Pnicogen Bonds Pairing Anionic Lewis Acid with Neutral and Anionic Bases.

The complexation process between anionic ZCl (Z = P, As, Sb) and neutral NCH and pyridine, as well as the CN anion, is studied in both the gas phase and aqueous solution by high-level ab initio calculations. Despite the absence of a positively charged σ-hole on ZCl, a pnicogen bond (ZB) holds all of these complexes together. The dimerization induces the ZCl to rearrange internally from a see-saw to a square geometry. The complexation process is endothermic for both HCN and CN in the gas phase but for different reasons. The approach of CN to ZCl must overcome anion-anion Coulomb repulsion, while HCN is a much weaker base. The intermediate nucleophilicity of pyridine leads to an exothermic dimerization reaction despite the deformation of the ZCl structure. The dimers must traverse an energy barrier in order to dissociate. Formation of the dianionic NC···ZCl complex becomes exothermic in aqueous solution. Complexation with HCN remains endothermic in water, although less so, and the exothermicity of ZB formation with pyridine is enhanced by solvation.