The crystal structure of the ammonium salt of 2-aminomalonic acid

2-Aminomalonic acid is particularly interesting for the synthesis of peptides and amino acids due to its two carboxyl groups. The crystal structure reported here is the first single-crystal X-ray diffraction measurement of this compound or one of its salts.


Introduction
The first synthesis of 2-aminomalonic acid was in 1864 and described by Bayer (Beaujon & Hartung, 1953).In 1902, Ruhemann and Orton investigated the preparation with nitromalonamide as a starting material and a reduction with amalgam (Beaujon & Hartung, 1953).In 1902, Lu ¨tz used halogenated malonic acid and ammonia as the starting materials to obtain 2-aminomalonic acid as the product (Beaujon & Hartung, 1953).To obtain a much purer product, Hartung invented in 1952 a distillation in a vacuum with a palladiumcharcoal catalyst.2-Aminomalonic acid was obtained in a yield of 80-90% (Beaujon & Hartung, 1953).
2-Aminomalonic acid is used as a complexone in medicine, environmental technology and chemistry due to it being a member of the amino polycarboxylic acid group of substances (Anderegg et al., 2005).In 1945, G. Schwarzenbach introduced the name 'complexones' for laboratory-synthesized compounds which are close to amino acids (Anderegg et al., 2005).Well-known representatives of complexones are, for example, EDTA (ethylenediaminetetraacetic acid), DTPA (diethylenetriamine pentaacetate) or TETA (triethylenetetramine) (Anderegg et al., 2005).These compounds are built with a nitrogen-containing moiety which enables their use as ligands.
The corresponding acids of 2-aminomalonic acid and its salts are of particular interest because of their two carboxyl groups, one of which can be decarboxylated to form a chiral centre (Zheng et al., 2023).Like other complexones, 2-aminomalonic has a nitrogen moiety and other functional groups that are very suitable for binding complexes (Anderegg et al., 2005).The zwitterionic character is similar to that of amino acids and makes it possible to use it as a ligand at different pH values.

Synthesis and crystallization
Malonic acid (10.4 g, 0.1 mmol) and diethyl ether (100 ml) were added to a dried Schlenk flask.The mixture was cooled to 273 K and bromine (16.0 g, 0.1 mol) was added under stirring over a period of 40 min.The mixture was warmed to room temperature and stirred for a further 40 min.Aqueous ammonia (100 ml, 25%) was added slowly under stirring.The solvent was removed in a vacuum.The product was obtained as a white-to-light-yellow solid product.The synthesis route is shown in Scheme 1.

Analysis (X-ray, Raman and NMR)
We investigated and characterized salt (I) by single-crystal X-ray diffraction, Raman spectroscopy and NMR spectroscopy.Complete data and devices for the X-ray measurements are listed in the CIF in the supporting information.Lowtemperature Raman spectroscopic studies were performed using a Bruker MultiRAM FT-Raman spectrometer with an Nd:YAG laser excitation (� = 1064 cm À 1 ) under vacuum at 77 K.For a measurement, the synthesized compound was transferred to a cooled glass cell.A Bruker AV400TR spectrometer was used for the 1 H, 13 C and 14 N NMR measurements.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 1.

Figure 1
The asymmetric unit of salt (I), with displacement ellipsoids drawn at the 50% probability level.
The carbon chain has a C1-C2-C3 angle of 113.00 (10) � and is only slightly magnified compared to the starting material The crystal structure of salt (I) displays a three-dimensional network built of moderate N-H� � �O hydrogen bonds, according to the classification of Jeffrey (1997) The low-temperature Raman spectrum of malonic acid and (I).

Figure 4
The 1 H NMR spectrum of (I) in D 2 O.

Raman spectroscopy
The Raman spectrum of (I) is shown in Fig. 3, together with that of the starting material malonic acid.The N-H stretching vibrations are detected at 3032 and 2809 cm À 1 .The C-H stretching vibration is observed at 2977 cm À 1 .The polarized C O stretching vibration is detected at 1684 cm À 1 and that of C-O at 1328 cm À 1 .

NMR spectroscopy
The 1 H, 13 C and 14 N NMR spectra of salt (I) were measured in D 2 O at room temperature.The 1 H NMR spectrum (Fig. 4) shows one singlet at 4.18 ppm (s, CH).Compared to the starting material, the proton is significantly less acidic and deshielded by 0.76 ppm.The starting material has an H/D exchange in D 2 O, which is recognizable by the triplet at

Figure 6
The 13 C NMR spectrum of (I) in D 2 O.
3.40 ppm and the singlet at 3.42 ppm (Fig. 5).The 13 C NMR analysis of (I) detected the carboxyl C atom at 170.1 ppm and the C2 atom at 59.1 ppm (Fig. 6); compared to the starting material, the carboxy moieties are not significantly shifted (Fig. 7).The protons of atom C2 of the malonic acid are much more acidic, resulting in the 13 C NMR spectrum in a triplet at 40.7 ppm (t, J = 20.0Hz) and a quintet at 40.2 ppm (p, J = 20.3Hz) splitting.In salt (I), the C2 carbon is much more deshielded and a singlet is seen at 59.1 ppm.The 14 N NMR spectrum (Fig. 8) shows the ammonium cation at À 340.6 ppm and the -NH 3 + moiety at À 361.5 ppm as singlets.

Conclusion
Herein we present the first single-crystal X-ray diffraction and Raman and NMR spectroscopy study of the salt ammonium 2-aminomalonate.The 13 C NMR spectrum of malonic acid (C 3 H 4 O 4 ) in D 2 O.

Figure 8
The 14 N NMR spectrum of (I) in D 2 O.

2004
).Also, we describe the H/D exchange of the CH 2 moiety in D 2 O of malonic acid for the first time.

sup-2
Acta Cryst.(2024).C80, 291-296 Refinement.Refinement of F 2 against ALL reflections.The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 .The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement.R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Figure 5
Figure 5The 1 H NMR spectrum of malonic acid (C 3 H 4 O 4 ) in D 2 O.

Table 1
Experimental details.