Reaction of nitrogen oxides with salts of metal-substituted heteropolyanions

Abstract

We have used IR spectroscopy to study the reaction with NO and NO₂ of solid tetramethylammonium and cesium salts of heteropolyanions (HPA) PW₁₁M(L) ⁿ⁻_0.39 [M=V(V), Cr(III), Mn(II), Fe(II, III), Co(II), Ni(II), Cu(II); L=H₂O, OH⁻ or O²⁻], preevacuated at 110°C or 300°C. Only in the case of Fe(II)-substituted heteropolyanions are nitrosyl complexes formed: PW₁₁Fe(NO)O ⁵⁻₃₉ (vNO=1730 cm⁻¹), which leads to stabilization of the NO molecules with respect to oxidation by oxygen. We observed reversible reaction with NO₂ by the listed heteropoly complexes. The vibrational frequencies of adsorbed NO₂ are virtually independent of the metal M and its coordination environment in the heteropolyanion (v₁=1335−1360) and v₃=1620−1640 cm⁻¹). This provides a basis for assuming that the NO₂ groups are bonded to the oxygen atoms in the heteropoly anion with formation of the fragments O_HPA...NO₂.