Cation-Controlled Assembly of Polyoxotungstate-Based Coordination Networks.

The Preyssler polyoxoanion, [NaP5 W30 O110 ]14- ({P5 W30 }), is used as a platform for evaluating the role of nonbridging cations in the formation of transition-metal-bridged polyoxometalate (POM) coordination frameworks. Specifically, the assembly architecture of Co2+ -bridged frameworks is shown to be dependent on the identity and amount of alkali or alkaline-earth cations present during crystallization. The inclusion of Li+ , Na+ , K+ , Mg2+ , or Ca2+ in the framework synthesis is used to selectively synthesize five different Co2+ -bridged {P5 W30 } structures. The influence of the competition between K+ and Co2+ for binding to {P5 W30 } in dictating framework assembly is evaluated. The role of ion pairing on framework assembly structure and available void volume is discussed. Overall, these results provide insight into factors governing the ability to achieve controlled assembly of POM-based coordination networks.

Tunable Metal Oxide Frameworks via Coordination Assembly of Preyssler-Type Molecular Clusters.

We present the synthesis of metal oxide frameworks composed of [NaP5W30O110]14- assembled with Mn, Fe, Co, Ni, Cu, or Zn bridging metal ions. X-ray diffraction shows that the frameworks adopt the same assembly regardless of bridging metal ion. Furthermore, our synthesis allows for the assembly of isostructural frameworks with mixed-metal ion bridges, or with clusters that have been doped with Mo, providing a high degree of compositional diversity. This consistent assembly enables investigation into the role of the building blocks in the properties of the metal oxide frameworks. The presence of bridging metal ions leads to increased conductivity compared to unbridged frameworks, and frameworks bridged with Fe have the highest conductivity. Additionally, Mo-doping can be used to enhance the conductivities of the frameworks. Similar structures can be obtained from clusters in which the central Na+ has been replaced with Bi3+ or Sm3+. Overall, the optical and electronic properties are tunable via choice of bridging metal ion and cluster building block and reveal emergent properties in these cluster-based frameworks. These results demonstrate the promise of using polyoxometalate clusters as building blocks for tunable complex metal oxide materials with emergent properties.

Co2+-Linked [NaP5W30O110]14-: A Redox-Active Metal Oxide Framework with High Electron Density.

A new metal oxide framework based on the redox-active Preyssler anion linked with Co(H2O)42+ bridging units is presented. The framework can be photochemically reduced, allowing the storage of multiple electrons under mild conditions. Titrations with molecular redox species show that this reduction is reversible and can accommodate up to 10 electrons per Preyssler cluster (corresponding to an electron density on the order of 1021 cm-3) without changing the crystal structure. This addition of delocalized electrons is accompanied by a 1000-fold increase in the conductivity. These results demonstrate that the ability to add delocalized electrons to polyoxometalate clusters can be incorporated into self-assembled extended solids, enabling the development and tuning of metal oxide materials with emergent or complementary properties.

Dual-mode crystal-bound and X-type passivation of quantum dots.

In this report, we present a new path to the control of quantum dot surface chemistry that can lead to a better understanding of nanoscale interfaces and the development of improved photocatalysts. Control of the synthetic methodology leads to QDs that are concomitantly ligated by crystal-bound organics at the surface anion sites and small X-type ligands on the surface cation sites.

Crystal-bound vs surface-bound thiols on nanocrystals.

The use of thiol ligands as a sulfur source for nanocrystal synthesis has recently come en vogue, as the products are often high quality. A comparative study was performed of dodecanethiol-capped Cu2S prepared with elemental sulfur and thiol sulfur reagents. XPS and TGA-MS provide evidence for differing binding modes of the capping thiols. Under conditions where the thiol acts only as a ligand, the capping thiols are "surface-bound" and bond to surface cations in low coordination number sites. In contrast, when thiols are used as a sulfur source, "crystal-bound" thiols result that sit in high coordination sites and are the terminal S layer of the crystal. A (1)H NMR study shows suppressed surface reactivity and ligand exchange with crystal-bound thiols, which could limit further application of the particles. To address the challenge and opportunity of nonlabile ligands, dodecyl-3-mercaptopropanoate, a molecule possessing both a thiol and an ester, was used as the sulfur source for the synthesis of Cu2S and CuInS2. A postsynthetic base hydrolysis cleaves the ester, leaving a carboxylate corona around the nanocrystals and rendering the particles water-soluble.