Alkali metal salts of 4-hydroxybenzoic acid: a structural and educational study

Although 4-hydroxybenzoic acid (H2hba) is a relatively simple organic molecule, it displays remarkable coordinative flexibility in its reactions with alkali metal hydroxides, forming ionic networks containing the dianion (hba2−), the monoanion (Hhba−) or the neutral acid species (H2hba). A common feature of the structures of the lattices is their layered arrangement: alternating hydrophilic layers made up of closely packed metal–oxygen polyhedra are separated by the hydrophobic nonpolar components of the hydroxybenzoate linking units.


Introduction
In the study of chemistry, an understanding of the various types of chemical bonding is essential and thus it is not surprising that introductory chemistry courses at the secondary school level tend to have a strong emphasis on primary and secondary bonding. Many of the ideas that are presented to students have come from the analysis of bonds within and between molecules, the structures of which have ISSN 2053ISSN -2296 been determined by the technique of single-crystal X-ray diffraction. It is therefore somewhat surprising that the role of X-ray crystallography in providing detailed representations of molecules is poorly recognized in many secondary school chemistry courses worldwide.
The reason that crystallography does not tend to form part of school chemistry curricula is perhaps due to the traditional inaccessibility of the technique. It would be fair to say that for a large part of the 20th century, crystal structures were determined by expert crystallographers who had extensive training in the technique and possessed a detailed understanding of the theory that underpinned the collection of data, the structural solution and the refinement process. For many chemists it was an unavailable technique unless one was able to collaborate with a crystallographer.
The 21st century has seen rapid development in both crystallographic hardware (sources and detectors) and software. Data sets can now be collected in minutes and improvements in both computers and crystallographic programs with easy-to-use GUI interfaces have allowed rapid structure solution and refinement of structures, particularly for routine structures. These advancements have allowed crystallographic novices to measure their own data and determine their structure quickly and with relative ease. Of course, there are traps for the inexperienced crystallographer, but nevertheless the technique of X-ray crystallography has never been so widely accessible.
As research chemists with a particular interest in crystallography and chemical education, we recognized an opportunity to expose secondary school students to crystallography. As part of a pilot elective program, students in the penultimate year of secondary education (Year 11; average age 16 years) from Scotch College, a school in the suburbs of Melbourne, were invited to participate in a seven-week afterschool research investigation. The program consisted of weekly one hour sessions that included the basic principles of crystallography, workshops on the use of the OLEX2 software package (Dolomanov et al., 2009) and experimental work to make new crystalline compounds in the school laboratory.
In order for school students to participate in a research project that involves synthesis and structure determination, we recognized that the synthetic work would need to be straightforward. Our research group has had a long-standing interest in coordination networks in which Zn II and Co II centres are linked by the dianion of 4-hydroxybenzoic acid (H 2 hba) (White et al., 2015) and we thought an investigation of alkali metal salts of the acid might be easily performed.
The carboxyl group of H 2 hba is deprotonated with relative ease (pK a 4.5 compared with 4.2 for benzoic acid) to form the Hhba À anion. Under some conditions, the phenolic group can also be deprotonated (pK a 9.7 compared with 10.0 for phenol), to form the dianion hba 2À . Given the coordinative versatility of the O-donor atoms of H 2 hba and its ability to form ions with either 1À and 2À charges, it was recognized that there may be an interesting systematic variation in the structures that could be obtained.
Transition-metal compounds of aromatic polycarboxylate ligands, such as benzene-1,4-dicarboxylate and benzene-1,3,5tricarboxylate, have been widely studied as a consequence of the potential applications for coordination polymers in drug delivery, gas storage, catalysis, separation and electrochemical applications (Li et al., 2020;Shi et al., 2019). On the other hand, relatively few carboxylate compounds of the s-block metals have been studied (Alnaqbi et al., 2021;Banerjee & Parise, 2011) and phenolate/carboxylate ligands have received little attention.
The Cambridge Structural Database (CSD; Version 5.42, February 2021 release; Groom et al., 2016) lists just four structures made only from H 2 hba and alkali metals. Skinner & Speakman (1951) isolated a potassium salt containing a proton placed symmetrically between adjacent carboxylate groups with a formula that can be represented as K(H 2 hba)-(Hhba)ÁH 2 O. The structure of this compound is discussed further in this article and improved structural data with more accurate molecular geometries are also provided. Skinner and Speakman mention the existence of an isostructural rubidium compound but no further details are given.
A sodium salt containing the anion Hhba À has been made by reacting H 2 hba and sodium metal in tetrahydrofuran (Dinnebier et al., 1999). The salt can be represented by the formula Na(Hhba) and, using powder X-ray diffraction methods, the compound was shown to consist of layers of distorted NaO 6 prisms with arene rings perpendicular to these layers and pointing up and down. The network is held together by hydrogen bonding between the phenolic hydroxy groups.
Finally, a lithium metal oxide framework containing the hba 2À dianion of formula Li 2 (hba)(CH 3 OH) 2 has been prepared by heating t-BuOLi and H 2 hba in a mixture of methanol and hexane (Zhao et al., 2018). It is composed of parallel helical chains of Li-O rings. These chains are bridged by hba 2À units to form channels with a triangular cross section.
This article reports the structures of nine new alkali metal salts of H 2 hba and also provides data for the salt Rb(H 2 hba)(Hhba)ÁH 2 O, which has been mentioned in the literature but not characterized previously by single-crystal X-ray diffraction. The investigations that resulted in the structures described in this article were successful in generating interest and enthusiasm among the students who performed the experimental work and initial crystallographic processing, as well as enhancing their understanding of chemical bonding. Furthermore, we believe the research will be of interest to the wider scientific community. Not only are the structures of the individual compounds inherently interesting, but collectively they demonstrate the effects of reaction research papers Table 1 Experimental details. Experiments were carried out using a Rigaku XtalLAB Synergy-S diffractometer at 100 K, except for the data for compounds 1 and 2, which were collected on a Rigaku Supernova diffractometer at 130 K. Cu K radiation was employed, with the exception of the data collection for compound 8, which used Mo K radiation. H atoms were treated by a mixture of independent and constrained refinement. Multi-scan (CrysAlis PRO; Rigaku OD, 2018) Analytical [CrysAlis PRO (Rigaku OD, 2018), based on expressions derived by Clark & Reid (1995)] Multi-scan (CrysAlis PRO; Rigaku OD, 2018) T min , T max 0. stoichiometry, ion size, hydrogen bonding and the nature of the ligand and solvent in the formation of ionic networks involving metal ions and organic anions.

