Characterizing Complexes with Pnicogen Bonds Involving sp2 Hybridized Phosphorus Atoms: (H2C═PX)2 with X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2
Ab initio MP2/aug′-cc-pVTZ searches of the potential surfaces of (H2C═PX)2 complexes, with X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2, have been carried out to identify and characterize the properties of complexes with P···P pnicogen bonds. All (H2C═PX)2 form equilibrium conformation A dimers with C2h symmetry in which A–P···P–A approaches a linear alignment, with A the atom of X directly bonded to P. Conformation A dimers containing the more electronegative substituents are stabilized by a P···P pnicogen bond, have shorter P–P distances, and have binding energies which correlate with the P–P distance. Dimers stabilized by a P···P pnicogen bond and two P···Hb interactions consist of those with the more electropositive substituents, have shorter P–Hb distances, and have binding energies which are too high for their P–P distances. Conformation A complexes with P···Hbinteractions in addition to the P···P bond are more stable than the corresponding (PH2X)2complexes, while with only one exception, complexes stabilized by only a P···P bond are less stable than the corresponding (PH2X)2 complexes. In the region of the potential surfaces with C–P···P–C approaching linearity (conformation B), the only planar equilibrium complex is (H2C═POH)2, which is stabilized primarily by two O–H···P hydrogen bonds. The remaining (H2C═PX)2 complexes are not stabilized by pnicogen bonds, but by π interactions between the two H2C═PX monomers which are in parallel planes. When A–P···P–C approaches linearity, two types of equilibrium structures with P···P bonds exist. Of the conformation C dimers, (H2C═POH)2 is planar and the most stable, with a P···P pnicogen bond and an O–H···P hydrogen bond. (H2C═PH)2 and (H2C═PCH3)2 are also planar, and stabilized by a P···P pnicogen bond and a P···Hb interaction. The absence of a P···Hb interaction results in nonplanar C′ conformations with structures in which the monomers essentially retain their symmetry plane, but the plane of one molecule is rotated about the P···P bond relative to the other. C and C′ dimers are less stable than the corresponding A dimers, except for (H2C═PCH3)2. 31P chemical shielding patterns are consistent with the changing nature of the interactions which stabilize (H2C═PX)2 complexes. EOM-CCSD 31P–31P spin–spin coupling constants increase quadratically as the P–P distance decreases.