Crystal structure of potassium triethylhydridoborate (‘superhydride’)

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


Chemical context
The title compound KHBEt 3 was first prepared by Ziegler and Lehmkuhl from NaBEt 3 H and potassium amalgam (Ziegler & Lehmkuhl, 1963), but a more convenient approach was reported a few years later using KH and BEt 3 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. 1987Yoon et al. , 1989, for the generation of low-valent transition-metal complexes (Bö nnemann & Korall, 1992), and as a hydride transfer reagent resulting in well-defined metalhydride 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 KHBEt 3 has so far remained elusive. The few reported examples of structures containing KHBEt 3 include its adducts with polydentate amines such as N,N,N 0 ,N 0 -tetramethylethylenediamine (TMEDA) and N,N,N 0 ,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 KHBEt 3 unsupported by any further ligands (see Synthesis and crystallization) and here report its structure. ISSN 2056-9890

Structural commentary
The asymmetric unit of KHBEt 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 2 1 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 2 1 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 KÁ Á ÁH 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 107.6 (10) H01-K1-H01 i 104.0 (4) C5-B1-H01 109.5 (10) Symmetry codes: (i) x À 1 2 ; Ày þ 3 2 ; Àz þ 1; (ii) x þ 1 2 ; Ày þ 3 2 ; Àz þ 1; (iii) Àx þ 1 2 ; Ày þ 1; z À 1 2 .

Figure 2
The environment of the potassium atom in KHBEt 3 , 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 1 2 + x, 3 2 À y, 1 À z and À 1 2 + x, 3 2 À y, 1 À z, respectively. The contact to H2B of a fourth anion (at 1 2 À x, 1 À y, À 1 2 + z) is omitted for clarity.

Figure 1
The asymmetric unit of KHBEt 3 . Ellipsoids are drawn at the 50% level.
Only the shortest K1-H contact is drawn explicitly.

Figure 3
Simplified packing diagram of KHBEt 3 viewed parallel to the b axis. Hydrogen atoms except for H01 are omitted.
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 multicentre 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.

Synthesis and crystallization
We attempted the preparation of a rare-earth metal hydride by salt metathesis between [{( 5 :-N-pdl*SiMe 2 NtBu)-La(thf)} 2 (-Cl)] (Jones et al., 2021) and 2 equiv. of KHBEt 3 (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 [{( 5 :-Npdl*SiMe 2 NtBu)La(thf)} 2 (-H)], but of the starting reagent KHBEt 3 .

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 (   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.