Synthesis and crystallization
In a series of reactions, H 2 hba was combined with the hydroxides of lithium, sodium, potassium, rubidium and caesium in different stoichiometric ratios.
Typically, this involved the addition of 0.10 g (0.73 mmol) of H 2 hba to the appropriate amount of metal hydroxide in 5 ml of warm water (50 C). Crystals of the alkali metal salts suitable for single-crystal X-ray diffraction formed upon cooling and evaporation of the solvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The H atoms of the water molecules, phenolic groups and carboxylic acid groups were located in difference Fourier maps and refined with O-H distances restrained to 0.85 Å , except for the H atoms involved in the short strong hydrogen bonds in K(H 2 hba)-(Hhba)ÁH 2 O and Rb(H 2 hba)(Hhba)ÁH 2 O, which were located in difference Fourier maps and refined independently. The U iso values of the H atoms bonded to O atoms were allowed to refine. Other H atoms were placed in calculated positions and refined as riding atoms, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for aromatic H atoms. The uncoordinated water molecules are disordered in compound 10 and the H atoms of the water molecules are disordered in both 10 and 11. As a consequence, their positions have not been assigned. The phenolic H atoms in 10 and 11 are disordered over two positions. Details of the refinements can be found in the embedded instruction files in the CIF files.

Results and discussion
Each of the compounds formed by the reaction of the alkali metal hydroxides and H 2 hba in aqueous solution can be classified within one of three categories according to the metal-H n hba ratio in the crystal structure (n = 0, 1 or 2).
Type I: M 2 (hba)(H 2 O) x (M = Li, x = 2 and 3). Computer programs: CrysAlis PRO (Rigaku OD, 2015, 2018, 2021, SHELXT2014 (Sheldrick, 2015a), SHELXL2018 (Sheldrick, 2015b), OLEX2 (Dolomanov et al., 2009) and CrystalMaker (Palmer, 2020). Whilst H 2 hba can lose protons to form either the 1À or 2À ions, i.e. Hhba À or hba 2À , lithium is the only group I metal that yielded salts containing the dianion under the reaction conditions employed in this investigation. The loss of the weakly acidic phenolic proton in the presence of the lithium ion may be a consequence of the relatively small size of the Li + ion resulting in a strong Li-O interaction, which in turn promotes the loss of the proton of the hydroxy group.
A dihydrate, 1, and a trihydrate, 2, crystallized from aqueous 2:1 molar mixtures of LiOH and H 2 hba. The structures of their asymmetric units are shown in Fig. 1.
Compound 1 contains Li-O layers (hydrophilic sheets) that are formed from lithium ions linked by carboxylate and phenolate groups from hba 2À , creating helical chains which run in the a direction (Fig. 2a). Bridging water molecules (shown in green in Fig. 2b) link these chains to form twodimensional (2D) layers. Each Li + ion is four-coordinate and bonded to two bridging water molecules, a bridging phenolate O atom and a carboxylate O atom.
The extended packing arrangement in 1 is shown in Fig. 3(a). The hba 2À ligands extend above and below the Li-O sheets to form a pillared-type three-dimensional (3D) network. The hydrophilic sheets are separated by the hydrophobic sections of hba 2À , with a sheet-to-sheet separation of approximately 9.3 Å (half the length of the c axis). Hydrophilic M-O layers separated by hydrophobic regions is an arrangement common to all of the structures of the alkali metal-H n hba compounds described in the current work. This layered architecture is characteristic of many of the structures previously reported for coordination polymers of alkali metals (Banerjee & Parise, 2011).
The hba 2À pillars form stacks arranged in a face-to-face pattern (Fig. 3b), with alternating orientations of the ligand.
Whilst the trihydrate, 2, is also composed of hydrophilic Li-O sheets separated by hydrophobic organic regions, there are marked differences in its structure compared with the structure of 1. Each carboxylate O atom is bonded to one Li + ion in compound 1, whereas one of the carboxylate O atoms is bonded to two metal ions in 2, forming a 2D network in which four-coordinate lithium centres form discrete intrasheet Li 4 units within a sheet involving four-and six-membered rings (Fig. 4a). Fig. 4(b) shows the arrangement of these Li 4 units within a hydrophilic sheet. Although the Li 4 units within each sheet are not linked by strong bonds, neighbouring Li 4 units are linked to other Li 4 units via bonds to atoms in the adjacent hydrophilic sheets, resulting in a 2D network which extends in the bc plane (Fig. 4c). The sheet-to-sheet separation is about 9.15 Å (half the length of the c axis). Unlike the dihydrate salt, 1, the hba 2À pillars are packed in an edge-to-face arrangement, inverted along the a axis, as shown in Fig. 4(d). The asymmetric units of Li 2 (hba)(H 2 O) 2 (1) and Li 2 (hba)(H 2 O) 3 (2), showing the atom-labelling schemes. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented by spheres of arbitrary size.

