Principles of crystal packing and intermolecular motifs for Ph3XSnXPh3, X⊕=⊕C, Si, Ge, Sn

Abstract

The crystallisation and crystal structure of Ph₃CS₃CPh₃1 are reported and analysed in the context of the known crystal structures of the class of compounds Ph₃XS_nXPh₃, n⊕=⊕1–6, X⊕=⊕C, Si, Ge, Sn. The goal is to understand the crystal packing forces and patterns. A general pattern of intermolecular motifs is evident. Molecules Ph₃XS_nXPh₃ with n⊕>⊕1 normally crystallise with sixfold phenyl embraces (6PEs) of XPh₃ at both ends, forming parallel but curved strands of molecules ·Ph₃XS_nXPh₃·6PE·Ph₃XS_nXPh₃·6PE·. For n⊕=⊕3, 4, 5 and 6 these strands are further associated by a mixed sulfur phenyl embrace with composition S₂(Ph)₄, named 2S4PE. This embrace includes local interactions between phenyl groups and the lone-pair electron density of sulfur, and is calculated to have a stabilising energy within the range 4–8 kcal mol⁻¹, compared with 10.5–13.0 kcal mol⁻¹ for the 6PEs in these molecules. In addition to the 6PEs and 2S4PEs there are some local herringbone arrays of phenyl groups, generating efficient crystal packing. The exception to this pattern, 1, does not form 6PEs or a 2S4PE, but instead forms lateral and end-to-end motifs with less concerted multiple phenyl embraces. This set of compounds demonstrates substitutional polymorphism, that is polymorphism where the molecules are not identical but have internal iso-stereochemical substitution. Recrystallisations of 1 have not yet yielded its expected dimorph.