Crystal structure of rubidium methyldiazotate

The title compound has been crystallized in liquid ammonia as a reaction product of the reductive ammonolysis of the natural compound streptozocin. Elemental rubidium was used as reduction agent as it is soluble in liquid ammonia, forming a blue solution. Reductive bond cleavage in biogenic materials under kinetically controlled conditions offers a new approach to gain access to sustainably produced raw materials.


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 starting material, streptozocin, was commercially available and used as shipped.

Structural commentary
The methyldiazotate anion is found to exist in the cis configuration, which is in correspondence with the equivalent potassium species (Mü ller et al., 1963;Huber et al., 1965). The structure of the diazotate anion has been further discussed by Suhr (1963) and by Kü bler & Lü ttke (1963).
The title compound does not contain any solvent molecules, which is unusual for ionic species crystallized from liquid ammonia. The anion is nearly planar, having an O1-N1-N2-C1 torsion angle of À0.4 (2) . Five direct anion-cation contacts can be observed, with maximum bond lengths of d(Rb-O) = 2.9871 (12) Å and d(Rb-N) = 3.1656 (15) Å . The rubidium cation has a coordination number of seven, in which five anions can be observed in its direct environment (Fig. 1). The coordination to the cation is both side-on and terminal: one anion is bound via both its N atoms, one by both O and N, two anions are bound only via O, and the remaining anion is bound via the N atom adjacent to the methyl group.

Supramolecular features
The diazotate anions are bridged by cations and do not exhibit any direct contacts to each other. The cations are found to form a corrugated-layer like arrangement within the structure, propagating in the (101) plane (Fig. 2). Although the oxygen atom can act as a hydrogen-bridge acceptor, no such interactions can be found in the structure as the C-H bonds are not sufficiently polarized. As the compound is of an ionic nature, electrostatic interactions are the dominant driving force towards the arrangement of the ionic species. An aggregation of methyl groups is therefore not observed.

Computational analysis
To get a more detailed understanding of the bonding situation in the anion, quantum chemical calculations were carried out at the DFT level (B3LYP functional) using def2-TZVP basis sets. To embed the results in a meaningful frame of reference, diazene and methylnitrosamine were used for comparison ( Fig. 3). It was found that the methyldiazotate anion tends to have properties most similar to methylnitrosamine. This indicates a high ability to delocalize its sp 2 electrons.
By analyzing the rotational potential, the energy barrier of the transition between the cis and trans form was determined to be 173.57 kJ mol À1 . The energetic difference between the two forms is 14.30 kJ mol À1 , wherein the cis form is energetically preferred. For comparison, the rotational barriers of diazene and methylnitrosamine are calculated to be 317.44 kJ mol À1 and 174.58 kJ mol À1 , respectively. The various computational methods employed have been described by Neese (2012) The extended arrangement formed by the cations in the crystal structure. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
Rotational potentials of diazene, methylnitrosamine and methyldiazotate. The energetic minima were geometrically optimized and are drawn as thick circles.

Synthesis and crystallization
250 mg (0.94 mmol) of streptozocin and 322 mg (3.8 mmol) of rubidium were placed under an argon atmosphere in a reaction vessel and 20 ml of dry liquid ammonia was condensed. The mixture was stored at 237 K for two weeks to ensure that all substances were completely dissolved. The flask was then stored at 161 K for several months. After that period, clear colorless crystals of the title compound could be found at the bottom of the flask.

Rubidium methyldiazotate
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.