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

The combination of the redox-active mesogenic anion, [NiII(Bdt)(BdtSQ)]- (Bdt = 1,2-benzenedithiolato and BdtSQ = 1,2-dithia-semi-benzoquinonato), with alkyl-substituted ammonium cations afforded a series of redox-active ionic complexes of [NR4][NiII(Bdt)(BdtSQ)] (R = nC16H33 (NC164Ni) and C8,10 (NC8,104Ni); C8,10 = 6-octylhexadecyl) or [NMe2R2][NiII(Bdt)(BdtSQ)] (R = nC16H33 (NMe2C162Ni) and C8,10 (NMe2C8,102Ni)). The X-ray crystallographic analyses of NMe2C162Ni and NC164Ni revealed the formation of cation-dependent integrated ionic layers separated by interdigitated alkyl chains. Complexes NMe2C162Ni and NC164Ni commonly form crystalline phases at room temperature, while complexes NMe2C8,102Ni and NC8,104Ni which contain branched alkyl chains, form metastable mesophase and 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, i.e., when these complexes were cast on an ITO working electrode. The present study demonstrates the tunability of the mesomorphism of ionic molecular assemblies composed of alkyl-substituted quaternary ammonium cations, while the well-defined redox responses of the anions are maintained even in the condensed phases.