B4H4 and B4(CH3)4 as Unique Electron Donors in Hydrogen-Bonded and Halogen-Bonded Complexes

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on B4H4 and B4(CH3)4 to investigate the base properties of these molecules with Td symmetry. Each face of the tetrahedral structure of B4H4 and B4(CH3)4 is stabilized by a two-electron, three-center B-B-B bond. The face uses these two electrons to act uniquely as an electron-pair donor for the formation of stable hydrogen-bonded and halogen-bonded complexes with C3v symmetry. The hydrogen-bonded complexes are B4H4:HY and B4(CH3)4:HY, with HY = HNC, HF, HCl, HCN, and HCCH; the halogen-bonded complexes are B4H4:ClY and B4(CH3)4:ClY, with ClY = ClF, ClCl, ClNC, ClCN, ClCCH, and ClH. The absolute values of the binding energies of the hydrogen-bonded complexes B4(CH3)4:HY and of the halogen-bonded complexes B4(CH3)4:ClY are significantly greater than the binding energies of the corresponding complexes with B4H4. The binding energies of each series correlate with the distance from the hydrogen-bonded H atom or halogen-bonded Cl atom to the centroid of the interacting face. Charge transfer stabilizes all complexes and occurs from the B2-B3-B4 orbital of the face to the antibonding H-X orbital of HY in hydrogen-bonded complexes and to the antibonding Cl-X orbital of ClY in halogen-bonded complexes, with X being the atom of Y that is directly bonded to either H or Cl. For fixed HY, EOM-CCSD spin-spin coupling constants J(X-B1) are greater than J(X-Bn) for complexes B4H4:HY, even though the X-B1 distances are longer. B1 and Bn are the atoms at the apex and in the interacting face, respectively. Similarly, for complexes B4H4:ClY, J(Cl-B1) is greater than J(Cl-Bn). In the halogen-bonded complexes, both coupling constants correlate with the corresponding distances.