Bis(glycinium) oxalate: evidence of strong hydrogen bonding

In the title 2:1 salt, 2C₂H₆NO₂⁺·C₂O₄^2-, the glycine mol­ecule is in the cationic form with a positively charged amino group and an uncharged carboxylic acid group. The doubly charged oxalate anion lies across a crystallographic inversion centre. One of the reasons why the 1:1 glycinium oxalate salt has a higher melting point than the title compound may be the difference in their hydrogen-bonding patterns. A database search for salts formed between amino acids or substituted amino acids and oxalic acid revealed that, in most of the structures, the conformation about the O=C-OH bond is synplanar. D-Tryptophan oxalate is the only example where the OH group of a semi-oxalate adopts an anti­planar conformation. The 2:1 stoichiometry seen in the present salt is observed only in the salts of DL-serine, DL-aspartic acid and betaine with oxalic acid.

S1. Comment

The salts of amino acids with simple carboxylic acids are believed to have existed in the prebiotic earth (Miller & Orgel, 1974: Kvenvolden et al., 1971). These salts are held together by an extensive network of hydrogen bonds that could provide insight into the principles of bimolecular aggregation and recognition. Amino acid salts exhibit structural phase transitions of various types, and they also exhibit ferroelectric, antiferroelectric or ferroelastic behaviour (Albers, 1988; Schaack, 1990). Glycine is the simplest and the only non-chiral amino acid in nature. It can exist in cationic, zwitterionic or anionic forms. Oxalic acid is the simplest dicarboxylic acid, and it can exist as the oxalate, semioxalate or oxalic acid form. A 1:1 salt of glycine and oxalic acid has already been reported (Subha Nandini, Krishna Kumar, Malathi et al., 2001; Subha Nandini, Krishna Kumar & Natrajan, 2001a,b,c), in which glycine exists in the cationic form with a positively charged amino group and an uncharged carboxylic acid group, while oxalic acid exists as the semioxalate. Salts of oxalic acid with many other natural and substituted amino acids, such as DL-alanine (Subha Nandini, Krishna Kumar, Malathi et al., 2001; Subha Nandini, Krishna Kumar & Natrajan, 2001a,b,c), L-alanine (Subha Nandini et al., 2001, 2001a,b,c), DL-arginine and L-arginine (Chandra et al., 1998), DL-aspartic acid (Alagar et al., 2003), L-histidine and DL-histidine (Prabu et al., 1996), DL-lysine and L-lysine (Venkatraman et al., 1997), D-tryptophan (Bakke & Mostad, 1980), DL-threonine (Subha Nandini et al., 2001, 2001a,b,c), L-leucine (Rajagopal et al., 2003), DL-serine (Alagar et al., 2002), β-alaninuim (Krishna Kumar et al., 2002; Godzisz et al., 2003), and betaine (Rodrigues et al., 2001), have been reported. In these salts, oxalic acid predominantly occurs as the semioxalate ion, except in the case of DL-serine, L-lysine and DL-aspartic acid, where one of the species was in the oxalate form.

Using the November 2004 update 5.26 of the Cambridge Structural Database (Allen, 2002), an analysis of the geometry and hydrogen bonding in salts of amino acids or substituted amino acids with oxalic acid has been carried out. Among all these salts, a short and very strong O—H···O hydrogen bond involving the carboxyl group of the amino acid has been observed only in the salt with betaine. The O—H···O hydrogen bond in the betaine salt is found between the carboxyl groups of two amino acid molecules; one H atom is shared between two amino acid molecules. The present X-ray study of the of 2:1 glycinium oxalate salt, (I), carried out at room temperature, also reveals a strong and short hydrogen bond involving the amino acid carboxylic group, but in this case the O—H···O bond is between the protonated carboxyl group of the amino acid and the oxalate anion. All the other three structures containing an amino acid and the oxalate ion contain similar interactions but with longer O···O distances [i.e. 2.531 (1) Å and 2.516 (1) Å for DL-serine, 2.565 (2) Å for DL-aspartic acid, and 2.497 (2) Å for L-lysine, cf. 2.454 (1) Å for (I)]. The asymmetric unit consists of half an oxalate ion lying across an inversion centre and one glycinium ion (Fig. 1). The glycine has a positively charged amine group and an uncharged carboxylic acid group, while the oxalic acid exists as a doubly charged oxalate anion. Fig. 2 shows the packing of the molecules viewed approximately along the a axis. The carboxylate O atoms of the oxalate anion are involved as hydrogen-bonding acceptors. The protonated glycine molecules are linked by N—H···O hydrogen bonds forming a three dimensional network running along all three principal axes.

