A Bowl-Type Dodecavanadate as a Halide Receptor

The
dodecavanadate framework, [V12O32]4–, exhibits a unique bowl-type structure with an open molecular oxide
cage having a cavity diameter of 4.4 Å, and different synthetic
paths were required to construct the bowl-type structure with a different
guest. A new dodecavanadate, {(n-C4H9)4N}4[V12O32(CH3NO2)] (1), is synthesized with a nitromethane
guest, which is stacked above the entrance of the hemisphere rather
than fully occupying the cavity, and it enables a guest-capturing
reaction, while retaining the anionic cage structure. Compound 1 is a good precursor for halide-centered dodecavanadates,
{(C2H5)4N}5[V12O32(X)] (X = Cl– (2), Br– (3), and I– (4)). The position of the halide inside the cavity correlates
with the ionic radius of the guest; the small chloride ion sat at
the far bottom, and the large iodide floated at the entrance. The
inclusion reaction rates were estimated through 51V NMR
time-course measurements in nitromethane. The reaction rates increase
in the order I– < Br– <
Cl–.