Boron as an Electron-Pair Donor for B⋅⋅⋅Cl Halogen Bonds.

MP2/aug'-cc-pVTZ calculations were performed to investigate boron as an electron-pair donor in halogen-bonded complexes (CO)2 (HB):ClX and (N2 )2 (HB):ClX, for X=F, Cl, OH, NC, CN, CCH, CH3 , and H. Equilibrium halogen-bonded complexes with boron as the electron-pair donor are found on all of the potential surfaces, except for (CO)2 (HB):ClCH3 and (N2 )2 (HB):ClF. The majority of these complexes are stabilized by traditional halogen bonds, except for (CO)2 (HB):ClF, (CO)2 (HB):ClCl, (N2 )2 (HB):ClCl, and (N2 )2 (HB):ClOH, which are stabilized by chlorine-shared halogen bonds. These complexes have increased binding energies and shorter B-Cl distances. Charge transfer stabilizes all complexes and occurs from the B lone pair to the σ* Cl-A orbital of ClX, in which A is the atom of X directly bonded to Cl. A second reduced charge-transfer interaction occurs in (CO)2 (HB):ClX complexes from the Cl lone pair to the π* C≡O orbitals. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants,1x J(B-Cl), across the halogen bonds are also indicative of the changing nature of this bond.1x J(B-Cl) values for both series of complexes are positive at long distances, increase as the distance decreases, and then decrease as the halogen bonds change from traditional to chlorine-shared bonds, and begin to approach the values for the covalent bonds in the corresponding ions [(CO)2 (HB)-Cl]+ and [(N2 )2 (HB)-Cl]+ . Changes in11 B chemical shieldings upon complexation correlate with changes in the charges on B.