research communications
of potassium triethylhydridoborate (`superhydride')
aInstitut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany
*Correspondence e-mail: p.jones@tu-bs.de
In the title compound, formally K+·C6H16B−, the contact sphere of potassium consists of eleven hydrogen atoms from three different anions, assuming an arbitrary cut-off of 3 Å. The shortest interaction, 2.53 (2) Å, involves the hydridic hydrogen H01, which fulfils a bridging function in the formation of chains of KHBEt3 units parallel to the a axis [K1—H01i 2.71 (2) Å, K1—H01—K1ii 126.7 (9)°, operators x∓1/2, −y + , −z + 1].
Keywords: crystal structure; potassium; hydridoborate; superhydride.
CCDC reference: 2081809
1. Chemical context
The title compound KHBEt3 was first prepared by Ziegler and Lehmkuhl from NaBEt3H and potassium amalgam (Ziegler & Lehmkuhl, 1963), but a more convenient approach was reported a few years later using KH and BEt3 in toluene (Binger et al., 1968). Alternatively, the latter reaction may also be performed in THF (Brown & Krishnamurthy, 1978). Since its original synthesis this so-called `superhydride' reagent has found widespread applications, e.g. as a reducing reagent in organic synthesis (Brown & Hubbard, 1979; Ito et al., 1985; Yoon et al. 1987, 1989), for the generation of low-valent transition-metal complexes (Bönnemann & Korall, 1992), and as a hydride transfer reagent resulting in well-defined metal–hydride complexes (Smith et al., 2003; Pfirrmann et al., 2008; Walter et al., 2011; Maekawa et al., 2012). Despite it being a reagent in frequent use, the structure of KHBEt3 has so far remained elusive. The few reported examples of structures containing KHBEt3 include its adducts with polydentate such as N,N,N′,N′-tetramethylethylenediamine (TMEDA) and N,N,N′,N",N"-pentamethyldiethylenetriamine (PMDETA) (Haywood & Wheatley, 2009). During our study on the coordination chemistry of enantiomerically pure constrained-geometry complexes of the rare-earth metals bearing a dianionic N-donor functionalized pentadienyl ligand, we accidentally obtained crystals of solvent-free KHBEt3 unsupported by any further ligands (see Synthesis and crystallization) and here report its structure.
2. Structural commentary
The 3 is shown in Fig. 1. Selected interatomic distances and angles are shown in Table 1. The shortest contact involving the potassium atom is K1—H01 at 2.53 (2) Å, but K1—H5B (not drawn explicitly) is not much longer at 2.69 Å. If the neighbouring asymmetric units generated by the 21 screw axis parallel to the a axis (see next section) are considered, there are a total of eleven K1—H distances shorter than 3 Å, with no clear limit as to what might be considered a `bonding' distance. One further such distance involves the 21 screw axis parallel to the c axis. The environment of the potassium atom is shown in Fig. 2. For comparison, one may note the K—H distance of 2.85 Å in potassium hydride (Kuznetsov & Shkrabkina, 1962), which, however, is regarded as an essentially ionic compound, crystallizing in the NaCl lattice type with 6 (cf. the ionic formulation of the title compound in Table 2, which is certainly a considerable oversimplification). Some K⋯H contacts of ca 2.8–2.9 Å, involving methyl hydrogen atoms, have been postulated as structurally significant in a TMEDA complex of potassium diisopropylamide (Clegg et al., 1998). Similarly, the distances from K1 to carbon and boron atoms range upwards from 3.103 (2) and 3.205 (2) Å, respectively. The bonding to CHn and BH moieties may involve multi-centre interactions, but we do not wish to speculate on their exact nature. The coordination geometry at the boron atom is as expected tetrahedral to a good approximation.
of KHBEt
|
3. Supramolecular features
To a first approximation, ignoring all interactions at K1 except for K1—H01, the molecules are connected by the appropriate 21 operators to form chains parallel to the a axis (Fig. 3). The hydridic hydrogen atom acts as the main bridging group, with K1—H01i = 2.71 (2) Å, H01—K1—H01i = 104.0 (4)°, K1—H01—K1ii = 126.7 (9)°. The distance between adjacent potassium atoms in the chain is 4.6839 (6) Å.
