Synthesis and characterization of alkylammonium hyponitrites and base-stabilized hyponitrous acid salts

A number of alkylammonium and bipyridinium salts of hyponitrous acid, namel y, N,N,N',N'-tetraethylethylene-diammonium (3 and 4), N,N,N',N'-tetramethyl ethylenediammonium (5), triethylenediammonium (6), diquinuclidinium (7), 2, 2'-bipyridinium (8), 4,4'-bipyridinium (9), 4,4'-trimethylenebis(1-methylpi peridinium) (10), 4,4'-trimethylenepiperidinium (11), and bis(triethylammon ium) (12) hyponitrite salts, have been synthesized by the reaction of the c orresponding amine in either anhydrous diethyl ether or absolute ethanol wi th hyponitrous acid solution in anhydrous diethyl ether. Single-crystal X-r ay crystallographic data were obtained for sodium hyponitrite (1), and the thermal decomposition behavior of the salt was examined. The new salts were characterized by LR and Raman spectroscopic data and elemental analyses. C ompounds 4 and 7-9 were also characterized by single-crystal X-ray crystall ography. The hyponitrite anions in 1 and 7-9 exhibit similar structural fea tures. The anions are planar with average N=N and N-O bond distances of 1.2 37 and 1.380 Angstrom, respectively. The monoprotonated hyponitrite anion a nd solvated hyponitrous acid molecules present in 4 also exhibit close stru ctural similarity to the dianion in the crystals of 1 and 7-9. Most of thes e salts are soluble in organic solvents. The UV-vis spectra of the sodium a nd alkylammonium salts (1, 3-7, and 10-12) in aqueous 0.1 M NaOH exhibit an absorption peak at 248 nm (lambda(max)) with a molar extinction coefficien t of 7033 +/- 153 M-1 cm(-1). Differential scanning calorimetric data for t he salts reveal exothermic decomposition of the hyponitrite species. Signif icantly, the pentahydrate and anhydrous forms of Na2N2O2 exhibit distinct t hermal decomposition behavior. The thermogram of the pentahydrate exhibits two exotherms at ca. 96 and 382 degrees C, whereas that of the anhydrous Na 2N2O2 exhibits a single exotherm at ca. 382 degrees C. The exotherm at 96 d egrees C observed for the pentahydrate is explained in terms of a coupled p hase transition-dehydration process. The alkylammonium and bipyridinium sal ts undergo exothermic decomposition at a considerably lower temperature in the range 67-170 degrees C. Electrochemistry of the salts in acetonitrile s olvent reveal an irreversible oxidation at similar to 1.80 V vs an Ag/AgCl reference electrode.