σ**-Bonded** *p***-Dioxolene Transition Metal Complexes**

Anastasios D. Keramidas1, Chryssoula Drouza2 and Marios Stylianou1 *1University of Cyprus 2Cyprus University of Technology Cyprus* 

#### **1. Introduction**

136 Current Trends in X-Ray Crystallography

Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G. & Terraneo, G. (2008). Halogen bonding in

Nishio, M.; Umezawa, Y.; Honda, K.; Tsuboyama, S. & Suezawa, H. (2009). CH/π hydrogen

Petragnani, N. & Stefani, H. A. (2005). Advances in organic tellurium chemistry. *Tetrahedron*,

Pimentel, G. C. & McClellan, A. L. (1960). *The Hydrogen Bond*, W. H. Freeman and Company,

Rowland, R. S. & Taylor, R. (1996). Intermolecular Nonbonded Contact Distances in Organic

Salonen, L. M.; Ellermann, M. & Diederich, F. (2011). Aromatic rings in chemical and

Schneider, H. J. (2009). Binding Mechanisms in Supramolecular Complexes. *Angew. Chem.* 

Shanks, D.; Engman, L.; Mayte Pach, M. & Thomas Norrby, T. (2006). Probing the

Singh, A. K. & Sharma, S. (2000). Recent developments in the ligand chemistry of tellurium.

Srivastava, P. C.; Bajpai, S.; Lath, R.; Bajpai, S. M.; Kumar, R. & Butcher, R. J. (2004).

Steigerwald, M. L. & Sprinkle, C. R. (1987). Organometallic synthesis of II-VI

Steiner, T. (2002). The Hydrogen Bond in the Solid State. *Angew. Chem. Int. Ed.*, Vol. 41(1),

Zukerman-Schpector, J. & Haiduc, I. (2001). Diorganotellurium(IV) Dihalides and Secondary

Zukerman-Schpector, J.; Haiduc, I.; Camillo, R. L.; Comasseto, J. V., Cunha, R. L. O. R. &

1433-7581

1757-1788, ISSN 1466-8033

pp. 4808-4842, ISSN 1433-7581

7201, ISSN 0002-7863

pp. 48-76, ISSN 1433-7581

*Elem.*, Vol. 171(1), pp. 73-112, ISSN 1042-6507

*Chem.*, Vol. 80(11), pp. 1530-1537, ISSN 0008-4042

Vol. 61(7), pp. 1613-1679, ISSN 0040-4020

ISBN 978-0198558835, San Francisco, USA

*J. Phys. Chem.*, Vol. 100(18), pp. 7384-7391, ISSN 1089-5639

and thiol in oil. *Lubr. Sci.*, Vol. 18(2), pp. 87-94, ISSN 0954-0075

bonding. *Polyhedron*, Vol. 23(9), pp. 1629-1639, ISSN 0277-5387

*Coord. Chem. Rev.*, Vol. 209(1), pp. 49-98, ISSN 0010-8545

 *Int. Ed.*, Vol. 48(22), pp. 3924-3977, ISSN 1433-7581

supramolecular chemistry. *Angew. Chem. Int. Ed.*, Vol. 47(33), pp. 6114-6127, ISSN

bonds in organic and organometallic chemistry. *Cryst. Eng. Comm.*, Vol. 11(9), pp.

Crystal Structures: Comparison with Distances Expected from van der Waals Radii.

biological recognition: energetics and structures. *Angew. Chem. Int. Ed.*, Vol. 50(21),

antioxidative properties of combinations of an organotellurium compound, BHT

Molecular aggregates, zig-zag 2D-stairs, -ribbons and 3D-supramolecular networks of cyclic telluranes assisted by intermolecular Te···Cl and Te···Br secondary

 semiconductors. 1. Formation and decomposition of bis(organotelluro)mercury and bis(organotelluro)cadmium compounds. *J. Am. Chem. Soc.*, Vol. 109(23), pp. 7200–

Bonding: Revisiting the Coordination Polyhedra. *Phosphorus, Sulfur, Silicon, Relat.* 

Jorge, A. (2002). Supramolecular self-assembly through tellurium···halogen secondary bonds: A hexagonal grid of Te2Cl2 and Te6Cl6 rings in the solid state structure of 1,1,3-trichloro-2,4,5,6-tetrahydro-1*H*-1λ4-benzo[b]tellurophene. *Can. J.*  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*quinone(Q) species as illustrated in figure 1.

