Crystal structure of rubidium peroxide ammonia disolvate

The title compound, Rb2O2·2NH3, has been obtained as a reaction product of rubidium metal dissolved in liquid ammonia and glucuronic acid. As a result of the low-temperature crystallization, a disolvate was formed. The peroxide bond length is 1.530 (11) Å


Chemical context
The crystal structure of the title compound was determined in the course of investigations regarding the reactivity of carbohydrates towards alkali metals and NH 3 in solutions where liquid ammonia itself is used as solvent. The source of the peroxide anion could not be explicitly traced back but it seems to have its origin in oxygen gas from intruding atmosphere due to undetected leakage in the reaction vessel.

Structural commentary
The asymmetric unit contains one peroxide anion, two chargecompensating rubidium cations and two ammonia molecules ( Fig. 1). Except for one nitrogen atom (N1, showing halfoccupancy) and one hydrogen atom (H2B), all other atoms are located on mirror planes. The anion is surrounded by four rubidium cations located around the girth of the peroxide ion (Fig. 2). This unit forms one-dimensional infinite strands by sharing one common edge of a distorted plane of four Rb ions The asymmetric unit of the title compound, with the atom labeling and displacement ellipsoids drawn at the 50% probability level.

Figure 2
The environment of the peroxide anion. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry code: (i) À1 + x, y, z.]

Figure 4
Comparison of peroxide bond lengths in different compounds. The vertical line shows the peroxide bond length commonly used in the literature. Each data point is shown with its standard uncertainties.
sphere in a reaction vessel and 25 ml of dry liquid ammonia were condensed. The mixture was stored at 237 K for five days.
The flask was then stored at 161 K for several months. After that period, clear needle-shaped colorless crystals of the title compound could be found at the wall of the flask.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The nitrogen atom N1 is disordered with 0.5 as the site occupation factor. All hydrogen atoms could be located in difference map and and their positions were refined freely with a common U iso (H) parameter. The isotropic displacement parameters were fixed to 0.025.

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.   (14) 3.07 (16) 3.597 (16) 125 (11) N2-H2A···N2 iv 0.74 (16) 3.03 (12) 3.57 (2) 131 (6)