Dimethylammonium 2,4,5-tricarboxybenzoate: an example of the decarbonylation of N,N-dimethylformamide in the presence of a metal and a benzenepolycarboxylic acid. Is zirconium(IV) the Tsotsi?

The paper reports the molecular and crystal structure of the salt (CH3)2NH2 +·C10H5O8 −, with the cation formed by the decarbonylation of DMF solvent.


Chemical context
The term Tsotsi is South African township slang for a street gangster or hoodlum who is known to mug unsuspecting passers-by and steal their goods. Identifying a possible Tsotsi on the street is part of everyday township life. We attempted to grow a single crystal of a zirconium-based metal-organic framework incorporating 1,2,4,5-benzenetetracarboxylic acid that we had previously synthesized in powder form, in a dimethylformamide, DMF, solution. Instead this yielded crystals of the unanticipated title compound (I). The unexpected decarbonylation of N,N-dimethylformamide (DMF) has led us to ponder the possible characteristics of the reagents used that led to this 'plundering' of the DMF. Decarbonylation of DMF has previously been shown to occur under slow evaporation conditions in the presence of coordination complexes (Siddiqui et al., 2012;Chen et al., 2007;Karpova et al., 2004). In these reports, the nitrate salts of Mg II (Siddiqui et al., 2012), Pb II (Chen et al., 2007), Ho III and the chloride salt of Nd III (Karpova et al., 2004) ions were suggested to play a unique catalytic role in the observed decarbonylation reaction. The form of the metals in these reactions was thought to be as sixcoordinate metal complexes This suggests that, in the decarbonylation reaction observed here, the active decarbonylation agent could be the chloride salt of Zr IV as this is also likely to be six-coordinate in solution.
Clearly this further implicates the zirconium(IV) as the Tsotsi in this decarbonylation reaction, stealing CO from the DMF and forming the dimethylammonium cation (Fig. 1). While the detailed mechanism of the decarbonylation process remains unclear, it is most likely that the formation of this salt is initiated by the zirconium(IV) Tsotsi.

Structural commentary
The asymmetric unit of the title salt C 2 H 8 N + C 10 H 5 O 8 À , (I), consists of two anions, 1 and 2 and two cations, 3 and 4, differentiated by the leading numbers in the numbering scheme, Fig. 2. Within the asymmetric unit, both cations and anions are linked by strong N-HÁ Á ÁO and weaker C-HÁ Á ÁO hydrogen bonds, Table 1, Fig. 3. Bond distances and angles in the approximately tetrahedral dimethylammonium cations are unremarkable.
The benzene rings of the anions are inclined to one another by 6.56 (3) . In both anions, the two carboxylate substituents lie reasonably close to the benzene ring planes, inclined at 16.16 (19) for 1 and 6.01 (5) for 2. One carboxylic acid substituent in each cation also lies close to these planes, [5.85 (8) for 1 and 6.25 (9) for 2]. This planarity is doubtless aided by the two intramolecular O17-H17AÁ Á ÁO16 and O22-H22AÁ Á ÁO23 hydrogen bonds that form between a carboxylate oxygen and the OH group of an adjacent carboxylic acid substituent in each of the discrete anions, Fig. 2. Each encloses an S7 ring. The other two carboxylic acid substituents in both anions lie well out of the benzene ring planes with dihedral angles ranging from 75.4 (4) to 37.23 (15) . The reaction procedure used in the preparation of (I). Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; y À 1; z; (ii) x À 1; y; z; (iii) y À 1; Àx þ 1; z À 1 4 ; (iv) x; y þ 1; z; (v) x þ 1; y; z.

Figure 2
The asymmetric unit of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Intramolecular hydrogen bonds are drawn as dashed lines. classical C-HÁ Á ÁO hydrogen bonds. These are detailed in Table 1. Each individual anion of type 1 binds to four other type 1 anions through O-HÁ Á ÁO hydrogen bonds. Each also binds to four cations, two of type 3 and two of type 4, through N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, Fig. 4. Similarly, each type 2 anion binds to four other discrete type 2 anions and to three cations one of type 3 and two of type 4, Fig

Database survey
A search of the Cambridge Structural Database (Version 5.37, update November 2015; Groom et al., 2016) for 1,2,4,5-H 3 B4C À anion yielded 46 hits and of these 35 are purely organic compounds. One particular compound, YIRFOV, reports a 1,2,4,5-H 3 B4C À salt with a tetramethylammonium The asymmetric unit of (I), showing the hydrogen bonds formed between the cations and anions. In this and subsequent figures, hydrogen bonds are shown as blue dashed lines.

Figure 4
The immediate environment of anion 1.

Figure 5
The immediate environment of anion 2.

Figure 6
Sheets formed in the ab plane by the cations and anions of (I).
cation (Cunha-Silva et al., 2008). This is very similar to the structure reported here. The principal difference between these structures is that the asymmetric unit of YIRFOV comprises one tetramethylammonium cation, one 1,2,4,5-H 3 B4C À anion co-crystallized with half a fully protonated 1,2,4,5-H 4 B4C molecule that lies on a centre of inversion. In YIRFOV, the crystal packing is also mediated by an extensive hydrogen-bonding network.

Synthesis and crystallization
A 2 mL aqueous solution of ZrOCl 2 Á8H 2 O (0.04 g, 0.124 mmol) was suspended in 0.5 mL N,N-dimethylformamide (DMF). A 2 mL aqueous solution of 1,2,4,5-H 4 B4C 0.032 g, 0.124 mmol) was similarly suspended in 0.5 mL DMF and the two solutions were combined in a small sample vial. This was placed inside a larger sample vial. 0.5 mL of deionized water was added before it was covered and left until crystallization was complete. After three weeks, yellowbrown cubic crystals formed. These were isolated and used for the X-ray crystallographic analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All non-hydrogen atoms were refined anisotropically. Carbon-bound hydrogen atoms were placed in calculated positions and were included in the refinement in the riding-model approximation, with U iso (H) Overall packing for (I), viewed along the a-axis direction. set to 1.2U eq (C). The hydrogen atoms of the methyl groups were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density, with U iso (H) = 1.5U eq (C). The H atoms of the hydroxyl groups were allowed to rotate with a fixed angle around the C--O bond to best fit the experimental electron density with U iso (H) set to 1.5U eq (O). Mercury (Macrae et al., 2008); software used to prepare material for publication: PLATON (Spek, 2009).

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. Refinement. Carbon and nitrogen-bound H atoms were placed in calculated positions and were included in the refinement in the riding model approximation, with U(H) set to 1.2 U eq (C) and U eq (N) respectively. The H atoms of the methyl groups were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density (HFIX 137 in the SHELX program suite (Sheldrick, 2008)), with U(H) set to 1.5Ueq(C). The H atoms of the hydroxyl groups were allowed to rotate with a fixed angle around the C-O bond to best fit the experimental electron density (HFIX 147 in the SHELX program suite (Sheldrick, 2008)), with U(H) set to 1.5U eq (C).