Structures of five salt forms of disulfonated monoazo dyes

The structures of five s-block metal salt forms of three disulfonated monoazo dyes are presented. The coordination behaviour of the azo ligands and the water ligands, the dimensionality of the coordination polymers and the overall packing motifs of these five structures are contrasted to those of monosulfonate monoazo congers. It is found that two of the compounds adopt similar structural types to those of monosulfonate species but that the other three structures do not.


Introduction
Azo compounds have a long history of use as both dyes and pigments. One of the commonest subclasses is that of sulfonated azo species, where the sulfonate group is typically added to aid water solubility and/or to decrease toxicity (Hunger et al., 2003). Despite being widely referred to as organic colourants, the commercial products of sulfonated azo species are commonly metal complexes and often s-block metal salt forms (Christie & Mackay, 2008). Even before large-scale crystallographic studies were available, it was recognized that small structural changes systematically changed the colour and material properties of such dyestuffs (Greenwood et al., 1986). These structure-property relationships led to an interest in more detailed structural investigations. A reasonable number of crystal structures of the salt forms of monosulfonated azo dyes and even pigments are now known (e.g. Kennedy et al., 2000Kennedy et al., , 2004Kennedy et al., , 2009Tapmeyer et al., 2020;Aiken et al., 2013). However, far fewer relevant structures of disulfonated azo species are known, despite these being commercially commonplace. The only azobenzene-based disulfonate structures that we are aware of are those of azobenzene-4,4 0 -disulfonate (Soegiarto & Ward, 2009;Soegiarto et al., 2010, ISSN 2053-2296 2011). In these structures, the disulfonate ions are utilized as framework hosts for a series of functional organic guests and thus they are not of particular relevance to commercial colourant materials. Some s-block metal salt structures of more complicated disulfonated dyes, with naphthalene-rather than azobenzene-based azo fragments, are also known (e.g. Black et al., 2019;Kennedy et al., 2006;Ojala et al., 1994). The azo moiety in all these examples exists in the hydrazone tautomeric form and in all cases both sulfonate groups lie on only one ring system at one end of the azo bond. The only colourant relevant disulfonate structures with sulfonate groups on both the ring systems, at either end of an azo bond, are the Ca lake structures of Pigment Yellow 183 and Pigment Yellow 191 determined by Schmidt and co-workers Schmidt et al., 2009). These are relatively complex materials with pyrazolone groups between the two sulfonated aryl rings. Herein we present five new structures of s-block metal salt forms of azobenzene disulfonate derivatives (Scheme 1), namely, [Na 2 L1(OH 2 ) 4 ] n , (I), and [CaL1(OH 2 ) 4 ] n , (II), where L1 is azobenzene-3,3 0disulfonate; {[NaL2(OH 2 ) 2 ]Á2H 2 O} n , (III), and [Mg(OH 2 ) 6 ]-[L2] 2 Á8H 2 O, (IV), where L2 is 4-aminodiazeniumylbenzene-3,4 0 -disulfonate; and {[BaL3(OH 2 ) 4 ]Á2H 2 O} n (V), where L3 is 4-amino-2-methyl-5-methoxyazobenzene-2 0 ,4 0 -disulfonate. Structure (III) is notable as it was obtained from recrystallizing the commercial dyestuff Acid Yellow 9 [74543-21-8].

