Thiophene-benzoquinones: synthesis, crystal structures and preliminary coordination chemistry of derived anilate ligands

Abstract

2,5-Bis(thiophene) and 2,5-bis(ethylenedioxy-thiophene) (EDOT) derivatives of 3,6-diethoxy-1,4-benzoquinone (para isomers) were prepared by Stille coupling between the 2,5-dibromo-3,6-diethoxy-1,4-benzoquinone precursors and (n-Bu)₃Sn-R (R = 2-thiophenyl or 3,4-ethylenedioxy-2-thiophenyl) reagents. In a parallel series of experiments 2,6-bis(thiophene) and 2,6-EDOT-3,5-diethoxy-1,4-benzoquinone (meta isomers) were synthesized by the same strategy. The four compounds were structurally characterized. The thiophene derivatives show essentially planar conformation thanks to the conjugation and establishment of S⋯O 1,5-nonbonded interactions, while in the EDOT derivatives the thiophene moieties are twisted with respect to the benzoquinone ring because of the steric hindrance. TD-DFT calculations were performed on both para and meta thiophene isomers in order to explain the differences observed in the UV-Vis absorption spectra. The 2,5-derivatives are valuable precursors for thiophene containing anilate (An) ligands, as the first examples of electron rich substituent based anilates. The Cu(II) complex [Cu(Th₂An)(tbbpy)]·2H₂O (Th₂An = thiophene-anilate; tbbpy = 4,4′-bis(tert-butyl)-2,2′-bipyridine) was isolated and structurally characterized. The metal center lies within a square planar coordination geometry, while the ligands engage in a set of intermolecular contacts.