1-Methylpiperazine-1,4-dium bis(hydrogen oxalate)

In the crystal structure of the title compound, C5H14N2 2+·2HC2O4 −, the two crystallographically independent hydrogen oxalate anions are linked by strong intermolecular O—H⋯O hydrogen bonds, forming two independent corrugated chains parallel to the b axis. These chains are further connected by N—H⋯O and C—H⋯O hydrogen bonds originating from the organic cations, forming a three-dimensional network. The diprotonated piperazine ring adopts a chair conformation, with the methyl group occupying an equatorial position.

In the crystal structure of the title compound, C 5 H 14 N 2 2+ Á2HC 2 O 4 À , the two crystallographically independent hydrogen oxalate anions are linked by strong intermolecular O-HÁ Á ÁO hydrogen bonds, forming two independent corrugated chains parallel to the b axis. These chains are further connected by N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds originating from the organic cations, forming a three-dimensional network. The diprotonated piperazine ring adopts a chair conformation, with the methyl group occupying an equatorial position.
Supporting information for this paper is available from the IUCr electronic archives (Reference: ZL2578).  Piperazine and its derivatives have been intensively investigated owing to their interesting pharmacological, cardiovascular and autonomic properties (Conrado et al., 2008). Piperazine derivatives are found in biologically active compounds across a number of different therapeutic areas such as antifungal, antibacterial, antimalarial, antipsychotic, antidepressant and antitumour activity against colon, prostate, breast, lung and leukemia tumors (Brockunier et al., 2004;Bogatcheva et al., 2006;Essid et al., 2013). In the present work, we report the preparation and the crystal structure of an organic proton transfer salt (C 5 H 14 N 2 ) 2+ ·2(HC 2 O 4 ) -, (I). The asymmetric unit of (I) contains one 1-methylpiperazin-1,4diium dication and two semi-oxalate anions ( Fig. 1). 1-Methylpiperazine is diprotonated at atom N1 and N2 and oxalic acid is mono-deprotonated. The oxalate monoanions are essentially planar, with dihedral angles between the carboxylate groups of less than 4°. Two strong O-H···O (Table 1) hydrogen bonds generate linear oxalate chains running parallel to the b axis (Fig. 2). The geometrical parameters of these chains correlate well with the corresponding values found in related crystal structures (Essid et al., 2013;Vaidhyanathan et al., 2002;Ejsmont & Zaleski, 2006). Bond distances around atom C7 and C8 indicate a carboxylate group with delocalization of the negative charge between atoms O3 and O4, and between O7 and O8. In the hydrogenoxalate anion HC 2 O 4 -, the H atoms are located at O2 and O5. The position of protonation is also indicated by elongation of the corresponding C-O distances [O2-C6 = 1.306 (2) Å, O5-C9 = 1.306 (1) Å]. The bond lengths of C6-C7 and C8-C9 are relatively long [1.553 (2) Å, 1.544 (2) Å] as expected for an oxalate anion. Geometrical parameters of the methylpiperazin-1,4-dium dications are found to be in agreement with those of another similar structure of methylpiperazin-1,4-diium dipicrate (Dutkiewicz et al., 2011). The cyclic amine adopts a chair conformation with the methyl group occupying an equatorial position, with puckering parameters: Q = 0.5772 (11) A, θ = 2.85 (11)° and φ = -174 (2)° (Cremer & Pople, 1975) and atoms N1 and N2 deviating by -0.308 (2) and 0.333 (2) Å from the least-squares plane defined by the remaining atoms in the ring. In addition, the crystal structure of [C 5 H 14 N 2 ] (HC 2 O 4 ) 2 is stabilized by ionic interactions between the 1-methylpiperazin-1,4-dium dications and the oxalate monoanions chains, as well as by a network of N-H···O and C-H···O hydrogen bonds ( Fig. 3 and Table 1) such that all the hydrogen atoms bonded to nitrogen atoms participate in the formation of these hydrogen bonds, with donor-acceptor distances between 2.745 (2) and 3.433 (2) Å (Table 1).

Experimental
An aqueous solution containing 2 mmol of H 2 C 2 O 4 in 20 ml of water was added to 1 mmol of 1-methylpiperazine in 10 ml of ethanol.

Refinement
All H atoms were located in a difference map. Nevertheless, they were geometrically placed and refined using a riding    Projection of (I) along the b axis. The H-atoms not involved in H-bonding are omitted.

1-Methylpiperazine-1,4-dium bis(hydrogen oxalate)
Crystal data 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.