Unconventional connections

J. Am. Chem. Soc. 135, 2439–2442 (2013)

Hydrogen bonding can occur in a variety of situations and is not limited to interactions between polar hydrogens and electronegative atoms of adjacent molecules. Heteropolar dihydrogen interactions (X–H···H–Y) have been observed and, somewhat counter-intuitively, so have homopolar examples, such as between two CH groups. Rather than being governed by electrostatic interactions, these cases arise from attractive van der Waals forces.

By alkylating the NH group of LiNH₂BH₃, David J. Wolstenholme, G. Sean McGrady and colleagues from the University of New Brunswick have now suppressed the influence of the conventional proton–hydride N–H···H–B interaction to study in detail the homopolar dihydrogen interactions occurring in solid-state LiNMe₂BH₃. The compound was extensively characterized through X-ray crystallography and an analysis of the calculated electron distribution. It was found that [Li]⁺[NMe₂BH₃]⁻ adopts a one-dimensional chain structure (pictured; yellow, Li; orange, B; blue, N; grey, C; white, H) held together by inter-ion Li···N and Li···H–B interactions (shown as blue and green dashed lines, respectively).

Credit: © 2013 ACS

Within a given chain, a zigzag arrangement (black dashed lines) of supporting weak B–H···H–B interactions arising from mutual polarization of the BH groups was found between amidoborane moieties [NMe₂BH₃]⁻. A significant accumulation of electron density between the two hydrogen atoms confirmed that the BH groups do interact with one another, rather than simply being in close proximity. This conclusion is also supported by the fact that similar B–H···H–B distances were observed in the parent compound LiNH₂BH₃. The 1D chains are further connected to each other, albeit loosely, through weaker C–H···H–C dispersion forces (grey dashed lines between the chains) and C–H···H–B interactions (not shown; perpendicular to the plane pictured).

In contrast, the potassium analogue of the compound (KNMe₂BH₃) was found to have a sheet-like structure with layers connected to each other through C–H···H–C dispersion forces. This change in topology is caused by subtle differences in the main inter-ion interactions (K···N and K···H–B), which in turn prevent the formation of the weaker B–H···H–B and C–H···H–B bonds. These structural studies underscore the fact that weak homopolar dihydrogen interactions play a substantial supporting role in supramolecular structures, and may contribute to the stability and reactivity of hydrogen-rich compounds.