Crystal supramolecular motifs. Two- and three-dimensional networks of Ph4P+ cations engaged in sixfold phenyl embraces

Abstract

The sixfold phenyl embrace (6PE) which is a widespread supramolecular motif for Ph₄P⁺ cations in crystals can occur as a zig-zag chain motif (ZZI6PE) in which each Ph₄P⁺ engages two 6PE, and these ZZI6PE allow further multiple phenyl embraces at each Ph₄P⁺ cation. The Cambridge Structural Database has been explored for occurrences of the two- and three-dimensional networks of multiple phenyl embraces involving the common cation Ph₄P⁺. A prevalent motif involves parallel ZZI6PE chains linked by fourfold phenyl embraces (the parallel variant, P4PE) to form skewed hexagonal nets, called ZZI6PE/P4PE layers. Another type has ZZI6PE chains running in orthogonal directions and linked by the orthogonal variant of the 4PE: these are the orthogonal ZZI6PE/O4PE three-dimensional nets. An expanded layer involves eight-rings of Ph₄P⁺ built from finite sequences of 6PE. The compounds [Ph₄P⁺]₂[Te₄]^2– and [Ph₄P⁺]₄[(S₆)Cu(S₈)Cu(S₆)]^4– contain more elaborate three-dimensional nets comprised of 6PE, 4PE and 3PEs. These nets are more highly concerted crystal supramolecular motifs. The nets described are different from those based on 4PE, and the difference is related to the charge of the anion. Anions are usually located over or in the cycles of cations in the layered motifs, and in cavities of the three-dimensional motifs. The concerted supramolecular motifs of cations, rather than cation–anion attractions, are believed to be the dominant factors in the crystal supramolecularity. Calculated energies for the motifs are reported. It is concluded that a single Ph₄P⁺ cation can participate in up to four multiple phenyl embraces, and that the maximum attractive interaction energy per Ph₄P⁺ cation is ca. 9 kcal mol^–1. This is comparable with the energies of hydrogen bonds.