INTRODUCTION:

Metal complexes with fullerenes attract attention due to the prospects of their application in catalysis, as the materials for nonlinear optics, for designing artificial photosynthesis systems, and in the development of supra- and nanomaterials. These compounds can be divided into the following three classes: fullerene salts with alkali metals, endohedral compounds of fullerenes with metals, and exohedral complexes of fullerenes with transition metals in which fullerene acts as the ligand. Consideration of the electrochemical properties of the latter is the main subject of tshe present chapter. The wide variety of exohedral fullerene complexes with transition metals is determined by the structural and electronic properties of fullerene ligands such as their polyhedral structure, steric capacity, the existence of a large number of coordination sites and a wide diversity of possible coordination modes to metal atoms and also the considerable electron- withdrawing ability of fullerenes. According to quantum chemical calculations, fullerene should be first of all considered as conjugated electron-withdrawing polyolefins [1]. Figure 1 shows the possible modes of fullerene coordination to the metal atom the majority of which have been realized to date [2].

RESULTS AND DISCUSSION:

Studies of electrochemical properties of exohedral complexes of fullerenes provided quantitative data characterizing their susceptibility to redox transitions.

In so far as the redox potential variations with the structure of a complex may be decisive for the appearance of electrophysical and optical properties [3, 4], the electrochemical data play the key role in solving the problem of the use of complexes as the catalysts and also in the development of materials for nonlinear optics and materials with electron-conducting properties.

The literature contains reviews on the electrochemistry of various organic derivatives of fullerenes [5–7]; however, the data on electrochemistry of metal complexes with fullerenes were not summarized. Some reviews [8, 9] provide certain information on this subject. This is why the present study is aimed at consideration of electrochemical properties of metal complexes with fullerenes С60 and С70 as a function of the nature, composition and structure of the metal containing fragment and also of the mode of metal atom coordination with the fullerene ligand.

Electrochemical Properties of C60 and C70

For studying the electrochemical properties of fullerenes and their derivatives including their complexes with metals, the method of cyclic voltammetry (CVA) on solid electrodes such as glassy carbon and platinum was widely used. Solvents such as tetrahydrofuran, methylene chloride, o dichlorobenzene or solvent mixtures (toluene/acetonitrile, toluene/DMFA, etc.) in different ratios proved to be optimal. As the standard electrode system to which the redox potentials shown in different studies were related, the ferrocene/ferrocenium (Fc/Fc+) system was used, which made is possible to compare the potential values shown in different studies.

The number of electrons transferred per molecule in each reduction stage was assessed using the first peak of oneelectron reduction of fullerene or the height of the one-electron peak of ferrocene oxidation as the reference. Electrochemical studies have shown that fullerenes exhibit electron withdrawing properties and are capable of stepwise addition of six electrons to form fulleride anions from Cn- to Cn6-, (for n = 60, 70) [10]. This agrees with their electronic structure according to which fullerenes have three degenerate lowest unoccupied molecular orbitals (LUMO).

Complex of VIII Group metal with C60 and C70

π-Complexes coordinated as olefins in the η2-mode according to which a metal atom is coordinated to one of –C=C bonds of fullerene were synthesized for VI–VIII group transition metals. The other ligands bound to the metal atom were phosphines, phosphites, isonitriles, CO, dienes, hydride hydrogen, and substituted pyridines. Fullerene complexes with VIII-group metals exhibited the highest stability. This and also the fact that transition metal complexes were the first complexes with fullerene ligands explains the observed predomination of electrochemical studies devoted to these complexes.

Nickel, palladium, and platinum complexes.

The electrochemical properties of Ni, Pd, and Pt complexes with fullerenes were studied most thoroughly and systematically; this is why the analysis of these results allows one to elucidate the dependence of the electronic structure of complexes on such factors as the nature of metal and the metal-bound ligands, the number of added metallofragments, etc. Complexes (η2-С60)ML2 (M = Pt, Pd; L = mono- or bidentate phosphorus-containing ligands) were synthesized by either the substitution of ligands in metal complexes or in the reaction of a fullerene with metallocarbinoid complexes of metals [12].The electrochemical reduction of phosphine complexes of nickel, palladium, and platinum, namely, (Ph3P)2Pt(η2-C60), (Et3P)2M(η2-C60), [(Et3P)2M]2(η2-C60) (M = Ni, Pd, Pt), [(Et3P)2Pt]n(η2-C60) (n = 1–4) was studied in a THF solution on a Pt electrode [13] or in a mixture of solvents toluene-acetonitrile [14]. The oxidation peaks of fullerene anions were also observed in CVA curves at reverse potential scanning (Figure 2).