Synthesis and crystallization
The Raman spectra of solid samples were measured using a Reinshaw Ramascope 2000 instrument with excitation at 785 nm. IR samples were prepared as KBr discs and spectra were measured using a Nicolet Avatar 360 FT-IR.
The monosodium salt of Acid Yellow 9 was purchased from Sigma-Aldrich and recrystallized from water to give fibrous red crystals of (III). The Mg salt (IV) was prepared by adding an equimolar amount of MgCl 2 to an aqueous solution of the monosodium salt of Acid Yellow 9. After filtering off the initial dark precipitate, allowing the remaining solution to evaporate to dryness gave red crystals of (IV). IR (KBr): 1625,1574,1528,1392,1162,1008, 879 cm À1 .
The free acid equivalent of (V) was provided by Dystar UK. Treatment of an aqueous solution with Ba(OH) 2 gave an orange solution. After several attempts, a simple slow evaporation (approximately four weeks) from water gave a few suitable orange crystals of (V).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Data for (III) were measured at the Daresbury SRS Station 9.8 (Cernik et al., 1997) and for (V), data were measured by the UK National Crystallography Service (Cole & Gale, 2012).
Disorder models were used for one non-metal-bound water molecule of both (III) and (IV), and also for one SO 3 group of (IV). In all cases, a two-site model was used and site-occupancy factors were refined. Suitable restraints and constraints were applied to the bond lengths and displacement parameters of the disordered units to ensure that they displayed approximately normal behaviour.
For all structures, H atoms bound to C atoms were placed in the expected geometric positions and treated in riding mode, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for C-H groups, and C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for CH 3 groups. H atoms bound to N or O atoms were located by difference synthesis and placed accordingly. For (III) and (IV), H atoms bound to N atoms were refined freely and isotropically. For (V), the N-H distances were restrained to 0.88 (1) Å . All water H atoms were restrained such that O-H = 0.88 (1) Å and HÁ Á ÁH = 1.33 (2) Å . For the water H atoms of (V) and the H atoms of the disordered groups, U iso values were allowed to

Results and discussion
Previous work on the salt forms of monosulfonated dyes and pigments has shown that many structural features can be predicted from knowledge of the cation identity and the position of the sulfonate group (Kennedy et al., 2009(Kennedy et al., , 2012 (Kennedy et al., 2004). In all cases, the overall packing should feature simple alternating layers of hydrophilic groups (e.g. cations, SO 3 and H 2 O) and hydrophobic groups (the aryl azo body of the anions) (Kennedy et al., 2009). The structure of disulfonate (I) fits well with these predictions from monosulfonates. It is indeed a three-dimensional coordination polymer with both SO 3 and H 2 O groups bridging between metal centres, and it forms a simple layered structure as expected. In more detail, the asymmetric unit of (I) contains two separate Na sites, both of which occupy special positions (Na1 sits on a twofold axis and Na2 on a centre of symmetry in the space group C2/c). It also contains two water ligands and half of an L1 dianion. A crystallographic centre of symmetry is located at the centre of the azo bond, giving a planar dianion with mutually anti SO 3 groups (Fig. 1). As can be seen from The asymmetric unit of (I) expanded to show the coordination shell about Na1 and Na2, and the conformation of L1. Non-H atoms are shown as 50% probability displacement ellipsoids and H atoms are drawn as small spheres of arbitrary size.

