Characterizing Traditional and Chlorine-Shared Halogen Bonds in Complexes of Phosphine Derivatives with ClF and Cl2
Ab initio MP2/aug’-cc-pVTZ calculations have been carried out on the halogen-bonded complexes H2XP:ClF and H2XP:Cl2, with X = F, Cl, OH, NC, CN, CCH, CH3, and H. H2XP:ClF complexes are stabilized by chlorine-shared halogen bonds with short P–Cl and significantly elongated Cl–F distances. H2XP:Cl2 complexes with X = OH and CH3 form only chlorine-shared halogen bonds, while those with X = H, NC, and CN form only traditional halogen bonds. On the H2FP:Cl2, H2(CCH)P:Cl2, and H2ClP:Cl2 potential surfaces small barriers separate two equilibrium structures, one with a traditional halogen bond and the other with a chlorine-shared bond. The binding energies of H2XP:ClF and H2XP:Cl2 complexes are influenced by the electron-donating ability of H2XP and the electron accepting ability of ClF and ClCl, the nature of the halogen bond, other secondary interactions, and charge-transfer interactions. Changes in electron populations on P, F, and Cl upon complex formation do not correlate with changes in the chemical shieldings of these atoms. EOM-CCSD spin–spin coupling constants for complexes with chlorine-shared halogen bonds do not exhibit the usual dependencies on distance. 2XJ(P–F) and 2XJ(P–Cl) for complexes with chlorine-shared halogen bonds do not correlate with P–F and P–Cl distances, respectively. 1XJ(P–Cl) values for H2XP:ClF correlate best with the Cl–F distance, and approach the values of 1J(P–Cl) for the corresponding cations H2XPCl+. Values of 1XJ(P–Cl) for complexes H2XP:ClCl with chlorine-shared halogen bonds correlate with the binding energies of these complexes. 1J(F–Cl) and 1J(Cl–Cl) for complexes with chlorine-shared halogen bonds correlate linearly with the distance between P and the proximal Cl atom. In contrast, 2XJ(P–Cl) and 1XJ(P–Cl) for complexes with traditional halogen bonds exhibit more normal distance dependencies.