[Se(CH2C(O)CH3)3][B12F11NH3]: The first selenium cation with three β-ketone substituents

The cation [Se(CH2C(O)CH3)3]+ represents the first example for a cationic selenium compound with three ketone functional groups located in the β-position with respect to the selenium atom. The cation possesses almost trigonal–pyramidal C 3 symmetry and forms hydrogen bonds to the ammonio group of the anion


Structural commentary
The salt [Se(CH 2 C(O)CH 3 ) 3 ][B 12 F 11 NH 3 ] crystallizes solventfree in the orthorhombic crystal system in space group Pbca (Fig. 1). The cation is close to being C 3 symmetric and the selenium atom is bonded to three chemically equivalent methylene groups with essentially identical bond lengths of 1.947 (2) to 1.951 (2) Å (Table 1). These distances are in the expected range when compared to the simple [SeMe 3 ] + cation (Hope, 1966) with Se-C bond lengths from 1.94 (2) to 1.96 (2) Å . This indicates that the electron-withdrawing effect of the ketone groups has no influence on the Se-C bond lengths.
Furthermore, there are additional contacts between the central selenium cation and the three oxygen atoms of the ketone groups (Fig. 2a). The oxygen-selenium contacts (2.810 Å on average) are much shorter than the sum of the van der Waals radii of selenium and oxygen (3.42 Å ; Bondi, 1964). This interaction can be considered as mainly electrostatic, since the oxygen atoms are partially negatively charged and the selenium cation carries a positive charge. Thus, the 222 Jenne and Nierstenhö fer C 9 H 15 O 3 Se + ÁB 12 F 11 H 3 N À Acta Cryst. (2020). E76, 221-224 research communications Table 1 Selected geometric parameters (Å , ).    selenium atom forms six short contacts, i.e. it is covalently bonded to three carbon atoms and forms three ionic interactions to the three oxygen atoms. The carbon atoms span a small triangular face, which is essentially parallel to a larger triangular face formed by the oxygen atoms (Fig. 2b). This results in a flat distorted trigonal prism surrounding the selenium atom.
The structure of the anion [B 12 F 11 NH 3 ] À is less interesting and reveals bond distances in the expected range. The boronboron bond lengths in the anion are in the range 1.777 (3) to 1.803 (4) Å and the average boron-fluorine bond length is 1.38 Å , which are very similar to those in other fluorinated dodecaborates such as [B 12 F 11 NMe 3 ] À  or [B 12 F 12 ] 2À . The B-N bond length of 1.538 (3) Å is essentially equal to that in [B 12 H 11 NH 3 ] À (Nachtigal et al., 1997) but slightly shorter than in [B 12 F 11 NMe 3 ] À  and in [B 12 Cl 11 NMe 3 ] À (Bolli et al., 2014).

Supramolecular features
The [Se(CH 2 C(O)CH 3 ) 3 ] + cations and the [B 12 F 11 NH 3 ] À anions are connected by intermolecular hydrogen-bonding interactions, resulting in a polymeric network. The hydrogen atoms of the positively charged ammonio group of the cation interact with the partially negatively charged oxygen atoms of the ketone groups of the cation (Fig. 3a). The oxygen-nitrogen distances are between 2.841 (2) and 2.865 (2) Å (Table 2), which is in the range of typical N-HÁ Á ÁO hydrogen bonds (Huheey, 1988). Every ketone group of the cation is coordinated to an ammonio group of a different boron cluster anion. Likewise every ammonio group is coordinated threefold by the ketone groups of three different cations, as shown in Fig. 3b.

Synthesis and crystallization
[Se 8 ][B 12 F 11 NH 3 ] 2 was prepared by a salt metathesis reaction of Na[B 12 F 11 NH 3 ] with [Se 8 ][AsF 6 ] 2 in liquid sulfur dioxide as a solvent in an H-shaped glass vessel with an incorporated frit. The insoluble by-product Na[AsF 6 ] was removed by filtration. The soluble black residue was dissolved in acetone in order to obtain single crystals of the intended compound [Se 8 ][B 12 F 11 NH 3 ] 2 . The title compound was obtained as a byproduct from the reaction of [Se 8 ][B 12 F 11 NH 3 ] 2 with acetone. Single crystals were grown by slow diffusion of diethyl ether into a saturated solution of acetone at room temperature. During crystallization a red precipitate formed, which hints at the formation of elemental red selenium. We assume that the formation of the title cation is the result of a C-H activation of acetone by the [Se 8 ] 2+ cation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were positioned geometrically (N-H = 0.91 or C-H = 0.95-0.99 ) and refined using a riding model with U iso (H) = 1.2U eq (C, N) or 1.5U eq (Cmethyl).  (Rigaku OD, 2015); cell refinement: CrysAlis PRO (Rigaku OD, 2015); data reduction: CrysAlis PRO (Rigaku OD, 2015); program(s) used to solve structure: ShelXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). 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.