Synthesis, crystal structure and Hirshfeld surface analysis of sodium bis(malonato)borate monohydrate

The asymmetric unit of the title salt, Na+·C6H4BO8 −·H2O, comprises a five-coordinate sodium cation, a bis(malonato)borate anion and a water molecule of crystallization.


Chemical context
The review by Vaalma et al. (2018) provides a comprehensive overview of the cost and resource implications of sodium-ion batteries, which are a promising alternative to lithium-ion batteries for energy storage applications.The authors conclude that sodium-ion batteries have the potential to be significantly less expensive than lithium-ion batteries, due to the abundance of sodium and the lower cost of sodium-based materials.Allen et al. (2012) described the structure of lithium bis(2-methyllactato)borate monohydrate and there is a growing interest, as highlighted in various recent studies (Vaalma et al., 2018;Abraham, 2020;Li et al., 2019;Wang et al., 2018), in lithium bis(malonato)borate polymers as robust electrolytes.
The present study explores the substitution of 2-methyllactic acid and lithium carbonate with malonic acid and sodium carbonate, respectively, and presents the synthesis, crystal structure and Hirshfeld surface analysis of the title compound, Na + •[B(C 3 H 2 O 4 ) 2 ] À •H 2 O, (I).

Structural commentary
The asymmetric unit of (I) comprises a sodium cation, a bis-(malonato)borate anion and a coordinated water molecule (Fig. 1).The tetrahedral B atom is bonded to two malonate (C 3 H 2 O 4 ) ligands coordinated in an O,O 0 -bidentate mode.

Supramolecular features and Hirshfeld surface analysis
The crystal structure of (I) is consolidated by two C-H� � �O links with the acceptors being one water O9 atom and one carbonyl O4 atom, and three O-H� � �O links, one of which is bifurcated, with the acceptors being one borate O1 atom and carbonyl atoms O4 and O2 (Table 2).The packing of the structure is shown in Fig. 2.
The Hirshfeld surface analysis of (I) was performed with CrystalExplorer (Version 21.5; Spackman et al., 2021).Fig. 3 shows the d norm surface for the bis(malonato)borate anion plotted over the limits from À 0.66 to +0.93 a.u.The intense red spots represent the shortest intermolecular contacts (attractive interactions like hydrogen bonds) and the blue

Database survey
A search using CCDC ConQuest of the Cambridge Structural Database (CSD, Version 5.44, updated to June 2023; Groom et al., 2016) for the bis(malonato)borate anion gave one hit (CSD refcode PITQUF; Zviedre & Belyakov, 2007), in which the bis(malonato)borate unit is similar to that in (I) and is charge balanced by potassium cations.The K + coordination geometry in PITQUF is an irregular nine-vertex polyhedron formed by the O atoms of seven complex anions.

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.

Figure 1
Figure 1View of the molecular structure of (I), showing 50% probability displacement ellipsoids (arbitrary spheres for the H atoms).

Figure 2 A
Figure 2 A packing diagram of (I), viewed along the a-axis direction (projection onto the bc plane), showing the C-H� � �O and O-H� � �O hydrogen bonds as black dashed lines.

Figure 3 Figure 4 A
Figure 3The Hirshfeld surface for (I).The surface is drawn with transparency and simplified for clarity, and the regions with the strongest intermolecular interactions are shown in red.(d norm range: À 0.66 to +0.93 a.u.)

Table 3
Experimental details.