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

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

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

σ-Bonded *p*-Dioxolene Transition Metal Complexes 139

complexation. The first hydroquinone complex characterized by crystallography has been reported 30 years ago (Heistand, et al., 1982). However only the last 10 years the number of the characterized by crystallography *p-*dioxolene complexes has increased significantly, including the first *p*-semiquinone complex in 2002 (Drouza, et al., 2002). This is in marked contrast to the extensive structural chemistry of chelate stabilized *o*-dioxolene metal complexes reported in the literature(Pierpont, 2001). This is mainly due to the absence of a chelate coordination site in simple *p*-dioxolenes and their low pKa values. The oxygen atoms of *p*-dioxolenes act as unidentate donor atoms, as shown in figure 2 for hydroquinones. Hydroquinone may ligate one metal ion or two metal ions bridged from

Substituted in *o*-position *p*-dioxolenes, with substituents containing one or more donor atoms, stabilize metal ion ligation through the formation of chelate rings. A systematic collection of the substituents reported in the literature including their transition metal complexes characterized by X-ray crystallography is illustrated in figure 3. The transition metal complexes of these ligands together with some important crystallographic data are summarized in table 1. The type of the substituent is very important because it may control the stabilization of certain metal ions defining the oxidation states of the metal ions and of

Fig. 2**.** A) Coordination modes of hydroquinones **I**) monodentate, **II**) remote bridged, **III**) adjacent bridged, **IV**) remote and one adjacent bridges, **V**) remote and two adjacent bridges **VI**) protonated monodentate, **VII**) protonated bridged, **VIII**) monoprotonated bridged, B)

Fig. 3. Substituents of hydroquinone / *p*-semiquinone / *p*-quinones used for transition metal

/

1.330(7) --- 126.7(3)

1.35(2) --- 146.5(7)

129.6(3)

*Å* C-O-M (o) Arom. Substitution Ref.

R1=G2 R2=R3=R4=H

138.5(8) R1=R2=R3=R4=H (Errington, et

(Sanmartin, et al., 2004)

al., 2007)

two different or from the same oxygen donor atoms (figure 2).

the *p*-dioxolenes, as well the structure of the molecule.

Labeling of the M-O and C-O bonds

ion ligation

Comp Metal η

1a FeIII 1.903(4)

2a TiIII 1.828(8)

/

1.864(4)

1.775(8)

*Å* δ

/

1.374(8) 1.372(8)

1.38(3) 1.38(2)

*Å* ε

/

1.316(6)

1.34 (1)

*Å* ζ

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-Ohydroquinonate 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 *p*semiquinone 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 VIV/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 σ-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.

### **2. Structural studies of hydroquinonate/p-semiquinonate/p-quinone transition metal complexes**

Structural investigation has proven to be an essential tool for the characterization of *p*dioxolene complexes. Metal-oxygen bond lengths are often characteristic of a particular oxidation state of the metal, and the *p*-dioxolene carbon-oxygen lengths are sensitive to the charge of the ligand. Apart from providing indirect information on the charge distribution within the complex, crystallographic studies have revealed the donor-acceptor tendency for

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-Ohydroquinonate and the C-C bond lengths of the

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 *p*semiquinone 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 VIV/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

It is clear that σ-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

Structural investigation has proven to be an essential tool for the characterization of *p*dioxolene complexes. Metal-oxygen bond lengths are often characteristic of a particular oxidation state of the metal, and the *p*-dioxolene carbon-oxygen lengths are sensitive to the charge of the ligand. Apart from providing indirect information on the charge distribution within the complex, crystallographic studies have revealed the donor-acceptor tendency for

hydroquinones/*p*-semiquinones based functional bioinorganic models.

**2. Structural studies of hydroquinonate/p-semiquinonate/p-quinone** 

*p*-dioxolene ligands are strongly dependent on the formal charge of the ligands.

complexes.

