The low-temperature triclinic crystal structure of silver 3-sulfobenzoic acid

Silver(I) 3-carboxybenzenesulfonate, Ag(O3SC6H4CO2H), has been found to undergo a reversible phase transition from monoclinic to triclinic between 160 and 150 K. The low-temperature triclinic structure (space group P ) has been determined at 100 K.


Chemical context
Over the past two decades, organosulfonate and organocarboxylate anions have received significant attention as building blocks for metal-organic framework (MOF) structures (Dey et al., 2014;Shimizu et al., 2009). As a result of its soft nature, sulfonate tends to bond well with soft cations like silver(I) so a significant chemistry of silver sulfonates has developed during this period (Cô té & Shimizu, 2004;Hoffart et al., 2005). Having previously investigated some structures of silver sulfonate salts (Downer et al., 2006;Squattrito et al., 2019), we have continued this effort with the reaction of Ag + with the bifunctional 3-sulfobenzoate anion. The resulting monobasic salt has been found to have an unexpected lowtemperature structural modification that is reported here.

Structural commentary
The product of the reaction of silver nitrate and sodium 3-sulfobenzoic acid is Ag(O 3 SC 6 H 4 CO 2 H), (I), an anhydrous monobasic silver(I) salt of 3-sulfobenzoic acid. The roomtemperature (293 K) structure of (I) was previously reported in the monoclinic space group C2/c with one independent cation and anion in the asymmetric unit (Prochniak et al., 2008). We find the structure at 100 K to be triclinic (P1) with two independent cations and anions in the asymmetric unit (Fig. 1). The major features of the structure at 100 K are consistent with those at 293 K. The silver ions are coordinated ISSN 2056-9890 by six sulfonate O atoms with four shorter (ca 2.4-2.5 Å ) and two longer (ca 2.7 Å ) distances (  (7) to 164.88 (6) indicating at best a very distorted tetrahedron]. Not surprisingly, the Ag-O distances are shorter by an average of 0.02 Å at 100 K than at 293 K. This kind of pseudotetrahedral coordination geometry significantly distorted by two somewhat longer Ag-O interactions was previously observed in the silver salt of 6-ammonionaphthalene-1,3-disulfonate (Downer et al., 2006). The Ag-O distances are consistent with those seen in other silver arenesulfonates (Cô té & Shimizu, 2004). The extensive metal-sulfonate bonding is as expected given the softer nature of Ag + relative to most d-block transition-metal ions (Parr & Pearson, 1983), which generally show little tendency to bond directly to sulfonate groups (Ma et al., 2003). The carboxylate group remains protonated with the acidic H atoms unambiguously located on O2 and O7. The C-O distances in the carboxylate groups clearly distinguish the non-protonated (C O) and protonated (C-O) O atoms: C7-O1 1.232 (3), C7-O2 1.312 (3) Å ; C14-O6 1.231 (3), C14-O7 1.311 (3) Å .

Figure 1
The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are shown at the 75% probability level and hydrogen atoms are shown as small spheres of arbitrary radii. Symmetryequivalent oxygen atoms are included to show the complete coordination environments of the cations. [Symmetry codes: sulfonate and carboxylate groups directed to opposite sides of the layer is contrary to what was found in the silver salt of the isomeric 4-sulfobenzoic acid (Squattrito et al., 2019). In that compound, both functional groups are involved in metaloxygen bonding so the anions are positioned with both groups equally distributed with respect to each surface of the layer, in contrast to the segregated arrangement in (I).
Comparison of the 100 K and 293 K structures reveals that the key difference is in the carboxylate group. At 293 K, the C-O bond lengths are almost the same [1.250 (3) and 1.271 (3) Å ], indicating significant disorder between the protonated and non-protonated O atoms, while at 100 K the C-O and C O bonds are clearly distinguished and the placement of the acidic H atoms accordingly renders the two 3-sulfobenzoic acid moieties symmetry-inequivalent. Variabletemperature single-crystal X-ray measurements between 250 and 130 K show that the monoclinic-to-triclinic transition occurs on going from 160 to 150 K and that it is reversible.

Synthesis and crystallization
A 2.24 g (10.0 mmol) sample of sodium 3-sulfobenzoic acid (Aldrich, 97%) was dissolved in 45 ml of water. To this colorless solution was added a colorless solution of 1.69 g (9.95 mmol) of AgNO 3 (Baker) in 45 ml of water. The resulting clear colorless solution was stirred for about 30 minutes and transferred to a porcelain evaporating dish that was set out to evaporate in a fume hood. After several days, the water had completely evaporated leaving behind small colorless needle-shaped crystals, 0.75 g of which were collected by hand from the dish. These were identified as (I) through the single crystal X-ray study. Partial packing diagram of (I) showing the hydrogen-bonding scheme involving the carboxylic acid groups of neighboring anions. Hydrogen bonds are shown as dashed bonds. Displacement ellipsoids are drawn at the 90% probability level. [Symmetry codes: (#) 2 À x, 1 À y, 1 À z; ($) 3 À x, 2 À y, 2 À z; (&) x + 1, y + 1, z.] four relatively large peaks (1.22-1.46 e Å À3 ) in the final difference electron-density map that are located ca 0.9 Å on either side of the Ag atoms along the a axis. Attempted refinement of the extinction parameter resulted in a value near zero so it was not included in the final model. Although we cannot rule out an issue with the absorption correction, none is evident and the structure is otherwise well-behaved. The variable-temperature single crystal X-ray experiment was done by cooling in 10 K increments from 250 to 130 K and then heating back to 170 K. At each step once the desired temperature was reached, the crystal was maintained at that temperature for 15 minutes before data acquisition. A complete data collection and refinement were also conducted at 296 K to confirm the reported monoclinic structure (Prochniak et al., 2008). Our results were essentially identical to the reported ones so they are not included here. The low-temperature triclinic crystal structure of silver 3-sulfobenzoic acid

Poly[(µ 4 -3-carboxybenzenesulfonato)silver(I)]
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.