Pnicogen Bonds between X=PH3 (X = O, S, NH, CH2) and Phosphorus and Nitrogen Bases
Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the pnicogen bonded complexes formed between the acids O═PH3, S═PH3, HN═PH3, and H2C═PH3 and the bases NH3, NCH, N2, PH3, and PCH. All nitrogen and phosphorus bases form complexes in which the bases are lone pair electron donors. The binding energies of complexes involving the stronger bases NH3, NCH, and PH3 differentiate among the acids, but the binding energies of complexes with the weaker bases do not. These complexes are stabilized by charge transfer from the lone pair orbital of N or P to the σ*P═A orbital of X═PH3, where A is the atom of X directly bonded to P. PCH also forms complexes with the X═PH3 acids as a π electron donor to the σ*P═A orbital. The binding energies and the charge-transfer energies of the π complexes are greater than those of the complexes in which PCH is a lone pair donor. Whether the positive charge on P increases, decreases, or remains the same upon complex formation, the chemical shieldings of 31P decrease in the complexes relative to the corresponding monomers. 1pJ(P–N) and 1pJ(P–P) values correlate best with the corresponding P–N and P–P distances as a function of the nature of the base. 1J(P–A) values do not correlate with P–A distances. Rather, the absolute values of 1J(P–O), 1J(P–S), and 1J(P–N) decrease upon complexation. Decreasing 1J(P–A) values correlate linearly with increasing complex binding energies. In contrast, 1J(P–C) values increase upon complexation and correlate linearly with increasing binding energies.