Hydrogen/dihydrogen-bonded systems

It is also remarkable that M- H-X systems show proton transfer which is mechanistically close to that of classical hydrogen bonds. Protonation of the metal atom occurs via M- H bonds and ion pairs (Scheme 2.4) lying as intermediates on the reaction coordinate. As we will see below, a very similar mechanism operates in the case of dihydrogen-bonded complexes. 

Since A-H- -H-X systems can lose H2, it is interesting to rationalize Would the forces within a crystal be enough to generate molecular hydrogen from a dihydrogen bond To provide an answer, the following neutral and charged systems. 

It is obvious that these molecular systems can be prepared specially or can form spontaneously as self-assemblies when hydrogen or dihydrogen bonds dictate the processes of molecular aggregation. The latter is particularly importaut in supramolecular chemistry and crystal engineering, where the primary attention is focused on mastering weak intermolecular interactions [1]. 

In contrast, the HF/6-31G calculations by Rozas and co-workers have resulted in the dihydrogen-bonded complex L-H H-F as a stationary strucmre [2], However, when even a weak electric field is applied to this complex, the system changes to L+- H-H- F with an H H distance of 0.734 A, very close to that of molecular hydrogen. 

For comparison, the authors have probed a complex formed by the same proton-donor molecule and molecular hydrogen. In this very weak complex, HCCH- - (H2), the H- - (H2) distance has been calculated as 2.606 A (i.e., significantly larger than the sum of the van der Waals radii of H). It is extremely interesting that a topological analysis of the electron density also leads to the appearance of the bond critical point in the H- - (H2) direction. However, the Pc and V pc values are very small (0.0033 and 0.0115 au, respectively) compared with those in the HCCH- - -HLi complex (0.0112 and 0.0254 au, respectively). The most important conclusion of this comparison is There is no evident borderline between the dihydrogen-bonded complexes and the van der Waals systems. 

In the first report on the beryllinm dihydrogen-bonded complex, pnblished in 1996 [5], the H-Be-H- H-N(H3)+ system was located at an energy minimnm of the potential energy surface calcnlated at the MP2/6-311-H-G level. By analogy with classical hydrogen bonds, this dihydrogen complex can be said to be charge-assisted. 

One of the important aspect of the use of dihydrogen bonds as driving forces in molecular associations is their cooperativity or anticooperativity when increasing the number of H- H contacts in the self-association of molecular systems leads to additional energy, calculated on the basis of one dihydrogen bond, or vice versa. As we have shown in Chapter 2, classical hydrogen bonds can be highly cooperative, due to their mutual polarization [5]. 

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