Polyoxometalate Nanostructures, Superclusters, and Colloids: From Functional Clusters to Chemical Aesthetics

It has often been stated that polyoxometalates (POMs) represent a class of molecules of almost unrivaled versatility, which results from their enormous structural diversity, range of sizes, tunable redox properties, and fascinating catalytic activity. These properties were very much in evidence at a recent fourday “International Symposium on Nano-structures and Physicochemical Properties of Polyoxometalate Superclusters and Related Colloid Particles” in Kanagawa (Japan), organized by T. Yamase, M. T. Pope, and A. M ller with the support of the JSPS, CREST, the Rigaku Corporation, and the Chemical Society of Japan. The previous conference in this series had taken place in 1999 at the Universit t Bielefeld (Germany). POMs represent unique polymerizable entities that are often based on common structural fragments, or building blocks. This research area has undergone an important transition from classical structural and coordination chemistry towards the chemistry of “smart”, “functional” materials, which may even have properties of artificial cells/porous capsules. There has been a dramatic increase in the number of interdisciplinary projects in a reasonably short period of time. The field also continues to surprise with new findings at a fundamental scientific level which open avenues for future research.[1–7] The diverse and exciting nature of this field became apparent immediately from the opening lecture of F. S cheresse (Universit Versailles St-Quentin, France), who described interesting developments in the synthesis of discrete polyoxomolybdates and polyoxotungstates. In particular the careful use of simple, directing building blocks based on MoV dimers of the form [Mo2O2(m-E)2(H2O)6] (E = S or O) to produce an impressive array of larger cluster structures was demonstrated.[1] S cheresse then described effectively how POM-based building blocks (this time lanthanide-functionalized e Keggin anions) can be taken to the next level by linking complete molecular cluster anions with carboxylate ligands to networks of different topologies and dimensionalities, as well as strategies for the formation of “superclusters” by carefully considering how POM-based building blocks can be connected geometrically and chemically. This concept was also a focus of M. T. Pope s (Georgetown University, USA) review of general routes to ever larger POMs by using lacunary fragments. Pope also proposed the development of large clusters based on reduced polytungstate moieties, thereby applying concepts that are successfully used in polyoxomolybdate chemistry.[2] P. Gouzerh (Universit Pierre et Marie Curie, France) extended the discussions on polytungstate-based moieties by presenting the special abilities of the hexavacant {P2W12} anion. This species readily cocondenses with 3d cations to form large and symmetrical assemblies of {P2W12} fragments that are supported by a {M4O6} core (M = FeIII). The self-assembly of a range of polytungstate clusters was also explored by U. Kortz (Universit t Bremen, Germany), who showed some exciting new approaches to polytungstate systems by using novel linking groups, including a polyoxotungstate wheel [{b-Ti2SiW10O39}4] . In this case the use of Ti4+ to connect the {W10}-based building blocks is key.[3] Recent developments in polyoxoniobate chemistry were presented by M. Nyman (Sandia National Laboratories, USA), who demonstrated how this field could be expanded from the {Nb6O19} Lindqvist ion to a variety of Keggin-based compounds, despite the considerable challenges faced in determining the correct conditions for the formation and isolation of these materials. In one of several highlights of the conference C. Hill (Emory University, USA) presented a route to break through the “oxo wall” and stabilize a single (H2O)Pt=O moiety by using electron-withdrawing {PW9} building blocks as ligands to sandwich the Pt=O moiety. The resulting complex is the first discrete metal–oxo complex of a late transition metal (Figure 1).[4] Because such species are thought to represent intermediates in O2 activation at platinum surfaces, this finding is of significant consequence and will surely spark extensive follow-up work. The nanoscopic nature of POMbased clusters and their great potential as mesoporous materials were discussed by T. Yamase (Tokyo Institute of Technology, Japan), who showed how the photoreduction-induced self-assembly of molybdenum blue type polyoxomolybdates can be controlled by the incorporation of different Ln3+ ions into the reaction system. This process leads to the formation of nanowheel structures