Crystal structure of 4,4′-(disulfanediyl)dibutanoic acid–4,4′-bipyridine (1/1)

A distinctive feature of the crystal structure is the geometry of the dtba moiety, which appears to be stretched and acts as an hydrogen-bonding connector, forming linear chains along [-211] with the 4,4′-bpy moiety by way of O—H⋯N hydrogen bonds and C—H⋯O interactions. The influence of the molecular shape on the hydrogen-bonding pattern is analysed by comparing the title compound and two other 4,4′-bpy co-crystals, showing the way in which this correlates with the packing arrangement.


Chemical context
The object of the present study, the 4,4 0 -(disulfanediyl)dibutanoic acid molecule C 8 H 12 O 4 S 2 (dtba), consists of a tenmembered C(H 2 ) 4 S 2 C(H 2 ) 4 chain setting apart the carboxylic acid groups at each end. This suggests that the molecule may be a good candidate for a 'spacer' in the design of compounds with metal-organic framework (MOF) structures, provided that the molecule connects the metal centres in an 'extended' fashion. However, the 'solid-state shape' of molecules such as dtba is not directly discernible from first principles, as the chain includes many sp 3 carbon atoms, which may possibly lead to twisted linkages.
In addition, dtba is a rather uncommon ligand. The Cambridge Structure Database (Version 5.4, including June 2014 upgrades;Allen, 2002) does not at present include any entry whatsoever with the molecule, either in its coordinating or free forms, for which any direct evidence of its shape is available. We have been trying for a while to coordinate the acid to some transition metals; however, so far we have been unsuccessful. During one of these numerous attempts, a cocrystal of dtba with 4,4 0 -bipyridine (4,4 0 -bpy), C 10 H 8 N 2 , was obtained instead. This serendipitous synthesis ended up being unique, since all subsequent attempts to obtain crystals with dtba ligand(s) in a more orthodox way have proved ineffective. We thus present herein the structural analysis of the 4,4 0bpy:dtba 1:1 co-crystal, C 8 H 14 O 4 S 2 ÁC 10 H 8 N 2 (I), which to our knowledge is the first crystal structure to be reported surveying the dtba group. Fig. 1 (top) presents an ellipsoid plot of the asymmetric unit of (I). The 'topological' (non-crystallographic) symmetry of the dtba molecule with a twofold rotation axis located at the center of the S1-S2 bond is obvious from inspection, and it is somehow reflected in the bond-length sequence, presented in Table 1 (corresponding bonds are presented in the same line). In fact, the pseudo-symmetry goes a bit further: the group presents a non-crystallographic C 2v symmetry involving the molecular core (C2-C7), which is reflected in the central torsion angles, viz. those involving S atoms (Table 1). Fig. 1 (bottom) shows the least-squares fit of this core and its C 2vrelated image, with deviations falling in the tight range 0.011-0.015 Å . The outermost parts of the molecule (the carboxylic functions at the ends) deviate significantly from this trend, probably as a result of the strong O-HÁ Á ÁN interactions with neighbouring 4,4 0 -bpy molecules (see discussion below), a fact also reflected in the torsion angles involved (last two lines in Table 1). The double bonds in the -COOH groups are nondelocalized, with the C-O(H) bonds being distinctly longer than the C O bonds (Table 1).

Structural commentary
In spite of the unavoidable twisting due to the individual sp 3 carbon atoms in the chain, the molecule can be considered to be stretched, with a C1Á Á ÁC8 span of 9.98 (1) Å and the terminal OH groups being almost anti-parallel to each other, subtending an angle of 175.5 (1) . Thus, at least in the present structure, the molecule can be considered as a potentially adequate spacer for MOF construction.

