Soluble metal-oxide frameworks: Transport of large organic guests through smaller subnanoscale pores and the effects of nano-confinement on hydrophobic self-assembly and organic reactions

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In zeolites and rigid functional materials, substrates whose sizes exceed those of the pore dimensions are rigorously excluded. Meanwhile, cavities within flexible metal-organic frameworks (MOFs; another class of porous solid-state structures) can accommodate substrates by reversible, stepwise structural expansion (“breathing”). Now, using a “capsule”-like molybdenum-oxide-based framework as a soluble analog of porous solid-state (rigid) oxides, we recently reported a new phenomenon: the passage of branched-alkane “guests” through flexible sub-nanometer Mo9O9 apertures whose geometrical diameters are smaller than the entering species (“Flexible Pores of a Metal-Oxide-Based Capsule Permit Entry of Comparatively Larger Organic Guests” J. Am. Chem. Soc., 2009, 131, 6380-6382). This new phenomenon likely reflects the greater flexibility of molecular versus solid-state structures, and represents a distinct departure from traditional models for diffusion through rigid porous solid-oxides. The immediate goal of proposed research is to investigate the fundamental mechanism(s) responsible for the uptake of large guests by comparatively smaller metal-oxide pores. The next objective is to use the metal-oxide capsules as “nano-reactors” for catalytic accelerations of reaction rates through the templating effects of proximal binding, and/or hydrophobic self-assembly within the capsule’s nano-confined interiors. In all the above, the metal-oxide frameworks will be deployed as molecular analogs for porous solid-state metal oxides, whose reactions and their mechanisms can now be studied in detail by solution-state methods.