Received 26 October 2012
An anisaldehyde sodium bisulfite derivative: poly[[4-(hydroxy)(4-methoxyphenyl)methanesulfonato]sodium]
The title complex, [Na(C8H9O5S)]n, is polymeric and consists of broad layers parallel to (100) made up of an inner hydrophilic core of Na+ cations and polar SO3C(OH)- groups, padded on both sides by two hydrophobic layers of pendant methoxyphenyl groups. The Na+ cations in the inner core are six-coordinated into highly distorted NaO6 octahedra by four symmetry-related (hydroxy)(4-methoxyphenyl)methanesulfonate anions, leading to a tightly woven two-dimensional structure. While there are some hydrogen bonds providing interplanar cohesion, interactions between adjacent layers are weak hydrophobic ones. The present compound appears to be the first reported structure containing the (hydroxy)(4-methoxyphenyl)methanesulfonate ligand.
Aldehydes and methyl ketones are known to undergo nucleophilic addition of NaHSO3 in aqueous solution (Clayden et al., 2012). The resulting derivatives could act as potentially good ligands in coordination complexes through the O atoms of the sulfite group, which normally exhibits several binding modes. In order to obtain these coordination compounds, we have first prepared and determined the crystal structure of the title compound, (I). A search of the Cambridge Structural Database (CSD, Version 5.33, August 2012 update; Allen, 2002) revealed that this is the first crystal structure containing this otherwise well known commercially available (hydroxy)(4-methoxyphenyl)methanesulfonate (mbs) derivative [Chemical Abstracts Service (CAS) number 33402-67-4]. However, a few closely related compounds have been reported. Perhaps the most interesting one for comparison purposes is the potassium salt of (hydroxyphenyl)methanesulfonate, (II) (Kuroda et al., 1967), a ligand similar to mbs but lacking the terminal methoxy group. The two solids are almost isostructural, and here we shall analyse their similarities and differences.
The asymmetric unit of (I) consists of one Na+ cation and one mbs anion. Fig. 1 shows the complete Na environment, while Table 1 presents Na-O coordination bond lengths and some NaNa distances (see below). Compound (I) has a two-dimensional polymeric structure built up around the six-coordinated Na+ cations, which show a highly distorted NaO6 octahedral environment. This is provided by four symmetry-related mbs anions (Fig. 1), two of them through two different chelating bites (atoms O1 and O5, and O2iii and O3iii) and the remaining two by way of two further O atoms (O1i and O2ii), acting now in a bridging mode [symmetry codes: (i) -x + 1, -y + 1, -z + 1; (ii) -x + 1, y - , -z + ; (iii) x, y - 1, z]. The Na-O coordination bond lengths span a narrow range [2.3060 (19)-2.439 (2) Å for five of the O-atom donors, the sixth being an outlier of 2.736 (2) Å]. The fact that all four O atoms of the SO3C(OH)- group are involved in coordination provides multiple bridging paths connecting the Na+ cations, though this is not necessarily reflected in particularly short NaNa distances (see below).
The mbs anion presents a 44 binding mode not shown by any previous ligand containing an SO3C(OH)- group. Fig. 2 presents a summary of the different coordination modes found in the CSD, ranging from the very simple 1 up to the extremely complex 74. This diversity seems to confirm the potential of mbs in structural design.
The -SO3 group in the ligand is extremely regular, with completely delocalized double bonds [S-O = 1.4533 (18)-1.4594 (18) Å], and the very small angular deviations from regularity are due solely to chelation. Thus, the O2-S1-O3 angle of 110.49 (11)° is 3% smaller than the remaining two O-S-O angles. A similar relationship is encountered between C1-S1-O1 [104.83 (11)°] and the other C-S-O angles.
The bonding scheme results in a tightly woven two-dimensional array which can be described as a three-layered sandwich-like structure parallel to (100), viz. the B-A-B motif shown in Fig. 3. The inner part (A) is a hydrophilic core centred at x 0.50 and about a/4 wide, built up by Na+ cations and sulfite anions. The methoxyphenyl groups, in turn, evolve upwards and downwards to form two limiting hydrophobic layers (B) sandwiching the core (Fig. 3). Fig. 4 displays a simplified version of the central type `A' layer, built up of Na+ cations and sulfite anions. The wide diversity of loops linking the cations is apparent, and, as expected, those involving direct (O-atom mediated) Na-O-Na bridges lead to the shortest NaNa distances. In the following, we refer the reader to Fig. 4 for geometric details and to Table 1 for symmetry codes. The nearest approach appears between atoms Na1 and Na1i [dark-grey shaded loop; NaNa = 3.729 (2) Å] built up around an inversion centre and, accordingly, this results in two such Na-O-Na bridges. The second nearest are those linking 21-related cations, Na1Na1ii [light-grey shading; NaNa = 4.006 (1) Å], with only one such bridge. The minimum approach distance in (I) is in the range of the average found in the CSD for normal Na-O networks [3.47 (18) Å in a sample of 220 cases] but appears rather long if compared with, for instance, that in pure Na2SO3 [NaNa = 3.090 (2) Å; Larsson & Kierkegaard, 1969]. As expected, Na-O-S-O-Na bridges are noticeably less effective in promoting close NaNa contacts (see Fig. 4 and Table 1 for details).
