Insights into Metal Borohydride and Aluminohydride Bonding: X-Ray and Neutron Diffraction Structures and a DFT and Charge Density Study of [Na(15-crown-5)][EH₄] (E = B, Al)

Abstract

The neutron and X-ray structures of [Na(15-crown-5)][BH(4)] and [Na(15-crown-5)][AlH(4)], respectively, are reported, along with a topological analysis of their DFT-computed charge densities that explores the bonding between the anionic complex hydride [EH(4)](-) (E = B, Al) and the counterion [Na(15-crown-5)](+). In each case, the interaction is weak and mainly electrostatic in nature; however, notable differences are observed in the manner in which [BH(4)](-) and [AlH(4)](-) bind to the metal, which explains their different coordination modes. A range of unconventional E-H···H-C contacts is revealed to play an important role in the overall bonding and crystal packing of both complexes. These interactions can be classified as weak dihydrogen bonds based on the atoms in molecules approach.

Structure and Bonding of KSiH3and Its 18-Crown-6 Derivatives: Unusual Ambidentate Behavior of the SiH3–Anion

“…This feature indicates that the [SiH 3 ] À moieties interact globally with the K + ions, in an analogous manner as the [BH 4 ] À anions in [Na(15-crown-5)BH 4 ] and [K(18-crown-6)BH 4 ]. 14,15 In contrast, the tet structures of 1 and 2 display a single K 3 3 3 Si bond path between the cation and the anion (Figure 2b). The F b (r) values for the K 3 3 3 Si interactions in this conformation are considerably larger than those of their inv counterparts, revealing a stronger interaction between the K and Si atoms.…”

“…In these instances, the electron density accumulated at the bond critical points (BCP), ρ b ( r ), for both the K···Si and K···H interactions turn out to be nearly identical, indicating that the electron density in the vicinity of the anion–cation interface is flat and delocalized (Table ). This feature indicates that the [SiH 3 ] − moieties interact globally with the K + ions, in an analogous manner as the [BH 4 ] − anions in [Na(15-crown-5)BH 4 ] and [K(18-crown-6)BH 4 ]. , In contrast, the tet structures of 1 and 2 display a single K···Si bond path between the cation and the anion (Figure b). The ρ b ( r ) values for the K···Si interactions in this conformation are considerably larger than those of their inv counterparts, revealing a stronger interaction between the K and Si atoms.…”

Structural characterization of three hydride-bridged sodium aluminate compounds

“…Searching the CSD for the 'Na-H-Al' bridging unit returned just nine hits, with two of these results being NaAlH 4 with 15-crown-5 solvating the sodium atom (Sirsch et al, 2010;Olbrich & Trzaska, 2005). This small number of structural precedents showcases the relative novelty of the newly reported structures.…”

Non-classical hydrogen bonding in [K(1-aza-18-crown-6)]BH4 and its 18-crown-6 counterpart

“…This type of bonding motif has previously been observed in the related hypervalent [(triphos)Fe(BH 6 )Fe(triphos)] + [X]complex (triphos = MeC(CH 2 PPh 2 ) 3 ), in which each Fe atom displays an h 4 -interaction with the BH 6 moiety through Fe ◊ ◊ ◊ B and Fe ◊ ◊ ◊ H contacts. 14 Although the ion-pair structures of III and IV indicate broadly similar bonding interactions, variations in the orientation of the borohydride anion with respect to the crown ether ligand result in significantly different secondary stabilising interactions for the ion pair. Thus, the BH 4 group in IV is canted towards a C-H atom of the crown ether, resulting in a weak B-H…”