Crystal structures of sodium-, lithium-, and ammonium 4,5-dihydroxybenzene-1,3-disulfonate (tiron) hydrates

The first solid-state structures of the Na+, Li+, and NH4 + salts of the tiron dianion are reported. The structures reveal significant changes in local and long-range ionic interactions with variation of the cation, with consequences for fields as disparate as bioinorganic chemistry and fuel cell technologies.

The solid-state structures of the Na + , Li + , and NH 4 + salts of the 4,5-dihydroxybenzene-1,3-disulfonate (tiron) dianion are reported, namely disodium 4,5-dihydroxybenzene-1,3-disulfonate, 2Na + ÁC 6 H 4 O 8 S 2 2À , -4,5-dihydroxybenzene-1,3-disulfonato-bis[aqualithium (I)] hemihydrate, [Li 2 (C 6 H 4 O 8 S 2 )(H 2 O) 2 ]Á0.5H 2 O, and diammonium 4,5-dihydroxybenzene-1,3disulfonate monohydrate, 2NH 4 + ÁC 6 H 4 O 8 S 2 2À ÁH 2 O. Intermolecular interactions vary with the size of the cation, and the asymmetric unit cell, and the macromolecular features are also affected. The sodium in Na 2 (tiron) is coordinated in a distorted octahedral environment through the sulfonate oxygen and hydroxyl oxygen donors on tiron, as well as an interstitial water molecule. Lithium, with its smaller ionic radius, is coordinated in a distorted tetrahedral environment by sulfonic and phenolic O atoms, as well as water in Li 2 (tiron). The surrounding tiron anions coordinating to sodium or lithium in Na 2 (tiron) and Li 2 (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, (NH 4 ) 2 (tiron) exhibits extensive hydrogen-bonding arrays between NH 4 + and the surrounding tiron anions and interstitial water molecules. This series of structures may be valuable for understanding charge transfer in a putative solid-state fuel cell utilizing tiron.

