Crystal structure of a (carboxymethyl)triethylazanium bromide–2-(triethylazaniumyl)acetate (1/1) hydrogen-bonded dimer

The crystal structure of a hydrogen-bonded dimer of (triethylazaniumyl)acetic acid and (triethylazaniumyl)acetate bromide features a carboxylic acid hydrogen atom that is engaged in an asymmetric hydrogen bond with the carboxylate oxygen. The crystal also features intramolecular C—H hydrogen bonds and a layer of bromide ions that is surrounded by alkyl groups.


Chemical context
The �-carbonylphosphonate moiety is commonly used as a reagent in the Horner-Wadsworth-Emmons reaction (Horner et al., 1958;Wadsworth & Emmons, 1961;Bisceglia & Orelli, 2015).These molecules are reacted with aldehydes or ketones to prepare �,�-unsaturated esters, where a preference for the Z-configuration of the alkene group is often observed.When the phosphonate group is replaced with a phosphine oxide, these sets of compounds have found use as ligands and extraction agents for f-elements (Babecki et al., 1989(Babecki et al., , 1990(Babecki et al., , 1992)).Our research group has also characterized the ability of these types of compounds to sensitize the luminescence of lanthanide ions (Leach et al., 2017;Sartain et al., 2015).To this end, our group has been working to develop a synthetic route to the target compound shown in Fig. 1, following the procedure reported by Ando (1999).The title compound was an undesired byproduct of this reaction, and serendipitously crystallized from the aqueous washings upon standing.A proposed mechanism for the formation of the title compound I is also shown in Fig. 1.

Structural commentary
The title salt crystallizes as a 50:50 mixture of the ammonium carboxylate zwitterion and the ammonium bromide.The molecular entities of this compound are shown in Fig. 2 along with the atom-numbering scheme.The asymmetric unit is composed of all of the atoms shown in Fig. 2 where the carboxylic acid hydrogen atom H1 has a 0.50 occupancy, and the Br À anion is located on a twofold rotation axis (Wyckoff position 4e) of space group I2/a.The ammonium group has C-N bond lengths ranging from 1.514 (3) to 1.526 (3) A ˚with a nearly perfect tetrahedral arrangement of alkyl groups around the nitrogen atom with a � 4 descriptor for fourfold coordination of 0.97 (where 0.00 = square-planar, 0.85 = trigonal-pyramidal, and 1.00 = tetrahedral; Yang et al., 2007).The carboxylic acid group has C-O bond lengths of 1.286 (3) and 1.224 (3) A ˚.When the molecule is viewed down the C2-N1 bond the groups adopt a staggered conformation with the carboxylic acid group being anti to the C5-C6 ethyl group.The torsion angle between these two groups (C1-C2-N1-C5) is 168.8 (2) � .Two intramolecular C-H� � �O hydrogen bonds are present between the carbonyl oxygen atom O2 and hydrogen atoms H3A and H7B of the gauche alkyl groups (Table 1, Fig. 3).

Supramolecular features
Molecules of the title compound exist as hydrogen-bonded dimers in the solid state.The carboxylic acid hydrogen atom H1 is a half-occupied bridging hydrogen atom (Fa ´bry, 2018), and within this dimer it is either bonded to oxygen atom O1 or to its symmetry derived counterpart O1 i [symmetry code: (i)x + 3 2 , -y + 1 2 , -z + 1 2 ; Fig. 3].When this atom H1 is covalently bonded to O1, it is engaged in a very strong asymmetric hydrogen bond with the symmetry-derived oxygen atom O1 i (Table 1).The bromide counter-ions are located away from the carboxylate/carboxylic acid sites and occupy a layer that lies parallel to the ab plane.These layers are bordered by the ethyl chains of the ammonium groups (Fig. 4).

Figure 2
The molecular structure of compound I, with the atom-labeling scheme.Displacement ellipsoids are drawn at the 50% probability level using standard CPK colors.Atom H1 shows half-occupancy.

Figure 3
A depiction of the hydrogen-bonding interactions present in the crystal of compound I using a ball-and-stick model with standard CPK colors.
Hydrogen-bonding interactions are depicted with blue dashed lines and all hydrogen atoms not involved in a hydrogen bond are not shown for clarity.

Figure 4
A view of the crystal structure down the b axis showing a cross section of the layers of bromide ions.This figure was drawn with a ball-and-stick model using standard CPK colors.Only one position of hydrogen atom H1 is shown, and all other hydrogen atoms have been omitted for clarity.

Database survey
A search of the Cambridge Structural Database (CSD version 5.43, Jun. 2022;Groom et al., 2016) for structures with a hydrogen atom shared between two carboxylate sites resulted in 274 hits.One of the simplest structures in this set is that of ammonium diacetate (ACAMAC; Nahringbauer, 1969).The structures ALUNIE (Dega-Szafran et al., 2003) and CIVKUQ (Ghazaryan et al., 2018) are similar to the title compound with either a piperidinium ring or a trimethylammonium group, respectively, in the place of the triethylammonium groups.
Both compounds were isolated with a halide counter-ion: ALUNIE was isolated with one chloride anion and CIVKUQ was isolated as the iodide salt.

Synthesis and crystallization
Dibutyl phosphite (1.4 ml, 1.4 g, 7.17 mmol) was added via syringe to a two-necked 25 ml round-bottom flask under an atmosphere of nitrogen.The reagent was dissolved in 7.0 ml of dichloromethane and the flask was placed in an ice-water bath.Benzyl bromoacetate (1.1 ml, 1.6 g, 6.94 mmol) and triethylamine (1.4 ml, 1.0 g, 10.0 mmol) were added and the reaction mixture was stirred for 15 minutes in the ice bath followed by one hour at room temperature.Water (10 ml) was added to the reaction, and the aqueous layer was washed with ethyl acetate (3�10 ml).The organic extracts were combined and washed with 1 M HCl (3�10 ml) and brine (1�10 ml), then dried over MgSO 4 .The title compound crystallized serendipitously from the combined aqueous washings after standing for ca three days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.All hydrogen atoms bonded to carbon atoms were placed in calculated positions and refined as riding: C-H = 0.95 -1.00A ˚with U iso (H) = 1.2U eq (C) for methylene hydrogen atoms and U iso (H) = 1.5U eq (C) for the hydrogen atoms of the methyl groups.The carboxylic acid hydrogen atom H1 was located using electron-density difference maps.The position of this hydrogen atom was fixed and the occupancy constrained to 0.5.Its isotropic displacement parameter was refined as suggested by Fa ´bry (2018).

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 1 (top) The reaction carried out in this work, along with structures of the target �-carbonylphosphonate and the title compound I. (bottom) A proposed mechanism for the formation of the title compound.

Table 2
Experimental details.