Isolation of a Nitromethane Anion in the Calix-Shaped Inorganic Cage.

A calix-shaped polyoxometalate, [V12O32]4- (V12), stabilizes an anion moiety in its central cavity. This molecule-sized container has the potential to control the reactivity of an anion. The highly-reactive cyanate is smoothly trapped by V12 to form [V12O32(CN)]5-. In the CH3NO2 solution, cyanate abstracts protons from CH3NO2, and the resultant CH2NO2- is stabilized in V12 to form [V12O32(CH2NO2)]5- (V12(CH2NO2)). A crystallographic analysis revealed the double-bond characteristic short bond distance of 1.248 Å between the carbon and nitrogen atoms in the nitromethane anion in V12. 1H and 13C NMR studies showed that the nitromethane anion in V12 must not be exchanged with the nitromethane solvent. Thus, the V12 container restrains the reactivity of anionic species.

Synthesis and structural characterization of tube-type tetradecavanadates.

By the reaction of ammonium perchlorate with anion-incorporated bowl-type dodecavanadates, viz. [V12O32(X)]5- [X = N3- (1), OCN- and NO3-], tube-type tetradecavanadates, viz. (NH4)7[V14O38(X)] [X = N3- (2), OCN- (3) and NO3- (4)] were synthesized. The crystal structures of penta(tetraethylammonium) azidododecavanadate nitromethane monosolvate, (C8H20N)5[V12O32(N3)]·CH3NO2, 1, heptaammonium azidotetradecavanadate dimethyl sulfoxide hexasolvate, (NH4)7[V14O38(N3)]·6C2H6OS, 2, heptaammonium cyanatotetradecavanadate dimethyl sulfoxide hexasolvate, (NH4)7[V14O38(OCN)]·6C2H6OS, 3, and heptaammonium nitratotetradecavanadate dimethyl sulfoxide hexasolvate, (NH4)7[V14O38(NO3)]·6C2H6OS, 4, were determined. The tube consists of two layers of V7 rings with a guest anion at the centre. The distances between the incorporated anions and the nearest V atoms are 3.058 (3), 3.039 (6) and 2.811 (9) Šfor 2, 3 and 4, respectively, showing that the incorporated anions are stabilized via noncovalent interactions. Two ammonium cations cap both ends of the tube to stabilize the structures via hydrogen-bonding interactions. Linear OCN- and N3- anions sit on the twofold rotation axes of the tube frameworks and the triangular plane of the NO3- anion deviates from the equatorial plane of the tube by ca 30°.

Solid-State Umbrella-type Inversion of a VO5 Square-Pyramidal Unit in a Bowl-type Dodecavanadate Induced by Insertion and Elimination of a Guest Molecule.

Design of cavities for a target molecule and the elucidation of the corresponding host-guest interactions are important for molecular manipulation. A discrete dodecavanadate bowl, [V12 O32 ]4- (V12), with an entrance diameter of 4.4 Šand an electron-rich guest at the center of the bowl, was stabilized by electrostatic interactions. A characteristic of V12 is a solid-state polytopal rearrangement during guest elimination and recapture. A guest-free dodecavanadate, [V12 O32 ]4- (V12-free), was prepared by removal of the guest from CH2 Cl2 -inserted V12 under vacuum at 50 °C. Single-crystal X-ray analysis revealed that one of the VO5 square pyramids at the bottom of V12-free was inverted to fill the void of the bowl cavity. The exposure of V12-free to the guest molecule vapors of CH2 Cl2 , 1,2-dichloroethane, MeNO2 , MeCN, and MeBr resulted in the selective insertion of the guest to reform the guest-inserted V12 structure. Whereas CO2 could be inserted in the V12 bowl, CH4 and CO could not.

Small-Molecule Anion Recognition by a Shape-Responsive Bowl-Type Dodecavanadate.

A dodecavanadate, [V12 O32 ]4- , is an inorganic bowl-type host with a cavity entrance with a diameter of 4.4 Šin the optimized structure. Linear, bent, and trigonal planar anions are tested as guest anions and the formation of host-guest complexes, [V12 O32 (X)]5- (X=CN- , OCN- , NO2- , NO3- , HCO2- , and CH3 CO2- ), were confirmed by X-ray crystallographic analyses and a 51 V NMR spectroscopy study. The degree of distortion of the bowl from a regular to an oval shape depends on the type of guest anion. In 51 V NMR spectroscopy, all chemical shifts of the host-guest complexes are clearly shifted after guest incorporation. The incorporation reaction rates for OCN- , NO2- , HCO2- , and CH3 CO2- are much larger than those of NO3- and halides. The incorporated nonspherical molecular anions in the dodecavanadate host are easily dissociated or exchanged for other anions, whereas spherical halides in the host are preserved without dissociation, even in the presence of the tested anions.

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-.

A chloride capturing system via proton-induced structure transformation between opened- and closed-forms of dodecavanadates.

Chloride-incorporated dodecavanadates show two distinct structures of the monoprotonated-form [HV12O32(Cl)](4-) (closed-V12) with a spherical closed-structure and the opened-form [V12O32(Cl)](5-) (opened-V12). The reaction of closed-V12 with a stoichiometric amount of ethylenediamine drives the structure transformation reaction to opened-V12, quantitatively. From time dependent observations of (51)V NMR, a tube-type intermediate [V12O32(Cl)](5-) (tube-V12) was observed in the transformation process. Isolation of the intermediate was achieved by the deprotonation reaction of closed-V12 with diethylamine, and the structure transformation was confirmed by using the isolated intermediate. The reverse transformation from opened-V12 to closed-V12 was also achieved by addition of trifluoroacetic acid. The geometrical difference between closed-V12 and opened-V12 is reflected in the reactivity difference to the external reagents, and this was demonstrated by examining the chloride removal reaction by using a silver cation. The incorporated chloride was preserved in the closed-V12 cage even in the presence of a silver cation. In contrast, the chloride in opened-V12 was removed as AgCl by the silver cation. In addition, by the reaction of chloride-free opened-V12 with a quantitative amount of {Et4N}Cl retrieved opened-V12, showing the capability of opened-V12 to recapture a guest chloride in the cavity. This transformation between two isomeric dodecavanadate structures is regarded as the movement of a molecular mitt to catch a ball and secure it.