**transition metal complexes** 

complexation. The first hydroquinone complex characterized by crystallography has been reported 30 years ago (Heistand, et al., 1982). However only the last 10 years the number of the characterized by crystallography *p-*dioxolene complexes has increased significantly, including the first *p*-semiquinone complex in 2002 (Drouza, et al., 2002). This is in marked contrast to the extensive structural chemistry of chelate stabilized *o*-dioxolene metal complexes reported in the literature(Pierpont, 2001). This is mainly due to the absence of a chelate coordination site in simple *p*-dioxolenes and their low pKa values. The oxygen atoms of *p*-dioxolenes act as unidentate donor atoms, as shown in figure 2 for hydroquinones. Hydroquinone may ligate one metal ion or two metal ions bridged from two different or from the same oxygen donor atoms (figure 2).

Substituted in *o*-position *p*-dioxolenes, with substituents containing one or more donor atoms, stabilize metal ion ligation through the formation of chelate rings. A systematic collection of the substituents reported in the literature including their transition metal complexes characterized by X-ray crystallography is illustrated in figure 3. The transition metal complexes of these ligands together with some important crystallographic data are summarized in table 1. The type of the substituent is very important because it may control the stabilization of certain metal ions defining the oxidation states of the metal ions and of the *p*-dioxolenes, as well the structure of the molecule.

Fig. 2**.** A) Coordination modes of hydroquinones **I**) monodentate, **II**) remote bridged, **III**) adjacent bridged, **IV**) remote and one adjacent bridges, **V**) remote and two adjacent bridges **VI**) protonated monodentate, **VII**) protonated bridged, **VIII**) monoprotonated bridged, B) Labeling of the M-O and C-O bonds

Fig. 3. Substituents of hydroquinone / *p*-semiquinone / *p*-quinones used for transition metal ion ligation


σ-Bonded *p*-Dioxolene Transition Metal Complexes 141

1.865(2) 1.878(2)

1.880(2) 1.879(2)

1.8512(2) 1.913(2)

1.882(4) 1.874(6)

1.937(8) 1.935(8) 1.92(1)

2.464(2)

41b MoVI 1.922(8) 1.35(1) 1.35(1) 1.922(8) 136.1(7) R1=R2=R3=R4=H (Ung, et al.,

42b WVI 1.93(1) 1.36(2) 1.36(2) 1.927(1) 137(1) R1=R2=R3=R4=H (McQuillan,

45b FeIII 1.874(8) 1.27(1) 1.27(1) 1.874(8) 169.4(7) R1=R2=R3=R4=Cl (Rheingold &

1.914(8) 1.953(8)

1.31(1) --- 121.1(5),

2.030 (3)

51c TiIV 2.048(3), 1.373(7) 1.371(6) --- 126.2(3), R1=G12 (Vaid, et al.,

1.527(6) 1.347(4) --- 132.8(2),

132.3(1)

137.8(2), 132.3(2) 137.2(2), 135.9(2)

138.1(2), 136.4(2) 134.8(2), 131.8(2)

134.3(2), 134.8(2)

137.4(4),

137.3(9), 142.3(9) 127.5(9),

127.2(2)

111.7(2), 112.1(2)

140.9(8),

127.6(5),

123.2(2), 1233(2)

121.7(2)

1203(2)

R1=R2=G1 R3=R4=H

R1=R2=G1 R3=R4=H

R1=R2=G1 R3=R4=H

137.4(4) R1=R2=R3=R4=H (Vaid, et al.,

144.8(1) R1=R2=R3=R4=H (Evans, et al.,

1.429(3) R1=R2=R3=R4=H (Tanski, et al.,

R1=G15 R2=R3=R4=H

169.6(2) R1=R2=R3=R4=H (Horacek, et

155.2(2) R1=R2=R3=R4=H (Kunzel, et

R1=R2= G1, R3=R4=H

R3=R4=H

R3=R4=H

135.6(8) R1=R2=R3=R4=H (Ung, et al.,

R1= G4 R2=R3=R4=H

> R1=R2=G6 R3=R4=H

R1=G22 R2=R3=R4=H

R1=G12 R2-R3=R4=H

R1=R2=R3=R4=H (McQuillan,

129.9(8) R1=R2=R3=R4=H

2008)

(Drouza & Keramidas, 2008)

(Drouza & Keramidas, 2008)

