A Q- and X-Band Pulsed Electron Nuclear Double Resonance Study of the Structure and Location of the Vanadyl Ions in the Cs Salt of Heteropolyacid PVMo₁₁O₄₀

The location and coordination geometry of vanadium(IV) ions in the cesium salt of molybdovanadophosphoric heteropolyacid Cs₄PVMo ₁₁O₄₀ were studied using orientation-selective pulsed ENDOR (electron nuclear double resonance) experiments. To enhance the orientation selectivity for the paramagnetic vanadyl species, these investigations were done at Q-band frequencies. It was possible to resolve interactions of the paramagnetic vanadyl ions (VO²⁺) with all relevant nuclei, ¹H, ³¹P, ⁵¹V, and ¹³³Cs. The location of the vanadyl species was studied by determination of the complete ³¹P hyperfine tensor. This approach was done for both the fresh and the calcined Cs₄PVMo ₁₁O₄₀ materials, and no differences in the structures of the VO²⁺ complexes were found. The ENDOR results give experimental evidence for the location of the V(IV) ions. For both samples, it was possible to exclude the incorporation of V(IV) at the Mo sites. The VO²⁺ species are directly attached to the outer surface of the heteropolyanion and coordinated to four of the outer oxygen atoms with a V-P distance of 3.96 Å.