Crystal structures of sodium-, lithium-, and ammonium 4,5-di-hydroxy-benzene-1,3-di-sulfonate (tiron) hydrates

The solid-state structures of the Na⁺, Li⁺, and NH₄⁺ salts of the 4,5-di-hydroxy-benzene-1,3-di-sulfonate (tiron) dianion are reported, namely disodium 4,5-di-hydroxy-benzene-1,3-di-sulfonate, 2Na⁺·C₆H₄O₈S₂^2-, μ-4,5-di-hydroxy-benzene-1,3-di-sulfonato-bis-[aqua-lithium(I)] hemihydrate, [Li₂(C₆H₄O₈S₂)(H₂O)₂]·0.5H₂O, and di-ammonium 4,5-di-hydroxy-benzene-1,3-di-sulfonate monohydrate, 2NH₄⁺·C₆H₄O₈S₂^2-·H₂O. Inter-molecular inter-actions vary with the size of the cation, and the asymmetric unit cell, and the macromolecular features are also affected. The sodium in Na₂(tiron) is coordinated in a distorted octa-hedral environment through the sulfonate oxygen and hydroxyl oxygen donors on tiron, as well as an inter-stitial water mol-ecule. Lithium, with its smaller ionic radius, is coordinated in a distorted tetra-hedral environment by sulfonic and phenolic O atoms, as well as water in Li₂(tiron). The surrounding tiron anions coordinating to sodium or lithium in Na₂(tiron) and Li₂(tiron), respectively, result in a three-dimensional network held together by the coordinate bonds to the alkali metal cations. The formation of such a three-dimensional network for tiron salts is relatively rare and has not been observed with monovalent cations. Finally, (NH₄)₂(tiron) exhibits extensive hydrogen-bonding arrays between NH₄⁺ and the surrounding tiron anions and inter-stitial water mol-ecules. This series of structures may be valuable for understanding charge transfer in a putative solid-state fuel cell utilizing tiron.