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Three salts of diazonium cation (C 4 H 5 N 2 + ) encapsulated R-Keggin phosphododecamolybdate, (pyridazinium) 3 [PMo 12 O 40 ]‚ 1 / 2 H 2 O (1), (pyrimidinium) 3 [PMo 12 O 40 ]‚3H 2 O (2), and (pyrazinium) 3 [PMo 12 O 40 ]‚3H 2 O (3) have been synthesized at room temperature and atmospheric pressure in an organic-aqueous medium. Compounds 1-3 crystallize in the trigonal space group R3 h. Crystal data for compound 1 is as follows: a ) 17.704(6) Å, c ) 22.918(3) Å, V ) 6221(3) Å 3 , Z ) 6, R ) 0.028 for 3708 observed reflections with I > 3σ(I); for compound 2, a ) 19.022(1) Å, c ) 21.524(1) Å, V ) 6745(1) Å 3 , Z ) 6, and R ) 0.043 for 2143 observed reflections with I > 2σ(I); for compound 3, a ) 17.820(2) Å, c ) 24.369(2) Å, V ) 6701(1) Å 3 , Z ) 6, and R ) 0.047 for 3699 observed reflections with I > 3σ(I). These compounds contain the Keggin clusters arranged in a three-dimensional tunnel structure where the organic templates are sandwiched by two polyanions. Although the three salts are isostructural, the arrangement of organic cations and water molecules relative to the polyanion framework shows some significant differences that have been analyzed by means of ab initio calculations. Differences in the thermal behavior of the three compounds have also been explained on the basis of the different crystal arrangements.

Key indicatorsSingle-crystal X-ray study T = 293 K Mean '(C±C) = 0.004 A Ê R factor = 0.058 wR factor = 0.155 Data-to-parameter ratio = 19.6For details of how these key indicators were automatically derived from the article, see

The compound tetrakis(tert-butylammonium)-cyclo-tetrametavanadate(V), the first oxovanadate salt isolated from aqueous solution which contains a discrete unprotonated [V 4 O 12 ] 4anion, undergoes an irreversible, reconstructive, continuous, and heterogeneous phase transition in the solid state by means of a nucleation and growth mechanism. This transformation yields a polymeric metavanadate of tert-butylammonium and it has been followed by X-ray powder diffraction and FTIR techniques. The polymeric compound has also been isolated from aqueous solution and its crystal structure resolved by single-crystal X-ray diffraction methods, this compound crystallizes in the monoclinic space group P2 1 , a ) 10.397(2) Å, b ) 5.873(1) Å, c ) 13.879(4) Å, β ) 111.02(3)°, V ) 791.1(3) Å 3 , Z ) 4, and R ) 0.026 for 1926 observed reflections with I g 3σ(I). Comparison of the polymeric and cyclic metavanadate structures reveals several structural similarities such as V‚‚‚V contacts and V-O-V angles. A possible transformation mechanism has been suggested on the basis of these structural analogies.

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