Crystal structure of 2,3,5,6-tetrakis(pyridin-2-yl)pyrazine hydrogen peroxide 4.75-solvate

The structure of title co-crystal consists of a 2,3,5,6-tetrakis(pyridin-2-yl)pyrazine coformer and hydrogen peroxide solvent molecules in a ratio of 1:4.75.


Chemical context
Peroxosolvates are solids that contain H 2 O 2 molecules in a manner analogous to the water in crystalline hydrates. Nowadays, some peroxosolvates find widespread use as environmentally friendly decontaminating and bleaching compounds (Jakob et al., 2012), and as oxidizing agents in organic synthesis (Ahn et al., 2015). Hydrogen bonding in peroxosolvates is of particular interest because it may be used for modelling of hydrogen peroxide behaviour in various significant biochemical processes, especially oxidative stress and transport through cellular membranes (Kapustin et al., 2014).
In the organic molecule, all four pyridin-2-yl substituents are significantly inclined with respect to the central pyrazine ring (Fig. 3), such that the N-C-C-N torsion angles range between 130.8 (6) and 140.0 (4) . Similar conformations have been observed for all three known polymorphs of the pure coformer (Bock et al., 1992;Behrens & Rehder, 2009;Malecki, 2010). Of structural significance, the pairs of pyridinyl nitrogen atoms N1, N4 and N2, N3 are located at opposite sides of the central pyrazine ring. This arrangement clearly facilitates the organization of hydrogen-bonded chains in the structure (see below). All four pyridinyl nitrogen atoms are involved as hydrogen-bond acceptors, but neither of the pyrazine N atoms participate in hydrogen bonding, presumably because of steric hindrance.

Figure 1
Labelling scheme for organic coformer and six crystallographically independent peroxide molecules. Displacement ellipsoids are shown at the 50% probability level. Hydrogen bonds are drawn as dashed lines.
The ordered molecules Per4 and Per5 form four hydrogen bonds (two as donor and two as acceptor) in [2,2] mode ( Fig. 4). This coordination environment of the peroxide molecules is the most common arrangement in organic peroxosolvates (Prikhodchenko et al., 2011). In contrast, the disordered or partially occupied molecules Per1, Per2, Per3, and Per6 are involved in just two or three hydrogen bonds with adjacent peroxide molecules, but not with the organic coformer. It should be noted that the maximum number of hydrogen bonds possible for H 2 O 2 is six (two as donor and four as acceptor), but such cases are quite rare (Chernyshov et al., 2017).

Supramolecular features
In the crystal, all six peroxide molecules are linked into hydrogen-bonded chains that propagate parallel to the a-axis (Table 1, Fig. 5). To the best of our knowledge, this is only the second example of hydrogen-bonded chains formed exclusively from peroxide molecules. Recently we reported the structure of thymine peroxosolvate obtained from 98% hydrogen peroxide (Chernyshov et al., 2017). However, in the latter compound, the peroxide chains are very simple (see Scheme below), belonging to the C1 type according to the Infantes-Motherwell notation of water clusters (Infantes & Motherwell, 2002). In the title structure, the chains represent the more complicated T4(0)A1 motif (Fig. 5).
The peroxide chains are interconnected via the organic molecules by moderate HOO-HÁ Á ÁN hydrogen bonds. Despite the aromatic nature of organic coformer, nostacking or T-shaped C-HÁ Á Á intermolecular interactions are observed in the structure. Thus, hydrogen bonding plays the predominant role in the crystal packing.
Several crystals were examined. All of them exhibited poor crystallinity, presumably as a result of the rather extensive disorder of the peroxide molecules.
Handling procedures for concentrated hydrogen peroxide have been described in detail (danger of explosion!) by Schumb et al. (1955).
The centrosymmetric peroxide molecule modelled as H61/ O61/O61 i /H61 i [symmetry code: (i) 1 À x, Ày, 1 À z] was Peroxide hydrogen-bonded chains parallel to the a-axis. Minor components of disorder are not shown for clarity. Hydrogen bonds are drawn as dashed lines. Table 1 Hydrogen-bond geometry (Å , ). found to be partially occupied. Simultaneous refinement of occupancy and thermal parameters for atom O61 was not stable and resulted in oscillating occupancies between 0.46 and 0.53 for consecutive cycles of refinement. It was therefore fixed at 0.5 for the final refinement. Aromatic H atoms were placed in calculated positions with C-H = 0.95 Å and refined as riding atoms with relative isotropic displacement parameters U iso (H) = 1.2U eq (C). Peroxide hydrogen atoms were placed on the lines connecting hydrogen-bonded atoms at a distance of 0.80 Å from the corresponding O atoms. They were refined as riding atoms with relative isotropic displacement parameters U iso (H) = 1.5U eq (O).

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. 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.