The strengths of O—H···O hydrogen bonds are generally correlated with geometric parameters such as the O—H, H···O and O···O distances and the O—H···O angle (Jeffery, 1997). Very strong hydrogen bonds are observed to have H···O distances of 1.2–1.6 Å, O···O separations of 2.4–2.55 Å and O—H··· O angles of 175–180°, with concomitant lengthening of the covalent O—H bond. In the present salt, the carboxylic OH group of glycine makes a very strong hydrogen bond with an O atom of the doubly ionized oxalate. The two carboxyl groups involved in the strong hydrogen bond also satisfy the essential criterion for the formation of the strong hydrogen bond, namely that the donor and acceptor atoms have identical proton affinities in the media in which they coexist. The larger value for C1—O1 (Table 1) in the present structure could be attributed to the presence of a very strong O—H···O hydrogen bond involving this O atom (or the symmetry equivalent O4). Apart from these interactions, C—H···O contacts of 2.47 and 2.69 Å have also been observed for both Cα H atoms of glycine.

The number of 1:1 amino acid and substituted amino acid oxalates is much higher than that of other stochiometeries. The 2:1 stochiometery seen in the present salt is observed in the salts of DL-serine, DL-aspartic acid and betaine with oxalic acid. Interestingly, in DL-serine, DL-aspartic acid and the present structure, the 2:1 stochiometery has been achieved through identical protonation states of the components. The substituted amino acid betaine has a different protonation state (protonated and zwitterionic betaine and semioxalate), which could possibly be due to chemical modification of the amino acid.

The conformation of the oxalate or semioxalate ion is essentially determined by the torsion angle around the C—O and C—C bonds. In all of these 18 structures [the 18 structures found in the CSD?] with 22 oxalate or semioxalate ions between them, the conformation of the OH group about the O═C—OH bond is synplanar, except in the case of the D-tryptophan salt (Bakke & Mostad, 1980). Although an earlier analysis (Leiserowitz, 1976) found the antiplanar conformation to be associated with an intramolecular hydrogen bond, there is no such intramolecular hydrogen bond in the structure of D-tryptophan oxalate. The distribution of torsion angles around the C—C bond of the oxalic acid moiety was also examined. It was observed that the conformation typically adopted was planar (Chandra et al., 1998). However, in the cases of betanium oxalate and DL-arginine oxalate these torsion angles are 70.1 (4) and 95.2 (1)°, respectively. In the case of DL-arginine, there are additional hydrogen-bond donors, which form additional hydrogen bonds to the semioxalate ions. Twisting of the semioxalate ion might therefore reasonably be attributed to the formation of a stable hydrogen-bond network. In the case of betaine, as there is complete methylation of the amine group, there are no hydrogen bonds between the amino acid and oxalate moieties·The twisting in this case may be due to optimization of electrostatic O⁻···N⁺contacts. Hence these atypical torsion angles cannot be attributed to one factor. The melting points of 2:1 and 1:1 glycinium oxalates were found to be 428 and 449 K, respectively. One of the possible reasons for the higher melting point of glycinium oxalate is the difference in the hydrogen-bonding patterns of the two salts (Table 2).

S2. Experimental

Colourless single crystals of glycinium oxalate (2:1) were grown by slow evaporation from an aqueous solution containing glycine and oxalic acid in a 2:1 molar ratio. The melting points of crystalline samples were determined using differential scanning calorimeter (Metler toledeo make). AUTHOR: What does "Mettler/Metler Toledeo make" mean? FP62? The samples were heated at a rate of 10 K min⁻¹ from 123 K up to the melting point.

S3. Refinement

All H atoms were found initially in difference Fourier maps. H atoms bonded to C3 were placed at geometrically idealized positions (C—H = 0.91 and 0.93 Å) and refined using a riding model, with U_iso(H) = 1.5U_eq(C). All other H atoms were freely refined.

Computing details

Data collection: SMART (Bruker, 2004); cell refinement: SMART; data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. An ellipsoid plot of the title salt, with the atom-numbering scheme and 50% probability displacement ellipsoids. [Symmetry code: (i) −x, 1 − y, −z.] ##AUTHOR: please check the symmetry code.
	Fig. 2. Packing diagram for the title salt, viewed approximately along the a axis, showing the three-dimensional hydrogen-bonding network.

bis(glycinium) oxalate

Crystal data

2C2H6NO2^+·C2O4^2^−	Dx = 1.512 Mg m^−3
Mr = 240.16	Melting point: 428 K
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 4.9199 (18) Å	Cell parameters from 962 reflections
b = 9.959 (4) Å	θ = 2.8–25.4°
c = 10.859 (4) Å	µ = 0.14 mm^−1
β = 97.513 (5)°	T = 300 K
V = 527.5 (3) Å^3	Irregular WHAT?, colourless
Z = 2	0.03 × 0.02 × 0.01 mm
F(000) = 252

Data collection

Bruker SMART CCD area-detector diffractometer	854 reflections with I > 2σ(I)
Radiation source: sealed tube	Rint = 0.029
Graphite monochromator	θmax = 25.4°, θmin = 2.8°
ϕ and ω scans	h = −4→5
2712 measured reflections	k = −11→11
962 independent reflections	l = −13→9

