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 absorptions 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.

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 a 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 exist in certain compounds between two neutral non-bonding hydrogen atoms.

References

1. ^ Brown, M. P.; R. W. Heseltine (1968). "Co-ordinated BH3 as a proton acceptor group in hydrogen bonding". Chemical Communications (London) (23): 1551–1552. doi:10.1039/C19680001551. 
2. ^ Crabtree, Robert H.; Per E. M. Siegbahn, Odile Eisenstein, Arnold L. Rheingold, Thomas F. Koetzle (1996-01-01). "A New Intermolecular Interaction: Unconventional Hydrogen Bonds with Element−Hydride Bonds as Proton Acceptor". Accounts of Chemical Research 29 (7): 348–354. doi:10.1021/ar950150s. PMID 19904922. 
3. ^ Custelcean, Radu; James E. Jackson (2001-07-01). "Dihydrogen Bonding: Structures, Energetics, and Dynamics". Chemical Reviews 101 (7): 1963–1980. doi:10.1021/cr000021b. PMID 11710237. 
4. ^ Kubas, Gregory J. (2001-08-31). Metal Dihydrogen and -Bond Complexes - Structure, Theory, and Reactivity (1 ed.). Springer. ISBN 0306464659.