Figure 2
Packing diagram of (I), viewed down the b axis. Note the alternating hydrophobic and hydrophilic layers that lie parallel to the bc plane. Symmetry codes: (i) x; y À 1; z; (ii) Àx; y À 1; Àz þ 1 2 ; (iii) Àx; y; Àz þ 1 2 ; (iv) Àx; Ày þ 1; Àz þ 1; (v) Àx þ 1 2 ; Ày þ 1 2 ; Àz þ 1. Table 3 Hydrogen-bond geometry (Å , ) for (I).  Fig. 2, the layers expand parallel to the bc plane, with the disulfonate dianions bridging between neighbouring hydrophilic layers to give the overall three-dimensional coordination polymer. The hydrogen-bond details for (I) are given in Table 3. The asymmetric unit of (II) contains half of an L1 dianion, two water ligands and a Ca site. Both the Ca1 site and the centre of the azo N N bond occupy crystallographic inversion centres. As with (I), this gives a planar dianion with anti SO 3 groups and an octahedral metal centre ( Fig. 3 and Table 4). Ca1 forms bonds to O atoms from two trans SO 3 groups and to four terminal water ligands. Each SO 3 group makes a single Ca-O bond and thus the disulfonate dianion links Ca centres into a one-dimensional coordination polymer (Fig. 4). These features combine to give the layered structure shown in Fig. 5. Within the hydrophilic layers, hydrogen bonding between the water ligands and the two noncoordinating O atoms of SO 3 link neighbouring coordination chains (Table 5). Thus, structure (II) also follows the rules proposed for monosulfonated azo dye salts. There are Ca-O 3 S bonds, but these are relatively few in number and, even with the twoheaded nature of the disulfonate ligand, they combine to give only a one-dimensional coordination polymer. The H 2 O ligands take no part in bridging between metal centres and the overall packing motif is one of simple alternating hydrophobic and hydrophilic layers. Structure (III) was obtained from aqueous recrystallization of the commercial product called 'Acid Yellow 9, monosodium salt'. An interesting problem here was to discover the protonation site. The crystal structures of three acidic sulfonated azobenzene-based dyes with amino substituents are known. 4-Aminoazobenzene-4 0 -sulfonic acid crystallizes with protonation of the amino group, giving an -NH 3 -bearing zwitterion, whilst the other two known structures crystallize with protonation of the azo N atom furthest from the neutral -NH 2 group (Lu et al., 2009;Miyano et al., 2016; Table 5 Hydrogen-bond geometry (Å , ) for (II). Symmetry codes: (iii) Àx þ 2; Ày þ 1; Àz þ 1; (iv) Àx þ 2; Ày þ 2; Àz þ 1; (v) Àx þ 1, Ày þ 2; Àz þ 1. Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz þ 1; (ii) Àx; Ày þ 1; Àz.

Figure 4
Part of the one-dimensional coordination polymer of (II).

Figure 3
The asymmetric unit of (II) expanded to show the coordination shell about Ca1 and the conformation of L1. Non-H atoms are shown as 50% probability displacement ellipsoids and H atoms are drawn as small spheres of arbitrary size. 2020). The azo group is the commonest protonation site for the free acid forms of sulfonated azo dyes that do not bear a more basic substituent (Kennedy et al., 2001(Kennedy et al., , 2020. The asymmetric unit of (III) was found to contain an Na centre, a monoanionic L2 ligand with protonation at azo atom N1, two metal-coordinated water ligands and two non-bound water molecules, one of which is disordered (Fig. 6). Unusually for an Na salt of an aryl sulfonate, only one of the six independent SO 3 O atoms is involved in bonding to Na. This Na1-O6 interaction involves the SO 3 group meta to the azo bond. Na1 exists in a distorted square-pyramidal and hence five-coordinate environment, where one bond is to a terminal water ligand and the other four bonds (from two water ligands and two SO 3 groups) all bridge to neighbouring Na centres (see Table 6 for geometric details). The Na-O bond lengths of (III) [range 2.275 (2)-2.425 (2) Å ] are understandably shorter than those of the six-coordinate Na centres of (I). An interesting detail is that in (III) the Na-to-OH 2 distances are shorter that the Na-to-SO 3 distances. This is the opposite of the case in (I

Figure 5
Packing diagram of (II), viewed down the a axis. Note the alternating hydrophobic and hydrophilic layers that lie parallel to the ab plane.

Figure 6
The asymmetric unit of (III) expanded to show the coordination shell about Na1. The minor-disorder component at O4W is not shown. Non-H atoms are shown as 50% probability displacement ellipsoids and H atoms are drawn as small spheres of arbitrary size.

Table 7
Hydrogen-bond geometry (Å , ) for (III). Symmetry codes: (i) x; Ày þ 1 2 ; z þ 1 2 ; (ii) x À 1; y; z; (iii) Àx; Ày þ 1; Àz; (iv) x; Ày þ 1 2 ; z À 1 2 ; (v) Àx; Ày; Àz; (vi) Àx; y þ 1 2 ; Àz þ 1 and propagate parallel to the crystallographic c direction. Each chain is asymmetric, with the L2 anions on one side and the water ligands on the other (Fig. 7). This structure is thus unlike those of the monosulfonated azo Na salts as, despite having an extra potential metal-bonding group in the form of the second SO 3 substituent, it does not form a higher-dimensional coordination polymer. A further difference is highlighted by Fig. 8, which shows that (III) is not a simple alternating layer structure. Note the hydrate channels running parallel to c. A reason for this may be that the simple alternate layering seen elsewhere is a function of the azo anions' approximation to linear spacers, with hydrophilic head and tail groups separated by a hydrophobic central region (Kennedy et al., 2009). As L2 is protonated on the azo group, this introduces a hydrophilic group and strong hydrogen-bond donor to the centre of the azo anion. It may be that the need to provide a hydrogen-bond acceptor to this formally charged N-H group is what breaks the otherwise common simple layering Part of the one-dimensional coordination polymer of (III).

