Redox properties of hybrid Dawson type polyoxometalates disubstituted with organo-silyl or organo-phosphoryl moieties Mohammed Boujtita a,1 , Julien Boixel c , Errol Blart c , Ce ´dric R. Mayer b, * , Fabrice Odobel c, * a Universite ´ de Nantes, Nantes Atlantique Universite ´s, CNRS, Faculte ´ des Sciences et des Techniques, Laboratoire d’Analyse, Isotopique et Electrochimique, de Me ´tabolismes (LAIEM), UMR CNRS 6006, 2, rue de la, Houssinie `re, BP 92208, 44322 NANTES Cedex 3, France b Institut Lavoisier de Versailles, UMR-CNRS 8180, Universite ´ de Versailles St-Quentin, Ba ˆtiment Lavoisier, 45 avenue des Etats Unis, 78035 Versailles Cedex, France c Universite ´ de Nantes, Nantes Atlantique Universite ´s, CNRS, Faculte ´ des Sciences et des Techniques, Laboratoire de Synthe `se Organique (LSO), UMR CNRS 6513, 2, rue de la, Houssinie `re, BP 92208, 44322 NANTES Cedex 3, France Received 24 July 2007; accepted 30 October 2007 Available online 11 December 2007 Abstract This paper reports on the electrochemical study of Dawson-type polyoxometalates (POMs) disubstituted with phenyl-phosphonate, phenyl silyl or tertbutylsilyl moieties. The polyanions were studied by cyclic voltammetry and on rotating disk electrode in acetonitrile with tetraphenylphosphonium chloride as supporting electrolyte. We found that these hybrid polyoxometalates can accept up to five elec- trons and that the organic substituents modify the first and second reduction potential of the POM compared to the unsubstituted POM [P 2 W 18 O 62 ] 6À . Phenyl-phosphonate lowers the reduction potential, whereas the phenyl silyl or tertbutylsilyl moieties shift them cathod- ically. We also recorded by spectroelectrochemistry the spectrum of the monoreduced and the bisreduced species of the polyoxometalate disubstituted with phenyl-phosphonate. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Dawson-type polyoxometalate; Functionalization; Phosphoryl; Silyl; Electrochemical study; Spectroelectrochemistry 1. Introduction Polyoxometalates (POMs) are large inorganic polyani- ons generally based on oxides of molybdenum or tungsten. Each metal is located at the center of an octahedron, MO 6 , and these octahedra are all bonded together through their corners or their edges and are assembled around an inter- nal tetrahedrally coordinated heteroatom, typically PO 4 3À or SiO 4 4À . Polyoxometalates are a valuable class of com- pounds because they span a wide range of potential appli- cations in material science, medicine, catalysis and pigments owing to their rich electrochemical, photochemi- cal and catalytic properties [1–4]. One appealing character- istic of POMs is their capacity to accept several electrons [3,5], which makes them particularly well-suited to act as electron reservoirs. As a result, POMs are attractive candi- dates to be used as multielectron acceptors for the develop- ment of photochemical devices aiming at photocumulative electron transfer and as electrocatalysts for multielectron reduction processes such as the reduction of water into hydrogen gas. Indeed, there are several reports showing that some POMs can act as homogenous catalysts for the reduction of water into dihydrogen from aqueous solution [3,6–9]. The possibility to covalently connect POMs to organic molecules via phosphonates [10–13], silanes [14–16], imido [17–21] or other binding units [22,23] opens up the possibil- 0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.10.022 * Corresponding authors. Tel.: +33 1 39 25 43 97; fax: +33 1 39 25 43 81 (C.R. Mayer); tel.: +33 2 51 12 54 29; fax: +33 2 51 12 54 02 (F. Odobel). E-mail address: Fabrice.Odobel@chimie.univ-nantes.fr (F. Odobel). 1 Tel.: +33 2 51 12 57 23; fax: +33 2 51 12 57 12. www.elsevier.com/locate/poly Available online at www.sciencedirect.com Polyhedron 27 (2008) 688–692