4. Database survey
A CSD search with ConQuest (Bruno et al., 2002) for organic hydridoborate derivatives involving K—H bonds led to the above-mentioned complexes [K(TMEDA)Et3BH]2 and [K(PMDETA)Et3BH]2 (Haywood & Wheatley, 2009, refcodes CUNNEF and CUNNIJ) with K—H distances of 2.52, 2.58 (3) and 2.64, 2.69 (3) Å, respectively, in the central K2H2 rings. A similar structure (refcode OZAZAR), but with 1,3,5-trimethyl-1,3,5-triazanonane, was reported by Krieck et al. (2010), with K—H = 2.56, 2.59 (3) Å. Somewhat more complex structures, involving cyclic and additional aromatic ligands at the potassium atom, have been reported by Grigsby & Power (1996; refcode TIZYAC, K—H = 2.54, 2.68 Å) and Chen et al. [2007; refcode MITWUI, K—H = 2.65–2.92 (1) Å].
5. Synthesis and crystallization
We attempted the preparation of a rare-earth metal hydride by salt metathesis between [{(η5:κ-N-pdl*SiMe2NtBu)La(thf)}2(μ-Cl)] (Jones et al., 2021) and 2 equiv. of KHBEt3 (1 M in THF) in n-hexane. The standard work-up procedure included removal of the solvent under dynamic vacuum, extraction of the residue with n-hexane and filtration. The filtrate was concentrated and cooled to 243 K. After several days, a few pale-yellow crystals were harvested. However, in contrast to our expectations, these did not consist of [{(η5:κ-N-pdl*SiMe2NtBu)La(thf)}2(μ-H)], but of the starting reagent KHBEt3.
6. Refinement
Crystal data, data collection and structure . The BH hydrogen atom was refined freely. The methyl groups were refined as idealized rigid groups allowed to rotate but not tip (AFIX 137; C—H = 0.98 Å, H—C—H = 109.5 °). The methylene hydrogens were included using a riding model starting from calculated positions (C—H = 0.99 Å). The Uiso(H) values were fixed at 1.2 (for methylene groups) or 1.5 (for methyl groups) times the equivalent Ueq value of the parent carbon atoms.
details are summarized in Table 2Supporting information
CCDC reference: 2081809
https://doi.org/10.1107/S2056989021004734/yz2007sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989021004734/yz2007Isup2.hkl
Data collection: CrysAlis PRO (Agilent, 2013); cell
CrysAlis PRO (Agilent, 2013); data reduction: CrysAlis PRO (Agilent, 2013); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: XP (Siemens, 1994); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015).K+·C6H16B− | Dx = 1.081 Mg m−3 |
Mr = 138.10 | Mo Kα radiation, λ = 0.71073 Å |
Orthorhombic, P212121 | Cell parameters from 2443 reflections |
a = 7.4758 (3) Å | θ = 2.8–26.1° |
b = 7.6682 (6) Å | µ = 0.54 mm−1 |
c = 14.8010 (12) Å | T = 100 K |
