Crystal engineering with short-chained amphiphiles: decasodium octa-n-butanesulfonate di-μ-chlorido-bis[dichloridopalladate(II)] tetrahydrate, a layered inorganic–organic hybrid material

The preparation and crystal structure of the layered inorganic–organic hybrid material are presented. The crystal structure determination of Na10(C4H9SO3)8[Pd2Cl6]·4H2O is the first of a metal n-butanesulfonate and confirms a unique lamellar amphiphilic bilayered structure with the hexachloridodipalladate(II) ions unexpectedly placed within the ‘organic’ hydrophobic layer region but primarily bonded to the neighbouring ‘inorganic’ hydrophilic layers via hydrogen bonding and ‘local’ ionic interactions.

In the course of crystal-engineering experiments, crystals of the hydrated title salt, Na 10 [Pd 2 Cl 6 ](C 4 H 9 SO 3 ) 8 Á4H 2 O, were obtained from a water/2-propanol solution of sodium n-butanesulfonate and sodium tetrachloridopalladate(II). In the crystal, sodium n-butanesulfonate anions and water molecules are arranged in an amphiphilic inverse bilayered cationic array represented by the formula {[Na 10 (C 4 H 9 SO 3 ) 8 (H 2 O) 4 ] 2+ } n . Within this lamellar array: (i) a hydrophilic layer region parallel to the bc plane is established by the Na + cations, the H 2 O molecules (as aqua ligands in Na,Na 0 -bridging coordination mode) and the O 3 S-groups of the sulfonate ions, and (ii) hydrophobic regions are present containing all the n-butyl groups in an almost parallel orientation, with the chain direction approximately perpendicular to the aforementioned hydrophilic layer. Unexpectedly, the flat centrosymmetric [Pd 2 Cl 6 ] 2À anion in the structure is placed between the butyl groups, within the hydrophobic regions, but due to its appropriate length primarily bonded to the hydrophilic 'inorganic' layer regions above and below the hydrophobic area via Pd-Cl t Á Á ÁNaand Pd-Cl t Á Á ÁH-O(H)-Na-type (Cl t is terminal chloride) interactions. In addition to these hydrogen-bonding interactions, both aqua ligands are engaged in chargesupported S-OÁ Á ÁH-O hydrogen bonds of a motif characterized by the D 4 3 (9) graph-set descriptor within the hydrophilic region. The crystal structure of the title compound is the first reported for a metal n-butanesulfonate.

