Synthesis and electronic properties of low-dimensional bis(benzene) vanadium reduced mesoporous niobium oxide composites

2005 Pure or Applied Inorganic Chemistry Award Lecture — Host–guest inclusion chemistry of electroactive, mesoporous transition metal oxides oxidation and 1-D confinement in one step and why amorphous is better

The flexible oxidation states of mesoporous Nb, Ta, and Ti oxides make them unique amongst porous materials allowing reaction pathways and cascades that are not possible for mesoporous silica or microporous materials such as zeolites. This electronic activity coupled with the 20–30 Å pores and the amorphous wall structure, which provides greater bandwidth (W) and hence an even greater range of redox potentials, leads to a rich variety of host–guest inclusion chemistry, which serves as an unprecedented 1-D analogue to layered 2-D host–guest inclusion reactions studied for decades. In this paper we survey a series of reactions between these mesoporous hosts and a wide variety of organic and organometallic guest species including alkali fullerides, cobaltocene, and other organometallic sandwhich species, and discuss the electronic and magnetic properties of the resulting composites.Key words: mesoporous materials, semiconductors, fullerides, superconductors, oxides, nanomaterials, metallocenes.

Compositional and 2H NMR studies of bis(benzene)chromium composites of mesoporous vanadium-niobium mixed oxides

New mesoporous niobium oxides with 5, 10, and 15 mol% vanadium(V) doped into the walls of the structure were synthesized by the ligand-assisted templating method with an octadecylamine template. These materials were characterized by XRD, XPS, EPR, elemental analysis, and nitrogen adsorption before being treated with excess bis(benzene)chromium to give new composites with an organometallic phase in the walls. All materials were also characterized by EPR, XRD, nitrogen adsorption, XPS, SQUID magnetometry, and elemental analysis. The materials with higher percentages of vanadium absorbed more bis(benzene)chromium, because this process depends largely on the electron transfer between the organometallic and the walls of the mesostructure and vanadium(V) is a stronger oxidant than niobium(V). Conductivity studies on these materials revealed that the ratio of Cr(0) to Cr(l) in the pores was more important than the absolute Cr loading level in governing electron transport properties but that increasing the V content led to more insulating behavior regardless of the Cr concentration. Solid-state 2H NMR studies on perdeuteriobenzene analogues of these composites showed the presence of the neutral and cationic Cr species in different ratios depending on the loading. Tumbling of these species was also slow on the NMR time scale, indicating that the charge-carrying Cr species are not rapidly moving through the pore channels of the mesostructure. This suggests that the walls of the structure may play a key role in charge transfer in these composites, contrary to what was previously believed.