Abstract
The crystal structures of the salts [Li(1,2-F2C6H4)] [B(C6F5)4] (1) and Cs[B(C6F5)4] (2) comprise six Li···F contacts (1.965(3) − 2.312(3) Å) and twelve Cs···F contacts (3.0312(1) − 3.7397(2) Å), respectively, which are significantly shorter than the sum of van der Waals radii (3.29 and 4.90 Å).
Polyfluorinated tetraarylborate ions, such as [B{3,5-(CF3)2C6H3}4]– “BArF” and [B(C6F5)4]– (Scheme 1), have received numerous applications as weakly coordinating anions (Krosssing and Raabe, 2004; Riddlestone et al., 2018), which also stimulated research into their alkali metal salts. Very recently, for the former, the crystal structures of [Li(H2O)][B{3,5-(CF3)2C6H3}4], M[B{3,5-(CF3)2C6H3}4] (M = Na, K) were described (Martínez-Martínez and Weller, 2019). For the latter, the crystal structures of [Li(C6H6)][B(C6F5)4]·benzene (Bolte et al., 2005), [Li(MeC6H5)][B(C6F5)4]·toluene, [Li(Et2O)4][B(C6F5)4], [Li(Et2O)4][B(C6F5)4]·CH2Cl2 (Kuprat et al., 2010; Martin et al., 2010), [Li(MeCN)4][B(C6F5)4] (Zhang et al., 2012) and K[B(C6F5)4] (Protchenko et al., 2016) were reported. In this work we convey two new crystal structures of the same anion.
Crystals of [Li(1,2-F2C6H4)][B(C6F5)4] (1) were obtained by recrystallization of [Li(Et2O)4][B(C6F5)4] from 1,2-difluorobenzene. The key feature of 1 (Figure 1) is the presence of six Li···F contacts (1.965(3) - 2.312(3) Å) that are significantly shorter than the sum of van der Waals radii (3.29 Å) and the absence of π-interactions between cation the electron poor aromatic rings. By contrast, in the crystal structure of [Li(C6H6)][B(C6F5)4]·benzene (Bolte et al., 2005) and [Li(MeC6H5)][B(C6F5)4]·toluene (Kuprat et al., 2010), the cations show also π-interactions with the electron rich solvate molecules. The cesium salt Cs[B(C6F5)4] (2) crystallized in the rare cubic space group
![Figure 1 Crystal structure of [Li(1,2-F2C6H4)][B(C6F5)4] (1) showing 50% probability ellipsoid and the essential atomic numbering. Cation anion contacts [Å]: Li1-F1 2.053(3), Li1-F2 2.090(3), Li1-F11 1.968(3), Li1-F21 1.965(3), Li1-F44a 2.312(3), Li1-F45a 2.018(3).](/document/doi/10.1515/mgmc-2020-0011/asset/graphic/j_mgmc-2020-0011_fig_002.jpg)
Crystal structure of [Li(1,2-F2C6H4)][B(C6F5)4] (1) showing 50% probability ellipsoid and the essential atomic numbering. Cation anion contacts [Å]: Li1-F1 2.053(3), Li1-F2 2.090(3), Li1-F11 1.968(3), Li1-F21 1.965(3), Li1-F44a 2.312(3), Li1-F45a 2.018(3).
![Figure 2 Crystal structure of Cs[B(C6F5)4] (2) showing 50% probability ellipsoid and the essential atomic numbering. Cation anion contacts [Å]: Cs1-F1 3.0312(1), Cs1-F2 3.7397(2), Cs1-F3 3.1943(1), Cs1-F5 3.3292(1).](/document/doi/10.1515/mgmc-2020-0011/asset/graphic/j_mgmc-2020-0011_fig_003.jpg)
Crystal structure of Cs[B(C6F5)4] (2) showing 50% probability ellipsoid and the essential atomic numbering. Cation anion contacts [Å]: Cs1-F1 3.0312(1), Cs1-F2 3.7397(2), Cs1-F3 3.1943(1), Cs1-F5 3.3292(1).
1 X-ray crystallography
Single crystals of 1 were obtained by drying [Li(Et2O)4] [B(C6F5)4] in high vacuum (5·10-3 bar) at 140°C for 24 h and recrystallization from 1,2-difluorobenzene and n-hexane (Romanato et al., 2010). Single crystals of 2 were obtained by ion exchange of 1 in 1,2-difluorobenzene with Cesium fluoride followed by filtration and addition of n-hexane (Mon et al., 2013). Intensity data were collected on a Bruker Venture D8 diffractometer with graphite-monochromated Mo-Kα (0.7107 Å) radiation. The structure was solved by direct methods and difference Fourier synthesis with subsequent Full-matrix least-squares refinements on F2, using all data (Dolomanov, 2009). All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms attached to carbon atoms were included in geometrically calculated positions using a riding model. Crystal and refinement data are collected in Table 1. Figures were created using DIAMOND (Brandenburg and Putz, 2006). Crystallographic data for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, CCDC numbers 1974618 (1) and 1974619 (2).
Crystal data and structure refinement of [Li(1,2-F2C6H4) B(C6F5)4] (1) and [CsB(C6F5)4] (2).
| 1 | 2 | |
|---|---|---|
| Formula | C30H4BF22Li | C24BCsF20 |
| Formula weight, g mol–1 | 800.08 | 394.74 |
| Crystal system | monoclinic | cubic |
| Crystal size, mm | 0.5 × 0.5 × 0.4 | 0.3 × 0.3 × 0.3 |
| Space group | P21/n | I‾43d |
| a, Å | 13.9256(5) | 19.213(1) |
| b, Å | 12.4375(5) | 19.213(1) |
| c, Å | 15.9947(5) | 19.213(1) |
| α, ° | 90 | 90 |
| β, ° | 92.016(1) | 90 |
| γ, ° | 90 | 90 |
| V, Å3 | 2768.6(2) | 7092(1) |
| Z | 4 | 12 |
| ρcalcd, Mg m–3 | 1.920 | 2.281 |
| T, K | 100 | 100 |
| m (Mo Kα), mm–1 | 0.215 | 1.741 |
| F(000) | 1560 | 4608 |
| θ range, deg | 2.20 to 33.23 | 2.60 to 30.43 |
| Index ranges | –21 ≤ h ≤ 21 | –27 ≤ h ≤ 23 |
| –18 ≤ k ≤ 19 | –27 ≤ k ≤ 23 | |
| –24 ≤ l ≤ 24 | –20 ≤ l ≤ 27 | |
| No. of reflns collected | 108154 | 23264 |
| Completeness to θmax | 99.9% | 99.9% |
| No. indep. Reflns | 10630 | 1805 |
| No. obsd reflns with (I>2σ(I)) | 8391 | 1695 |
| No. refined params | 487 | 105 |
| GooF (F2) | 1.037 | 1.110 |
| R1 (F) (I > 2σ(I)) | 0.0444 | 0.0277 |
| wR2 (F2) (all data) | 0.1272 | 0.0749 |
| (Δ/σ)max | < 0.001 | < 0.001 |
| Largest diff peak/hole, e Å–3 | 0.706 / –0.505 | 0.514 / –0.632 |
Weakly coordinating polyfluorinated tetraarylborate “BArF” ions.
Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336033; e-mail: deposit@ccdc.cam. ac.uk or http://www.ccdc.cam.ac.uk)
Conflict of interest: One of the authors (Jens Beckmann) is a member of the Editorial Board of Main Group Metal Chemistry.
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