(Drouza & Keramidas, 2008)

(Tanski & Wolczanski, 2001)

1997)

1998)

2000)

et al., 1998)

(Kretz, et al., 2006)

al., 2010)

al., 1996)

(Stylianou, et al., 2008)

(Caldwell, et al., 2008)

1996)

et al., 1996)

(Kretz, et al., 2006)

Miller, 2003)

1999)

(Zharkouskay a, et al., 2005)

(Rosi, et al., 2005)

(Song, et al., 2007)

(Sreenivasulu , et al., 2006)

137.0(1),

1.335(3) 1.327(3)

1.308(3) 1.345(4)

1.350(4) 1.303(4)

1.373(8) 1.36(1)

1.33(2) 1.38 (2) 1.34(2)

1.867(2) 1.349(4) 1.348(4) 1.867(2) 165.3(2),

1.86(4) 1.353(3) 1.353(3) 1.864(4) 155.2(2),

40b PdII 1.981(2) 1.341(4) 1.341(4) 1.981(2) 118.3(2) R1=R2= G7,

43b CuII 1.880(3) 1.337(5) 1.337(5) 1.880(3) 127.2(2) R1=R2= G15,

1.38(2) 1.35(2)

1.32(1)

1.930(3) 1.364(3) 1.371(4) --- 134.4(2),

2.030 (3) 1.347(5) 1.347(5) 2..021(3),

28b VV 1.827(2)

30b VIV/V 1.884(2)

32b TiIV 1.882(4)

37b TiIII 1.865(2),

38b TiIII 1.864(4),

39b CuII 2.370(3),

46b MoV 1.948(9)

47c CuII 1.889(7),

48c ZnII 2..021(3),

49c CuII 1.924(2),

50c CuII 1.934(2),

1.954(8)

2.326(7)

1.980(2)

29b VIV/V

35b MoIV

1.823(2)

1.937(2)

1.898(3)

1.915(6)

1.924(8) 1.974(8) 1.924(8)

1.879(2) 1.314(3)

1.346(3) 1.325(3)

1.350(4)

1.338(4) 1.302(4)

1.373(8) 1.37(1)

1.34(2) 1.37 (2) 1.33 (2)

1.36 (2) 1.36 (2)

1.40(2) 1.36(2)

31b VIII 1.877(9) 1.38(2) 1.34(2) 1.886(7) 134.1(9),

33b ZrIV 1.978(2) 1.357(3) 1.357(3) 1.978(2) 144.8(1),

34b TiIII 1.870(3), 1.360(7) 1.369(7) 1.898(4) 148.5(3),

36b CuII 1.880(3) 1.337(5) 1.337(5) 1.88(3) 127.2(2),

2.464(2) 1.386(4) 1.380(4) 2.370(3),


1.322(4) --- 127.8 (2)

1.325(7) --- 127.3(3),

1.35(5) --- 119.8(8),

1.348(5) --- 137.8(2),

1.878(3) 1.865(3)

1.91(2) 1.916(2)

1.951(3) 1.952(3)

1.824(2) 1.866(2) R2=R3=R4=H

R2=R3=R4=H

R1=G10 R2=R3=R4=H

> R1=G10 R3= tBu R2=R4=H

R2=R3=R4=H

R2=R3=R4=H

R1= G17, R2=R3=R4=H

R1= G10, R2=R3=R4=H

R2=R3=R4=H

R1= G7, R2=R3=R4=H

R1= G6, R2=R3=R4=H

R3=R4=H

R1=R2=G1 R3=R4=H

R1=R2=G3 R3=R4=H

R1=R2=G5 R3=R4=H

R3=R4=H

R1=R2=G8 R3=R4=H

R1=R2=G3 R3=R4=H

R2=R3=H

R3=R4=Cl

R1=R2=G1 R3=R4=H

R1=R2=G1 R3=R4=H

165.1 (4) R1=R4=Me

133.3(5) R1=R2=R3=R4=H (Stobie, et al.,

132.13(3) R1=R2=R3=R4=H (Heistand, et

157(3), R1=R2=R3=R4=H (Vaid, et al.,

127.2(1)

127.6(2)

126.4(2)

127.4(3)

118.9(8)

133.5(2)

