Lithium and sodium 3-(3,4-dihydroxyphenyl)propenoate hydrate

The Li cation in the crystal structure of the lithium salt LiC9H7O4·H2O shows a coordination number of four whereas the Na cation in the crystal structure of the sodium salt NaC9H7O4·H2O shows a coordination number of seven.


Chemical context
trans-3-(3,4-Dihydroxyphenyl)-2-propenoic acid (caffeic acid) is ubiquitous in plants and plays a role as an intermediate in the biosynthesis of lignin (Boerjan et al., 2003).The first X-ray crystal-structure analysis of caffeic acid dates back to the year 1987 (Garcı ´a-Granda et al., 1987), and a more recent study was published in 2015 (Kumar et al., 2015).In current research, caffeic acid is used as a co-crystallizing agent, particularly for pharmaceutically relevant compounds such as 5-fluorouracil (Yu et al., 2020).The simultaneous presence of the carboxyl and the catechol moieties renders caffeic acid a versatile ligand in coordination chemistry, in particular after deprotonation of the acidic groups (Petrou et al., 1993).However, transition-metal complexes of caffeic acid derivatives have not yet been structurally investigated.Even for simple alkali metal caffeates, reports are rare and, up to now, only potassium caffeate has been studied in detail as the potassium caffeate/ caffeic acid co-crystallization product (Lombardo et al., 2011).
Here we report the crystallization and crystal-structure analysis of the lithium and sodium salts of caffeic acid

Structural commentary
The asymmetric unit of 1 comprises one Li cation, one 3-(3,4dihydroxyphenyl)propenoate anion and one water molecule (Fig. 1).The Li cation is coordinated nearly tetrahedrally by three carboxylate O atoms of three caffeate anions and one water molecule.The Li-O distances range from 1.908 (2) to 2.005 (3) A ˚and the O-Li-O angles from 105.35 (12) to 112.20 (11) � (Table 1).These values are similar to those reported for other lithium carboxylate compounds such as lithium acetate monohydrate [Li-O: 1.920 (2) to 2.031 (2) A ˚, O-Li-O: 99.78 (10) to 124.21 (11) � ; Martı ´nez Casado et al., 2011].The carboxylate group adopts a � 3 -� 3 O:O 0 ,O 0 coordination mode.This leads to the formation of six-membered Li 2 O 3 C rings that are catenated parallel to the b axis by edgesharing (Fig. 2).Alternatively, the chain structure can be derived from condensation of corner-sharing LiO 4 tetrahedra (Fig. 3).The translational period of the 2 1 -type helix corresponds to the length of the b axis and one repeating unit comprises two LiO 4 tetrahedra.Chains of corner-sharing LiO 4 tetrahedra are not unusual for lithium carboxylate monohydrates, and similar patterns were observed for a lithium chloride glycine adduct (Mu ¨ller et al., 1994) and lithium 2,4,6trifluorobenzoate hydrate (Lamann et al., 2012), which may serve as representative examples.
In the crystal structure of 2, the sodium cation adopts a sevenfold coordination from one water oxygen atom, two carboxylate and four catechol oxygen atoms (Fig. 4).According to a SHAPE analysis (SHAPE 2.1; Llunell et al., 2013), the NaO 7 polyhedron is roughly related to the facecapped octahedron (CShM 2.807) and to the face-capped trigonal prism (CShM 3.593) with some preference to the former (Llunell et al., 2013;Pinsky & Avnir, 1998;Casanova et al., 2004;Cirera et al., 2005).Numerical data for this analysis Molecular structure of the (3,4-dihydroxyphenyl)propenoate anion in compound 1 along with the coordination sphere of the lithium cation.The asymmetric unit is displayed with filled bonds.Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.Symmetry codes refer to Table 1.

Figure 2
Section of the crystal structure of 1 showing the six-membered Li 2 O 3 C rings.are given in Table 2.The centre of Fig. 5 displays the observed NaO 7 polyhedron and the idealized CTRP-7 (left) and COC-7 (right) polyhedra in order to illustrate the degree of distortion.
The Na-O distances are in the range from 2.3185 ( 14) to 2.7897 (17) A ˚(Table 3) and are comparable to those found in monosodium tartrate hydrate [2.3331 (12) to 2.6740 (12) A ˚], which also displays coordination number seven for the sodium cation (Al-Dajani et al., 2010).Generally, sodium carboxylates with coordination number seven for the cation are rather rare.
A search of the Cambridge Structural Database (CSD, version 2022.5.43;Groom et al., 2016) gave 20 matches.In this selection, the Na-O distances range from 2.299 to 3.017 A ˚with a median value of 2.44 A ˚(lower quartile: 2.380 A ˚, upper quartile: 2.557 A ˚). Regarding the coordination mode of the carboxylate unit, the sodium salt 2 differs from the lithium salt 1 in the way that only one carboxylate O atom (O2) is involved in coordination.Furthermore, the coordination mode of the catechol groups is also different in the two structures.In the case of 1, the catechol groups are part of the hydrogenbonding network and there is no direct Li-O coordination from these groups.In contrast, the crystal structure of 2 reveals a direct coordination by the catechol oxygen atoms.
Here, the catechol group acts as a chelating ligand and connects two sodium cations.The coordination mode can be described as �-� 4 O,O 0 :O,O 0 .Up to now, sodium compounds with chelating catechol groups have been observed only rarely.