Chemical context
Catechols play important roles across many areas of chemistry and biology. Their rich coordination chemistry with metal ions (Pierpont & Lange, 1994;Sever & Wilker, 2004) emerges for example in siderophores (Boukhalfa & Crumbliss, 2002;Raymond et al., 2015;Springer & Butler, 2016). One catecholcontaining siderophore, enterobactin (ent) has the strongest characterized Fe III complex to date (K a = 10 49 ) (Loomis & Raymond, 1991). Catechols are also key to the function of some marine bioadhesives (Lee et al., 2011); in one recent example, a protein in sessile marine organisms uses a cooperation between surface residues containing 3,4-dihydroxyphenylalanine (DOPA) and lysine to bind strongly to mineral surfaces (Rapp et al., 2016). Some species of ascidians produce a polyphenol-containing molecule called tunichrome that has been implicated in metal binding and/or metal function (Sugumaran & Robinson, 2012). ISSN 2056-9890 Upon binding to metal cations such as Fe III and Ti IV , catechols typically form brightly colored complexes (Sever & Wilker, 2004;Pierpont & Lange, 1994). In solution, however, some catechols can oxidize and form polymers, thus forming metal complexes that are more difficult to characterize. Compared to unsubstituted catechol, tiron (4,5-dihydroxy-1,3benzenedisulfonic acid, Fig. 1) allows for improved water solubility as well as reduced polymerization by substituting electron-withdrawing sulfonic acid moieties (Sommer, 1963a,b). Tiron has long been used for colorimetric determination of both Ti IV and Fe III (Yoe & Armstrong, 1945, 1947, hence its name. The free acid of tiron has been used in an aqueous flow battery because of its two-electron redox couple within range of an aqueous system, high water solubility, and low cost (Yang et al., 2014). When crystallized, tiron molecules can form a network through coordination of the counter-cation to the sulfonate or protonated or deprotonated hydroxide of the tiron (Cô té & Shimizu, 2001Sheriff et al., 2003;Guan & Wang, 2016, 2017. These networks can range from onedimensional networks, which form a linear polymer (Cô té & Shimizu, 2003;Sheriff et al., 2003), to three-dimensional networks in which each tiron anion is coordinated to a metal cation and forms an interconnected lattice among all tiron anions in the crystal (Cô té & Shimizu, 2001Guan & Wang, 2016). Many of these tiron-containing crystal structures exhibit counter-cation-dependent luminescent properties (Guan & Wang, 2016, 2017. The three-dimensional networks with tiron can absorb H 2 S gas after interstitial and coordinated H 2 O are liberated with heat (Cô té & Shimizu, 2003).
Currently, examples of three-dimensional networks formed by tiron and cations are relatively rare. Presented here are the first two examples of the preparation and characterization of the Li + tiron salt and Na + tiron salt, which forms a threedimensional network. In addition to Li 2 (tiron) and Na 2 (tiron), the preparation and crystallization of the NH 4 + tiron salt is reported. This species is the first tiron salt which utilizes a counter-cation capable of hydrogen bond (H-bond) donation to allow for a complex H-bonding network.
All asymmetric units (Fig. 2) contain two of their respective cations on general positions. Water is included in all asymmetric units, however in different amounts. Both sodium and ammonium tiron have one water molecule in the asymmetric unit, whereas lithium tiron has 2.5 water molecules in the asymmetric unit. The lithium tiron also exhibits rotational whole-molecule disorder leading to two possible placements of O1 on the phenyl, and representing a major and minor orientation [89.2 (3) and 10.8 (3)% occupancy, respectively].
The structure of Li 2 (tiron) is presented in the P2 1 /n space group. The lithium ion is coordinated by phenolic, sulfonate and water oxygen atoms. Lithium is bonded to only three sulfonate moieties, and one water molecule in a distorted tetrahedral geometry. An extensive H-bonding network with three types of solvate water molecules stabilizes the crystal structure (Table 1, Fig. 3). The geometrically frustrated water molecule containing O10 sits in a pocket surrounded by Hbond donors and acceptors from sulfonate (O6, O4), and water (O9), and phenol (O2). As a result of the frustration, O10 is highly disordered, modeled with a two-site split-atom model that additionally exhibits special-position disorder about the inversion element at Wyckoff position d. The result is a four-site disorder model for the water molecule containing O10. The lithium-bound water molecule containing O11 Hbonds with sulfonate oxygen atoms O4 and O8, but the oxygen atom is disordered, pyramidalized predominantly toward the phenolic O-H hydrogen atom of O2 due to H-bonding, but with a minor component pyramidalized toward O1, which is less available as an H-bond acceptor since the phenolic hydrogen of O1 is already involved in an intramolecular ortho-H-bond with its own O2 (Fig. 2). The water molecule containing O9 is also lithium bound, but not disordered, and interacts with O10/10A of the disordered water and with sulfonate oxygen O3 and the phenolic hydrogen atom of O1.
The sodium salt of tiron is also presented in the P2 1 /n space group. Each sodium atom is bonded to four sulfonate moieties, one hydroxide, and one water oxygen atom to give a distorted octahedral geometry (Fig. 4). The two types of Na atoms are bridged to one another along the crystallographic a axis by O9 of a water ligand and by phenolic oxygen residue O1 on one side, and by sulfonate residues O6 and O5 on the other.
Finally, (NH 4 ) 2 (tiron) is presented in the Pbca space group. Ammonium is oriented around the negatively charged sulfonates, and acts as an H-bond donor to both sulfonates and neighboring water molecules (Table 3, Fig. 5). The structure of (NH 4 ) 2 (tiron) is well-ordered with a clear H-bonding network, discussed in more detail in the next section.

Figure 3
Ball and stick representation of Li 2 (tiron)Á2.5H 2 O, including the Hbonding environment of neighboring tiron anions and water showing all disorder components. Atom labels with the suffix D are generated via inversion through the center of symmetry at Wyckoff position d. Atom labels with the suffix A represent minor components of two-site disorder models.

Figure 4
Displacement ellipsoid plot of Na 2 (tiron)ÁH 2 O illustrating the pseudooctahedral coordination geometry around the Na ions. Ellipsoids shown at the 50% probability level. H atoms are shown as spheres. Atom labels with the suffix A are related by translation by one unit cell.

