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Keywords:

  • Cluster compounds;
  • Ionic liquids;
  • Liquid crystals;
  • Luminescence;
  • Synthesis design

Abstract

The salts [Cnmim]2[Mo6Cl14] (mim = methylimidazolium; n = 4, 6, 12, 16, 18) have been obtained by reaction of CnmimCl with MoCl2. Thermal analysis shows the melting point decreases with increasing alkyl chain length of the cation. The imidazolium chloromolybdates(II) with n = 6–18 decompose above 340 °C; [C18mim]2[Mo6Cl14] is thermally stable up to 390 °C. All compounds are insensititve to the constituents of the atmosphere. Of the higher melting salts [Cnmim]2[Mo6Cl14] (n = 4, 6), high-quality single crystals could be obtained. Single-crystal X-ray structural analyses clearly show that the cluster complex anion [Mo6Cl14]2– has an electron-precise octahedral {Mo6} cluster. For [C4mim]2[Mo6Cl14], two polymorphs differing in the cation alkyl-side-chain conformation were obtained. The less thermodynamically stable modification with a gauche conformation undergoes monotropic phase transition to the more stable form with the all-trans conformation. Long alkyl-side-chain salts, [Cnmim]2[Mo6Cl14] with n = 16 and 18, exhibit thermotropic liquid-crystalline behaviour. All compounds show bright red luminescence centred at about 730 nm with a lifetime of about 0.15 ms, originating from the cluster complex anion, which can be sensitized by the imidazolium cations.