Regarding type `B' zones, they are linked internally by weaker noncovalent interactions (Table 2), where atoms O4 and O5 present quite different behaviours. Protonated atom O5 is not only involved in coordination but also takes part in a moderately strong O-HO hydrogen bond (first entry in Table 2), while due to its isolation in the hydrophobic region, atom O4 is neither coordinated nor involved in any relevant secondary interaction. The intralayer links are completed by significantly weaker interactions, viz. a nonconventional C-HO hydrogen bond (second entry in Table 2) and two C-H interactions (third and fourth entries in Table 2).
The whole three-layered B-A-B array fills one complete unit cell along the  direction. As suggested by Fig. 3, the interaction between neighbouring B-A-B structures is governed by weak hydrophobic BB interactions.
As stated, (I) and the closely related potassium analogue, (II), are quasi-isostructural, crystallizing in the same space group (P21/c) and having similar unit-cell parameters, although with a variable relative increase when going from (I) to (II) (1.7% in a, 0.2% in b and 7.1% in c). These values correlate closely with the influence of the bulky methoxy group in (I) in the direction of each lattice parameter; this group is mainly oriented along c, only slightly along a and has no significant component along b.
The crystal structure description in terms of B-A-B zones is applicable to both (I) and (II), although, strikingly, this is where the similarities end: the BB interactions and alignments are different in the two structures, as are the cation-sulfonate interactions in the hydrophilic zones A. This is easily revealed by inspection of the coordination modes of the ligands shown in Fig. 2, in which the modes for (I) and (II) have been encircled for clarity.
In summary, the present results concerning the ligand properties of the mbs anion are encouraging, and therefore a project aimed at the synthesis of possible transition metal complexes of this ligand is under way in our laboratory.
| || Figure 1 |
A view of (I), showing the atom-numbering scheme and the complete Na environment requiring four symmetry-related ligands. The symmetry-independent part of the structure is shown with full ellipsoids and bonds. Displacement ellipsoids are drawn at the 40% probability level. C-bound H atoms have been omitted for clarity. [Symmetry codes: (i) -x + 1, -y + 1, -z + 1; (ii) -x + 1, y - , -z + ; (iii) x, y - 1, z.]
| || Figure 2 |
The different coordination modes found in the CSD for ligands containing the SO3C(OH)- group. Those corresponding to structures (I) and (II) are circled. CSD refcodes: A = JAYPEE (Cameron et al., 1990) and QEQFIC (Abrahams et al., 2006); B = PEFDAF (Larsen et al., 1992); C = ABUMIU (Seki et al., 2004); D = PEFDAF (Larsen et al., 1992); E = QACCOO (Haines & Hughes, 2010); F = ODUWIT (Cole et al., 2001); G = (I) (this work); H = ODUWOZ (Cole et al., 2001); I = (II) and KHBSLF (Kuroda et al., 1967); J = KHMSUL (Cameron & Chute, 1979).
| || Figure 3 |
A side view of (I), projected down the  direction, showing the broad planar structures. The hydrophilic part (A) is shown with bold lines and the hydrophobic part (B) is shown with thin lines.
| || Figure 4 |
A schematic view of a type `A' layer, where only the Na+ cations and -SO3 groups are drawn, showing the variety of loops linking the cations. For intercationic distances, see Table 1. The significance of the light- and dark-grey shaded areas is discussed in the Comment. [Symmetry codes: (i) -x + 1, -y + 1, -z + 1; (ii) -x + 1, y - , -z + ; (iii) x, y - 1, z; (iv) -x + 1, -y, -z + 1; (v) x, -y + , z - ; (vii) x, -y + , z + ; (viii) x, y + 1, z; (ix) -x + 1, y + , -z + .]
Compound (I) was synthesized by mixing a saturated aqueous solution of NaHSO3 with 4-methoxybenzaldehyde at room temperature (molar ratio 2:1). The resulting white precipitate was first washed with the same bisulfite solution used in the preparation, then with ethanol (96%) and finally with diethyl ether. The solid powder thus obtained was dissolved in H2O, and methanol was added until saturation was achieved. After evaporation at room temperature for one month, a few crystals suitable for X-ray diffraction analysis were obtained.
All H atoms were visible in a difference map. The H atom on O5 was freely refined, whereas H atoms bonded to C atoms were idealized and allowed to ride, with C-H = 0.98 Å and Uiso(H) = 1.2Ueq(C) for tertiary, C-H = 0.93 Å and Uiso(H) = 1.2Ueq(C) for aromatic, and C-H = 0.98 Å and Uiso(H) = 1.5Ueq(C) for methyl H atoms.
Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009).
Supplementary data for this paper are available from the IUCr electronic archives (Reference: EG3104 ). Services for accessing these data are described at the back of the journal.
The authors acknowledge the Spanish Research Council (CSIC) for providing a free-of-charge licence to the Cambridge Structural Database. We also thank Ing. A. Ibañez, Physics Department, Universidad de Chile, for carrying out the data collection.
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