Crystal structure of potassium tri­ethyl­hydridoborate (‘superhydride’)

The structure of KHBEt₃ is polymeric, involving chains linked by K—H—K motifs via the hydridic hydrogen.

Abstract

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].

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 amines 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.

Structural commentary  

The asymmetric unit of KHBEt₃ 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 coordination number 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 CH_n 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.

Figure 1.

The asymmetric unit of KHBEt₃. Ellipsoids are drawn at the 50% level. Only the shortest K1—H contact is drawn explicitly.

Table 1. Selected geometric parameters (Å, °).

K1—C1i	3.103 (2)	K1—H2B iii	2.83
K1—B1	3.205 (2)	K1—H3B ii	2.93
K1—C5	3.310 (2)	K1—H3A	2.93
K1—C3ii	3.387 (2)	K1—H5A ii	2.94
K1—C5ii	3.396 (2)	K1—H3A ii	2.97
K1—B1i	3.465 (2)	K1—H5B ii	2.99
K1—H01	2.53 (2)	B1—C3	1.640 (3)
K1—H5B	2.69	B1—C5	1.640 (3)
K1—H01i	2.71 (2)	B1—C1	1.640 (3)
K1—H1A i	2.76	B1—H01	1.20 (2)
K1—H1B i	2.75
C3—B1—C5	109.90 (16)	C1—B1—H01	106.1 (10)
C3—B1—C1	112.16 (16)	K1—H01—B1	113.5 (13)
C5—B1—C1	111.40 (16)	K1—H01—K1ii	126.7 (9)
C3—B1—H01	107.6 (10)	H01—K1—H01i	104.0 (4)
C5—B1—H01	109.5 (10)

Symmetry codes: (i) x-{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1; (ii) x+{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1; (iii) -x+{\script{1\over 2}}, -y+1, z-{\script{1\over 2}}.

Figure 2.

The environment of the potassium atom in KHBEt₃, showing ten of the eleven K—H contacts < 3 Å to three neighbouring hydridotri­ethyl­borate units. Radii are arbitrary. K—H distances shorter than 2.8 Å are shown as thick dashed bonds, whereas those greater than 2.9 Å are shown as thin dashed bonds. The anion on the right corresponds to the asymmetric unit; the anions at top and bottom were generated by the operators  + x,  − y, 1 − z and − + x,  − y, 1 − z, respectively. The contact to H2B of a fourth anion (at  − x, 1 − y, − + z) is omitted for clarity.

Table 2. Experimental details.

Crystal data
Chemical formula	K+·C6H16B−
M r	138.10
Crystal system, space group	Orthorhombic, P212121
Temperature (K)	100
a, b, c (Å)	7.4758 (3), 7.6682 (6), 14.8010 (12)
V (Å3)	848.48 (10)
Z	4
Radiation type	Mo Kα
μ (mm−1)	0.54
Crystal size (mm)	0.3 × 0.2 × 0.15
Data collection
Diffractometer	Oxford Diffraction Xcalibur, Eos
Absorption correction	Multi-scan (CrysAlis PRO; Agilent, 2013 ▸)
T min, T max	0.976, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	13206, 2441, 2182
R int	0.060
(sin θ/λ)max (Å−1)	0.704
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.036, 0.068, 1.04
No. of reflections	2441
No. of parameters	80
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.20, −0.23
Absolute structure	Flack x determined using 806 quotients [(I +)−(I −)]/[(I +)+(I −)] (Parsonset al., 2013 ▸)
Absolute structure parameter	−0.05 (3)

Computer programs: CrysAlis PRO (Agilent, 2013 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2018/3 (Sheldrick, 2015 ▸) and XP (Siemens, 1994 ▸).

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.

Figure 3.

Simplified packing diagram of KHBEt₃ viewed parallel to the b axis. Hydrogen atoms except for H01 are omitted.

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 boranes 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].

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.

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. 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 U _iso(H) values were fixed at 1.2 (for methyl­ene groups) or 1.5 (for methyl groups) times the equivalent U _eq value of the parent carbon atoms.

Supplementary Material

CCDC reference: 2081809

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Crystal data

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

Data collection

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

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)

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. The compound is achiral and crystallizes only by chance in a chiral (Sohncke) space group.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	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*

Atomic displacement parameters (Ų)

	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)

Geometric parameters (Å, º)

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.