137.0(1),

131.9(3) 131.5(3)

126.8(1)

123.9(2) 122.6(2)

115.9(2)

121.4(1)

165.1 (4)

128.2(3), 128.2(3)

132.3(1), 137.0(1)

137.0(1) R1=R2=G1

(Sembiring, et al., 1999)

> (He, et al., 2003)

(Margraf, et al., 2006)

(Margraf, et al., 2006)

(Berthon, et al., 1992)

(Huang, et al., 2008)

(Becker, et al., 2010)

(Kondo, et al., 2003)

> (Li, et al., 2000)

(Sembiring, et al., 1992)

(Litos, et al., 2006)

2001)

(Drouza, et al., 2002)

(Drouza, et al., 2002)

2003)

(Dinnebier, et al., 2002)

al., 1982)

(Margraf, et al., 2009)

(Kumbhakar, et al., 2008)

(Kumbhakar, et al., 2008)

(Margraf, et al., 2006)

(Arévalo, et al., 2003)

(Brandon, et al., 1998)

(Drouza & Keramidas, 2008)

(Drouza & Keramidas,

3a PtII 2.030(3) 1.380(6) 1.334(5) --- 130.3 (3) R1=G7

4a CuII 1.900(4) 1.386(7) 1.327(7) --- 122.0(3) R1=G9

1.321(5)

7a PdII 1.940(7) 1.39(1) 1.34(1) --- 124.5(5) R1=G11

8a CrIII 1.924(2) 1.391(3) 1.362(3) --- 120.9(1) R1=G1

1.920(2) 1.379(4) 1.343(4) --- 127.7(2),

1.332(7)

1.37(2)

1.353(5)

1.352(6) 1.353(5)

1.322(3) 1.327(3)

21b RuIII 1.975(7) 1.38(1) 1.34 (1) 1.966(5) 115.9 (6) R1=R2=G8

25b MnIII 2.193(4) 1.253(7) 1.253(7) 2.193(4) 180.000 R1=R2=

1.364(5) 1.352(5)

1.338(3) 1.338(3)

11a CuII 1.870(4) 1.384(9) 1.321(7) --- 126.0(4) R1= G18,

5a CuII,I 1.924(1) 1.381(3) 1.329(2) --- 126.4(1)

1.397(5) 1.386(5)

1.386(9) 1.393(8)

1.38(2) 1.36(2)

1.381(6) 1.359(5)

1.353(6) 1.353(5)

1.322(3) 1.327(3)

24b TiII 1.785(5) 1.360(8) 1.360(8) 1.785(5)

1.364(5) 1.352(5)

1.346(3) 1.346(3)

14a WVI 1.88(3) 1.36(4) 1.36(4) --- 166(2),

1.887(4) 1.322(6) 1.322(6) ---

17b WV 1.948(6) 1.362(9) 1.362(9) 1.948(6) 133.3(5),

18b CuII 1.803(3) 1.300(2) 1.300(2) 1.803(3) 126.8(1),

19b FeIII 1.862(1) 1.3492(4) 1.3492(4) 1.8616(1) 132.13(3),

22b RuIII 1.983(2) 1.346(4) 1.346(4) 1.976(2) 118.4(2),

23b CuII 1.915(1) 1.322(2) 1.322(2) 1.915(1) 121.4(1),

6a NiII 1.854(2)

9a FeIII 1.927(2),

10a NiII 1.827(4)

12a NiII 1.90(1)

13a MoVI 1.945(2)

15b VIV 1.887(4)

16b VV 1.878(3)

20b CuI 1.91(2)

26b VIV 1.951(3)

27b VV 1.866(2)

1.952(3)

1.824(2)

1.860(3)

1.827(4)

1.87(1)

1.955(2)

1.865(3)

1.916(2)


σ-Bonded *p*-Dioxolene Transition Metal Complexes 143

1.908(2)

2.343(6)

1.99(2)

2.514(4)

