Bis(tetraethylammonium) oxalate dihydrate

The title compound, 2C8H20N+·C2O4 2−·2H2O, synthesized by neutralizing H2C2O4·2H2O with (C2H5)4NOH in a 1:2 molar ratio, is a deliquescent solid. The oxalate ion is nonplanar, with a dihedral angle between carboxylate groups of 64.37 (2)°. O—H⋯O hydrogen bonds of moderate strength link the O atoms of the water molecules and the oxalate ions into rings parallel to the c axis. The rings exhibit the graph-set motif R 4 4(12). In addition, there are weak C—H⋯O interactions in the crystal structure.

The title compound, 2C 8 H 20 N + ÁC 2 O 4 2À Á2H 2 O, synthesized by neutralizing H 2 C 2 O 4 Á2H 2 O with (C 2 H 5 ) 4 NOH in a 1:2 molar ratio, is a deliquescent solid. The oxalate ion is nonplanar, with a dihedral angle between carboxylate groups of 64.37 (2) . O-HÁ Á ÁO hydrogen bonds of moderate strength link the O atoms of the water molecules and the oxalate ions into rings parallel to the c axis. The rings exhibit the graph-set motif R 4 4 (12). In addition, there are weak C-HÁ Á ÁO interactions in the crystal structure.
The title compound, (Et 4 N) 2 C 2 O 4 .2H 2 O, synthesized by reaction of Et 4 NOH and H 2 C 2 O 4 .2H 2 O in a 2:1 mole ratio, is deliquescent and rapidly absorbs moisture from air. Previously prepared and used in situ or as a noncrystalline solid, it has been employed as a reductant in aprotic electrochemical cells (Engels et al., 1983) and in the synthesis of oxalatecontaining organometallic complexes (Diop et al., 1997;Demadis & Coucouvanis, 1995;Darensbourg et al., 1992).

Oxalate bond distances (C-C and C-O) and angles (O-C-O and O-C-C) are comparable to other reported
oxalate salts containing NH 4 + and alkali metal ions. The oxalate ion is planar in Li 2 C 2 O 4 (Beagley & Small, 1964) and Na 2 C 2 O 4 (Jeffrey & Parry, 1954), but the dihedral angle between planes of symmetrical carboxylate groups of the oxalate fragment is 26.6° in (NH 4 ) 2 C 2 O 4 .H 2 O (Robertson, 1965). The directional character of the hydrogen bonding pattern in the monohydrate is believed responsible for the observed non-planar stereochemistry of the oxalate ion. A dihedral angle of 64.37 (2)° is present in the title compound (Fig. 1). The non-planarity of the oxalate ion is maintained by moderate hydrogen bonds (Table 1) that link the oxygen atoms of the oxalate ion and the water molecules into a ring motif R 4 4 (12) (Etter et al., 1990). (For the classification of the hydrogen bonds, see Gilli & Gilli, 2009). In addition, there are also present weak C-H···O interactions in the structure (Tab. 1). Fig. 2 illustrates the packing diagram for the structure of the title compound.

Experimental
An aqueous 35 weight percent solution of Et 4 NOH (6.69 g, 15.9 mmol OH -) was added by syringe to a 50-ml Schlenk tube containing a 20-ml CH 3 CN solution of H 2 C 2 O 4 .2H 2 O (1.00 g, 7.93 mmol). The colorless solution was stirred under argon for 15 min followed by solvent removal under reduced pressure. Hot tetrahydrofuran (25 ml) was added to the greasy solid, the mixture was stirred for 10 min, and the solution was decanted from the product. The deliquescent white solid was dried under vacuum and crystallized from CH 3 CN/ THF. Yield: 1.98 g (65%). IR (υ(CO), CH 3 CN) 1565(s) cm -1 .
Oxalate was confirmed by the blue ring resorcinol test (Chernoff, 1920). The analyzed crystal was rapidly transferred from vacuum to a 100 K stream of dry air for X-ray analysis.

Refinement
Data were refined against F 2 . Because of the relatively high R int the determination of the absolute structure turned out to be meaningless and therefore 1509 Friedel pairs have been merged. All the hydrogen atoms appeared in the difference supplementary materials sup-2 Acta Cryst. (2012). E68, o2382-o2383 electron density map, nevertheless, those pertinent to the methyl and the methylene carbons were situated into the idealized positions and refined in the riding atom formalism. The positional parameters of the water hydrogens were refined freely. The applied constraints: C methyl -H methyl =0.98, C methylene -H methylene =0.99 Å. U iso (H methylene ) = 1.2U eq (C methylene ), U iso (H methyl ) = 1.5U eq (C methyl ), U iso (H water ) = 1.5U eq (O water ).

Figure 1
Ellipsoid plot of the title molecules. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
Mercury (Macrae et al., 2006) packing diagram of the title compound viewed down the b axis.

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. 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 > σ(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.