Figure 2
The Li-O layers   Four different structural arrangements are observed in this group of salts.
Compound 3, LiHhba, is a 2D ionic network formed in a 1:1 reaction of LiOH and H 2 hba. Within the Li-O layers, each carboxylate O atom bridges two lithium ions to form four-, sixand eight-membered rings (Fig. 6a). The Hhba À pillars of each layer are closely stacked in an edge-to-face pattern (Fig. 6b), with each Li-O layer about 14.9 Å apart (the length of the a axis).
Each phenolic OH group forms two hydrogen bonds with phenolic OH groups on an adjacent 2D framework (Fig. 6c), holding the hydrophobic regions from the two layers together in a bilayer motif. There are, therefore, two types of hydrophilic layers within the structure: the layers containing Li + ions and, between these layers, ones composed of phenolic OH groups. The presence of a bilayer packing motif has been observed in many other metal carboxylates, although it is more common in aliphatic salts than in salts of aromatic acids (Vela & Foxman, 2000). Dinnebier et al. (1999) reported the synthesis of a salt of formula Na(Hhba) which has a similar bilayer structure to that of Li(Hhba) described above. It was made by reacting H 2 hba and sodium metal in tetrahydrofuran and powder X-ray diffraction was used to determine its structure. The salt consists of layers of distorted NaO 6 prisms and the layers are held together by hydrogen bonding between the phenolic groups. Unlike Li(Hhba), the arene rings, which are orientated perpendicular to these layers, are arranged in parallel stacks.      The Hhba À pillars are arranged in an antiparallel slipped stacking pattern (Fig. 7b). They form hydrogen bonds with the water molecules coordinated to the Na + ions, with a layer-tolayer separation of approximately 12.2 Å (Fig. 7c).
The potassium salt 5, K(Hhba)(H 2 O) 3 , may be considered to be a 2D network. The hydrophilic K-O layer shown in Fig. 8(a) is composed of distorted KO 8 square antiprisms formed between the metal ions and the O atoms from water molecules and one carboxylate O atom of each Hhba À unit bridging K + centres. Fig. 8(b) shows the face-to-face and edgeto-edge close packing of the organic pillars;stacking interactions are present between the cofacial Hhba À ligands [the centroids of the face-to-face pairs are 3.503 (2) Å apart]. The arene rings of Hhba À are perpendicular to the hydrophilic K-O layers and point up and down; they are interleaved with the arene rings of adjacent layers, as shown in Fig. 8(c).
Phenolic OH groups participate in hydrogen-bonding interactions with the water molecules bonded to the K centres in adjacent parallel sheets, which are about 12.3 Å apart.
3D networks with the general formula M(Hhba)(H 2 O) are formed with potassium, rubidium and caesium. Compound 6 is orthorhombic and shares some structural features with the isostructural monoclinic compounds, 7 and 8. Taking the rubidium salt, compound 7, as the exemplar, each Rb + ion is seven-coordinate and each carboxylate group is linked to four Rb + ions (Fig. 9a). The phenolic O atom, even though protonated, bridges two Rb + ions. Each water molecule bonds to only one metal ion. The Hhba À pillars are packed in a face-toface and edge-to-edge arrangement (Fig. 9b), forming the 3D network shown in Fig. 9(c).
As seen for the rubidium salt in Fig. 9(b), there is a pronounced rotation of the atoms in the carboxylate groups      away from the plane of the aromatic ring in the potassium, rubidium and caesium M(Hhba)(H 2 O) compounds [K 25.13 (8),Rb 26.86 (8) and Cs 24.49 (6) ]. The metal cations bonded to the carboxylate O atoms in these compounds are also not in the plane of the carboxylate group. Such configurations are uncommon in transition-metal-carboxylate complexes because the transition-metal ion is generally located in the plane of the carboxylate group. In s-block compounds, the bonds are mainly ionic in nature with little or no directionality and thus crystal packing forces and other steric considerations can dominate.