It should be noted that for organic derivatives of fullerene with electron-donating substituents, the negative shift of reduction potentials and the lower stability of formed anions were observed [7]. Studying the oxidation of complexes containing several metallofragment (Et3P)2 Pt groups, the CVA curve (Figure 3) revealed oxidation peaks each corresponding to the successive cleavage of a metal fullerene bond in the two-electron oxidation [(Et3P)2 Pt]n(η2-C60) [(Et3P)2Pt]n– 1(η2-C60) + C60 + [(Et3P)2Pt]2+ + 2е.

Figure 2 CVA curves (Pt electrode, THF, 0.2 M Bu4NPF6, с = 0.5 mM; v = 200 mV/s): (a) (Ph3P)2Pt(η 2-C60), (b) C60 [13].с) was also observed. In contrast to the reduction potentials of complexes, the rate of metalofullerene bond rupture depended on the nature of the metal (decreased in the sequence Ni > Pd > Pt) and the phosphorusbound radical (Ph > Et for dianions formed by platinum complexes) and also on the charge of the formed complex anion: (Ph3P)2Pt(C60)3– > (Ph3P)2Pt(C60)2– > (Ph3P)2Pt(C60)– [13]. It should be noted that for organic derivatives of fullerene with electron-donating substituents, the negative shift of reduction potentials and the lower

stability of formed anions were observed [7]. Studying the oxidation of complexes containing several metallofragment (Et3P)2 Pt groups, the CVA curve (Figure 3) revealed oxidation peaks each corresponding to the successive cleavage of a metal fullerene bond in the two-electron oxidation

[(Et3P)2 Pt]n(η2-C60) [(Et3P)2Pt]n – 1(η2-C60) + C60 + [(Et3P)2Pt]2+ + 2е.

All studied fullerene complexes were oxidized easier as compared with fullerene; moreover, the stepwise addition of each metal-containing group made the oxidation easier, shifting linearly the oxidation potentials in the negative direction by 0.18–0.25 V . It is significant that in contrast to the reduction , the oxidation potentials depended on the metal nature and the susceptibility to oxidation varied in the sequence Ni > Pd > Pt. The oxidation potentials of phosphine complexes of nickel, palladium, and platinum with another η2-coordinated ligand, namely, methylmethacrylate, varied in the same sequence. This provided the grounds to assume [13, 15] that metal d-orbitals made the main contribution to the highest occupied molecular orbital (HOMO) of complexes.The cathodic shift of oxidation potentials and the irreversible two-electron oxidation were also observed for palladium complexes (η2- Cn)Pd[(–)Ме4BITIOP], where (–)Ме4BITIOP was a chelating enantiomeric ligand 2,2',5,5'tetramethyl-4,4'-bis (diphenylphosphino)-3,3'-bithienyl [16]. Fullerene complexes of palladium with metallocenyl radicals (η2-Cn) PdL1L2 [17, 18] demonstrated the same relationships as the fullerene complexes of platinum group metals containing PPh3 and PEt3 [13] . Semi-empirical computations of the energy of boundary orbitals by the ZINDO/1 method [18] have shown that for all complexes, the LUMO is localized on the fullerene and the palladium fragment makes virtually no contribution into this orbital.

Complexes of other VIII-group metals

Namely, Co, Rh, Ir, Ru and Os. shows the potentials of the electrochemical oxidation and reduction peaks for a series of VIII-group metals (Os, Ir, Rh) with fullerenes of the common composition С60[MLn] and С70[MLn]. The peaks of reduction of complexes to anions shifted to negative potentials as compared with the fullerene reduction peaks , which points to the partial transfer of electron density from the metal containing fragment to the fullerene ligand at complexation.

Complex of VI and VII Group metal with C60 and C70

The electrochemical properties of fullerene complexes with the VI-group metals. The electrochemical reduction of ((η2-C60) M (CO)2(phen)(dbm) complexes (M = Mo, W; dbm = dibutylmaleate, phen = 1,10- phenanthroline) [24, 25] included three reversible one-electron stages corresponding to their successive reduction to mono-, di-, and trianions. The fourth peak in CVA curves was two-electron and corresponded to the ECE mechanism, as illustrated by the following scheme (M = Mo (CO)2(phen)(dbm)): [(η2-C60) M]3– + e [(η2-C60)M]4– [M]– + [C60]3– +e [C60]4–.

