Abstract
A series of thio-phenes substituted in positions 2 and 5 by imine groups have been synthesized using a solvent-free approach, and their crystal structures determined. The substituents are chiral groups, and the expected absolute configuration for each mol-ecule was confirmed by refinement of the Flack parameter. The compounds are 2,5-bis-[(S)-(+)-(1,2,3,4-tetra-hydro-naphthalen-1-yl)imino]-thio-phene, C26H26N2S, (I), 2,5-bis-{[(R)-(-)-1-(4-meth-oxy-phen-yl)eth-yl]imino-meth-yl}thio-phene, C24H26N2O2S, (II), 2,5-bis-{[(R)-(-)-1-(4-fluoro-phen-yl)eth-yl]imino-meth-yl}thio-phene, C22H20F2N2S, (III), and 2,5-bis-{[(S)-(+)-1-(4-chloro-phen-yl)eth-yl]imino-meth-yl}thio-phene, C22H20Cl2N2S, (IV). A common feature of all four mol-ecules is the presence of twofold symmetry. For (I), which crystallizes in the triclinic space group P1, this symmetry is non-crystallographic, but for (II) in C2 and the isomorphous structures (III) and (IV) that crystallize in P21212, the twofold symmetry is crystallographically imposed with one half of each mol-ecule in the asymmetric unit. The comparable mol-ecular symmetry in the four structures is also reflected in similar packing, with mol-ecules aggregated to form chains through weak C-H⋯S inter-actions.
Highlights
A series of thiophenes substituted in positions 2 and 5 by imine groups have been synthesized using a solvent-free approach, and their crystal structures determined
A common feature of all four molecules is the presence of twofold symmetry
For (I), which crystallizes in the triclinic space group P1, this symmetry is non-crystallographic, but for (II) in C2 and the isomorphous structures (III) and (IV) that crystallize in P21212, the twofold symmetry is crystallographically imposed with one half of each molecule in the asymmetric unit
Summary
Thiophenedicarbaldehydes have a variety of applications (Dean, 1982a,b), for instance in the synthesis of annulenones and polyenyl-substituted thiophenes (Sargent & Cresp, 1975), in the preparation of macrocyclic ligands for bimetallic complexes that are able to mimic enzymes (Nelson et al., 1983), in crown ether chemistry (Cram & Trueblood, 1981). More recently, in the preparation of azomethines for photovoltaic applications (Bolduc et al, 2013a,b; Petrus et al., 2014) In regard to this latter application, most of the conjugated materials used in organic electronics are synthesized using time-consuming Suzuki-, Wittig-, or Heck-type coupling reactions that require expensive catalysts, stringent reaction conditions, and tedious purification processes. In order to afford a more economic route towards organic photovoltaic materials, Schiff bases derived from 2,5-thiophenedicarbaldehyde as the conjugated linker unit have recently been used. In the synthesis of the thiophenes reported here, the Schiff condensation generates a single by-product, water, and a onestep recrystallization affords the pure substituted thiophene in nearly quantitative yields. The central core containing the thiophene ring and the imine bonds is virtually planar, and the imine bonds are substituted by the tetralin ring systems, which present the same conformation.
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