Interactions of boranes and carboranes with aromatic systems: CCSD(T) complete basis set calculations and DFT-SAPT analysis of energy components.

The noncovalent interactions of heteroboranes with aromatic systems have only recently been acknowledged as a source of stabilization in supramolecular complexes. The physical basis of these interactions has been studied in several model complexes using advanced computational methods. The highly accurate CCSD(T)/complete basis set (CBS) value of the interaction energy for the model diborane···benzene complex in a stacking geometry exhibiting a B(2)H···π hydrogen bond was calculated to be -4.0 kcal·mol(-1). The DFT-SAPT/CBS approach, which is shown to reproduce the CCSD(T)/CBS data reliably asserted that the major stabilizing component was dispersion, followed by electrostatics. Furthermore, the effect of the benzene heteroatom- and exosubstitutions was studied and found to be small. Next, when aromatic molecules were changed to cyclic aliphatic ones, van der Waals complexes stabilized by the dispersion term only were formed. As the last step, interactions of two larger icosahedral borane cages with benzene were explored. The complex of the monoanionic CB(11)H(12)(-) exhibited two minima: the first stacked above the plane of the benzene ring with a C-H···π hydrogen bond and the second planar, in which the carborane cage bound to benzene via five B-H···H-C dihydrogen bonds. The DFT-SAPT/CBS calculations revealed that both of these binding motifs were stabilized by dispersion followed by electrostatic terms, with the planar complex being 1.4 kcal·mol(-1) more stable than the stacked one. The dianionic B(12)H(12)(2-) interacted with benzene only in the planar geometry, similarly as smaller anions do. The large stabilization energy of 11.0 kcal·mol(-1) was composed of dominant attractive dispersion and slightly smaller electrostatic and induction terms. In summary, the borane/carborane···aromatic interaction is varied both in the complex geometries and in the stabilizing energy components. The detailed insight derived from high-level quantum chemical computations can help us understand such important processes as host-guest complexation or carborane···biomolecule interactions.