The cleavage of the metal–fullerene ligand bond occurred with the addition of the fourth electron, which points to the involvement of the metallofragment in this reduction stage, where M = Mo, W, to C60 shifted the first three reduction potentials in the cathodic region by 0.13– 0.17 V as compared with fullerene.

The substitution of phosphine ligands for phenanthroline and dibutylmaleate [26] did not induce any substantial shifts of reduction potentials. However, the substitution of ligands led to a decrease in the stability of anions of complexes 9 and 10 as compared anions of complexes 3 and 6, because the former complexes undergo demetallation with the addition of the third electron[19–21].

Electrochemical Properties of complex of metal cluster

A fullerene can be bound with metal clusters by different modes, namely, η2-С60, -η2:η2-С60 and μ3-η2:η2:η2-С60 in which fullerene acts as the π- or σ–π-bound ligand (Figure 5).

Modification of the coordination sphere of a cluster can lead to the formation of new forms of С60 ligand coordination, namely, of the π- and σ types (μ3-η1:η1:η2-С60 and μ3-η1:η2:η1-С60). The electrochemical reduction of complexes formed by triosmium clusters with a fullerene ligand, where only one osmium atom was coordinated to the fullerene in the η2 mode (Os3 (CO) 11(η2-C60), Os3(CO)10(PPh3)(η2-C60), and Os3(CO)9(PPh3)2(η2- C60), were carried out on a platinum electrode in a dichloromethane–toluene mixture with the ratio 4 : 1 [28]. Figure 5 Structure of fullerene complexes with Rhenium clusters [31]. It was shown that the electrochemical reduction of clusters without fullerene.

CONCLUSION:

The results included in this chapter show that the electron-withdrawing properties of the fullerene ligand, even if, considerably affect the electrochemical properties of its π complexes with transition metals does not exhaust their diverse electrochemical behavior. The nature of metal and other ligands affects not only the redox potentials but also the stability of products formed in the reduction or oxidation of complexes. The progress in the synthesis of various fullerene complexes in tandem with studying their electrochemical properties make it possible to choose the best objects for studying the prospects of their application as the catalysts of different processes and also as the components for the development of materials and devices for molecular electronics.

REFERENCES:

1. Sokolov, V.I. and Stankevich, I.V., Usp. Khim. vol. 62, (1993)p. 455.

2. Sokolov, V.I., Koord. Khim., vol. 33,( 2007) no. 10, p. 1.

3. Prato, M., J. Mater.Chem., , vol. 7, (1997)p. 1097.

4. Prato, M. and Maggini, M., Acc. Chem. Res., vol. 31, (1998,) p. 519.

5. Echegoyen, L. and Echegoyen, L.E., Acc. Chem. Res., vol. 31,( 1998), p. 593.

6. Sidorov, L.N. and Yurovskaya, M.A., Fullereny: Uchebnoe posobie (Fullerenes: Study Book), Moscow: Ekzamen, (2005).

7. Yanilkin, V.V., in Elektrokhimiya organicheskikh soedinenii (Electrochemistry of Organic Compounds in the Beginning of 21st Century), Moscow: Sputnik, (2008).

8. Reed, C.A. and Bolskar, R.D., Chem. Rev., vol. 100,( 2000) p. 1075.

9. Balch, A.L. and Olmstead, M.M., Chem. Rev., , vol. 98,( 1998) p. 2123.

10. Xie, Q., Petez_Cordero, E., and Echegoyen, L., J. Am.Chem. Soc., vol. 114,(1992) p. 3978.

11. Xie, Q., Arias, F., and Echegoyen, L., J. Am. Chem.Soc., vol. 115, (1993), p.9818.

12. Bashilov, V.V., Tumanskii, B.L., Petrovskii, P.V., andSokolov, V.I., Izv. Akad. Nauk, Ser. Khim., (1999), p. 575.

13. Lerke, S.A., Parkinson, B.A., Evans, D.H., and Fagan, P.J.J. Am. Chem. Soc., vol. 114, (1992), p. 7807.

14. Magdesieva, T.V., Bashilov, V.V., Gorel’skii, S.I.,Sokolov, V.I., and Butin, K.P.,Izv. Akad. Nauk, Ser.Khim., , no. 12, (1994)p. 2153.

15. Lerke, S.A., Evans, D.H., and Fagan, P.J., J. Electroanal. Chem., vol. 383, (1995)p. 127.

16. Bashilov, V.V., Dolgushin, F.M., Petrovskii, P.V.,Sokolov, V.I., Sada, M., Benincori, T., and Zotti, G.,J. Organomet. Chem.,vol. 690, (2005), p. 4330.

