An ab initio study on the concerted interaction between chalcogen and pnicogen bonds.

We analyzed cooperation between chalcogen-bonding and pnicogen-bonding interactions in XHS···NCH2P···NCY (X = F, Cl; Y = H, OH, NH2, CN and NC) complexes at the MP2/6-311++G** level. These effects were studied in terms of geometric and energetic properties, harmonic frequencies, and nuclear magnetic resonance (NMR). A cooperativity factor was adopted to measure the cooperativity between the two types of interaction in triads based on S-X and P-CN stretching frequencies. The size of the cooperative effect in each complex depends on the strength of S···N and P···N interactions. It is largest for FHSN⋯CH2P⋯NCNH2 and smallest for ClHS⋯NCH2P⋯NCCN and ClHS⋯NCH2P⋯NCNC complexes. The total spin-spin coupling constants across the chalcogen and pnicogen bonds in the ternary complexes are always larger than those in the binary systems. This trend can be also interpreted as a cooperative effect between chalcogen and pnicogen bond interactions. The enhancing mechanism was analyzed in terms of electron redistribution effects in XHS···NCH2P···NCY complexes.