Figure 5
Displacement ellipsoid plot of asymmetric-unit contents for the crystal structure of (NH 4 ) 2 (tiron)ÁH 2 O, showing intramolecular H-bonding with ammonium ions and solvate water. Ellipsoids shown at the 50% probability level. Hydrogen atoms shown as spheres.
which -stack with one another about a crystallographic inversion center (Wyckoff position b), hence the rings are perfectly parallel. The other lithium ion, Li2, links through a sulfonate group (S1) of one tiron to the other sulfonate group S(2) of a third tiron, generating a 'square' assembly of tiron anions and two Li2 ions around a crystallographic inversion element (Wyckoff position a, Fig. 6). The distance of 3.718 (10) Å between the centroids of neighboring arene rings is consistent with a strong -stacking interaction, and suggests the interaction is augmented by the array of H-bonding interactions among phenolic hydroxyl and sulfonate groups and water (Table 1, Fig. 3).
The H-bonding in the sodium complex is entirely intermolecular (Table 2 Two types of sodium atoms arrange in channels along the crystallographic a-axis direction, and are bridged by sulfonyl oxygen atoms O3, O4, O5, and O6, by phenolic oxygen atom O1, and by water oxygen atom O9 (Fig. 7). Neighboring tiron arenes -stack along the crystallographic a-axis direction, related by crystallographic inversion centers (Wyckoff letters a and b), also requiring the arene rings to be parallel, as in the lithium salt. The -stacking interactions are further augmented by H-bonding interactions between the phenolic hydrogens of O1 and O2 with sulfonate oxygens O3 and O4 respectively. The -stacking distance of 3.753 (18) Å is similar to that observed in the lithium salt with its corresponding dense array of H-bonding interactions, but slightly shorter due to the more acute O-Na-O bond angles in octahedrally coordinated sodium atoms, in contrast to tetrahedrally coordinated lithium atoms in the lithium salt. This arrangement of Na + and tiron ions results in an ordered array of sodium channels interspersed between columns of strongly  Table 2 Hydrogen-bond geometry (Å , ) for Na 2 (tiron)ÁH 2 O.  (15) 158 (3) Symmetry codes: (i) Àx þ 2; Ày þ 1; Àz þ 1; (ii) Àx þ 1; Ày þ 1; Àz þ 1; (iii) x À 1; y; z.

Figure 6
Illustration of selected nearest neighbor lithium linkages of neighboring tiron anions in Li 2 (tiron)Á2.5H 2 O. Atom labels with the suffix B are generated via inversion through the center of symmetry at the center of the cell (Wyckoff b) and those with the suffix A are generated via inversion through the center of symmetry in the a-face (Wyckoff b). Ellipsoids shown at the 50% probability level. Hydrogen atoms are shown as spheres.

Figure 7
Packed structures of Na 2 (tiron)ÁH 2 O. Left: packing arrangement in the unit cell. Right: displacement ellipsoid plot showing alternating units in a -stacked arrangement in the crystallographic a-axis direction. Ellipsoids are shown at the 50% probability level, hydrogen atoms are shown as spheres.

Figure 8
Extended packing diagram of Na 2 (tiron)ÁH 2 O, showing the columnar arrangement of tiron aryl groups and parallel sodium ion channels.
Unlike Na + and Li + , NH 4 + cannot be coordinated by any atoms on tiron or water. Because of this inability, NH 4 + interactions with the surrounding molecules are primarily Hbond based. Both ammonium ions H-bond to three sulfonate moieties and an oxygen atom from a phenolic hydroxyl or a water molecule (Tables 3 and 4, Fig. 9). The ammonium ion containing N1 forms H-bonds with two tiron molecules that are horizontally next to each other in the unit cell as well as a tiron above and a tiron below. The ammonium ion containing N2 also forms a similar H-bonding network with the tiron molecules but also stabilizes an interstitial water. This water H-bonds to two first position sulfonate moieties in alternating layers of tiron molecules [O-HÁ Á ÁO-S 1.97 (2)  Regarding intramolecular H-bonding, because the protons on the hydroxyls are pointed away from each other to allow for H-bonding to N1, the phenolic hydroxyl containing O1 is directed to H-bond with O3 of the sulfonate (Fig. 9).
The (NH 4 ) 2 (tiron) packs such that the tiron units connect to one another along the crystallographic c-axis direction via a six-membered H-bonding array of two lattice water molecules, two ammonium ions (containing N2), and two sulfonate oxygen atoms (Fig. 9). The ammonium ions containing N1 further serve to link tiron units along the crystallographic baxis direction by H-bonding with sulfonate oxygen O8 and phenolic oxygen atom O1. Further, the arene -stacking