17. Magdesieva, T.V., Bashilov, V.V., Kravchuk, D.N., Dolgushin, F.M., Butin, K.P., and Sokolov, V.I., Izv. Akad.Nauk, Ser. Khim., no. 4, (2004), p. 759.

18. Bashilov, V.V., Magdesieva, T.V., Kravchuk, D.N.,Petrovskii, P.V., Ginzburg, A.G., Butin, K.P., andSokolov, V.I., J. Organomet. Chem., vol. 599,(2000), ,p. 37.63

19. Usatov, A.V., Peregudova, S.M., Denisovich, L.I.,Vorontsov, E.V., Vinogradova, L.E., and Novikov, Y.N.,J. Organomet. Chem. vol. 599, (2000), p. 87.

20. Denisovich, L.I., Peregudova, S.M., Usatov, A.V.,Sigan, A.L., and Novikov, Yu.N., Izv. Akad. Nauk, Ser.Khim., no. 7, (1997), p. 1308.

21. Peregudova, S.M., Denisovich, L.I., Martynova, E.V.,Tsikalova, M.V., and Novikov, Yu.N., Elektrokhimiya, vol. 44, (2008), p. 268.

22. Chernega, A.N., Green, M.L.H., Haggitt, J., and Stephens, A.H.H., J. Chem. Soc., Dalton Trans., no. 5, ( 1998), p. 755.

23. Koefod, R.S., Xu, C., Lu, W., and Shapley, J.R., J. Phys. Chem., vol. 96, (1992), p. 2928.

24. Zanello, P., Laschi, F., Fontani, M., Mealli, C., Lenco, A., Tang, K., Jin, X., and Li, L., J. Chem. Soc., Dalton Trans., (1999), p. 965.

25. Zanello, P., Laschi, F., Cinquantini, A., Fontani, M.,Tang, K., Jin, X., and Li, L., Eur. J. Inorg. Chem., (2000),p. 1345.

26. Zanello, P., Laschi, F., Fontani, M., Song, L. C., andZhu, Y.H., J. Organomet. Chem., (2000), vol. 593–594, p. 7.

27. Iikura, H., Mori, S., Sawamura, M., and Kuninobu, Y.,J.Org. Chem., vol. 62,(1997), p. 7912.

28. Park, J.T., Cho, J._J., Song, H., Son, Y., and Kwak, J.,Inorg. Chem., vol. 36,(1997),p. 2698.

29. Song, H., Lee, K., Park, J.T., and Choi, M.G., Organometallics, vol. 17,(1998) p.4477.

30. Kim, K.H., Jung, J., and Han, YK., Organometallics, vol. 23,(2004) p. 3865.

31. Song, H., Lee, Y., Choi, Z.H., Lee, K., Park, J.T.,Kwak, J., and Choi, M.G.,organometallics, vol. 20,(2001) p. 3139.

32. Lee, K., Song, H., Kim, B., Park, J.T., Park, S., and Choi, M.G., J. Am. Chem. Soc., vol. 124,(2002)p. 2872.

33. Lee, K., Song, H., and Park, J.T., Acc. Chem. Res., vol. 36, (2003)p. 78.

34. Lee, G., Cho, Y. J., Park, B.K., Lee, K., and Park, J.T.,J. Am. Chem. Soc., vol. 125,(2003) p. 13920. 64

35. Lee, K., Choi, Y.J., Cho, YJ., Lee, C.J., Song, H.,Lee, C.H., Lee, Y.S., and Park, J.T., J. Am. Chem. Soc., vol. 126,(2004) p. 9837.

36. Balch, A.L., Costa, D.A., Fawcett, R., and Winkler, K.,J. Phys. Chem. B, , vol.100,(1996) p. 4823.

37. Fujiwara, K. and Komatsu, K., Org. Lett., vol. 4, (2002) p. 1039.

38. Zanello, P., de Biani, F.F., Cinquantini, A., and Grigiotti, E., C. R. Chim., vol. 8, 2005) p. 1655.

39. Cho, Y.J., Ahn, T.K., Song, H., Kim, K.S., Lee, C.Y.,Seo, W.S., Kim, S.K., Kim, D., and Park, J.T.,J. Am. Chem. Soc.,vol. 127,(2005) p. 2380.

40. Babcock, A.J., Li, J., Lee, K., and Shapley, J.R., Organometallics, vol. 21,(2002) p. 3940.

Received on 28.11.2012 Modified on 14.12.2012

Asian J. Research Chem. 6(1): January 2013; Page 11-16