70a ZnII 2.222(6) 1.233(9) 1.242(9) --- 135.7(5) R1= G23, R2= R3=

71b RhII 2.25(1) 1.24(2) 1.24(2) 2.25(1) 136.8(8) R1= R2= CH3, R3=

73a MoII 2.569(6) 1.21(1) 1.24(1) --- 140.6(5) R1= R3= tBu, R2=

mode VI, g mode VII, h mode VIII according to figure 2

Table 1. Summary of structurally characterized hydroquinone/*p*-semiquinone/*p*-quinone transition metal complexes. Some important crystallographic data are also included. Abbreviations are according to figures 2 and 3. a Mode I, b mode II, c mode III, d mode IV, e

The first crystallographic report on a transition metal hydroquinone appeared in 1982 (Heistand, et al., 1982). Heistand et.al reported the structure of a binuclear iron(III) complex containing two iron-salen units bridged together with a simple deprotonated hydroquinone (coordination mode **II**, figure 4). Although the C-Ohydroquinonate bond length [1.349(3) Å] is shorter than the C-O bond of free hydroquinone (1.398 Å), it is within the limits expected for this *p-*dioxolene's oxidation state. It is worth noticing here that the respective catecholate complex found in the crystal structure is ligated with FeIII in a monodentate fashion, in contrast to the hydroquinone complex which even in 50 fold excess of [Fe(salen)]+ crystallizes as dimer. Heistand et al. have assigned the formation of the dimer to the crystallization process. However, the fist coordination of FeIII to *p*-hydroquinone enhances the acidity of the second OH, and this may account for the stabilization of the dinuclear complex. In contrast, the intra molecular H-bond stabilization in catechols reduces the acidity of the second OH favoring the formation of the mononuclear complex. Nevertheless,

this is the first example showing that hydroquinone can function as bridging ligand.

reported with simple hydroquinone to bridge two Zr(acac)3

Other examples of dinuclear complexes following a mode II coordination have been

FeIII(5,10,15,20-tetraphenylporphyrinato)+ (**45**) (Rheingold & Miller, 2003), or TiIVCl(CP\*)2

(**38**) (Kunzel, et al., 1996), or TiIII(CP\*)2 (**37**) (Horacek, et al., 2010) or WVOCl[hydrogen

173.0(3), 125.9(3)

128.76(6), 125.70(6), 128.76(6), 125.70(6),

116.2(5), 124.1(5),

130(1)

125(1), 129(1)

124.6(3), 113.6(3)

141.7(8)

1999)

1999)

(Phan, et al., 2011)

(Phan, et al., 2011)

(Dietzel, et al., 2008)

(Dietzel, et al., 2008)

(Kretz, et al., 2006)

(Senge, et al., 1999)

(Handa, et al., 1996)

(Handa, et al., 1995)

(Handa, et al., 1995)

129.3(3) R1=R2=R3=R4=H (Vaid, et al.,

R1= R2= G14, R3= R4= H

R1= R2= G14, R3= R4= H

> R1= R2= G6, R3=R4=H

> R1= R2= G6, R3=R4=H

R1= R2= G14, R3=R4=H

CH3, R4= H

R4= H

R1= R3= CH3, R2= R4= H

R4= H

+ (**33**) (Evans, et al., 1998), or

+

1.782(3) 1.387(6) 1.361(6) 2.208(4) 125.9(3)

b 1.794(4) 1.376(7) 1.351(6) 1.923(3) 164.1(5),

1.908(2) 1.403(8) 1.4025(8) 1.921(2),

2.338(6) 1.36(1) 1.34(1) 1.920(7),

1.99(2) 1.38(3) 1.38(3) 1.96(1),

2.514(4) 1.350(6) 1.350(6) 1.922(4),

72b MoII 2.619(9) 1.28() 1.28(2) 2.594(9) 152.7(8),

67h ZnII 2.00(2) 1.36(3) 1.36(3) 2.00(2) 128(1),

64h CuII 1.921(2),

66h CuII 1.931(7),

68h ZnII 1.96(1),

69h CuII 1.922(4),

mode V, f

**2.1 Simple hydroquinones** 


1.38(1) 1.32(2) --- 130.1(5),

1.35(1) --- 126.8(5)


1.782(3) 1.387(6) 1.361(6) 2.208(4) 173.0(3),

1.33(1)

1.343(9) 1.327(8)

