Ephemeral templating

The isolation of an intermediate species during the self-assembly of a giant molybdenum oxide wheel suggests that a smaller cluster templates the wheel's formation before being evicted.

Credit: © 2010 AAAS

Self-assembly processes have led to an extremely wide variety of architectures, but their typical one-pot syntheses have so far hindered mechanistic investigations. The formation of giant molybdenum oxide rings by reduction of an acidic molybdate solution has remained particularly intriguing. Now, by using a flow reactor system, Leroy Cronin and co-workers at the University of Glasgow and Bielefeld University in Germany have been able to study the assembly of a {Mo₁₅₀} wheel and isolate an intermediate structure in which its central cavity is occupied¹.

Through continuous addition of nitric acid — which is both an acid and an oxidant — the researchers precisely controlled the reduction of the reaction mixture. They observed the crystallization of an incompletely reduced {Mo₁₅₀} wheel hosting a {Mo₃₆} cluster. The two polyoxomolybdate anions, held together by intercalated sodium cations, have matching elliptical shapes. On further reduction, the {Mo₃₆} cluster is expelled to give the well-known hollow {Mo₁₅₀} ring, suggesting the smaller cluster acts as a transient template. This is also supported by the fact that adding the {Mo₃₆} cluster directly to the initial reaction mixture considerably helps the self-assembly process.

Such an ephemeral templating approach could be used in future strategies to prepare other open architectures. Furthermore, this flow system should enable chemists to probe other self-assembly mechanisms.