Organic–Inorganic Hybrid Materials DOI: 10.1002/anie.201006938 Real-Time Observation of the Self-Assembly of Hybrid Polyoxometalates Using Mass Spectrometry** Elizabeth F. Wilson, Haralampos N. Miras, Mali H. Rosnes, and Leroy Cronin* Polyoxometalate (POM) clusters constitute a wide and varied family of structures formed by condensation reactions of metal oxide anions of early transition metals, for example, V, Mo, and W, in high oxidation states. [1] Their size can vary from sub-nanoscale to protein-sized clusters, [2] and their interesting electronic and molecular properties [3] have led to many potential applications for POMs, in areas as diverse as catalysis, [3, 4] medicine, [1] and materials science. [5] One of the most interesting aspects of POM chemistry lies with the fact that the clusters can be viewed as transferable building blocks or synthons, [6a] the controlled assembly of which is desirable in order to synthesize novel architectures with functionality. However, despite the increasingly inten- sive research in this area, understanding of the complex formation mechanisms and self-assembly processes which govern POM structure formation remains limited. [6] In practice, this lack of understanding leads to experimentation where manipulation of reaction parameters in the commonly used “one-pot” POM syntheses often leads to the formation of new POM structures, albeit by a somewhat serendipitous approach. [2] For example, in the area of organic–inorganic POM hybrid compounds, although solid-state investigations using tris(alkoxo) ligands with POMs have been carried out, that is, using Anderson, [7, 8] Lindqvist, [9] and Dawson [5a, 10] structural types, along with investigations of other POM architec- tures, [11–13] there has been very little research into the self- assembly processes which govern the formation of these structures in solution. In particular, the use of mass spec- trometry (MS) to aid elucidation of the rearrangements and aggregation processes involved has, so far, been neglected. Thus, there is a clear and urgent need to develop approaches to bridge the gap [14, 15] between solid-state and solution studies so that the key features of the self-assembly mechanisms of polyoxometalate clusters can be revealed. Herein, the use of electrospray ionization mass spectrom- etry (ESI-MS) has allowed real-time, “in-solution” monitor- ing of the formation of a complex, organic–inorganic POM hybrid system. The reaction system chosen for investigation was found by Hasenknopf and co-workers [7b] and involves the rearrangement of [a-Mo 8 O 26 ] 4À , coordination of Mn III , and coordination of two tris(hydroxymethyl)aminomethane mol- ecules (TRIS), to form the symmetrical Mn-Anderson cluster ((n-C 4 H 9 ) 4 N) 3 [MnMo 6 O 18 ((OCH 2 ) 3 CNH 2 ) 2 ](1; also written as TBA 3 [MnMo 6 O 18 ((OCH 2 ) 3 CNH 2 ) 2 ]; Figure 1). This work builds upon our earlier studies of using mass spectrometry to investigate a simpler reaction system, wherein the for- mation of the silver-linked b-octamolybdate structure ((n-C 4 H 9 ) 4 N) 2n [Ag 2 Mo 8 O 26 ] n was investigated. [16] The reaction system forming POM hybrid 1 was selected for investigation for a number of reasons. First, it presents a conveniently accessible, complex cluster rearrangement form- ing an organic–inorganic POM hybrid structure. Also, as such organic–inorganic POM hybrids can be used as synthons in the design of nanoscale hybrid POM architectures, increasing our knowledge of the aggregation processes which form such building blocks will make the field of nanoscale functional materials more accessible for further exploration. Moreover, this reaction takes place in acetonitrile solution which is a volatile, moderately polar solvent suitable for use in MS investigations and which usually provides MS data of polyoxometalates with a good signal-to-noise ratio and Figure 1. Illustration to visualize the prominent, intermediate fragment ions identified in this study (labeled b–f), which are involved in the rearrangement of the [a-Mo 8 O 26 ] 4À anion (labeled a), into the sym- metrical Mn-Anderson anion [MnMo 6 O 18 ((OCH 2 ) 3 CNH 2 ) 2 ] 3À of 1 (la- beled g). [7b] Structures (b–f) (formal representations based on crystallo- graphic data [7b] are shown here) represent the following fragment ions identified in these ESI-MS investigations: b) [Mo 4 O 13 TBA] À , c) [Mo 2 O 7 H] À , d) [Mo 3 O 10 TBA] À , e) [Mo 2 O 5 ((OCH 2 ) 3 CNH 2 )] À , f) [Mn III Mo 3 O 8 ((OCH 2 ) 3 CNH 2 ) 2 ] À . Color scheme: Mo green polyhedra, Mn orange polyhedra, O red, N blue, C gray spheres. H atoms are omitted for clarity. [*] Dr. E. F. Wilson, [++] [+] Dr. H. N. Miras, [+] M. H. Rosnes, Prof. L. Cronin WestCHEM, Department of Chemistry, University of Glasgow Glasgow, G12 8QQ, Scotland (UK) Fax: (+ 44) 141-330-4888 E-mail: l.cronin@chem.gla.ac.uk Homepage: http://www.croninlab.com [ ++ ] Current address: Department of Chemistry and Pharmacy Inorganic Chemistry, University of Erlangen-Nürnberg Egerlandstrasse 1, 91058 Erlangen (Germany) [ + ] These authors contributed equally to this work. [**] We acknowledge financial support from the EPSRC, the Royal Society of Edinburgh, and Marie Curie actions, and collaborative work with Bruker Daltonics Ltd. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201006938. Communications 3720  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2011, 50, 3720 –3724