Na+[Me3NB12Cl11]−·SO2: a rare example of a sodium–SO2 complex

In the title compound, the SO2 molecule is η 1-O-coordinated to the Na+ cation. The Na+ cation has a coordination number of eight in a distorted twofold capped trigonal prism and makes contacts to three individual boron cluster anions resulting in an overall three-dimensional network.

In the title compound, Na + [Me 3 NB 12 Cl 11 ] À ÁSO 2 [systematic name: sodium 1-(trimethylammonio)undecachloro-closo-dodecaborate sulfur dioxide], the SO 2 molecule is 1 -O-coordinated to the Na + cation. Surprisingly, the SO 2 molecule is more weakly bound to sodium than is found in other sodium-SO 2 complexes and the SO 2 molecule is essentially undistorted compared to the structure of free SO 2 . The Na + cation has a coordination number of eight in a distorted twofold-capped trigonal prism and makes contacts to three individual boron cluster anions, resulting in an overall three-dimensional network. Although the number of known 1 -O-coordinated SO 2 complexes is growing, sodium-SO 2 complexes are still rare.

Chemical context
Liquid sulfur dioxide is a polar but only very weakly coordinating solvent (Waddington, 1965), which is frequently used in organic and inorganic synthesis. The coordination chemistry in and of sulfur dioxide has been the topic of various reviews (Mingos, 1978;Ryan et al., 1981;Mews et al., 2000). Initially, only S-and 2 -S,O-coordination of SO 2 with soft transitionmetal centers were investigated, but it was subsequently shown that 1 -O-coordination of SO 2 is preferred with hard main-group and transition-metal cations. Theoretical studies established that the oxygen-metal cation bonds are purely ionic (Decken et al., 2009;Derendorf et al., 2010). Mews and co-workers crystallized metal hexafluoro arsenates M[AsF 6 ] (M = alkaline-earth and transition-metal cations) from liquid sulfur dioxide to obtain their SO 2 complexes (Mews et al., 2000). Unfortunately, the alkali-metal hexafluoro arsenates, M[AsF 6 ] (M = Li, Na, K), are almost insoluble in liquid sulfur dioxide and the corresponding SO 2 complexes remained elusive. Until recently, only two examples of alkali-metal-SO 2 complexes were known; namely, the 2 -O,O bridged coordination complexes, [Li(OSO) 6/2 ][AlCl 4 ] (Simon et al., 1980) and [Na(OSO) 1.5 ][AlCl 4 ] (Peters et al., 1982), crystallized in the presence of the [AlCl 4 ] À anion. Only after the introduction of modern weakly coordinating anions into sulfur dioxide coordination chemistry could alkali-metal sulfur dioxide complexes be studied intentionally. By using a large fluorinated aluminate anion, the crystal structure of [(OSO) 2 Li{AlF(Al(OR) 3 }Li{Al(OR) 4 }] [R = C(CF 3 ) 3 ] (Cameron et al., 2010) was determined. In addition, use of halogenated closo-dodecaborates [B 12 X 12 ] 2À (X = F-I) led to a systematic study of alkali-metal sulfur dioxide complexes . Halogenated closo-dodecaborates belong to the growing class of modern weakly coordinating ISSN 2056-9890 anions (Knapp, 2013). The [Me 3 NB 12 Cl 11 ] À anion represents a recent modification of the halogenated closo-dodecaborates and possesses a reduced charge of À1 (Bolli et al., 2014). This anion has been utilized very recently to stabilize a variety of reactive cations in the solid state (e.g. Bertocco et al., 2016) and has been applied in silver-free gold catalysis (Wegener et al., 2015). From a failed attempt to prepare [Et 3 SiOS(H)OSiEt 3 ][Me 3 NB 12 Cl 11 ] À , we obtained single crystals of the title compound as a by-product. Na + [Me 3-NB 12 Cl 11 ] À ÁSO 2 is a rare example of a sodium-SO 2 complex, and its crystal structure is discussed herein.

