Janet E. Del Bene, I. Akorta, J. Elguero, G. Sánchez-Sanz
Lone-Pair Hole on P: P···N Pnicogen Bonds Assisted by Halogen Bonds
Ab initio MP2/aug’-cc-pVTZ calculations have been performed on the binary complexes XY:PH3 for XY = ClCl, FCl, and FBr; and PH3:N-base for N-base = NCH, NH3, NCF, NCCN, and N2; and the corresponding ternary complexes XY:PH3:N-base, to investigate P…N pnicogen bond formation through the lone-pair hole at P in the binary complexes, and P…N pnicogen-bond formation assisted by P…Y halogen bond formation through the σ-hole at Y. Although the binary complexes PH3:N-base which form through the lone-pair hole have very small binding energies, they are not equilibrium structures on their potential surfaces. The presence of the P…Y halogen bond makes PH3 a better electron-pair acceptor through its lone-pair hole, leading to stable ternary complexes XY:PH3:N-base. The halogen bonds in ClCl:PH3 and ClCl:PH3:NCCN are traditional halogen bonds, but in the remaining binary and ternary complexes they are chlorine- or bromine-shared halogen bonds. For a given nitrogen base, the P...N pnicogen bond in the ternary complex FCl:PH3:N-base appears to be stronger than that bond in FBr:PH3:N-base, which is stronger than the P…N bond in the corresponding ClCl:PH3:N-base complex. EOM-CCSD spin-spin coupling constants for the binary and ternary complexes with ClCl and FCl are also consistent with the changing nature of the halogen bonds in these complexes. At long P-Cl distances, the coupling constant 1xJ(P-Cl) increases with decreasing distance, but then decreases as the P-Cl distance continues to decrease, and the halogen bonds become chlorine-shared bonds. At the shorter distances, 1xJ(P-Cl) approaches the value of 1J(P-Cl) for the cation +(Cl-PH3). The coupling constants 1pJ(P-N) are small and with one exception, are greater in ClCl:PH3:N-base complexes compared to FCl:PH3:N-base, despite the shorter P-N distances in the latter.