Isomeric and Isostructural Oligothienylsilanes–Structurally Similar, Physicochemically Different: The Effect of Interplay between C–H···C(π), S···C(π), and Chalcogen S···S Interactions

The solid state and solution properties of tris­(2-thienyl)­methylsilane, I, tetrakis­(2-thienyl)­silane, III, and their positional isomers bearing 3-thienyl groups (II and IV) were investigated and compared. The tris­(thienyl)­silanes (I, II) crystallize in different space groups, but their respective structural motifs are very comparable. In turn, the tetrathienyl isomers are isostructural. Furthermore, in all studied systems the same set of C–H···C­(π), S···C­(π), S···S, C–H···S interactions are engaged in supramolecular structure formation. These interactions are interchangeable as thienyl rings (excluding structure II) are affected by 2-fold positional disorder. Despite the high level of structural similarity, the studied thienylsilanes show very different physicochemical behavior: (1) much higher melting points and larger enthalpies of fusion for II (mp = 71.3 °C, ΔH = 20.9 kJ mol–1) and IV (mp = 221.2 °C, ΔH = 29.1 kJ mol–1) with respect to their isomeric counterparts I (mp = 28.6 °C, ΔH = 16.0 kJ mol–1) and III (mp = 131.5 °C, ΔH = 27.0 kJ mol–1), (2) different temperature-dependence unit-cell evolution, and (3) much lower solubility of IV compared to III. The computations show that the strength of interactions decreases in the series C­(α)–H···C­(π) > C­(β)–H···C­(π) > S···C­(π) ≫ S···S. In a combination with crystal symmetry, this leads to a different distribution of energy within the corresponding crystal structures, and as a consequence, results in their different macroscopic behaviors. In addition, the solid–liquid equilibrium studies suggest that the specific S···S chalcogen bonding between molecules of IV is responsible for decreased solubilities of this compound. To characterize the specific interactions involving sulfur atoms (S···S and S···C­(π)), the quantum theory of atoms in molecules has been successfully applied.