2.463(2) 1.374(4) 1.345(3) --- 118.0(2),

1.36(1), 1.35(1) ---

1.934(6) 1.389(9) 1.350(8) --- 128.1(5),

2.13(1) 1.40(4) 1.36(3) --- 118(1),

1.955(7) 1.38(1) 1.32(1) --- 130.1(5),

2.014(7) 1.38(2) 1.37(2) 1.78(1) 126.6(7),

1.382) 1.37(2) 1.78(1) 126.6(7),

1.38(2) 1.39(2) 1.80(1) 147.0(7),

1.33(2) 1.33(2) 1.854(7) 134.2(8),

1.38(2) 1.39(2) 1.80(1) 145(1)

60e CuII 2.375(1) 1.382(2) 1.376(2) --- 109.22(9) R1=G1 (Zhang, et al.,

1.218(5) --- 106.7(2),

g 1.782(3) 1.387(6) 1.361(6) 2.208(4) 173.0(3), R1=R2=R3=R4=H (Vaid, et al.,

b 1.794(4) 1.376(6) 1.351(6) 1.923(3) 164.1(3),

1.37(1) 1.39(1)

1.377(9) 1.368(8)

1.39(1), 1.37(1)

b 1.836(8), 1.35(2) 1.37(2) 1.814(7)

b 1.814(7) 1.37(1) 1.35(1) 1.836(8) 127.8(7),

b 1.807(9) 1.38(2) 1.38(2) 1.807(9) 146.8(2)

1.217(4)

62f CuII 2.359(2) 1.382(3) 1.382(3) --- 108.6(1) R1= R2= G1,

63c TiIV 2.048(3) 1.373(7) 1.371(6) --- 126.8(3) R1=G12

1.216(5) 1.219(4)

g

52c CuII

55c CuII

53a ZnII 1.946(5)

<sup>c</sup>2.046(5)

54c CuII 1.893(2),

56c CuII 1.971(6),

57c PdII 2.02(2),

58c CuII 1.971(6),

59d TiIV 2.07(1),

<sup>d</sup>2.014(7),

<sup>d</sup>2.019(8),

<sup>b</sup>1.854(7),

<sup>d</sup>2.08(1),

61e CuII 2.653(3),

2.07(1)

2.08(1)

1.80(1)

2.019(8)

2.547(3)

1.971(6), 1.955(7) 1.922(7), 1.929(7)

1.962(6)

2.043(5)

1.936(6), 1.978(6), 2.358(6), 1.945(6)

2.043(3) 126.8(3) R2-R3=R4=H 1997)

130.1(6)

123.8(5)

126.1(5), 127.4(5) 124.0(5), 128.8(5)

126.8(2)

129.9(6), 119.2(5), 108(5), 128.8(5)

132.1(6)

122(1)

130.1(6)

126(1), 153(1)

126(1), 153(1)

140.5(7)

124.5(9), 145(1)

112.1(2)

125.9(3) R1=R2=R3=R4=H (Vaid, et al.,

129.3(3) R1=R2=R3=R4=H (Vaid, et al.,

R1=G14 R2=R3=R4=H

R1=G2 R2=R3=R4=H

R1=G2 R2=R3=R4=H

R1= G19, R2= R3=R4=H

R1= G1, R2= R3=R4=H

R1= G18, R2=R3=R4=H

R1= G7, R2=R3=R4=H

R1=R3= G14, R2=R4=H

R1=tBu R3=G9 G2=G4=H

R3=R4=H

R2-R3=R4=H

R1=R2=R3=R4=H (Vaid, et al.,

1997)

1997)

(Gelling, et al., 1990)

(Matalobos, et al., 2004)

(Matalobos, et al., 2004)

(Matalobos, et al., 2004)

(Stylianou, et al., 2008)

> (Li, et al., 2000)

(Sembiring, et al., 1995)

> (Gelling, et al., 1990)

> > 1999)

2009)

(Philibert, et al., 2003)

(Stylianou, et al., 2008)

(Vaid, et al., 1999)


Table 1. Summary of structurally characterized hydroquinone/*p*-semiquinone/*p*-quinone transition metal complexes. Some important crystallographic data are also included. Abbreviations are according to figures 2 and 3. a Mode I, b mode II, c mode III, d mode IV, e mode V, f mode VI, g mode VII, h mode VIII according to figure 2