V = 848.48 (10) Å3 | Prism, pale yellow |
Z = 4 | 0.3 × 0.2 × 0.15 mm |
F(000) = 304 |
Oxford Diffraction Xcalibur, Eos diffractometer | 2441 independent reflections |
Radiation source: Enhance (Mo) X-ray Source | 2182 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.060 |
Detector resolution: 16.1419 pixels mm-1 | θmax = 30.0°, θmin = 2.8° |
ω scans | h = −10→10 |
Absorption correction: multi-scan (CrysAlisPro; Agilent, 2013) | k = −10→10 |
Tmin = 0.976, Tmax = 1.000 | l = −20→20 |
13206 measured reflections |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: mixed |
R[F2 > 2σ(F2)] = 0.036 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.068 | w = 1/[σ2(Fo2) + (0.0251P)2 + 0.0172P] where P = (Fo2 + 2Fc2)/3 |
S = 1.04 | (Δ/σ)max < 0.001 |
2441 reflections | Δρmax = 0.20 e Å−3 |
80 parameters | Δρmin = −0.23 e Å−3 |
0 restraints | Absolute structure: Flack x determined using 806 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsonset al., 2013) |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: −0.05 (3) |
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. The compound is achiral and crystallizes only by chance in a chiral (Sohncke) space group. |
x | y | z | Uiso*/Ueq | ||
K1 | 0.22297 (6) | 0.87358 (6) | 0.42934 (3) | 0.01798 (12) | |
B1 | 0.2627 (3) | 0.6127 (3) | 0.59734 (14) | 0.0140 (4) | |
H01 | 0.368 (3) | 0.680 (3) | 0.5467 (14) | 0.020 (6)* | |
C1 | 0.3821 (3) | 0.5161 (3) | 0.67567 (13) | 0.0159 (4) | |
H1B | 0.457483 | 0.426196 | 0.646460 | 0.019* | |
H1A | 0.463245 | 0.603494 | 0.702910 | 0.019* | |
C2 | 0.2753 (3) | 0.4296 (3) | 0.75137 (15) | 0.0260 (5) | |
H2C | 0.200842 | 0.517218 | 0.781547 | 0.039* | |
H2B | 0.357773 | 0.377763 | 0.795257 | 0.039* | |
H2A | 0.198637 | 0.338312 | 0.725885 | 0.039* | |
C3 | 0.1433 (3) | 0.4737 (3) | 0.53862 (14) | 0.0184 (5) | |
H3B | 0.053383 | 0.420846 | 0.579383 | 0.022* | |
H3A | 0.077404 | 0.539343 | 0.491626 | 0.022* | |
C4 | 0.2465 (3) | 0.3262 (3) | 0.49238 (16) | 0.0271 (5) | |
H4C | 0.342593 | 0.375726 | 0.455348 | 0.041* | |
H4B | 0.164952 | 0.259304 | 0.453846 | 0.041* | |
H4A | 0.298115 | 0.249185 | 0.538342 | 0.041* | |
C5 | 0.1323 (3) | 0.7630 (3) | 0.64061 (14) | 0.0150 (4) | |
H5B | 0.067029 | 0.820394 | 0.590574 | 0.018* | |
H5A | 0.042278 | 0.704783 | 0.679326 | 0.018* | |
C6 | 0.2249 (3) | 0.9042 (3) | 0.69656 (16) | 0.0270 (5) | |
H6C | 0.284544 | 0.850412 | 0.748517 | 0.041* | |
H6B | 0.135728 | 0.988325 | 0.717930 | 0.041* | |
H6A | 0.313644 | 0.964283 | 0.659107 | 0.041* |
U11 | U22 | U33 | U12 | U13 | U23 | |
K1 | 0.01299 (19) | 0.0216 (2) | 0.0194 (2) | 0.00053 (18) | −0.00233 (19) | 0.0028 (2) |
