Templating A Molecular Donut

To construct an inorganic donut, nature appears to start with the donut hole—a strategy chemists might be able to use for future efforts at molecular assembly.

An international team reports that a molybdenum oxide "donut" structure forms by using as a template a smaller molybdenum cluster, which is then dislodged to create the hole (Science 2010, 327, 72). In the course of the study, the team also developed a new flow system that could be used to study chemistry at conditions far from equilibrium.

The template mechanism for forming the molecular donut suggests "new strategies for creating other open structures by judicious combinations of ions of different metals and/or the same metals in differing oxidation states," Kenton H. Whitmire, a chemistry professor at Rice University, says in a perspective accompanying the report.

The group, led by chemistry professors Leroy Cronin of the University of Glasgow, in Scotland, and Achim Müller of Bielefeld University, in Germany, studied the formation of a 3.6-nm-diameter, donut-shaped Mo₁₅₀ structure with the formula [Mo₁₅₀O₄₄₂(OH)₁₀(H₂O)₆₁]^14– from a sodium molybdate solution. The researchers used a flow system that allowed real-time control of pH and reagent concentrations.

By adjusting the system conditions, the researchers were able to isolate and crystallize an intermediate with a [Mo₃₆O₁₁₂(H₂O)₁₆]^8– cluster bound within the Mo₁₅₀ donut through 22 sodium cations, which balance the negative charges of the donut and cluster. When the solution is reduced, the Mo₃₆ cluster dissociates from the donut, and the two species can be isolated separately.

The Mo₃₆ cluster is known to form spontaneously in acidified molybdate solutions, the authors note. They hypothesize that the cluster serves as a structure-directing template for the formation of the donut. "The results show how a flow system can be used to understand and optimize a complex bottom-up assembly process that generates gram quantities of a nanomaterial with well-defined size, shape, and composition," Cronin says.

The flow system is also interesting because of its potential for studying chemistry at nonequilibrium conditions such as those found in responsive and adaptive biological systems, notes Jonathan W. Steed, a chemistry professor at Durham University, in England.