The structures of 6, 7 and 8 have similar connectivity and are compared in Fig. S1 of the supporting information.
There is extensive hydrogen bonding in each Na-O layer and the Na + ions are linked by a pair of bridging water molecules to form disodium units (Fig. 11a). Octahedral Na + ions are coordinated by three water molecules, two bridging and one terminal. The Na + ion is also coordinated by one phenolic O atom; the phenolic group on the other organic ligand is noncoordinated and does not participate in hydrogen bonding. Finally, the remaining cis sites on each Na + ion are occupied by an O atom of a protonated carboxylic acid group and an O atom of a deprotonated carboxylate group.
The disodium units are linked by the organic ligands to form a 2D network in the bc plane. Within each network, the organic units separate the Na + ions by approximately 9.7 Å , as shown in Fig. 11(b). Hydrogen bonds to intraplanar uncoordinated water molecules and between coordinated water molecules and phenolic groups on adjacent layers link the layers together. Alternate stacks of H 2 hba and Hhba À ligands are shown in Fig. 11(c).
Potassium and rubidium hydroxide react with H 2 hba to form compounds 10 and 11 with a general formula that can be represented as M(Hhba)(H 2 hba)ÁH 2 O. As indicated in the Introduction, the structure of the potassium salt was first determined by Skinner & Speakman (1951) and a more accurate study was performed in 1968 (Manojlović, 1968). As part of our investigation, single-crystal X-ray diffraction of a The structure of 5, showing (a) layers of distorted KO 8 prisms formed from the metal ions, the O atoms of water molecules (shown in green) and one carboxylate O atom from each Hhba À unit. (b) The edge-to-face and face-to-face stacking of the Hhba À ligands; note the inversion that occurs within face-to-face pairs. (c) The packing arrangement, viewed along the c axis. H atoms have been omitted in parts (a) and (c). Colour code: K blue, carboxylate and phenolate O red, water O green, C black and H pale pink. crystal of this compound has confirmed the previous results and provides more accurate molecular geometries. Furthermore, we have also obtained the first single-crystal X-ray diffraction data for the isostructural rubidium analogue that is mentioned in the article of Skinner and Speakman.
A feature of these potassium and rubidium compounds is the presence of an unusually strong interaction between two carboxylate O atoms involving a three-centre four-electron OÁ Á ÁHÁ Á ÁO hydrogen bond (Fig. 12). The O atoms involved are closely separated: 2.448 (3) Å in the potassium compound and 2.466 (4) Å in the rubidium compound. These bonds can be described as 'short strong' (SSHB) or 'low barrier' (LBHB) hydrogen bonds and may be considered partly covalent in character (Reiersølmoen et al., 2020;Saunders et al., 2019). On the basis of a single peak of electron density, we have elected to assign the H atom to the mid-point of the two O atoms. However, it is possible that the H atom is disordered over two closely separated positions. The crystallographic data does not allow us to clearly differentiate between a symmetrical or a disordered model.  The structure of 7, showing (a) the Rb-O layer, viewed along the a axis; C atoms in the arene rings and H atoms have been omitted. (b) The stacking of the Hhba À ligands, similar to that seen in compound 5; the centroids of the face-to-face pairs are approximately 3.83 Å apart. (c) The packing arrangement, viewed down the b axis, with H atoms omitted. Colour code: Rb purple, carboxylate and phenolate O red, water O green, C black and H pale pink.

