Concerted supramolecular motifs: linear columns and zigzag chains of multiple phenyl embraces involving Ph4P+ cations in crystals

Abstract

One-dimensionally infinite sequences of mutually attractive Ph₄P⁺ cations occur in crystals, as independent supramolecular motifs. These motifs involve concerted, multiple phenyl embraces, based on the intermolecular edge-to-face (ef) interaction between phenyl groups. The linear infinite translational quadruple phenyl embrace (LITQPE) has four ef interactions between each pair of Ph₄P⁺ ions, propagated in almost all cases by lattice translation. In the zigzag infinite sextuple phenyl embrace (ZZISPE), each Ph₄P⁺ presents three phenyl groups to each of its neighbours, and there are six ef interactions between each pair of cations which are commonly related by a centre of inversion: the infinite sequence of P atoms is planar with approximately tetrahedral angles. In crystals, LITQPE columns and ZZISPE chains are usually parallel to stacks of anions, which vary in size from Br^– to [(µ₆-C)Os₁₀(CO)₂₄HgCF₃]^–: the anion stack can contain solvent, or the anion can be one-dimensionally non-molecular. 52 Crystals containing isolated LITQPE columns and 36 crystals containing ZZIPSE chains have been described, and crystal packing types (supramolecular crystal isomers) classified. The cation motifs are not disordered, whereas associated anions can be. Strong attractions exist between Ph₄P⁺ cations within these two motifs. Calculated through-space energies for typical structures are ca. 13 kcal mol^–1 net attraction per [Ph₄P⁺]₂ pair within a LITQPE, and ca. 16 kcal mol^–1 net attraction within a ZZISPE. In contrast, the strongest interactions between these isolated motifs are ca. 1–3 kcal mol^–1 attraction for the closest intermotif pair of Ph₄P⁺ cations. The intramotif attractive energies have similar magnitude to the cation–anion attractive energies. The LITQPE and ZZISPE can be regarded as supramolecular crystal tectons, energetically comparable with hydrogen bonds.