Synthesis and Characterization of Alkylammonium Hyponitrites and Base-Stabilized Hyponitrous Acid Salts

A number of alkylammonium and bipyridinium salts of hyponitrous acid, namely, N,N,N‘,N‘-tetraethylethylenediammonium (3 and 4), N,N,N‘,N‘-tetramethylethylenediammonium (5), triethylenediammonium (6), diquinuclidinium (7), 2,2‘-bipyridinium (8), 4,4‘-bipyridinium (9), 4,4‘-trimethylenebis(1-methylpiperidinium) (10), 4,4‘-trimethylenepiperidinium (11), and bis(triethylammonium) (12) hyponitrite salts, have been synthesized by the reaction of the corresponding amine in either anhydrous diethyl ether or absolute ethanol with hyponitrous acid solution in anhydrous diethyl ether. Single-crystal X-ray crystallographic data were obtained for sodium hyponitrite (1), and the thermal decomposition behavior of the salt was examined. The new salts were characterized by IR and Raman spectroscopic data and elemental analyses. Compounds 4 and 7−9 were also characterized by single-crystal X-ray crystallography. The hyponitrite anions in 1 and 7−9 exhibit similar structural features. The anions are planar with average NN and N−O bond distances of 1.237 and 1.380 Å, respectively. The monoprotonated hyponitrite anion and solvated hyponitrous acid molecules present in 4 also exhibit close structural similarity to the dianion in the crystals of 1 and 7−9. Most of these salts are soluble in organic solvents. The UV−vis spectra of the sodium and alkylammonium salts (1, 3−7, and 10−12) in aqueous 0.1 M NaOH exhibit an absorption peak at 248 nm (λmax) with a molar extinction coefficient of 7033 ± 153 M-1 cm-1. Differential scanning calorimetric data for the salts reveal exothermic decomposition of the hyponitrite species. Significantly, the pentahydrate and anhydrous forms of Na2N2O2 exhibit distinct thermal decomposition behavior. The thermogram of the pentahydrate exhibits two exotherms at ca. 96 and 382 °C, whereas that of the anhydrous Na2N2O2 exhibits a single exotherm at ca. 382 °C. The exotherm at 96 °C observed for the pentahydrate is explained in terms of a coupled phase transition−dehydration process. The alkylammonium and bipyridinium salts undergo exothermic decomposition at a considerably lower temperature in the range 67−170 °C. Electrochemistry of the salts in acetonitrile solvent reveal an irreversible oxidation at ∼1.80 V vs an Ag/AgCl reference electrode.