Figure 10
The asymmetric unit of Na(Hhba)(H 2 hba)(H 2 O) 2 ÁH 2 O, 9, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented by spheres of arbitrary size. The red dotted line represents a hydrogen-bonding interaction.
Although we have chosen to represent the formulae of these compounds as M(Hhba)(H 2 hba)ÁH 2 O, if the proton is fixed in the symmetrical position, the organic ligand in these compounds could be regarded as the fusion of a H 2 hba and a Hhba À unit and represented as H(Hhba) 2 À , with each hydroxybenzoate unit having an average charge of À0.5.
The compounds form 2D ionic networks with both carboxylate O atoms bridging six-coordinate metal centres (Fig. 13a). The hydroxybenzoate pillars are closely packed in parallel stacks in a face-to-face and edge-to-edge pattern, as shown in Fig. 13 Hydrogen bonding between the phenolic OH groups and uncoordinated water molecules located between two adjacent hydrophobic regions holds the lattice together and creates a bilayer motif with a hydrophilic layer of water molecules and phenolic OH groups midway between the M-O hydrophilic layers (Fig. 13c). The M-O layers are about 16.4 Å apart.
The intralayer water molecules are disordered in compound 10 and the H atoms of the water molecules are disordered in both compounds 10 and 11, and their positions have not been assigned. The phenolic H atoms are disordered over two positions.

Packing of hydrophobic and hydrophilic layers
As described above, a common feature of the structures of the alkali metal salts of H 2 hba described in this work is the The structure of 9, showing (a) a disodium unit linked by a pair of bridging water molecules, (b) the packing arrangement, viewed down the a axis (H atoms have been omitted), and (c) the close-packed alternating stacking of the H 2 hba and Hhba À ligands. Colour code: Na yellow, carboxylate and phenolate O red, water O green, C black and H pale pink.

Figure 12
The asymmetric units of K(Hhba)(H 2 hba)ÁH 2 O, 10, and Rb(Hhba)-(H 2 hba)ÁH 2 O, 11, expanded to show the short strong hydrogen bonds present between carboxylate O atoms. Displacement ellipsoids of atoms other than hydrogen are drawn at the 50% probability level. H atoms are represented by spheres of arbitrary size. In the case of both 10 and 11, only one position of a disordered phenolic H atom is shown for clarity. The water molecule in 11 is disordered over two positions; the H atoms on O4 are disordered in both compounds and were not assigned.
hydrophilic M-O layers separated by hydrophobic regions of hydroxybenzoate units. It is perhaps surprising that similar layer structures are obtained regardless of the level of protonation of the hydroxybenzoate unit, i.e. the fully protonated H 2 hba, the monoanion Hhba À and the dianion hba 2À are arranged in such a way that hydrophilic groups participate in hydrogen bonding which appears to be important in the generation of layered structures. The hydroxybenzoate units adopt different packing arrangements in this series of compounds, including face-to-face, edge-to-face and a mixture of both. A slipped stacking arrangement is observed in compound 4 and three compounds contain a bilayer motif in which two discrete hydrophobic layers are held together by hydrogen bonds (compounds 3, 10 and 11).
Although the packing modes differ, the number of hydroxybenzoate ligands that pass through a cross-sectional plane through the hydrophobic layers and parallel to the M-O hydrophilic layer is calculated to be between 4.6 to 5.2 units per nm 2 for most compounds. As indicated in the spacefilling representations shown above, ligands are packed closely, with distances between adjacent molecules close to or less than that expected based on the van der Waals radii of their constituent atoms. The organic units in compound 2 are less closely packed (3.8 ligands per nm 2 ) due to penetration of coordinated water molecules into the 'hydrophobic' layers.
Close packing of the organic ligands seems to be a dominant factor in determining overall structure. As the ratio of metal ions, coordinated water molecules and hydroxybenzoate units changes in the compounds, the almost constant packing density of the hydroxybenzoate units influences the geometry and connectivity of the M-O hydrophilic layer.
In compounds 3, 4, 5, 10 and 11, the metal ions are in, or almost in, a plane and in compound 1 a sinusoidal pattern of metal ions is observed. Two planes of metal ions are present   within each hydrophilic layer in compounds 2, 6, 7, 8 and 9, an arrangement that allows the metal ions to be packed more densely in the layer than if only a single plane of metal ions were present. Fig. 14 shows the different topologies of the hydrophilic layers in the three lithium salts.

