No abstract

In this paper, we report a high-yield procedure for the synthesis of isolated W6S8(4-tert-butylpyridine)6 clusters from isolated tungsten chloride clusters. We also show that the 4-tert-butylpyridine ligand can be exchanged with triethylphosphine and piperidine.

The hydrothermal-electrochemical method has been used for the synthesis of manganese dioxides from acidic MnSO 4 or A 2 SO 4 /MnSO 4 (A ϭ Li, Na, K, NH 4 ) solutions. This method produced manganese dioxides of either the ␤-, ␥-or ␣-structure types, mixtures of these structure types, ␣/␥ or ␥/␤, or interconnected ␣-and ␥-phases ͑␣•␥͒. The structure of the material obtained can be controlled by adjusting the parameters of the synthesis: temperature, acidity of the solution, composition of the solution, or applied current density. The ␥-MnO 2 materials synthesized are characterized by exceptionally low amounts of microtwinning defects. These materials can be synthesized over a wide range of P r values, including in the range usually reserved for chemically prepared ͑chemical manganese dioxide, CMD͒ materials. Materials exhibiting lower P r values are favored when the pH or the temperature of the synthesis is decreased. The lowest P r values were found in materials synthesized at 92°C and pH 0. A study of the morphology of the deposits has shown that it can be controlled by changing the experimental conditions. Manganese dioxides are of technological interest due to their potential use as cathode materials for primary and secondary lithium and alkaline batteries. Many manganese dioxide polymorphs are known, including layered and tunnel structures. Two manganese dioxides of particular interest to the battery industry are ␣-MnO 2 ͑secondary batteries͒ and ␥-MnO 2 ͑primary and secondary batteries͒. Both structures are built up of chains of edge sharing MnO 6 octahedra ͑single or double͒ which share corners to form channels through the structure. In ␣-MnO 2 , with the Hollandite structure, the chains of octahedra connect to form 2 ϫ 2 as well as 1 ϫ 1 channels. In ␥-MnO 2 , it is widely accepted that the octahedra are connected such that channels of dimensions 2 ϫ 1 ͑characteristic of ramsdellite-MnO 2 ) and 1 ϫ 1 ͑characteristic of pyrolusite-MnO 2 ) are formed, as proposed by de Wolff. 1 ␣-MnO 2 is usually prepared by chemical methods utilizing two or more steps, with the large 2 ϫ 2 channels of the structure stabilized by large cations such as K ϩ or NH 4 ϩ . 2-8 However, for battery applications, these large cations are thought to inhibit the mobility of Li ϩ through the tunnels, and thus can be ion-exchanged for H ϩ ͑as H 3 O ϩ ) 9 or Li ϩ to improve performance. 10,11 Alternatively, ␣-MnO 2 containing only H 3 O ϩ in the channels can also be synthesized directly by chemical methods. 11-15 Lithium containing ␣-MnO 2 (Li x MnO 2 ; x р 0.25) have been prepared by heating H 3 O ϩ -16 or NH 4 ϩ -containing 6,17 ␣-MnO 2 with LiOH at temperatures у275°C.These proton-and lithium-containing ␣-MnO 2 materials have shown an increased cycled capacity compared to ␣-MnO 2 materials containing large cations. Synthesis of ␥-MnO 2 for commercial use ͑electrolytic manganese dioxide, EMD͒ is performed by oxidative deposition from Mn 2ϩ /H 2 SO 4 solutions. These samples characteristically contain a large amount of defects and their c...