Using Beryllium Bonds to Change Halogen Bonds From Traditional to Chlorine-shared to Ion-pair
Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the structures, binding energies, and bonding characteristics of binary complexes HFBe:FCl, R2Be:FCl, and FCl:N-base, and of ternary complexes HFBe:FCl:N-base and R2Be:FCl:N-base for R = H, F, Cl; N-base = NH3, NHCH2, NCH. Dramatic synergistic cooperative effects have been found between the BeF beryllium bonds and the ClN halogen bonds in ternary complexes. The ClN traditional halogen bonds and the BeF beryllium bonds in binary complexes become significantly stronger in ternary complexes, while the F–Cl bond weakens. Charge-transfer from F to the empty p(σ) orbital of Be leads to a bending of the XYBe molecule and a change in the hybridization of Be, which in the limit becomes sp2. As a function of the intrinsic basicity of the nitrogen base and the intrinsic acidity of the Be derivative, the halogen-bond type evolves from traditional to chlorine-shared to ion-pair bonds. The mechanism by which an ion-pair complex is formed is similar to that involved in the dissociative proton attachment process. EOM-CCSD spin–spin coupling constants 1XJ(Cl–N) across the halogen bond in these complexes also provide evidence of the same evolution of the halogen-bond type.