Molybdenum oxide-based porous capsules sound like something rigid, but researchers have found one that adapts to circumstance. Achim Müller of the University of Bielefeld, in Germany, earlier assembled 3-nm-diameter spheres from molybdenum oxide rings (which act as the pores), pentagons, and linking groups. When Ira A. Weinstock of Ben Gurion University of the Negev, in Beer Sheva, Israel, and coworkers resynthesized the capsules and studied their properties, they found that carboxylates larger than the pores can make their way to the interior (J. Am. Chem. Soc., DOI: 10.1021/ja900452d). They used four types of spectroscopic and kinetic experiments to demonstrate conclusively that carboxylates enter the complex through the smaller pores without changing the capsule structure irreversibly, suggesting that the pores adjust to accommodate the carboxylates. "The surprising behavior of this molecular open framework likely reflects the greater flexibility of molecular versus solid-state structures and represents a distinct departure from traditional models for diffusion through porous solid-state (rigid) oxide materials," the researchers write. The capsules might prove useful for promoting reactions of compounds that pass through the pores.