Crystal structure of the sodium salt of mesotrione: a triketone herbicide

The crystal structure of the sodium salt of mesotrione [2-(4-methylsulfonyl-2-nitrobenzoyl)cyclohexane-1,3-dione] is described. A one-dimensional polymer is formed by the coordination of all functional groups except the NO2 group. The coordination number of the sodium atom in the compound is 5.


Chemical context
Mesotrione, 2-(4-methylsulfonyl-2-nitrobenzoyl) cyclohexane-1,3-dione, is an organic compound classified as a triketone herbicide that is widely used in modern agriculture to control weeds and increase crop yields of corn (Mitchell et al., 2001).The coordination properties of triketone herbicides are dictated by the presence of three ketone functional groups, which act as ligands, forming stable coordination complexes with metal ions such as Cu 2+ , Co 2+ and Fe 3+ (Le Person et al., 2016).The stability of the chelates depends largely on the pH, as mesotrione is a weak acid that dissociates from the molecular to the anionic form at higher pH, which is more resistant to hydrolysis and photolysis processes (Reynolds et al., 2007).For a comparative study, the crystal structure of the sodium salt of mesotrione, NaL, as well as analogues structures were retrieved from the Cambridge Structural Database (CSD, vesion 5.44, update of September 2023; Groom et al., 2016) and their geometries and confirmations are discussed (Kang et al., 2015); Hou et al., 2010;Wu et al., 2002).

Structural commentary
Selected geometrical parameters of the sodium salt of mesotrione are summarized in Table 1.The ligand shows a polydentate function.Coordination to the sodium ion occurs through the formation of a 6-membered chelate involving two oxygen atoms from the two keto groups (Fig. 1).This leads to the occurrence of �-conjugation within the chelate ring, leading to a shortening of the C-C bonds by 0.06 (3) A ˚and lengthening of C O bonds by 0.062 (3) A ˚in comparison to the free ligand HL (Table 2).In turn, in the mesotrione sodium salt, the occurrence of conjugation in the triketonate ligand results in a decrease in the conjugation between the benzene ring and the chelate ring, as evidenced by a 0.014 (3) A ˚increase in the C4-C8 bond length (Table 2).
The chelate fragment tends towards a planar structure.Simultaneously, the oxygen atom O5 of the cyclohexane fragment serves as a bridge to a neighboring sodium ion, forming a flat quadrangle Na1-O5-Na1 i -O5 i constituting the linker that forms the polymer chain (Fig. 2).
The benzene and cyclohexane ring conformations in the structure of sodium salt and free ligand are similar.The benzene ring has a planar conformation, while the cyclohexane ring represents a semi chair with a bend in the line linking atoms C11-C13.The main geometrical characteristics of hydrogen bonds of the compound [NaL(EtOH)]•EtOH are given in Table 3.
The environment sphere of the sodium ion comprises the oxygen atoms O5 and O6 of the chelate, the bridging oxygen atom O5 i , the oxygen atom O8 from the coordinated ethanol molecule, and the oxygen atom O1 ii from the methyl sulfonyl group of a neighboring molecule (Fig. 2).Using the SHAPE program (Version 2.1; Llunell et al., 2013), it was determined that the environment of the sodium atom is close to D 3h symmetry (trigonal bipyramid) with a convergence factor of 1.6.
The fragment of the structure of the sodium salt of mesotrione, showing the atom-numbering scheme for non-hydrogen atoms and displacement ellipsoids at 50% probability level.

Figure 2
Coordination polyhedron of the sodium salt of mesotrione.

Supramolecular features
In the crystal structure of the sodium salt of mesotrione, the molecules are assembled in a polymer chain (Fig. 3).Two types of hydrogen bonds are observed: the first between the oxygen atom of the uncoordinated ethanol molecule (O9A) and the oxygen atom (O8) of the coordinated ethanol molecule  3).

Experimental
The FT-IR spectra of the solids were recorded in a KBr matrix in the range 4000-400cm À 1 using a Perkin-Elmer Spectrum BX2 spectrometer. 1 H NMR spectra were recorded using a WR-400 Bruker NMR spectrometer at room temperature in DMSO-d 6 , with TMS used as the internal standard.Studies on the thermal properties of the sodium salt of mesotrione were conducted using a synchronous TG/DTA analyzer, the Shimadzu DTG-60H.The sample was heated in an air atmosphere to 600 � C in aluminum crucibles at a heating rate of 10 � C min À 1 .

Synthesis and crystallization
Mesotrione was obtained commercially.Other chemicals and solvents used in this study were purchased from Aldrich and used without further purification.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Non-coordinated ethanol molecules forming hydrogen bonds with the coordination fragment are disordered at two positions H9-O9A-C17A-C18A with an occupancy ratio of 0.8 and 0.2 for H9-O9B-C17B-C18B.Both disordered molecules were refined anisotropically,  Crystal packing in a cell with projection onto the ac plane.Hydrogen bonds are highlighted in blue.
with certain constraints applied to bond lengths and the same U ij components in the minor constituent.C-bound H atoms were positioned geometrically (C-H = 0.95-0.99A ˚) and refined as riding with U iso (H) = 1.2U eq (C).

Thermogravimetric analysis
Four different stages of decomposition of the mesotrionebased sodium complex were observed in the investigated temperature range (Fig. 5).The first stage of thermal decomposition is characterized by a distinct exothermic effect and a mass loss of �12% in the temperature range of 25-182 � C. The exothermic effect is observed at a temperature of 147 � C (m.p. = 149-151 � C), corresponding to the loss of the first ethanol molecule.
At the second stage of the decomposition of the coordination compound in the temperature range 182-281 � C, the loss (�11%) of the second ethanol molecule occurs, which is accompanied by an endothermic effect.The third stage of thermal decomposition is characterized by exothermic effect and a mass loss of �8.5% in the temperature range 280-340 � C. The exothermic effect is observed at a temperature of 318.8 � C, corresponding to the combustion of the entire organic components.
The fourth stage begins at 500 � C and ends at 600 � C and cannot be detected by the Shimadzu DTG-60H.
The TGV analysis and calculations based on its results show that the third and fourth stages consist of the combustion of the entire organic component of the molecule and the formation of sodium pyrosulfate.
According to the thermal studies, the fourth stage is accompanied by a strong exothermic effect and includes the further transformation of Na 2 S 2 O 7 into Na 2 SO 4 , which is confirmed by the results of IR spectroscopy (Fig. 6).The DTA (red line), DrTGA (pink line) and TGA (blue line) weight loss trace for the sodium salt of mesotrione.

Figure 6
The IR spectrum for the final product after TGA (Na 2 SO 4 ).

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.

Table 2
Comparison between some geometrical parameters (A ˚) in the chelate ring for HL and NaL.

Table 4
Experimental details.