Synthesis and thermotropic behaviour of bis(imidazolium) salts bearing long-chain alkyl-substituents and of the corresponding dinuclear gold carbene complexes

• Au(I) complexes with di(N-heterocyclic carbene) ligands.
• Bis(imidazolium) salts bearing long-chain alkyl substituents.
• Study on thermotropic properties of bis(imidazolium) salts and carbene complexes.
• Bis(imidazolium) salts behave as thermotropic liquid crystals.

New propylene bridged bis(imidazolium) salts bearing at the wingtip positions a benzyl group functionalised with one or two long alkyl chains, as well as the corresponding dinuclear N-heterocyclic dicarbene gold(I) complexes of general formula [Au2(RIm–(CH2)3–ImR)2](X)2 (X = Br, PF6, BF4) have been synthesised and thermally characterised. All the compounds are stable up to 200 °C and the bis(imidazolium) salts [H2(RIm–(CH2)3–ImR)](X)2 (X = Br, PF6) behave as thermotropic liquid crystals in the temperature range 100–200 °C. By contrast, only the gold(I) diNHC complexes with eight aliphatic chains present mesomorphism, whereas those with four chains show numerous crystalline phases with structural disorder and presumably modified Au–Au distances, as indicated by solid state emission properties.

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