Dihydrogen bond

In chemistry, a dihydrogen bond is a kind of hydrogen bond, an interaction between a metal hydride bond and an OH or NH group or another proton donor. The first example of this phenomenon is credited to Brown and Heseltine.^[1] They observed intense absorption bands in the IR bands at 3300 and 3210 cm^-1 for a solution of (CH₃)₂NHBH₃. The higher energy band is assigned to a normal N-H vibration whereas the lower energy band is assigned to the same bond, which is interacting with the B-H. Upon dilution of the solution, the 3300 cm^-1 band increased in intensity and the 3210 cm^-1 band decreased, indicative of intermolecular association.

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Interest in dihydrogen bonding was reignited upon the crystallographic characterization of the molecule H₃NBH₃. In this molecule, like the one studied by Brown and Hazeltine, the hydrogen atoms on nitrogen have a partial positive charge, denoted H^δ+, and the hydrogen atoms on boron have a partial negative charge, often denoted H^δ-.^[2] In other words, the amine is protic acid and the borane end is hydridic. The resulting B-H^...H-N attractions stabilize the molecule as a solid. In contrast, the related substance ethane, H₃CCH₃, is a gas with a boiling point 285 °C lower. Because two hydrogen atoms are involved, this is termed a dihydrogen bond.

Formation of a dihydrogen bond is assumed to precede formation of H₂ from the reaction of a hydride and a protic acid. A very short dihydrogen bond is observed in NaBH₄^.2H₂O with H---H contacts of 1.79, 1.86, and 1.94 Å. ^[3]

This kind of H---H interaction is distinct from the H---H bonding interaction in transition metal complexes having dihydrogen bound to a metal.^[4] Another so-called hydrogen hydrogen bond is postulated to exists in certain compounds between two neutral non-bonding hydrogen atoms.

References

1. ^ Brown, M. P.; Heseltine, R. W. "Co-ordinated BH3 as a proton acceptor group in hydrogen bonding" Chemical Communications 1968, page 1551-2. DOI: 10.1039/C19680001551
2. ^ A New Intermolecular Interaction: Unconventional Hydrogen Bonds with Element-Hydride Bonds as Proton Acceptor" Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle, T. F. Acc. Chem. Res. 1996, 29(7), 348 - 354.
3. ^ Custelcean, R.; Jackson, J. E. "Dihydrogen Bonding: Structures, Energetics, and Dynamics" Chemical Reviews 2001, volume 101, page 1963-1980.
4. ^ Kubas, G. J., "Metal Dihydrogen and σ-Bond Complexes", Kluwer Academic/Plenum Publishers: New York, 2001.

See also

• Hydrogen bond