Structural commentary
The title salt crystallizes with one SO 2 molecule per formula unit (Fig. 1) Grabowsky et al., 2012] or in the gas phase [1.4343 (3) Å ; Holder & Fink, 1981]. This behaviour is in contrast to that observed in other coordination compounds with 1 -O-coordinated terminal SO 2 ligands (e.g. Mews et al., 2000) where lengthening of the S-O c bond and shortening of the S-O t bond occurs, as predicted by theoretical concepts (Decken et al., 2009;Derendorf et al., 2010). Thus, the current finding is in accord with weaker coordination of the SO 2 molecule to Na + than is found in other SO 2 complexes of hard metal cations. In addition to coordinating to the SO 2 molecule, each sodium cation coordinates to seven of the eleven chlorine atoms of the boron cluster ( Fig. 1).
In the packed structure, two sodium cations are coordinated in an 2 -fashion to two chlorine atoms, while a third sodium cation is coordinated in an 3 -manner to three chlorine atoms (Fig. 2). The ClÁ Á ÁNa + distances range from 2.870 (2) to 3.209 (2) Å , and are, on average, longer than those in Na 2 [B 12 Cl 12 ]Á4SO 2 (i.e. 3.052 vs 2.929 Å ) , and are in accord with the sum of the van der Waals radius of chlorine (1.75 Å ; Mantina et al., 2009) and the ionic radius of sodium (1.18 Å ; Shannon, 1976) of 2.93 Å . However, when anisotropy of the van der Waals radius (Batsanov, 2001) is taken into account, the intermolecular distances are still in the expected range. The B-Cl bond lengths of the chlorine atoms coordinating to Na + lie in the range 1.796 (5) to 1.803 (4) Å (av. 1.799 Å ) and are only slightly longer than those of the non-coordinating chlorine atoms [1.779 (5) to 1.797 (4) Å , av. 1.786 Å ]. It has previously been noted that the presence of strong Lewis acids, such as Me + or R 3 Si + , leads to a significant elongation of the B-Cl bonds by up to 0.1 Å (Bolli et al., 2010(Bolli et al., , 2014Kessler et al., 2010). Therefore, in the title compound, the ClÁ Á ÁNa + interaction can be classified as weak and the singly charged [Me 3 NB 12 Cl 11 ] À anion is more weakly coordinating towards Na + than the doubly charged [B 12 Cl 12 ] 2À anion. 608 Jenne and Lessen Na + C 3 H 9 B 12 Cl 11 N À ÁSO 2 Acta Cryst. Coordination sphere around the Na + cation. The Na + cation makes a total of eight contacts (dashed lines) to three individual boron cluster anions and to one sulfur dioxide molecule. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are shown with arbitrary radii. Symmetry codes: (a) 1 + x, y, z; (b) À1 + x, À 1 2 + y, 1 2 À z. Table 1 Selected geometric parameters (Å , ).

Supramolecular features
The Na + cation is surrounded by seven chlorine atoms from three different boron clusters and one oxygen atom from a SO 2 molecule, resulting in a total coordination number of 8 ( Fig. 1) and giving rise to a three-dimensional network. The polyhedron around Na + may be best described as a distorted twofold-capped trigonal prism (Fig. 3). The structure of the title compound is reminiscent of that of Ag[Me 3 NB 12 Cl 11 ]ÁSO 2 (Jenne & Wegener, 2018), although the coordination sphere around the metal cations is different in the two structures. The Na + Á Á ÁCl contacts are weaker than the Ag + Á Á ÁCl contacts and there is also only one SO 2 molecule per cation present in the title compound. The [Me 3 NB 12 Cl 11 ] À anions are placed in a body-centered cubic arrangement ( Fig. 4) with some of the intermolecular ClÁ Á ÁCl distances being shorter than the sum of the van Waals radii (3.50 Å ; Mantina et al., 2009). The [Me 3 NB 12 Cl 11 ] À anions pack quite efficiently in the solid state and unlike in Na 2 [B 12 Cl 12 ]Á4SO 2 , where the structure contains two molecules of SO 2 per sodium cation to separate the doubly charged anions, only one SO 2 molecule is required in this case.

Database survey
The [Me 3 NB 12 Cl 11 ] À anion was first reported in 2014 (Bolli et al., 2014) and a variety of crystal structures containing this anion have been published (e.g. Saleh et al., 2016;Bertocco et al., 2016;Bolli et al., 2017;Jenne & Wegener, 2018), in which the [Me 3 NB 12 Cl 11 ] À anion is essentially identical to that reported in this study. Sodium complexes of fluorinated closododecaborates were studied recently by Strauss and coworkers (Bukovsky et al., 2017a,b Distorted twofold-capped trigonal prism around the Na + cation. Displacement ellipsoids are drawn at the 50% probability level.

Synthesis and crystallization
The crystals were obtained as a by-product from a reaction of [CPh 3 ][Me 3 NB 12 Cl 11 ] with Et 3 SiH and SO 2 in 1,2-difluorobenzene designed to give [Et 3 SiOS(H)OSiEt 3 ]-[Me 3 NB 12 Cl 11 ] À in analogy to a published procedure . Crystallization of the red-brown product from 1,2-difluorobenzene/n-pentane yielded the title compound as colorless crystals. The source of the sodium cation remains uncertain, but it may arise from an incomplete conversion of Na[Me 3 NB 12 Cl 11 ] to [CPh 3 ][Me 3 NB 12 Cl 11 ] (Bolli et al., 2014).

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.