Figure 9
The asymmetric unit of (IV) expanded to show the coordination shell about Mg1. The minor-disorder components of the sulfonate groups of S2 and the O7W water molecule are not shown. Non-H atoms are shown as 50% probability displacement ellipsoids and H atoms are drawn as small spheres of arbitrary size.

Figure 8
Packing diagram of (III), viewed down the c axis. H atoms have been omitted for clarity. Note the hydrate channels that extend parallel to the c axis.
motif (Table 7). In this respect, the packing of (III) is more similar to the packing of free acid sulfonated azo structures than it is to the packing of equivalent salt forms (Kennedy et al., 2020). All known Mg salt forms of sulfonated azo dyes and pigments are solvent-separated ion pairs, with no direct bond between Mg and SO 3 (Kennedy et al., 2006(Kennedy et al., , 2009(Kennedy et al., , 2012. As is shown in Fig. 9, the structure of (IV) is also of this type. Its asymmetric unit contains an L2 anion that is protonated at the azo N1 atom, half of an octahedral [Mg(OH 2 )] 6 dication (with Mg1 situated at a crystallographic inversion centre) and four noncoordinated water molecules (Table 8). One of the water molecules and the SO 3 group ortho to NH 2 are disordered. As shown in the packing diagram ( Fig. 10), there are hydrophilic layers that extend parallel to the bc plane. The organic anions lie between these but their azobenzene cores do not form continuous hydrophobic layers -instead water molecules are dispersed within these layers. Thus, rather than true twodimensional layers, the hydrophobic azobenzene units form stacks parallel to the b direction surrounded by [Mg(OH 2 ) 6 ] 2+ ions and water molecules. As with (III) above, the protonation of the azo unit at the centre of the anion appears to mitigate against the simple alternating layer structures seen elsewhere. In both (III) and (IV), the protonated azo group acts as a hydrogen-bond donor to water molecules (see Tables 7 and 9). Fig. 11 shows the contents of the asymmetric unit of (V) extended to give the complete coordination geometry (Table 10). The asymmetric unit consists of an azo dianion, a Ba II cation with four coordinated water ligands and two nonbound water molecules. The Ba centre is nonacoordinated, with three bonds to O atoms of SO 3 groups and six bonds to water ligands. The Ba-O-Ba bridges all involve water O atoms. Both SO 3 groups interact with the Ba atom, with the group ortho to the azo group making two Ba-O bonds and the para SO 3 group making one bond. This is notable as ortho SO 3 groups are generally unfavourable coordination sites compared to para SO 3 groups (Kennedy et al., 2009). As with both L2 structures, here the amino group of L3 takes no part in coordination to the metal atom.
Complex (V) forms a two-dimensional coordination polymer. Ba-O-Ba bridges involving the water molecules extend the polymer parallel to the a direction, whilst parallel to the b direction, the polymer propagates through the coordination of the two SO 3 groups to give the large [Ba(OH 2 ) 4 -Ba(L3)] 2 cyclic structures shown in Fig. 12. The overall packing (Fig. 13) shows a layered structure with hydrophobic and hydrophilic layers parallel to the ab plane. As with (III) and (IV), the amine group of (V) is essentially planar rather than pyramidal. However, it differs by acting as a hydrogenbond donor to only SO 3 groups (Table 11), whilst the amine groups of (III) and (IV) donate hydrogen bonds to both SO 3 and water groups. None of the amine groups act as hydrogen- Packing diagram of (IV), viewed down the b axis. H atoms have been omitted for clarity. Note the solvent water molecules lying within the layers of azo dianions that lie parallel to the bc plane.

