Tuning the Mesomorphism and Redox Response of Anionic‐Ligand‐Based Mixed‐Valent Nickel(II) Complexes by Alkyl‐Substituted Quaternary Ammonium Cations.

Abstract: The combination of the redox‐active mesogenic anion [Ni^II(Bdt)(BdtSQ)]⁻ (Bdt=1,2‐benzenedithiolato; BdtSQ=1,2‐dithia‐semi‐benzoquinonato) with alkyl‐substituted ammonium cations afforded a series of redox‐active ionic complexes of the type [NR₄][Ni^II(Bdt)(BdtSQ)] [R=nC₁₆H₃₃ (NC16₄Ni) and C8,10 (NC8,10₄Ni); C8,10=6‐octylhexadecyl] or [NMe₂R₂][Ni^II(Bdt)(BdtSQ)] [R=nC₁₆H₃₃ (NMe₂C16₂Ni) and C8,10 (NMe₂C8,10₂Ni)]. X‐ray crystallographic analyses of NMe₂C16₂Ni and NC16₄Ni revealed the formation of cation‐dependent integrated ionic layers separated by interdigittated alkyl chains. Complexes NMe₂C16₂Ni and NC16₄Ni commonly form crystalline phases at room temperature, whereas complexes NMe₂C8,10₂Ni and NC8,10₄Ni, which contain branched alkyl chains, form a metastable mesophase and an amorphous phase at the same temperature, respectively. Furthermore, complexes NMe₂C16₂Ni, NMe₂C8,10₂Ni, and NC16₄Ni commonly form a smectic A phase (SmA) at 375, 317, and 342 K, respectively. For the four complexes, well‐defined cyclic voltammetry responses, derived from ligand‐based oxidation and reduction, were observed in solution and the condensed phases, that is, upon casting these complexes on an indium‐doped tin oxide working electrode. The present study demonstrates the tunability of the mesomorphism of ionic molecular assemblies composed of alkyl‐substituted quaternary ammonium cations, while maintaining the well‐defined redox responses of the anions even in the condensed phases. [ABSTRACT FROM AUTHOR]