2,5-Dibromothiophenes: Halogen Bond Involving Packing Patterns and Their Relevance to Solid-State Polymerization

A series of 2,5-dibromo-3-R-thiophenes (R = COONa 1, CN 2, CONH2 3, CON­(H)­Me 4, CON­(H)­Bn 5, CON­(CH2CH2)2O 6, CON­(H)­NH2 7, CON­(H)­OH 8) were prepared and studied via high-resolution ESI-MS, 1H and 13C­{H} NMR, IR, and X-ray diffraction. The analysis of the X-ray structures revealed Br···Br halogen bonds (XBs) as structure-directing interactions. A search of the Cambridge Structural Database showed an additional 12 structures of 2,5-dibromothiophenes whose structures featured Br···Br XBs. The crystal packing patterns for all these 2,5-dibromothiophenes were classified, and three general types were identified: (i) chains held by XBs, where each Br acts either as an XB donor or as an XB acceptor; (ii) zigzag arrays, where each Br functions simultaneously as an XB donor and acceptor; and (iii) rhombic assemblies with Br atoms at each vertex. DFT calculations, followed by topological analyses of the electron density distribution (QTAIM analysis) at the M06-2X/DZP-DKH level of theory, were performed to evaluate the strength of these XBs. The established energies were in the range from 0.5 to 2.2 kcal/mol, regardless of the packing type.