Comment

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 [100] 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.

Experimental

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 U_iso(H) = 1.2U_eq(C) for tertiary, C-H = 0.93 Å and U_iso(H) = 1.2U_eq(C) for aromatic, and C-H = 0.98 Å and U_iso(H) = 1.5U_eq(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).

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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.

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