Chemical context
Sodium alkanesulfonates are artificial soaps (anionic tensides) with a widespread use (Schramm et al., 2003). They are known to have a bilayered structure like 'natural' soaps, with an extreme tendency for disorder in the crystalline state Buerger et al., 1942). Compounds containing alkanesulfonate ions of the general formula C n H 2n+1 SO 3 À with n = 1-4 may be defined as short-chained alkanesulfonates (SCAS). In contrast to methanesulfonates (n = 1) and ethanesulfonates (n = 2), there is only rare structure information for the next higher homologues (n = 3, 4) (Frank & Jablonka, 2008;Russell et al., 1994). Solid sodium methanesulfonate is described as an inorganic-organic three-dimensional network (Wei & Hingerty, 1981). However, closer inspection shows the compound to have a bilayered soap-like structure with only one of five CH 3 SO 3 À anions connecting in the third dimension. In crystal-engineering experiments, we successfully exchanged this connecting anion by selected other ionic moieties and were able to retain the lamellar structure (Thoelen & Frank, 2017, 2018Verheyen & Frank, 2009). An ISSN 2056-9890 aim of subsequent attempts was to include chloridopalladate(II) anions Pd n Cl 2n+2 2À that are known to be catalytically active (Bouquillion et al., 1999;Jimeno et al., 2012;Lassahn et al., 2003;Mu et al., 2012), by using [PdCl 4 ] 2À in the form of its sodium salt as a typical precursor in aqueous palladium(II) chemistry.
In the investigation described herein, the incorporation of hexachloridodipalladate(II) anions into the sodium n-butanesulfonate layered system was realized, resulting in the title compound (1) having the typical brown colour of palladium complexes with a square-planar coordination environment. According to the results of elemental analysis and vibrational spectroscopic investigations, hydrated sodium cations, n-butanesulfonate and hexachloridodipalladate (II) anions are present in the solid. The crystal structure determination of this compound is the first of a metal n-butanesulfonate and eventually confirmed the composition Na 10 (C 4 H 9 SO 3 ) 8 [Pd 2 Cl 6 ]Á4H 2 O and a lamellar amphiphilic structure. Fig. 1 shows the asymmetric unit of the crystal structure that contains (all in general positions) five sodium cations, two water molecules, four n-butanesulfonate anions and, close to a center of inversion, one half of a hexachloridodipalladate anion. The five Na + cations are in quite different coordination environments (Fig. 2), defined by five sulfonato ligands (Na4, Na5), four sulfonato ligands and one aqua ligand (Na3), four sulfonato ligands and two aqua ligands (Na2) and four sulfonato ligands, one aqua ligand and one terminal chlorido ligand of the [Pd 2 Cl 6 ] 2À anion (Na1). Bond lengths and angles of the n-butanesulfonate anions are as expected (see supplementary Tables). All these anions are found with an entirely anti-periplanar conformation of the alkyl groups, without any disorder. Altogether, n-butanesulfonate anions, Na + cations and water molecules form a tenside-like inverse bilayered cationic array, which can be described by the formula {[Na 10 (H 2 O) 4 (C 4 H 9 SO 3 ) 8 ] 2+ } n . In this arrangement, the layerlike regions are oriented parallel to the bc plane of the unit cell. As visualized by the blue and the red sections of the transparent background of Fig. 3, hydrophilic and hydrophobic regions are given, reminiscent of sections of the structures of 'pure' short-chained sodium alkanesulfonates (Frank & Jablonka, 2008;Wei & Hingerty, 1981  Coordination environments of sodium cations. For clarity, n-butylgroups of the n-butanesulfonate anions are not shown. [Symmetry codes: (i) x, 1 2 À y, 1 2 + z; (ii) 1 À x, 1 À y, 1 À z; (iii) x, 1 + y, z; (iv) x, À1 + y, z; (v) 1 À x, À 1 2 + y, 1 2 À z; (vi) 1 À x, 1 2 + y,  The asymmetric unit of 1, chosen to give a compact segment with all nbutyl groups of the hydrophobic layer region oriented in one direction. In addition, the symmetry-related second half of the hexachloridodipallate(II) anion is shown in transparent mode [symmetry code: (i) Àx, 1 À y, 1 À z.]. The direction of coordinative bonding to atoms of neighbouring moieties is given by sharpened sticks, and hydrogen bonds are shown as segmented solid bonds. Displacement ellipsoids are drawn at the 50% probability level, hydrogen atoms are drawn with an arbitrary radius. Note the coordination of the hexachloridodipalladate(II) ion to hydrophilic moieties by hydrogen bonding and 'local' ionic interactions. the C 4 -chains in an approximately parallel orientation, the butyl groups are arranged on both sides of the hydrophilic region to complete the amphiphilic double layer with an inverse bilayer thickness according to unit-cell parameter a. The centrosymmetric [Pd 2 Cl 6 ] 2À anions in the structure of 1 are placed between the n-butyl groups within the hydrophobic regions. In a first view, this position seems to be unexpected; however, the length of the dipalladate(II) anion is appropriate to allow for pronounced bonding to the hydrophilic 'inorganic' layered regions above and below the hydrophobic area (Fig. 3). To interact with the inorganic areas above and below the hydrophobic region, a building block is needed that fits to the thickness of the hydrophobic double layer. In the concrete case of 1, the thickness is determined by the lengths of two 'end-facing' n-butyl groups.

