Oxovanadium phosphates constitute a crystallochemically very rich family that, in turn, results in a seemingly intricate magnetochemistry including from isolated dimers to 3-D systems. This magnetic diversity is due, in part, to the possible participation of phosphate groups in the spin transfer between VIV centers. This way, 31P solid-state NMR becomes a key tool in determining the exchange paths involving phosphorus orbitals. The magnetic behavior of several layered oxovanadium phosphates M(VOPO4)2·4H2O (M = Na+, Ca2+, Ba2+, and Pb2+) has been investigated. Like it occurs in the case of other previously studied lamellar derivatives, the best fit of the temperature-dependent magnetic susceptibility data is obtained for a 2-D model. This is consistent with chairlike V(OPO)2V exchange pathways (i.e., through di-μ-(O,O‘)phosphate bridges). Both these results and those previously available for other oxovanadium phosphates can be nicely correlated with the structural data focusing the attention on the topological features concerning the environment of the phosphate bridging entities, a result that is consistently rationalized on the basis of extended Hückel calculations. The reliability of our approach is supported not only by its ability to interpret the diversity of experimental magnetic behavior but also by its predictive character concerning the approximate magnitude of the superexchange interactions in other related derivatives.

A unified synthetic strategy has allowed us to rationalize the preparative chemistry of the layered oxovanadium phosphates M(VOPO(4))(2).nH(2)O. Thus, we have been able to isolate as single phases with reasonable yields both all the previously characterized phosphates and a new solid containing Ba(2+) cations as guest species as well as to prepare new related derivatives involving arsenate anions. In order to organize the experimental results, we have used two complementary models: a simple restatement of the partial charge model (PCM), and the valence matching principle (VMP) (derived from the bond valence method). The crystal structure of the new barium lamellar derivative, Ba(VOPO(4))(2).4H(2)O, has been solved from X-ray single crystal data. The cell is monoclinic (space group Pn; Z = 1) with a = 6.3860(3) Å, b = 12.7796(9) Å, c = 6.3870(5) Å, and beta = 90.172(6) degrees. Its structure, like it occurs with the other members of the M(VOPO(4))(2).nH(2)O family, can be thought of as derived from that of the well-known lamellar solid VOPO(4)(.)2H(2)O. Ba(2+) cations are located in the interlamellar space with an environment defined by 12 oxygen atoms. A comparative study of this family shows significant crystallochemical correlations with the radius of the guest cations.

Hexagonal mesostructured mixed-valence oxovanadium phosphates [CTA] x VOPO4·zH2O, in short ICMUV-2, have been synthesized through a S+I- cooperative mechanism using cationic surfactant (cetyltrimethylammonium, CTAB) rodlike micelles as a template. On the lines of the hypothesis that the driving force leading to the formation of mesostructured solids is the charge density matching at the interface between the supramolecular−organic and supramolecular−inorganic moieties, the self-assembling process between CTA+ micelles and VOPO4 q - planar anions can be thought of as consequence of the adequate adjustment of the metal mean oxidation state. X-ray powder diffraction and TEM techniques show that the solids consist of hexagonal arrays of mesopores filled with surfactant, whereas spectroscopic results allow us to propose an oxovanadium phosphate bond topology similar to that observed in the Na x VOPO4·nH2O layered derivatives. The thermal behavior of the mesostructured materials has also been investigated, given that both the easy elimination of CTA+ species and their V:P = 1:1 molar ratio make ICMUV-2 solids adequate pyrolytic precursors of the (VO)2P2O7 catalyst.

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