Crystal structure of di-μ-trihydro(pentafluorophenyl)borato-tetrakis(tetrahydrofuran)disodium

The title compound, [Na2(μ-C6F5BH3)2(C4H8O)4], represents a dimeric structure of sodium and organoborohydride, located about a centre of inversion. In the crystal, the molecules are stacked along the b axis via a π–π interaction between the benzene rings.


Chemical context
A series of alkali-metal borohydride salts are known as the most important, reliable and commercially available reducing agents, especially for carbonyl compounds (Magano & Dunetz, 2012). The reducing ability of borohydrides can easily be tuned by introducing functional groups on boron or by changing their counter-cation. To understand the relationship between reactivity and composition of borohydride species, structural understandings based on crystallographic analysis would be important. The structures of these borohydride compounds are largely affected by the number of hydrides, bulkiness of substituents on boron, and metal. For example, sodium triethylmonohydroborate forms a cubic tetramer (Bell et al., 1980) and lithium trihydroborate with a bulky alkyl group on boron gives a monomeric structure (Eaborn et al., 1984). Reports of the structures of sodium alkyl/aryltrihydroborates are very scarce, although some dimeric lithium organotrihydroborates (Knizek et al., 2000;Franz et al., 2011;Pospiech et al., 2015;Murosaki et al., 2016), monomeric lithium organotrihydroborate (Molitor & Gessner, 2013) and potassium aryltrihydrobotate (Kaese et al., 2016) have previously been characterized by X-ray crystal analyses. The only example of structurally characterized sodium alkyltrihydroborate is a compound bearing three methoxyethoxy groups, and no interaction between the hydrides and the sodium atom was observed in this case, because the sodium cation is trapped into the cage structure of the methoxyethoxy groups and no longer forms contacts with the borohydride anion (Thalangamaarachchige et al., 2019).
Herein, we report the first crystal structure analysis of sodium aryltrihydroborate, which bears a pentafluorophenyl substituent on the boron centre.
The distance between the sodium atom and fluorine atom F5 at the 2-position on the benzene ring [Na1 i -F5 = 2.6373 (12) Å ] is much shorter than the sum of van der Waals radii (3.74 Å ), indicating the presence of a sodium-halogen interaction. Such a halogen-metal interaction is also observed in bromoaryl-substituted lithium trihydroborate (Seven et al., 2014). The environment around the sodium atom can therefore be seen as having a distorted trigonal-bypiramidal geometry with one fluorine atom, two boron atoms and two THF molecules. The C-B bond [C1-B1 = 1.614 (2) Å ] is significantly longer than the previously reported C-B bond lengths of lithium organotrihydroborates (1.597-1.613 Å ), probably because of the electron-withdrawing property of the C 6 F 5 group.

Supramolecular features
In the crystal, the dimeric molecules are stacked along the b axis via ainteraction between the neighbouring C 6 F 5 rings as shown in Fig. 2. The plane-to-plane distance, the centroidto-centroid distance and the slippage are 3.388 (4), 3.582 (2) and 1.160 Å , respectively. The C 6 F 5 rings are stacked in an anti-parallel manner, so that the boron atom on one C 6 F 5 ring is close to the fluorine atom at 4-position on the other ring. However, the BÁ Á ÁF distance [B1Á Á ÁF3 ii = 3.589 (2) Å ; symmetry code: (ii) Àx + 1, Ày À 1, Àz] is slightly longer than the sum of van der Waals radii (3.39 Å ), suggesting that the BÁ Á ÁF interaction is weak. The distance between the closest hydrogen atom (H4) and centroid of the C 6 F 5 ring is 3.343 Å , indicating the absence of C-HÁ Á Á interactions.

Database survey
As described above, there is only one example of structural analysis on a sodium alkyltrisboronate complex (Thalangamaarachchige et al., 2019). This complex exhibits a monomeric twitterionic structure without any interaction between the borohydride and the sodium atom. Other examples of sodium trihydroborates bearing a carbon-based substituent on boron, the sodium salt of boranocarbamates (Pitchumony et al., 2010), cyanoborohydride (Custelcean et al., 1998(Custelcean et al., , 2002 and (isothiocyanato)trihydroborate (Nö th & Warchhold, 2004) have been structurally characterized by X-ray crystallographic analyses. In these salts, the sodium cation exists as an adduct of 146 Tanaka    The molecular structure of the title compound, with atom labels and 50% probability displacement ellipsoids for non-H atoms. H atoms other than hydrides have been omitted for clarity. polyethers or polyamine and is located distant from the borohydride anion.

Synthesis and crystallization
The title compound was prepared by the reaction of NaH (60% oil dispersion, 1.21 g, 30 mmol, washed twice with hexane prior to use) and (C 6 F 5 )BH 2 ÁS(CH 3 ) 2 (2.10 g, 8.7 mmol) in THF (20 mL) at 333 K for 5 h. The supernatant solution of the reaction mixture was separated and dried under vacuum. The obtained colourless solid was redissolved into 1 mL of THF, and 10 mL of hexane was layered on it. This solution was stored at 243 K overnight and 1.55 g (51%) of colourless crystals were obtained. 19

Di-µ-trihydro(pentafluoropheny)borato-tetrakis(tetrahydrofuran)disodium
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. Refinement. Reflections were merged by SHELXL according to the crystal class for the calculation of statistics and refinement. _reflns_Friedel_fraction is defined as the number of unique Friedel pairs measured divided by the number that would be possible theoretically, ignoring centric projections and systematic absences.