Polyoxometallate clusters

R. Schroden, C. Blanford, B. Melde, B.J.S. Johnson, and A. Stein Direct Synthesis of Ordered Macroporous SUica Materials Functionahzed with Polyoxometalate Clusters. Chem. Mater. 13, 1074 (2001). 

B. J.S. Johnson and A. Stein Surface Modification of Mesoporous, Macroporous, and Amorphous Silica with Catalytically Active Polyoxometalate Clusters. Inorg. Chem. 40, 801 (2001). 

The preparation of large clusters is certainly not exclusive to organic molecules. Inorganic polyoxometallate clusters containing 154 molybdenum atoms per cluster or more have been obtained (Muller et al, 1998). 

In 1989, Isobe and coworkers reported on an organometallic polyoxometalate cluster [(Rhcp )4V60i9] (cp = /j -CsMcs) that catalyzes the oxidation of cyclohexene with TBHP as oxidant to give mainly ally lie oxidation products (l-ferf-butylperoxycyclohex-2-ene 42% and cyclohex-2-en-l-one 21%) and only little epoxide (15%) (equation 62). The yield of 1 -ferf-butylperoxy cyclohex-2-ene increased with decreasing molar ratio of cyclohexene to TBHP, while the yield of cyclohex-2-en-l-one has a maximum at the ratio of 0.2. 

Scheme 13.1 Reaction of a hydrogen terminated surface with an alkene to generate surface mono-layers (a) monolayer formation by reaction of silicon with H2C=CH(CH2)90Ph(C6H40Me)2, (b, c) monolayer derivatisation to give a surface layer of tethered magnetic TiW5 polyoxometalate clusters.4...

Here we start to examine the pivotal role that polyoxometalate clusters can play in the development of nanoscale devices that utilize POM components, and start to conceptualize some example systems in which POM components could have a crucial role [13, 19]. This is because such functional nanosystems can exploit the building block principle already established in this area of chemistry, coupled with the range of physical properties, and the fact that POM systems can really be seen as molecular metal oxides [20]. To demonstrate this point, a number of examples have been selected across the area of POM chemistry, including our contributions, to help highlight new directions and concepts. It should also be noted that metal oxides already play an important role in the electronics and semiconductor industry today and their solid-state properties have been studied extensively [21, 22]. Many of these concepts are not new in isolation, but the possibility of using molecular design in metal oxides to produce... 

Fig.10 Polyoxometallate clusters anchored to soluble support via ionic interactions...

Schroden R. C., Stein A., et al.. Direct synthesis of ordered macroporous silica materials functionalized with polyoxometalate clusters, Chem. Mater. 13 (2001) pp. 1074-1081. 

Figure 5.17 The two-dimensional layer in [(C6H5N02)2Ln(H20)4]2[IMo6024][N03] [59]. (Reprinted with permission from H.Y. An, et al., Self-assembly of extended high-dimensional architectures from Anderson-type polyoxometalate clusters, Crystal Growth and Design, 6, no. 5, 1107-1112 (Figure 3), 2006. 2006 American Chemical Society.)...

Perspectives for Materials Science and Nanotechnology I 469 Tab. 14.2. Nanoscaled polyoxometalate clusters for materials science problems. 

J. L. Stark, A. L. Rheingold, E. A. Maatta, Polyoxometalate clusters as building-blocks. Preparation and structure ofbis (hexamolybdate) complexes covalently bridged by organodiimido ligands, J. 

M. Clemente-Juan, M. Aebersold, H. U. Giidel, E. Coronado, H. Biittner, G. Kearly, J. Melero and R. Burriel Magnetic Excitations in Polyoxometalate Clusters Observed by Inelastic Neutron-Scattering-Evidence for Anisotropic Ferromagnetic Exchange Interactions in the Tetrameric Cobalt(II) Cluster [Co- 4(H20)2(PWi 034)2]" - Comparison with the Magnetic and Specific- Heat Properties J. Am. Chem. Soc. 121 10028(1999). 

Polyoxometalate Cluster Synthesis Using Building Blocks Foundations and the Special Mon Motif. 205... 

Polyoxometalate Cluster Synthesis Using Building Blocks ... 

Use of a surfactant allows solubilization of the polyoxometalate cluster K6[Vi5As6042(H20)] 8H20 (V15) in the organic solvent chloroform. Spin echo measurements revealed a phase memory time of Tm = 340 ns, which was attributed to resonances in the 5 = 3/2 excited state of the cluster [166]. No quantum coherence was detected in the pair of 5 = 1/2 ground states [151]. By measurement of the z-magnetization after a nutation pulse, and a delay to ensure decay of all coherences, Rabi oscillations were observed. From the analysis of the different possible decoherence mechanisms, it was concluded that decoherence is almost entirely caused by hyperfine coupling to the nuclear spins. 

Figure 1 A representation of corner- and edge-sharing polyhedra found in polyoxometalate clusters. The metal ions at the center of the open polyhedra are shown by the black spheres and the oxygen ligands at the vertexes of the polyhedra are shown by smaller black circles. The top image shows exclusively a cornersharing mode whereas the bottom image shows a combination of edge and corner-sharing polyhedra.

Before outlining the general approach to the synthesis of polyoxometalate clusters it is informative to consider the most useful synthetic results thus far discovered for derivatization and functionalization of fragments leading to a huge variety of structures. These are given below ... 

The structural features of large polytungstate clusters can be visualized in terms of subunits based on lacunary fragments of the Keggin ion. In such representations many of the fragments may not actually exist independently from the cluster containing these subunits. This also applies to the building blocks identified as part of the polyoxometallate clusters discussed in Sections 7.1.4 and 7.1.5. 

In recent years there has been an unprecedented increase in the number of interesting polyoxometalate clusters that have been structurally characterized by single crystal X-ray crystallography and, of course, the absolute size of the molecules characterized. Indeed, the elucidation of the... 

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