The "dihydrogen bond" DรH d+ โขโขโข dร HรE-where D is a typical electrostatic atom such as nitrogen or oxygen, and E is a transition metal or boron-was first described by Crabtree and co-workers. [1] Both inter-and intramolecular versions of this bond have been cited, and with typical energies in the range of 12-28 kJ mol ร1 , [2] they are comparable in strength to conventional hydrogen bonds. In particular, the NรH d+ โขโขโข dร HรB intermolecular contact in solid-state BH 3 NH 3 has received special attention. [1][2][3][4] The absence of nonbonding valence electrons on boron negates a possible H d+ โขโขโข dร E interaction that could arise if E were a d-block element. Thus, for BH 3 NH 3 the HโขโขโขH contact must result from a sigma-type interaction.
To date, all computational studies investigating properties of this unusual intermolecular bond refer to isolated dimer (i.e., gas-phase) models, which do not resemble the crystal structure all that closely (Figure 1 a,b). The global minimum calculated by Crabtree and co-workers contains two HโขโขโขH contacts (PCI-80/B3 LYP, Figure 1 c), which are each assigned an average bond strength of 25.5 kJ mol ร1 (no basis set superposition error (BSSE) or zero-point energy (ZPE) corrections applied). [2] This value has been described as being "surprisingly large" by the authors, and places the N ร HโขโขโขH ร B contact at the upper end of the energy range quoted for dihydrogen bonds. Li et al. cite a different structure comprising four dihydrogen bonds (Figure 1 d), with an average bond strength of 11.6 kJ mol ร1 (after BSSE and ZPE correction; 6-31 + + G*/MP2), that is, at the lower end of the dihydrogen bond energy spectrum. [3] Popelier characterized the intermolecular interaction on the basis of the calculated electron density for a third dimer containing three dihydrogen bonds (Figure 1 e), with an average bond energy of 18.3 kJ mol ร1 (although bond