Figure 11
The asymmetric unit of (V) expanded to show the coordination shell about Ba1 and all dative bonds originating from the modelled dianion. Non-H atoms are shown as 50% probability displacement ellipsoids and H atoms are drawn as small spheres of arbitrary size.  (7) O2W-Ba1-O1W 72.69 (14) O1 136.99 (11) O3W iii -Ba1-O4W iv 120.18 (10) Symmetry codes: bond acceptors. Azo atom N1 of (V) does act as a hydrogenbond acceptor from water, as do both azo N atoms of (I), but this is not the case for any of the other azo N atoms, see hydrogen-bond tables for details. The literature on the Ba salt forms of monosulfonated azo dyes predicts structures with no bridging water ligands and with discrete coordination complexes or simple one-dimensional coordination polymers (Kennedy et al., 2004(Kennedy et al., , 2009). Neither prediction is true for disulfonate (V).
For L2, with its protonated azo group, the N N bond lengths of (III) and (IV) are 1.294 (3) and 1.294 (4) Å , respectively. The N2-C7 bond lengths are also equivalent at 1.341 (3) and 1.342 (4) Å . These values are as expected for a protonated azo unit bound to an aniline fragment and, despite being for an anionic ligand, are close matches to those found for the overall neutral but zwitterionic free acid forms of those monosulfonated azo dyes which also feature protonated azo groups (Kennedy et al., 2020). At 1.256 (3) and 1.432 (2) Å , the N N and N2-C7 bond lengths of L1 in (II) are clearly much shorter and longer, respectively, than their equivalents in L2. They fit well with the ranges found for the 4,4 0 isomer and with those found for monosulfonated azo species with no strong electron-donating ring substituents (Soegiarto et al., 2009(Soegiarto et al., , 2010(Soegiarto et al., , 2011Kennedy et al., 2001Kennedy et al., , 2020. The N N bond in (I) is 1.262 (4) Å and is thus outside the ranges of the literature Packing diagram of (V), viewed down the a axis. H atoms have been omitted for clarity.

Figure 12
Part of the two-dimensional coordination polymer of (V), viewed down the a axis, showing the coordination polymer extending by SO 3 coordination parallel to the b direction.

Conclusion
Compounds (I) and (II) both contain the simple disulfonate L1 and both have structures that fit with the structural types seen for equivalent monosulfonate salt species -they give the expected dimensionality coordination polymers in which the bonding roles of water ligands are predictable and their packing structures have the expected alternating layer motifs (Kennedy et al., 2004). However, the other three structures presented herein do not have the same structural features as their monosulfonate cognates. Structures (III) and (IV) both contain the monoanion L2. Neither adopts the expected simple alternating layer structure and Na salt (III) is a onedimensional coordination polymer rather than the expected two-or three-dimensional coordination polymer. The strong hydrogen-bonding N-H group at the centre of L2 is a feature not seen in other salt structures. This difference gives a rational explanation for the difference in packing behaviour. Finally, the Ba salt of L3, i.e. (V), does give the expected layered packing, but has metal-centre-bridging water ligands and an unexpected two-dimensional rather than a onedimensional coordination polymer structure. The extra dimensionality of the coordination polymer may simply be related to the extra SO 3 group in L3 compared to literature structures, but it is less clear why the coordination role of the water ligands should also change. Data collection: DENZO (Otwinowski & Minor, 1997) and COLLECT (Hooft, 1998)   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.

Crystal data
[Ca (C 12  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.40 e Å −3 Δρ min = −0.46 e Å −3 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.

Crystal data
[Na (C 12  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.35 e Å −3 Δρ min = −0.44 e Å −3 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.

Hexaaquamagnesium bis{2-(4-amino-3-sulfonatophenyl)-1-(4-sulfonatophenyl)diazenium} octahydrate (IV)
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.