Salicyl­aldoximium 3,5-di­nitro­salicyl­ate dihydrate

The crystal structure of the dihydrate of the 1:1 proton-transfer compound of 3,5-di­nitro­salicylic acid with 2-hydroxy­benz­aldehyde oxime (salicyl­aldoxime), C₇H₈NO₂⁺·C₇H₃­N₂O₇⁻·2H₂O, is a three-dimensional hydrogen-bonded polymer. In this, the oxime-N atom of salicyl­aldoxime is protonated and, together with the adjacent O—H group, gives primary cyclic asymmetric hydrogen-bonding associations with both carboxyl O atoms of the acid. These, together with hydrogen-bonding interactions among all species present, including the two water mol­ecules, give an essentially planar ten-membered ring. Peripheral hydrogen bonding also links the water mol­ecules to carboxyl and nitro-O atoms, giving rise to a three-dimensional polymer structure.