Supramolecular features
There are only two strong hydrogen-bond donors (the dtba carboxylic acid OH functions) and two hydrogen-bond acceptors (the pyridine N atoms of 4,4 0 -bpy) present, defining the supramolecular organization (first two entries in Table 2) in the form of linear chains running along [211] (Fig. 2) with graph-set descriptor C 2 2 (22) (for graph-set nomenclature, see Bernstein et al., 1995). Neighbouring chains, in turn, are connected into strips along [001] by a (notably weaker) C-HÁ Á ÁO contact involving the pyridyl C9A-H9A group and one of the two non-protonated carboxylato O atoms (third entry in Table 2, shown as nearly vertical broken lines in Fig. 2), giving rise to R 4 4 (16) centrosymmetric loops. The chains run parallel to each other, with no obvious second-order interactions linking them, either of the C-HÁ Á ÁO, C-HÁ Á Á ortypes. There is, however, a different type of contact present, namely a C-OÁ Á Á contact involving the nonprotonated O atom [C1A-O2AÁ Á ÁCg i where Cg1 is the centroid of atoms N1A, C1A-C5A, symmetry code (i): 1 À x, 1 À y, 1 À z, with O2AÁ Á ÁCg1 i = 3.619 (3) Å ; O2AÁ Á ÁCg1 i , : 165.25 ], which helps in connecting the strips together into a three-dimensional supramolecular structure (drawn in double dashed lines in Fig. 3). Thus, all potentially expected actors for the supramolecular building (OH, O and N functionalities) end up fulfilling a relevant role in the overall organization. Top: the asymmetric unit of (I), showing the H-OÁ Á ÁN linkages as dashed lines. Displacement ellipsoids are drawn at the 40% probability level. Bottom: the least-squares superposition of one dtba molecule and its C2 image, showing the pseudo-symmetry in its central core. Table 1 Selected geometric parameters (Å , ).

Figure 2
A packing view of (I), showing the slabs formed by neighbouring chains connected by C-HÁ Á ÁO contacts (shown as dashed lines).

Database survey
A brief search of the CSD confirmed that 4,4 0 -bpy:dicarboxylic acid adducts are rather frequent; among the most populated families, the one derived from alkanes/alkenes ranks on top.
However, examples of adducts with thiodicarboxilyc acids are notably more rare and only two reported co-crystals of the sort can be found in the literature. These involve thiodicarboxilyc acids closely related to dtba (see scheme below): thiodiglycolic acid (tdga) and thiodipropionic acid (tdpa), viz. 4,4 0 -bpy:tdga and 4,4 0 -bpy:tdpa (Pedireddi et al., 1998). Surprisingly, in these structures the linkers behave in a different way from dtba. Fig. 4 (left) shows the geometry of the three molecules under discussion, while Fig. 4 (right) presents the packing arrangements they give rise to.
In the first case, (tdga co-crystal) the molecule is shaped like a horseshoe, and the terminal OH functions end up being Packing view of (I) at right angles to the view in Fig. 2, showing the slabs in projection (one of them has been hightlighted). Single dashed lines denote the C-HÁ Á ÁO bonds. The C-OÁ Á Á contacts linking the slabs into a three-dimensional structure are shown as double dashed lines.

Figure 4
The three different molecular shapes for tdga, tdpa and dtba, and the packing arrangements they give rise to, as described in the text. almost parallel, subtending an angle of 12.5 (1) to each other. The 4,4 0 -bpy aggregation motifs with this particular geometry give rise to isolated closed dimers as shown in Fig. 4 (upper right).
The tdpa molecule presents a shape somehow similar to, but noticeably more open than tdga, with H-OÁ Á ÁO-H bonds almost at a right angle to each other [97.1 (1) ]. The resulting packing mimics this molecular geometry, in a tight herringbone pattern (Fig. 4, mid-right). Finally, and as already discussed, the present dtba co-crystal displays a fully stretched geometry [H-OÁ Á ÁO-H: 175.5 (1) ] and the basic packing unit is a linear chain.
From this analysis it can be concluded (at least for this type of terminal dicarboxilic acids) that the relative angular disposition of the outermost OH groups are relevant in defining the expected general aspect of the packing. In this context, dtba could be considered a potentially useful spacer for MOF construction, and further work to obtain transitionmetal complexes with this ligand is in progress.

Synthesis and crystallization
The reported 4,4 0 -bpy:dtba co-crystal was obtained serendipitously from an unsuccessful synthesis of a holmium complex, prepared from an Ho 2 O 3 :dtba:4,4-bpy solution (in a 1:2:1 ratio), dissolved in a mixture of water (200 ml) and ethanol (20 ml). After a few days of slow evaporation at room temperature, colourless block-like crystals were obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were originally found in a difference Fourier map, but treated differently in refinement: C-H H atoms were repositioned in their expected positions and thereafter allowed to ride with U iso (H) = 1.2U eq (host) (d = 0.93 Å for C-H aromatic and d = 0.97 Å for C-H methylene ), while OH H atoms were refined with a restrained distance of 0.85 (1) Å .

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.