C-H•••H-C interactions in organoammonium tetraphenylborates:: another look at dihydrogen bonds

The crystal structures of the tetraphenylborates of the dabcoH(+), guanidinium (MeCN solvate), and biguanidinium cations are shown to contain a variety of C-H...H-C dihydrogen (DB) bonds of nominally zero polarity, as well as a variety of N-H...N, C-H...N, N-H...Ph, and C-H...Ph hydrogen (HB) bonds. These intermolecular bonds have been characterized topologically after multipole refinement of the structures. The coexistence of the DBs and HBs in each of the structures makes it possible to establish their relative strength hierarchy. It also illustrates the importance of the DBs in satisfying the tendency of these structures to maximize the total intermolecular bonding engagement. To compare the above DBs with other DBs, the results of an extensive set of MP2/6-31G(d,p) calculations (supplied by I. Alkorta) were analyzed for reference correlations between the bond-critical parameters. Thus, for an X-H...H-Y bond, the difference Deltaepsilon(H)(m) between the Mulliken charges on the H atoms in the uncomplexed X-H and H-Y components correlates quite well with the X-H...H-Y parameters and can be used for predicting the topological strength of an X-H...H-Y bond. The use of the difference Deltaepsilon(H)(c) in the bond does not appear to change the correlation significantly; closer correlations are observed when the amount of charge transferred on formation of the H...H bond is used instead of Deltaepsilon(H)(m) or Deltaepsilon(H)(c). Bonding interactions are obtained even between like or symmetry-related H atoms as a consequence of induced-dipole interactions, which accounts for the existence of the above intermolecular C-H...H-C bonds with d(H...H) = 2.18-2.57 Angstrom, electron density at the bond-critical point of similar to0.05-0.08 e/Angstrom(3), and a rough estimate of the H...H binding energy of similar to1-5 kcal/mol. Examination of the bond-critical parameters of X-H...H-Y bonds also suggests a criterion of stability of these bonds with respect to the transition from non-shared (closed-shell) X-H...H-Y interaction to covalent (shared-shell) X...H-H...Y interaction. This transition appears to be discontinuous.