Supramolecular features
The supramolecular structure of lithium caffeate hydrate is governed by O-H� � �O hydrogen bonds (Table 4).Both hydrogen atoms H5A and H5B of the water molecule are involved in hydrogen bonds to adjacent catechol groups research communications Acta Cryst. (2024).E80, 401-407 Figure 4 Molecular structure of the caffeate anion in compound 2 along with the coordination sphere of the sodium cation.The asymmetric unit is displayed with filled bonds.Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.Symmetry codes refer to Table 2.Additional symmetry codes: (vii)

Figure 5
Representation of the NaO 7 polyhedron in 2 (centre) and the best fitting regular polyhedra TRP-7 (left) and COC-7 (right).

Figure 6
Linkage of the NaO 7 polyhedra by edge-and face-sharing in the crystal structure of 2.
(Fig. 8).H5B is part of a bifurcated hydrogen bond that connects the catechol oxygen atoms O3 and O4 with the water oxygen atom O5 (type a 1 and a 2 in Fig. 8).Hydrogen-bonding patterns in 1. Symmetry codes refer to Table 4.Additional symmetry codes: (

Figure 10
Position of the hydrogen bonds (dashed lines) along the LiO 4 tetrahedra chains in the crystal structure of 1.
hydrogen-bonding sequences starting from O5 or its centrosymmetric equivalent in the neighbouring chain.This leads to R 2 2 (24) motifs (Fig. 11).
Fig. 14 represents the position of the intra-chain hydrogen bonds.Furthermore, the catechol groups are involved in hydrogen-bonding interactions.The O3-H3 group acts as the donor with respect to the carboxylate oxygen atom O1 of a neighbouring chain (O iv in Fig. 12, type c).This leads to an R 2 2 (18) motif between adjacent 3-(3,4-dihydroxyphenyl)-2propenoate units (Fig. 13).Finally, the cross-linking of the Figure 14 Intra-chain hydrogen bonds in compound 2.

Figure 13
Inter-layer hydrogen bonds in 2. Symmetry codes refer to Table 5.

Figure 11
Section of the crystal structure of 1 with the complete hydrogen-bonding network.

Figure 12
Hydrogen-bonding patterns in the crystal structure of 2. Symmetry codes refer to Table 5.
chains is completed by hydrogen bonds of the type d with the O4-H4 group as the donor and the water oxygen atom O5 of a neighbouring chain as acceptor.The position of the different hydrogen-bonding types is displayed in Fig. 15.Fig. 16 shows a packing diagram with the complete hydrogen-bonding network of 2. In direct comparison with 1, the hydrogen bonds in 2 are significantly stronger, with the closest O� � �O distance being 2.6350 (19) A ˚(Table 5).Computer programs: X-AREA (Stoe & Cie, 2016), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), DIAMOND (Brandenburg & Putz, 2019) and OLEX2 (Dolomanov et al., 2009).

Figure 16
Section of the crystal structure of 2 showing the complete hydrogenbonding network (dashed lines).

Figure 15
Position of the hydrogen bonds (dashed lines) along the NaO 7 polyhedral chains in the crystal structure of 2. Symmetry codes refer to Table 3.Additional symmetry codes: (x) x À 1, y + 1, z; (xi) x + 1, y, z.

Synthesis and crystallization
2 mmol of the alkali hydroxide (48 mg, LiOH, 80 mg NaOH) and 2 mmol of caffeic acid (360 mg) were dissolved in 5 ml of water to give a clear solution.After slow evaporation the products were isolated as colourless solids in nearly quantitative yield.Single crystals were obtained by recrystallization from water.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6.All carbon-bound hydrogen atoms were positioned geometrically (C-H = 0.94 A ˚) and refined as riding, with U iso (H) = 1.2U eq (C).Hydrogen atoms bound to oxygen were found in difference-Fourier maps.The O-H distances of 2 were restricted to 0.82 A ˚.

sup-2
Acta Cryst.(2024).Geometric parameters (Å, º) Treatment of 3-(3,4-dihydroxyphenyl)propenoic acid (caffeic acid or 3,4-dihydroxycinnamic acid) with the alkali hydroxides MOH (M = Li, Na) in aqueous solution led to the formation of poly[aqua[�-3-(3,4-dihydroxyphenyl)propenoato]lithium], [Li(C 9 H 7 O 4 )(H 2 O)] n , 1, and poly[aqua[�-3-(3,4-dihydroxyphenyl)propenoato]sodium], [Na(C 9 H 7 O 4 )(H 2 O)] n , 2. The crystal structure of 1 consists of a lithium cation that is coordinated nearly tetrahedrally by three carboxylate oxygen atoms and a water molecule.The carboxylate groups adopt a � 3 -� 3 O:O 0 :O 0 coordination mode that leads to a chain-like catenation of Li cations and carboxylate units parallel to the b axis.Moreover, the lithium carboxylate chains are connected by hydrogen bonds between water molecules attached to lithium and catechol OH groups.The crystal structure of 2 shows a sevenfold coordination of the sodium cation by one water molecule, two monodentately binding carboxylate groups and four oxygen atoms from two catechol groups.The coordination polyhedra are linked by face-and edgesharing into chains extending parallel to the b axis.The chains are interlinked by the bridging 3-(3,4-dihydroxyphenyl)propenoate units and by intermolecular hydrogen bonds to form the tri-periodic network.

Figure 3
Figure 3Section of the crystal structure of 1 showing the linkage of the LiO 4 tetrahedra.

Table 6
Experimental details.
Special detailsGeometry.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.