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:PH3for 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 that 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 that in FCl:PH3:N-base, despite the shorter P–N distances in the latter.