Chemistry of the potassium, silver, and tetra(n-butyl)ammonium salts of sydnone N-oxide (Traube's anion)

Abstract

Traube's dianion, 4-methylcarboxysydnone-3-hydroxylate (2), and its monomethyl ester (1) are readily isolated as their di- and mono-potassium salts from the reaction of nitric oxide with dimethylmalonate and potassium methoxide in methanol. Metathesis reactions of 1 with silver nitrate and tetra(n-butyl)ammonium bromide yield the silver and tetra(n-butyl)ammonium salts of 4-methylcarboxysydnone-3-hydroxylate. In contrast to other known sydnone derivatives, the present sydnones are stable in both acidic and basic solutions. Single crystal X-ray diffraction data obtained for 2 and the silver salt (3) reveal planarity and delocalization of double bonds in the sydnone ring, indicating aromatic behavior. The tetra(n-butyl)ammonium salt (4) is soluble in a range of organic compounds. Cyclic voltammetric data measured for 4 in acetonitrile reveal an irreversible oxidation peak at +1.09 V vs. Fc–Fc+ couple suggesting that the sydnone derivatives can be oxidized with an appropriate oxidizing agent. Differential scanning calorimetry data reveal thermal stability at ambient conditions and exothermic decomposition at high temperatures (>229 °C). The sydnone derivatives are characterized by two strong electronic absorptions in the UV region and rich vibrational (IR and Raman) spectra in the 1800–1500 cm–1 region. Density functional theory (B3LYP/6-311+++G**) is applied to estimate the aromaticity of the sydnone ring and to assign the vibrational spectral data.Key words: sydnone, trans-diazeniumdiolate, N-oxide.