B1 | 0.0099 (10) | 0.0164 (10) | 0.0156 (10) | 0.0015 (10) | 0.0008 (8) | 0.0005 (8) |
C1 | 0.0112 (10) | 0.0187 (10) | 0.0178 (11) | 0.0003 (8) | 0.0013 (8) | 0.0028 (9) |
C2 | 0.0195 (11) | 0.0318 (12) | 0.0267 (12) | 0.0025 (10) | 0.0045 (10) | 0.0133 (9) |
C3 | 0.0145 (10) | 0.0199 (11) | 0.0207 (11) | 0.0033 (8) | −0.0003 (8) | −0.0018 (8) |
C4 | 0.0236 (13) | 0.0251 (11) | 0.0326 (12) | 0.0052 (9) | −0.0056 (10) | −0.0120 (9) |
C5 | 0.0130 (10) | 0.0163 (10) | 0.0158 (10) | −0.0002 (8) | −0.0018 (8) | −0.0013 (8) |
C6 | 0.0185 (10) | 0.0248 (12) | 0.0377 (13) | 0.0016 (10) | −0.0033 (11) | −0.0127 (9) |
K1—C1i | 3.103 (2) | B1—C1 | 1.640 (3) |
K1—B1 | 3.205 (2) | B1—H01 | 1.20 (2) |
K1—C5 | 3.310 (2) | C1—C2 | 1.527 (3) |
K1—C3ii | 3.387 (2) | C1—H1B | 0.9900 |
K1—C5ii | 3.396 (2) | C1—H1A | 0.9900 |
K1—B1i | 3.465 (2) | C2—H2C | 0.9800 |
K1—C2iii | 3.513 (2) | C2—H2B | 0.9800 |
K1—C3 | 3.518 (2) | C2—H2A | 0.9800 |
K1—H01 | 2.53 (2) | C3—C4 | 1.530 (3) |
K1—H5B | 2.69 | C3—H3B | 0.9900 |
K1—H01i | 2.71 (2) | C3—H3A | 0.9900 |
K1—H1Ai | 2.76 | C4—H4C | 0.9800 |
K1—H1Bi | 2.75 | C4—H4B | 0.9800 |
K1—H2Biii | 2.83 | C4—H4A | 0.9800 |
K1—H3Bii | 2.93 | C5—C6 | 1.529 (3) |
K1—H3A | 2.93 | C5—H5B | 0.9900 |
K1—H5Aii | 2.94 | C5—H5A | 0.9900 |
K1—H3Aii | 2.97 | C6—H6C | 0.9800 |
K1—H5Bii | 2.99 | C6—H6B | 0.9800 |
B1—C3 | 1.640 (3) | C6—H6A | 0.9800 |
B1—C5 | 1.640 (3) | ||
C1i—K1—B1 | 129.41 (5) | B1—C1—H1A | 108.4 |
C1i—K1—C5 | 112.01 (5) | K1ii—C1—H1A | 61.0 |
B1—K1—C5 | 29.10 (5) | H1B—C1—H1A | 107.5 |
C1i—K1—C3ii | 137.56 (5) | C1—C2—K1iv | 146.66 (14) |
B1—K1—C3ii | 91.22 (5) | C1—C2—H2C | 109.5 |
C5—K1—C3ii | 98.42 (5) | K1iv—C2—H2C | 96.6 |
C1i—K1—C5ii | 132.18 (5) | C1—C2—H2B | 109.5 |
B1—K1—C5ii | 87.74 (5) | K1iv—C2—H2B | 39.9 |
C5—K1—C5ii | 113.18 (5) | H2C—C2—H2B | 109.5 |
C3ii—K1—C5ii | 46.63 (5) | C1—C2—H2A | 109.5 |
C1i—K1—B1i | 28.23 (5) | K1iv—C2—H2A | 79.5 |
B1—K1—B1i | 101.50 (5) | H2C—C2—H2A | 109.5 |
C5—K1—B1i | 84.95 (5) | H2B—C2—H2A | 109.5 |
C3ii—K1—B1i | 157.94 (5) | C4—C3—B1 | 116.29 (17) |
C5ii—K1—B1i | 150.39 (5) | C4—C3—K1i | 141.80 (14) |
C1i—K1—C2iii | 78.93 (6) | B1—C3—K1i | 101.88 (11) |
B1—K1—C2iii | 99.69 (6) | C4—C3—K1 | 110.68 (14) |
C5—K1—C2iii | 122.65 (5) | B1—C3—K1 | 65.47 (10) |
C3ii—K1—C2iii | 109.29 (5) | K1i—C3—K1 | 85.41 (5) |
C5ii—K1—C2iii | 64.15 (5) | C4—C3—H3B | 108.2 |
B1i—K1—C2iii | 86.47 (5) | B1—C3—H3B | 108.2 |
C1i—K1—C3 | 109.21 (5) | K1i—C3—H3B | 55.0 |
B1—K1—C3 | 27.73 (5) | K1—C3—H3B | 138.8 |
C5—K1—C3 | 46.18 (5) | C4—C3—H3A | 108.2 |