Conclusion
Given that H 2 hba is a relatively simple organic molecule, the salts that crystallize from aqueous solution when H 2 hba reacts with alkali metal hydroxides might be expected to form only a few different structures. However, although the structures of the salts have features in common, they are also remarkably different. The ligand can bond to metal ions as a dianion (hba 2À ), a monoanion (Hhba À ) or as the neutral acid species (H 2 hba), allowing for considerable possible structural variation.
Most of the salts are either 2D or 3D ionic networks composed of alternating hydrophilic layers of closely packed M-O polyhedra separated by the hydrophobic nonpolar component of the pillar-like hydroxybenzoate linking units.
The hydroxybenzoate units in the hydrophobic sections of the lattices are usually closely packed; a feature that seems to impact on the arrangement of metal ions in the hydrophilic layers and hence the structures overall. Whilst the ligands are usually present in two distinct orientations within a hydrophobic layer [with the exception of M(Hhba)(H 2 hba)ÁH 2 O; M = K or Rb], the packing observed in different salts includes edge-to-face, face-to-face and a mixture of the two. A bilayer packing arrangement is formed in three compounds. The metal ions may be both in the plane and out of the plane of the coordinating carboxylate group. The carboxylate group generally remains in the plane of the arene rings, although significant twisting is noted in M(Hhba)(H 2 O) (M = K, Rb or Cs).
Hydrogen bonds play a key role in the structure of all the compounds. They are present between the hydrophilic layers, as in K(Hhba)(H 2 O) 3 , within layers, as in Li 2 (hba)(H 2 O) 2 , and also in the form of an SSHB in M(Hhba)(H 2 hba)ÁH 2 O (M = K or Rb).
Li + is the only metal ion to give the dianion form of the ligand under the reaction conditions used in this investigation, perhaps as a consequence of its high charge density and the strength of the Li-O interaction. This investigation of the alkali metal salts of H 2 hba proved to be highly successful in demonstrating the fundamental roles of strong and weak bonding interactions in the structure of materials to senior secondary school students. The salts display bonding types ranging from covalent bonds, ionic attractions, ion-dipole interactions, neutral and chargeassisted hydrogen bonding, and dispersion forces. Whilst the salts are readily synthesized, the structures of most of the compounds prepared in this study have not been reported previously, allowing students to experience genuine scientific discovery and also to appreciate the power, precision and convenience of the technique of X-ray crystallography for structure analysis.

Figure 14
The hydrophilic layer in the lithium salts: (a) sinusoidal pattern of Li + ions in compound 1, (b) two planes of Li + ions in 2 and (c) a single plane of Li + ions in 3. H atoms have been omitted. Colour code: Li purple, water O green, carboxylate and phenolate O red and C black.

Poly[di-µ-aqua-µ-4-oxidobenzoato-dilithium] (compound1)
Crystal data [Li 2  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.23 e Å −3 Δρ min = −0.28 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.

Poly[triaqua-µ-4-oxidobenzoato-dilithium] (compound2)
Crystal data [Li 2   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.

Poly[µ-4-hydroxybenzoato-lithium] (compound3)
Crystal data [Li(C 7  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.

catena-Poly[4-hydroxybenzoate [[diaquasodium]-di-µ-aqua]] (compound4)
Crystal data  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.33 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.

Poly[di-µ-aqua-aqua-µ-4-hydroxybenzoato-potassium] (compound5)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.46 e Å −3 Δρ min = −0.41 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.22701 (

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.5760 (2) 0.7691 (  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.

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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) x y z U iso */U eq Occ. (