biology. They undergo proton-coupled electron transfer to afford neutral *p*-semiquinone(SQ) and *p*-

**O**

**H+**

**.-**

**O**

**(a) HQ (b) SQ (c) Q**

Metal ions are known to lie in close proximity with these species in biological systems, thus resulting in immediate interaction. The two coupled, metal and organic redox centers have been found to participate in several biological processes such as, the oxidative maintenance of biological amine levels, (Klinman, 1996) tissue (collagen and elastin) formation, (Klinman, 1996) photosynthesis (Calvo, et al., 2000) and respiration (Iwata, et al., 1998). Although the crystal structures of many of these enzymes have been solved, the role of the metal ions in these reactions is still controversial. From another point of view, quinonoid metal complexes exhibit rich redox, magnetic and photochemical properties and thus can underpin key technological advances in the areas of energy storage, sensors, catalysis and "smart

Metal ions interact with hydroquinone systems, through σ-bonding to the oxygen atoms and/or through π-bonding to the carbocyclic ring. The structurally characterized σ-bonded hydroquinone metal complexes are surprisingly limited. Structures of metal ions with *p*semiquinones and quinones are even rarer, mainly due to the absence of a chelate coordination site in simple *p*-(hydro/semi)quinone and the low p*K* values of the semiquinone and quinone oxygen atoms. A strategy to synthesize stable metal complexes with hydroquinone species is to use substituted hydroquinones in *o*-position with substituents containing one or more donor atoms, enabling in this way the metal atom to

**H+, e-**

**H+, e-**

**1. Introduction** 

quinone(Q) species as illustrated in figure 1.

**OH**

**OH**

Fig. 1. Proton-coupled electron transfer in hydroquinone molecules

materials" (Evangelio & Ruiz-Molina, 2005; Stylianou, et al., 2008).

