pubs.acs.org/IC Published on Web 06/29/2010 r 2010 American Chemical Society Inorg. Chem. 2010, 49, 7001–7006 7001 DOI: 10.1021/ic100675h Influence of the Heteroatom Size on the Redox Potentials of Selected Polyoxoanions Isra € el-Martyr Mbomekall  e,* ,† Xavier L  opez,* ,‡ Josep M. Poblet, ‡ Francis S  echeresse, † Bineta Keita, § and Louis Nadjo § † Institut Lavoisier, UMR 8180, Universit e de Versailles St. Quentin, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France, ‡ Departament de Quı´mica Fı´sica i Inorg  anica, Universitat Rovira i Virgili, Marcel 3 li Domingo s/n, 43007 Tarragona, Spain, and § Laboratoire de Chimie Physique, Electrochimie et Photo electrochimie, Universit  e Paris-Sud, UMR 8000, CNRS, Orsay F-91405, France Received April 9, 2010 The apparent formal potentials for the one-electron redox process of most Keggin-type heteropolytungstates, XW 12 O 40 q- , have long been shown to linearly depend on their overall negative charges, in the absence of proton interference in the process. However, for a given overall negative charge, these formal potentials are also shown here to depend on the specific central heteroatom X. In the present work, cyclic voltammetry was used to study a large variety of Keggin-type anions, under conditions where their comparisons are straightforward. In short, apparent potential values get more negative (the clusters are more difficult to reduce) for smaller central heteroatoms within a given family of Keggin-type heteropolyanions carrying the same overall negative charge. Density functional theory calculations were performed on the same family of Keggin compounds and satisfactorily reproduce these trends. They show that internal XO 4 units affect differently the tungstate oxide cage. The electrostatic potential created by each internal anionic unit in a fragment-like approach (XO 4 q- @W 12 O 36 ) was analyzed, and it is observed that X atoms of the same group show slight differences. Within each group of the periodic table, X atoms with lower atomic numbers are also smaller in size. The net effect of such a tendency is to produce a more negative potential in the surroundings and thus a smaller capacity to accept electrons. The case of [BW 12 O 40 ] 5- illustrates well this conclusion, with the smallest heteroatom of the Keggin series with group III central elements and a very negative reduction potential with respect to the other elements of the same group. Particularly in this case, the electronic structure of the Keggin anion shows the effects of the small size of boron: the highest occupied molecular orbitals of [BW 12 O 40 ] 5- appear to be ∼0.35 eV higher than those in the other clusters of the same charge, explaining that the BO 4 unit is more unstable than AlO 4 or GaO 4 despite carrying the same formal charge. Introduction Polyoxometalates (POMs for short) are early-transition- metal anionic molecular nanoclusters that simultaneously exhibit many properties that make them attractive for appli- cations in catalysis, separations, imaging, materials science, and medicine. The chemistry of POMs 1 has become a very rich field, especially since the 1960s, and is still in constant academic and technological development. 2 With the recently summarized pioneering work of Baker and co-workers, 3 the study of the properties of POMs entered an era of systematic investigation that continues unabated. Among the plethora of beneficial properties, the importance of redox behaviors was soon recognized. In this domain, simple rules could be rapidly established. Keeping, for clarity, with Keggin-type XW 12 O 40 q- anions, the following main parameters appear to govern their voltammetric behaviors: (i) the basicity of the first reduced species and, occasionally, of the fully oxidized species; (ii) the overall negative charge of the POM, which is a function of the central heteroatom charge; (iii) the size of this central heteroatom; (iv) finally, it is worth noting that, in the vast majority of the most studied Keggin-type POMs, the *To whom correspondence should be addressed. E-mail: Israel. mbomekalle@chimie.uvsq.fr (I.-M.M.), javier.lopez@urv.cat (X.L.). (1) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag: Berlin, 1983. (2) (a) Pope, M. T.; M€ uller, A. Angew. Chem., Int. Ed. 1991, 30, 34. (b) Pope, M. T.; M€ uller, A. Polyoxometalates: From Platonic Solids to Anti- retroviral Activity; Kluwer: Dordrecht, The Netherlands, 1994. (c) Pope, M. T.; M€ uller, A. Polyoxometalate Chemistry: From Topology via Self-Assembly to Applications; Kluwer: Dordrecht, The Netherlands, 2001. (d) Hill, C. L. Chem. Rev. 1998, 98, 1. (e) Long, D.-L.; Burkholder, E.; Cronin, L. Chem. Soc. Rev. 2007, 36, 105. (f) Yamase, T., Pope, M. T., Eds. Polyoxometalate Chemistry for Nanocomposite Design; Kluwer Academic: New York, 2002. (e) Keita, B.; Nadjo, L. In Electrochemistry of Polyoxometalates, Encyclopedia of Electro- chemistry; Bard, A. J., Stratmann, M., Eds.; Wiley-VCH: New York, 2006; Vol. 7, pp 607-700. (3) Baker, L. C. W.; Glick, D. C. Chem. Rev. 1998, 98,3–49.