Synthesis and crystal structure of mixed metal(III) tungstenyl(VI) ortho-pyrophosphates

• In the quasi-binary system VVOPO4-WVOPO4 the new compound VIII(WVIO2)2(P2O7)(PO4) has been discovered.
• Internal redox and Lux-Flood acid-base reaction lead to stabilization during solid solution formation.
• VIII(WVIO2)2(P2O7)(PO4) represents the first anhydrous phosphate containing a tri- and a hexavalent transition metal.
• Vanadium(III) can be substituted by a wide range of trivalent cations.
• With titanium(III) the mixed ortho-pyrophosphate does not exist due to the redox reaction Ti3+ + W6+ → Ti4+ + W5+.

The series of isotypic anhydrous ortho-pyrophosphates MIII(WVIO2)2(P2O7)(PO4) (M: Sc, V, Cr, Fe, Mo, Ru, Rh, In, Ir) was obtained via vapor phase moderated solid state reactions in sealed ampoules. The crystal structure of the phosphates MIII(WVIO2)2(P2O7)(PO4) (M: V, Ru, Rh) was solved from single crystal X-ray data (C2/c, Z = 16). Fairly regular MO6 and distorted WO6 octahedra share vertices with PO4 and P2O7 units to form a 3D network. For the ortho-pyrophosphates with M: V3+, Cr3+, and Fe3+ the oxidation state of M is confirmed by magnetic measurements. 31P-MAS-NMR spectra of the diamagnetic phosphates MIII(WVIO2)2(P2O7)(PO4) (M: Sc, In, Ir) show surprisingly different isotropic chemical shifts for the seven phosphorus sites. VIII(WVIO2)2(P2O7)(PO4) occurs as equilibrium phase in the quasi-binary system (V1–xWx)OPO4 at x = 0.67 and exhibits a small homogeneity range 0.60 ≤ x ≤ 0.67. The scandium compound shows a fully inverted occupancy of the M sites according to the formulation W(Sc1/2W1/2O2)2(P2O7)(PO4).

VIII(WVIO2)2(P2O7)(PO4) is the first anhydrous phosphate containing a tri- and a hexavalent transition metal. Its formal composition (V1/3W2/3)OPO4 relates it to the quasi-binary system VVOPO4-WVOPO4. By internal redox and Lux-Flood acid-base reaction obviously a stabilization with respect to the crystal structures of the end members of the binary system is achieved. Vanadium(III) can be substituted by a wide range of trivalent cations (M3+: Sc, Cr, Fe, Mo, Ru, Rh, In, Ir).Figure optionsDownload as PowerPoint slide