C3ii—K1—C3 | 113.21 (5) | B1—C3—H3A | 108.2 |
C5ii—K1—C3 | 91.37 (5) | K1i—C3—H3A | 57.4 |
B1i—K1—C3 | 84.88 (5) | K1—C3—H3A | 47.0 |
C2iii—K1—C3 | 76.61 (5) | H3B—C3—H3A | 107.4 |
C1i—K1—H01 | 148.8 (5) | C3—C4—H4C | 109.5 |
B1—K1—H01 | 20.1 (5) | C3—C4—H4B | 109.5 |
C5—K1—H01 | 44.7 (5) | H4C—C4—H4B | 109.5 |
C3ii—K1—H01 | 73.1 (5) | C3—C4—H4A | 109.5 |
C5ii—K1—H01 | 69.0 (5) | H4C—C4—H4A | 109.5 |
B1i—K1—H01 | 121.4 (5) | H4B—C4—H4A | 109.5 |
C2iii—K1—H01 | 97.1 (5) | C6—C5—B1 | 116.11 (17) |
C3—K1—H01 | 40.9 (5) | C6—C5—K1 | 103.74 (13) |
C3—B1—C5 | 109.90 (16) | B1—C5—K1 | 71.90 (10) |
C3—B1—C1 | 112.16 (16) | C6—C5—K1i | 142.33 (14) |
C5—B1—C1 | 111.40 (16) | B1—C5—K1i | 101.54 (11) |
C3—B1—K1 | 86.80 (11) | K1—C5—K1i | 88.60 (5) |
C5—B1—K1 | 79.00 (11) | C6—C5—H5B | 108.3 |
C1—B1—K1 | 151.74 (13) | B1—C5—H5B | 108.3 |
C3—B1—K1ii | 119.99 (12) | K1—C5—H5B | 44.0 |
C5—B1—K1ii | 127.85 (13) | K1i—C5—H5B | 57.7 |
C1—B1—K1ii | 63.51 (10) | C6—C5—H5A | 108.3 |
K1—B1—K1ii | 89.12 (5) | B1—C5—H5A | 108.3 |
C3—B1—H01 | 107.6 (10) | K1—C5—H5A | 143.3 |
C5—B1—H01 | 109.5 (10) | K1i—C5—H5A | 54.9 |
C1—B1—H01 | 106.1 (10) | H5B—C5—H5A | 107.4 |
K1—B1—H01 | 46.4 (10) | C5—C6—H6C | 109.5 |
K1ii—B1—H01 | 42.8 (10) | C5—C6—H6B | 109.5 |
C2—C1—B1 | 115.49 (18) | H6C—C6—H6B | 109.5 |
C2—C1—K1ii | 156.25 (14) | C5—C6—H6A | 109.5 |
B1—C1—K1ii | 88.26 (11) | H6C—C6—H6A | 109.5 |
C2—C1—H1B | 108.4 | H6B—C6—H6A | 109.5 |
B1—C1—H1B | 108.4 | K1—H01—B1 | 113.5 (13) |
K1ii—C1—H1B | 60.2 | K1—H01—K1ii | 126.7 (9) |
C2—C1—H1A | 108.4 | H01—K1—H01i | 104.0 (4) |
C3—B1—C1—C2 | −66.0 (2) | K1ii—B1—C3—K1i | −166.41 (7) |
C5—B1—C1—C2 | 57.7 (2) | C5—B1—C3—K1 | 77.09 (14) |
K1—B1—C1—C2 | 164.9 (2) | C1—B1—C3—K1 | −158.41 (16) |
K1ii—B1—C1—C2 | −179.55 (19) | K1ii—B1—C3—K1 | −87.13 (10) |
C3—B1—C1—K1ii | 113.58 (14) | C3—B1—C5—C6 | −179.27 (18) |
C5—B1—C1—K1ii | −122.76 (14) | C1—B1—C5—C6 | 55.8 (2) |
K1—B1—C1—K1ii | −15.5 (3) | K1—B1—C5—C6 | −96.78 (17) |
B1—C1—C2—K1iv | 161.9 (2) | K1ii—B1—C5—C6 | −16.6 (3) |
K1ii—C1—C2—K1iv | −17.0 (6) | C3—B1—C5—K1 | −82.48 (14) |
C5—B1—C3—C4 | 179.06 (18) | C1—B1—C5—K1 | 152.57 (16) |
C1—B1—C3—C4 | −56.4 (2) | K1ii—B1—C5—K1 | 80.16 (12) |
K1—B1—C3—C4 | 101.97 (17) | C3—B1—C5—K1i | 2.19 (17) |
K1ii—B1—C3—C4 | 14.8 (2) | C1—B1—C5—K1i | −122.76 (14) |
C5—B1—C3—K1i | −2.19 (17) | K1—B1—C5—K1i | 84.67 (6) |
C1—B1—C3—K1i | 122.31 (13) | K1ii—B1—C5—K1i | 164.83 (9) |
K1—B1—C3—K1i | −79.28 (7) |
Symmetry codes: (i) x−1/2, −y+3/2, −z+1; (ii) x+1/2, −y+3/2, −z+1; (iii) −x+1/2, −y+1, z−1/2; (iv) −x+1/2, −y+1, z+1/2. |
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