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.034	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.090	w = 1/[σ^2(Fo^2) + (0.0423P)^2 + 0.1698P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max < 0.001
962 reflections	Δρmax = 0.21 e Å^−3
90 parameters	Δρmin = −0.18 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.088 (11)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.1464 (2)	0.33970 (10)	−0.00077 (10)	0.0304 (3)
O2	0.2942 (2)	0.52300 (10)	0.10586 (10)	0.0298 (3)
C1	0.1256 (3)	0.45859 (14)	0.02920 (13)	0.0228 (4)
O3	0.6960 (2)	0.38781 (12)	0.18572 (10)	0.0356 (4)
O4	0.7106 (2)	0.46925 (12)	0.37769 (11)	0.0426 (4)
C2	0.8025 (3)	0.40118 (15)	0.29946 (14)	0.0265 (4)
N1	1.1425 (3)	0.30420 (14)	0.46340 (13)	0.0287 (4)
H1	1.003 (4)	0.256 (2)	0.4938 (17)	0.048 (6)*
H2	1.304 (4)	0.254 (2)	0.4762 (17)	0.044 (5)*
H3	1.175 (4)	0.385 (2)	0.5056 (19)	0.053 (6)*
C3	1.0625 (3)	0.32122 (18)	0.32942 (15)	0.0366 (5)
H4	1.1998	0.3662	0.2980	0.055*
H5	1.0415	0.2358	0.2946	0.055*
H6	0.514 (6)	0.450 (3)	0.152 (3)	0.100 (9)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0234 (6)	0.0245 (6)	0.0416 (7)	0.0026 (4)	−0.0016 (5)	−0.0044 (5)
O2	0.0257 (6)	0.0271 (6)	0.0333 (6)	−0.0002 (4)	−0.0089 (5)	−0.0006 (5)
C1	0.0199 (7)	0.0242 (8)	0.0244 (8)	−0.0008 (6)	0.0027 (6)	0.0035 (6)
O3	0.0319 (6)	0.0457 (7)	0.0269 (6)	0.0120 (5)	−0.0047 (5)	−0.0002 (5)
O4	0.0412 (7)	0.0430 (7)	0.0398 (7)	0.0141 (5)	−0.0094 (6)	−0.0131 (6)
C2	0.0234 (8)	0.0246 (8)	0.0308 (8)	−0.0011 (6)	0.0003 (6)	0.0029 (7)
N1	0.0259 (7)	0.0257 (7)	0.0325 (8)	0.0030 (6)	−0.0039 (6)	0.0026 (6)
C3	0.0290 (9)	0.0502 (11)	0.0305 (9)	0.0128 (7)	0.0038 (7)	0.0120 (8)

Geometric parameters (Å, º)

O1—C1	1.2357 (18)	C2—C3	1.506 (2)
O2—C1	1.2697 (17)	N1—C3	1.467 (2)
O2—H6	1.34 (3)	N1—H1	0.93 (2)
C1—C1^i	1.551 (3)	N1—H2	0.93 (2)
O3—C2	1.2840 (19)	N1—H3	0.93 (2)
O3—H6	1.11 (3)	C3—H4	0.91
O4—C2	1.2189 (19)	C3—H5	0.93
C1—O2—H6	113.3 (12)	H1—N1—H2	109.1 (17)
O1—C1—O2	125.97 (13)	C3—N1—H3	113.4 (13)
O1—C1—C1^i	119.40 (15)	H1—N1—H3	110.8 (17)
O2—C1—C1^i	114.63 (15)	H2—N1—H3	107.2 (17)
C2—O3—H6	118.2 (14)	N1—C3—C2	112.68 (14)
O4—C2—O3	125.90 (14)	N1—C3—H4	108.1
O4—C2—C3	121.70 (14)	C2—C3—H4	108.0
O3—C2—C3	112.40 (13)	N1—C3—H5	107.4
C3—N1—H1	107.3 (12)	C2—C3—H5	110.5
C3—N1—H2	108.9 (11)	H4—C3—H5	110.2

Symmetry code: (i) −x, −y+1, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1^ii	0.93 (2)	2.01 (2)	2.9002 (19)	160.5 (17)
N1—H1···O3^ii	0.93 (2)	2.61 (2)	3.0641 (19)	110.9 (15)
N1—H2···O1^iii	0.93 (2)	1.92 (2)	2.8458 (19)	176.7 (17)
N1—H3···O4^iv	0.93 (2)	1.96 (2)	2.875 (2)	167.7 (19)
O3—H6···O2	1.11 (3)	1.34 (3)	2.4544 (15)	178 (3)
O3—H6···O1	1.11 (3)	2.54 (3)	3.1954 (17)	116.7 (18)
C3—H5···O2^v	0.93	2.47	3.112 (2)	126
C3—H4···O2^vi	0.91	2.69	3.458 (2)	142

Symmetry codes: (ii) x+1/2, −y+1/2, z+1/2; (iii) x+3/2, −y+1/2, z+1/2; (iv) −x+2, −y+1, −z+1; (v) −x+3/2, y−1/2, −z+1/2; (vi) x+1, y, z.