In zeolites and other rigid solid-state oxides, substrates whose sizes exceed the pore dimensions of the material are rigorously excluded. Now, using a porous 3 nm diameter capsule-like oxomolybdate complex [{Mo^VI₆O₂₁(H₂O)₆} 12{(Mo^V₂O₄)₃₀(OAc)₂₁(H₂O)₁₈}]^33- as a water-soluble analogue of solid-state oxides (e.g., as a soluble analogue of 3 Å molecular sieves), we show that carboxylates (RCO₂^–) can negotiate passage through flexible Mo₉O₉ pores in the surface of the capsule and that the rates follow the general trend R = 1° 2° > 3° phenyl (no reaction). Surprisingly, the branched alkanes (R = iso-Pr and tert-Bu) enter the capsule even though they are larger than the crystallographic dimensions of the Mo₉O₉ pores. Four independent lines of spectroscopic and kinetic evidence demonstrate that these organic guests enter the interior of the capsule through its Mo₉O₉ apertures and that no irreversible changes in the metal oxide framework are involved. This unexpected phenomenon likely reflects the greater flexibility of molecular versus solid-state structures and represents a sharp departure from traditional models for diffusion through porous solid-state (rigid) oxides.

(Cover)