dl-Piperidinium-2-carboxylate bis(hydrogen peroxide): unusual hydrogen-bonded peroxide chains

The title compound, C6H11NO2·2H2O2, is the richest (by molar ratio) in hydrogen peroxide among the peroxosolvates of aliphatic α-amino acids. Unusually for aliphatic α-amino acid peroxosolvates, the H2O2 molecules are linked, forming infinite hydrogen-bonded hydroperoxo chains running along the c-axis direction.


Chemical context
Peroxosolvates are crystalline adducts of hydrogen peroxide with various organic or inorganic compounds. Since they are convenient solid sources of active oxygen, some of them have become widely used commercial bleaching, disinfection and oxidation reagents (Jakob et al., 2012;Cronin et al., 2017). It is well known that their stability is strongly dependent on the hydrogen-bonded motifs formed by hydrogen peroxide (Chernyshov et al., 2017). On other hand, H 2 O 2 is one of the most important signalling molecules in biological systems (Li et al., 2020;To et al., 2020). The structures of amino acid peroxosolvates have been studied intensively as simple models of hydrogen peroxide binding with proteins (Prikhodchenko et al., 2011;Kapustin et al., 2014). Peroxide and water-peroxide clusters are now of special interest since they may simulate cooperative hydrogen-bonded switching in the transportation of hydrogen peroxide species through cell membranes (Grishanov et al., 2017;Varadaraj & Kumari, 2020;Wang et al., 2020). Recently, several structures of organic peroxosolvates with peroxide hydrogen-bonded 1D-aggregates have been reported (Chernyshov et al., 2017;Navasardyan et al., 2017Navasardyan et al., , 2018. ISSN 2056-9890

Structural commentary
The asymmetric unit of the title compound (I) comprises a pipecolinic acid molecule and two crystallographically independent peroxide molecules (Fig. 1). As expected, the amino acid coformer exhibits the zwitterionic form with almost equal C-O distances [1.2429 (11) and 1.2639 (11) Å ]. All bond lengths and angles in the organic coformer are close to those observed in the structures of pure pipecolinic acid [(II); Stapleton & Tiekink, 2001) and pipecolinic acid tetrahydrate [(III); Bhattacharjee & Chacko, 1979;Lyssenko et al., 2006]. As observed for (II) and (III), the pipecolinic acid molecule in (I) adopts a chair conformation with the carboxylate group occupying the equatorial position. It is of interest to note in all three structures (I), (II), and (III), the core amino acid fragments N-C-CO 2 are almost planar, with N-C-C-O torsion angles of less than 22 . This is obviously caused by electrostatic interactions between the oppositely charged amino and carboxylic groups.

Figure 5
Peroxide hydrogen-bonded chains parallel to the c axis. Hydrogen bonds are shown as dashed lines.

Figure 6
The mutual arrangement of syn and anti lone electron pairs of the carboxylate anion. (Mills & Dean, 1996). It is well known that syn and anti lone pairs exhibit noticeably different basicity (Gandour, 1981;Pal et al., 2018) and hydrogen-bonding properties as a result of electronic and steric effects (Gorbitz & Etter, 1992;Pranata, 1993). Nine hydrogen-bonded linkage modes are theoretically possible in the structures of amino acid peroxosolvates, taking into account that bifurcated HOOHÁ Á ÁO bonds are not known (Fig. 7). It has been shown that the ability of carboxylic anti-orbitals to form hydrogen bonds is strongly affected by steric hindrance caused by -substituents in the side chains of carboxylic acids (Gorbitz & Etter, 1992). It is clear that in (I) the unfeasibility of the fourth carboxylic hydrogen bond is the result of steric effects caused by the peroxide molecules hydrogen bonded with the ammonium group (Fig. 2). It should be noted that the spatial arrangement of the endocyclic amino group in (I) is predefined by the aforementioned planarity of the N-C-CO 2 amino acid fragment.

Synthesis and crystallization
96% Hydrogen peroxide was prepared by an extraction method from serine peroxosolvate (Wolanov et al., 2010). Colourless prismatic crystals of the title compound were obtained by cooling a saturated solution (r.t.) of pipecolinic acid (Aldrich) in 96% hydrogen peroxide to 255 K. Handling procedures for concentrated hydrogen peroxide have been described in detail (danger of explosion!) by Schumb et al. (1955).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were found in difference-Fourier maps and were refined with independent positional and isotropic displacement parameters.  Computer programs: APEX2 and SAINT (Bruker, 2008), SHELXL2018/3 (Sheldrick, 2015) and XP in SHELXTL (Sheldrick, 2008).

Figure 7
Possible hydrogen-bonded motifs in the structures of amino acid peroxosolvates.

DL-Piperidinium-2-carboxylate bis(hydrogen peroxide)
Crystal data 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.