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

The complexation process between anionic ZCl4- (Z = P, As, Sb) and neutral NCH and pyridine, as well as the CN- anion, are 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 ZCl4-, a pnicogen bond holds all of these complexes together. The dimerization induces the ZCl4- 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 ZCl4- 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 ZCl4- structure. The dimers must traverse an energy barrier in order to dissociate. Formation of the dianionic -NC∙∙∙ZCl4- complex becomes exothermic in aqueous solution. Complexation with HCN remains endothermic in water, although less so and the exothermicity of pnicogen bond formation with pyridine is enhanced by solvation.