Crystal supramolecularity. Multiple phenyl embraces by [PPN]+ cations

Abstract

Analysis of the 752 crystals in the Cambridge Structural Database containing [Ph₃PNPPh₃]⁺ ([PPN]⁺) cations has revealed mutually attractive interactions between the cations leading to the formation of supramolecular motifs. The cation is flexible due to P–N–P bending: calculation of single point energies for an idealised [H₃PNPH₃]⁺ fragment show a flat energy well for the cation, with there being less than 1 kcal mol⁻¹ difference in energy between P–N–P angles of 130–180°. The most populated conformation is that with a P–N–P angle in the range 140–145°. The types of inter-cation interaction can be classified by a combination of the N–P⋯P angle, the N–P⋯P–N torsion angle and the four intermolecular P⋯P distances between neighbouring cations. Interactions with an N–P⋯P angle of greater than 125° indicate a sixfold phenyl embrace (6PE), whilst those at more acute angles form expanded phenyl embraces with neighbouring cations either parallel (PEPE) or orthogonal (OEPE). The PEPE and OEPE are differentiated by torsion angle or intermolecular P⋯P separations. Computation of the energies of attraction between the cations gives values in the ranges 7.0–10.5, 7.9–11.1 and 8.6–13.0 kcal mol⁻¹ for the 6PE, OEPE and PEPE respectively. The individual embraces combine to form zigzag chains of cations leading to either columnar or layered structures. The crystal lattice formed does not depend on anion size or charge.