Structural commentary
As expected, the Pd-Cl bonds to the terminal chlorido ligands [2.2776 (12) and 2.2800 (10) Å ] are slightly shorter than the Pd--Cl bonds [2.3159 (11) and 2.3212 (12) Å ]. These geometric parameters, as well as the Cl-Pd-Cl bond angles of 86.20 (4) to 92.45 (4) and the Pd--Cl-Pd angle of 93.80 (4) , are in good agreement with those found in Cs 2 [Pd 2 Cl 6 ] (Schü pp & Keller, 1999) or in several hexachloridodipalladates with large organic cations (e.g. Chitanda et al., 2008;Gerisch et al., 1997;Makitova et al., 2007). Alternatively to the formula given above, compound 1 might be formulated as a hydrated double salt of sodium n-butanesulfonate and sodium hexachloridodipalladate(II): Na 8 (C 4 H 9 SO 3 ) 8 ÁNa 2 Pd 2 Cl 6 Á4H 2 O. This choice takes into account that the Na-Cl distance from the terminal chlorido ligand Cl2 of the hexachloridodipalladate(II) anion to the sodium cation Na1 [2.8560 (18) Å ] is close to the distances of 2.809 (3) to 2.821 (2) Å in Na 2 PdCl 4 (Schrö der & Keller, 1989). However, this is a singular similarity, and because all the sodium cations of 1 clearly are components of the layerlike hydrophilic region, there is a much closer structural relationship of 1 to sodium methanesulfonate (Wei & Hingerty, 1981) and sodium 1-propanesulfonate monohydrate (Frank & Jablonka, 2008). As in the structures of these compounds, the asymmetric unit in 1 contains five Na + cations, establishing a closely related Na-O coordination network, and the separation of hydrophilic layers and hydrophobic areas is similar to the most prominent structural feature of crystallized amphiphiles where the neighbouring hydrophobic areas in the layer-like structures are connected by van der Waals forces only.

Figure 3
Diagram displaying hydrophilic (blue) and hydrophobic sections ( is the entire path of hydrogen bonding described by the D 3 4 (9) graph-set descriptor (Russell et al., 1994;Grell et al., 1999), with the sulfonate oxygen atom O14 as the central double acceptor.

Database survey
A search in the Cambridge Structural Database (Version 5.40, update November 2018; Groom et al., 2016) for short-chained sodium alkanesulfonates Na(C n H 2n+1 SO 3 ) with n = 1-4 gave three hits, viz. the structures of sodium methanesulfonate (BAKLAA; Wei & Hingerty, 1981), sodium 1-propanesulfonate monohydrate (GOKHIY; Frank & Jablonka, 2008) and -cyclodextrin sodium 1-propanesulfonate nonahydrate (ACDPRS; Harata, 1977). For crystal structures with n-butanesulfonate anions, only one entry was found (WETNUE; Russell et al., 1994), describing the lamellar structure of guanidinium n-butanesulfonate. Searching for the hexachloridodipalladate(II) anion results in 46 entries. However, from a structural point of view, the role of the [Pd 2 Cl 6 ] 2À ion in 1 is completely different from the role of this species in all the other compounds. In addition to the reports on these compounds having organic components, there is one report on an inorganic ternary chloride containing the [Pd 2 Cl 6 ] 2À ion (CsPdCl 3 ; Schü pp & Keller, 1999).

Synthesis and Crystallization
Thin brown platelets of 1 were obtained by slow isothermal evaporation of the solvent from a solution of 5 ml of distilled water and 5 ml of isopropanol containing 3.203 g (20 mmol) of sodium n-butanesulfonate and 1.177 g (4 mmol) of sodium tetrachloridopalladate(II). The evaporation temperature of the solution was adjusted to 288 K with a thermostat. After three days, crystals suitable for X-ray crystal structure determination could be harvested (

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The positions of all hydrogen atoms were identified in difference-Fourier syntheses. In the course of the converging refinement, a riding model was applied using idealized C-H bond lengths (0.97-0.98 Å ) as well as H-C-H and C-C-H angles. In addition, H atoms of CH 3 groups were allowed to rotate around the neighboring C-C bonds. The U iso (H) values were set to 1.5U eq (C methyl ) and 1.2U eq (C methylene ), respectively. H-O distances of the water molecules were restrained to 0.83 (3)    SHELXL2018 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2016); software used to prepare material for publication: publCIF (Westrip, 2010).

Decasodium octa-n-butanesulfonate di-µ-chlorido-bis[dichloridopalladate(II)] tetrahydrate
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.