Electrochemical investigation of molecular growth of the {Mo 57 V 6 } polyoxometalate cluster Adrian Nicoara a,b, * , Adrian Patrut a,b , Dragos Margineanu b , Achim M€ uller a a Faculty of Chemistry, University of Bielefeld, P.O. Box 100 153, Bielefeld D-33501, Germany b Faculty of Chemistry, Babes-Bolyai University, 11 Arany Janos, Cluj-Napoca RO-3400, Romania Received 2 April 2003; received in revised form 2 May 2003; accepted 2 May 2003 Abstract The reduction of the {Mo 57 V 6 } polyoxometalate cluster which shows an interesting molecular growth process, modelling bio- logical molybdenum uptake into a storage protein, was investigated electrochemically. The electrochemical features of the growth process were studied by using cyclic voltammetry, either as such or associated with bulk electrolysis. Resonance Raman spec- troscopy was also used to identify the reduction products. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Polyoxometalates; Molecular growth; Cyclic voltammetry; Coulometry; Resonance Raman spectroscopy 1. Introduction Polyoxometalates form an important class of inor- ganic metal–oxygen clusters, which contain the largest synthesized and structurally characterized molecules, interesting also for their topological and electronic ver- satility. It is worthwhile to mention that numerous ap- plications of the polyoxometalate clusters are reported [1,2]. The investigated (NH 4 ) 21 [{V IV (H 2 O)O} 6 {Mo V (l-H 2 O) 2 (l-OH)Mo V } 3 {Mo 15 (MoNO) 2 O 58 (H 2 O) 2 } 3 ]  65H 2 O clus- ter compound, abbreviated as {Mo 57 V 6 }, is a member of a group of {Mo 57 M 6 }-type species, where different cat- ionic centers, such as {Fe III (H 2 O) 2 } 3þ , {Fe II (H 2 O) 2 } 2þ and {Cu II (H 2 O) 2 } 2þ can replace the {V IV (H 2 O)O} 2þ units [3]. The doughnut-shaped cluster of D 3h symmetry, having 276 non-hydrogen atoms, is built-up of three [{Mo 15 (MoNO) 2 O 58 (H 2 O) 2 } 3 ] 20 building blocks, noted as {Mo 17 }, which are connected by six {V IV (H 2 O)O} 2þ units and three {Mo V (l-H 2 O) 2 (l-OH)Mo V } 9þ moieties, see Fig. 1 [4,5]. The three {Mo 17 } blocks are constituted of two {Mo 8 } units linked by a {Mo 1 }-type center. The centers of the {Mo 8 } units are formed by pentag- onal {Mo(NO)O 6 } bypiramids, each connected to five {MoO 6 } octahedra via edges. Four {MoO 6 } octa- hedra are further connected with two additional {MoO 6 } octahedra via corners, which can be easier replaced. The synthesis of the investigated cluster is based on the acidification of an aqueous molybdate solution under reductive conditions, by using hydroxylammo- nium chloride. When higher concentrations of hydroxylammonium chloride or when a stronger re- ducing agent, i.e., hydrazine sulfate are used, further molecular growth is observed. The research suggests that every two-electron reduction of the {Mo 57 V 6 } cluster yields a molecular growth by one molybdenum center, more precisely by the addition of a ‘‘formal’’ {MoO} 4þ unit. From a structural point of view a molecular growth with a maximum six molybdenum atoms up to {Mo 63 V 6 } is possible, corresponding to a 12-electron reduction process. In this context the process is of relevance for the molybdenum storage protein of Azotobacter vinelandii. As both the syn- thesis of the parent cluster and the subsequent mo- lecular growth are based on electron transfer processes, an electrochemical study of {Mo 57 V 6 } is interesting to perform. Electrochemistry Communications 5 (2003) 511–518 www.elsevier.com/locate/elecom * Corresponding author. Fax: +40-264-190818. E-mail address: anicoara@chem.ubbcluj.ro (A. Nicoara). 1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00114-0