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 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.