Figure 9
Displacement ellipsoid plot of (NH 4 ) 2 (tiron)ÁH 2 O showing neighboring H-bonding interactions among six tiron anions, four ammonium ions, and two water solvate molecules. Ellipsoids are shown at the 50% probability level and hydrogen atoms are shown as open spheres. Atom labels with the suffix B are generated by symmetry about a crystallographic center of inversion (Wyckoff position b), and atom labels with the suffix A are generated by one or more glide operation.  (4) 3.113 (5) 114 (3) Symmetry codes:
interactions and additional H-bonding interactions between sulfonate oxygen atoms and both ammonium ions link these strands to one another in the crystallographic a-axis direction to give the three-dimensional H-bonding network (Fig. 10

Database survey
In the reported structures, interactions with sulfonate moieties and the protonated hydroxyl moieties together create a complex network formed through coordinate bonds or Hbonds. A search of the Cambridge Structural Database (Version 5.39, February 2018; Groom et al., 2016) yielded several structures that included tiron (Table 5). Of the structures reported, seven exhibited -stacking interactions between at least two tiron molecules as represented by their intercentroid distances. A rarer structural feature of these complexes is the formation of networks between tiron molecules and their corresponding counter-cations in which only HUCMOH, ADOXUP, and HUCMOH02 form threedimensional networks by eliciting multiple bonds to the cations (Cô té & Shimizu, 2003;Guan & Wang, 2016). Both presented Li + and Na + tiron salts are the first examples of tiron-containing structures with monovalent cations that form three-dimensional networks. Furthermore, the NH 4 + tiron salt presented is the first example of a tiron complex in which the counter-cation H-bonds to the tiron.

Synthesis and crystallization
Na 2 (tiron)ÁH 2 O Na 2 (tiron)ÁH 2 O was used as received from the commercial source (97%, Sigma Aldrich) and added to water until it was saturated. The slurry was filtered into a 1 dram scintillation vial and covered with a Kimwipe. The solution was allowed to sit undisturbed at room temperature until the water evaporated. The crystals which developed were off-white needles.
Li 2 (tiron)Á2.5H 2 O In 2.00 mL of water, 0.100 g of Na 2 (tiron)ÁH 2 O was dissolved. To this solution, 0.94 g of LiPF 6 was added. Once the lithium salt dissolved, 0.120 mL 15-crown-5 was added which immediately resulted in a white precipitate. The slurry was stirred while adding 1 mL of dichloromethane (DCM), then the DCM layer was removed by pipette. The aqueous solution was extracted twice more with DCM for a total of three times. The water was then evaporated by gentle heating. A white powder was obtained and triturated with 1 mL of diethyl ether three times. The resulting solid was dried, partially dissolved in ethanol, filtered, and allowed to sit undisturbed in a 1 dram scintillation vial at room temperature. The resulting crystals were off-white needles.
(NH 4 ) 2 (tiron)ÁH 2 O Na 2 (tiron)ÁH 2 O (0.100 g) was added and dissolved in 2.00 mL of water. After the Na 2 (tiron) had dissolved, 0.098 g of NH 4 PF 6 was added and dissolved. Upon adding NH 4 PF 6 , the solution turned rose pink. To this solution, 0.120 mL of 15crown-5 was added and a white precipitate formed. The slurry was extracted three times total with 1 mL of DCM each time. The aqueous solution was gently heated to dryness. The solid was then triturated with three separate 1 mL portions of diethyl ether. The solid was dried and dissolved in methanol, filtered, and allowed to sit undisturbed in a 1 dram vial at room temperature. The crystals which formed were off-white needles with a purple/rose-colored oily residue coating them.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. Water hydrogens were located in difference maps and refined wherever possible. H atoms bonded to C were placed in geometrically idealized positions based on sp 2 hybridization with C-H bond lengths of 0.95 Å and U iso (H) = 1.2U eq (C    prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Disodium 4,5-dihydroxybenzene-1,3-disulfonate (NaTiron)
Crystal data 2Na + ·C 6 H 6 O 9 S 2 2− M r = 332.21 Monoclinic, P2 1 /n a = 6.8156 (7)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

µ-4,5-Dihydroxybenzene-1,3-disulfonato-bis[aqualithium(I)] hemihydrate (LiTiron)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.