NHC Hg(ii) and Pd(ii) complexes based on 1,8-dihydroxy-9,10-anthraquinone: synthesis, structure and catalysis

Abstract

Three bis-azolium salts, 1,8-bis[3′-(N-R-azoliumyl)propoxy]-9,10-anthraquinone hexafluorophosphate L¹H₂(PF₆)₂–L²H₂(PF₆)₂ (R = ethyl or picolyl, azoliumyl = imidazoliumyl or benzimidazoliumyl) and 1,3-bis[8′-(3′′-(N-ethyl-imidazoliumyl)propoxy)anthraquinon-l-yloxy]propane hexafluorophosphate (L³H₂(PF₆)₂), as well as their three macrometallocycle N-heterocyclic carbene mercury(II) complexes [L^1′HgBr₂] (1), [L²Hg](PF₆)₂ (3) and [L³Hg](PF₆)₂ (3) were prepared and characterized by ¹H NMR and ¹³C NMR spectroscopy and X-ray crystallography. In complexes 1–3, each macrometallocycle (18-membered macrometallocycle for 1, one 18-membered and two 6-membered macrometallocycles for 2, and 28-membered macrometallocycle for 3) was formed by one biscarbene ligand (L^1′ for 1, L² for 2, and L³ for 3) and one Hg(II) ion. Interestingly, the carbonyl group on the 9-position in complex 1 reacted with two acetonitrile molecules in the presence of the strong base KO^tBu through losing one molecule of H₂O to form L^1′. As a result, two –CH₂CN units were introduced to anthraquinone, in which the two –CH₂CN units lie on two flanks of the anthraquinone plane. In the crystal packings of 1–3, 2D supramolecular layers and 3D supramolecular frameworks are formed via hydrogen bonds and π⋯π interactions. Additionally, the catalytic activity of the L¹–Pd(II) complex in situ in the Suzuki–Miyaura reaction was studied. The results show that this catalyst is efficient in the Suzuki–Miyaura reaction.