Synthesis and Structural Characterization of the Sodium Salt of a New Sulfonate-containing Water Soluble N-donor Ligand ⎯ Self-assembly in the Solid State by Π−π Stacking Interactions

The synthesis, spectroscopic characterization, and X-ray crystal structure of the sodium salt of a new sulfonated water-soluble ligand, sodium 2-(2-pyridin-2-yl-ethylamino)-benzenesulfonate (L) are described. involving interactions between the π-donor benzene and the π-acceptor pyridine systems reinforce and direct the self-assembly of the structural motifs in the solid state.


Introduction
Water-soluble ligands and the corresponding derived catalysts are of great interest for aqueous catalysis.Typically there are many more phosphine P-donor ligands than amine N-donor ligands specifically modified by incorporation of hydrophilic groups such as hydroxyl, carboxylate or sulfonate moieties to increase their water-solubility [1,2].We are interested in preparing water-soluble N-donor ligands containing sulfonate groups for synthesizing metal complexes that can catalyze the hydrolysis of organic molecules such as esters, phosphate esters and amides in water.Recently, we reported the synthesis of such sulfonate-containing N-donor ligands by a new route which involves the Michael-type addition reaction of primary or secondary amines with sodium vinylsulfonate in water [3].Herein, we report on the sodium salt of a new, sulfonate-containing N-donor ligand, sodium 2-(2pyridin-2-yl-ethylamino)benzenesulfonate (L), prepared via a similar synthetic pathway (Scheme 1).Compound L has been characterized by 1 H-and 13 C-NMR and IR spectroscopies, mass spectrometry and X-ray crystallography.Although the solid state superstructure of compound L is not a primary focus for our research on water-soluble ligands and complexes, an interesting π−π stacking effect, typically used for the generation of a wide variety of superstructures [4][5][6][7][8], was also observed in L in the solid state (vide infra).

Results and Discussion
Compound L was synthesized via a Michael-type addition reaction between 2-vinylpyridine and aniline-2-sulfonic acid in refluxing water and methanol similar to related reactions reported in the literature [9].Compared with other potential synthetic routes to L, such as unselective alkylation using alkyl halides [10], the selective conjugate addition reaction that was employed greatly decreased the amount of undesired side-products formed.As shown in Scheme 1, an aqueous solution of aniline-2sulfonic acid was mixed with 1.2 equivalents of 2-vinylpyridine, leading to the formation of L in 73% yield after 15 days.Only very little tertiary amine side-product was isolated in the reaction, probably due to the relatively slow kinetics and higher steric effect of the formation of tertiary amines compared to secondary amines in the reaction.Furthermore, no quaternary ammonium side-products resulted from the reaction, as might have been the case with an alkylation reaction using an alkyl halide [10].
White crystals suitable for X-ray crystallography were obtained by recrystallization of L from 50% aqueous methanol solution.The structure of L is shown in Figure 1, with 30% thermal ellipsoids probability.A more elaborate structural construction is showed in the 2D polymeric structure depicted in Figure 2. As can be seen in this figure, the organic portions of L are bridged by sodium ions (Na(1)−O(1), 2.3269(16) Å), and π−π stacking involving interactions between the π-donor benzene and the π-acceptor pyridine systems (ring … ring separation 3.847 Å) reinforce and direct the selfassembly of the structural motifs, which form a three-layer sandwich structure with an octahedrally-coordinated sodium cation in the middle and two ligand molecules on the sides.On the basis of the repeating pattern in the solid state, we can propose that π-stacking occurs preferentially over the suitable spatial arrangement.Transition metal complexes of L and the catalytic activities of the corresponding complexes are currently being investigated and will be reported soon.

Conclusions
A new, water-soluble N-donor ligand was synthesized in good yield and without major sideproducts.The result of X-ray crystallographic studies on the sodium salt of the ligand showed that π−π stacking effects play an important role in the self-assembly of the compound in the solid state.

Figure 1 .
Figure 1.The molecular structure of L with atomic labeling and thermal ellipsoids shown at the 30% probability level.Solvent molecules are omitted for clarity.

Table 1 :
Summary of Crystallographic Data and Parameters for L.