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; BdtSQ=1,2-dithia-semi-benzoquinonato) with alkyl-substituted ammonium cations afforded a series of redox-active ionic complexes of the type [NR4 ][NiII (Bdt)(BdtSQ)] [R=nC16 H33 (NC164 Ni) and C8,10 (NC8,104 Ni); C8,10=6-octylhexadecyl] or [NMe2 R2 ][NiII (Bdt)(BdtSQ)] [R=nC16 H33 (NMe2 C162 Ni) and C8,10 (NMe2 C8,102 Ni)]. X-ray crystallographic analyses of NMe2 C162 Ni and NC164 Ni revealed the formation of cation-dependent integrated ionic layers separated by interdigittated alkyl chains. Complexes NMe2 C162 Ni and NC164 Ni commonly form crystalline phases at room temperature, whereas complexes NMe2 C8,102 Ni and NC8,104 Ni, which contain branched alkyl chains, form a metastable mesophase and an amorphous phase at the same temperature, respectively. Furthermore, complexes NMe2 C162 Ni, NMe2 C8,102 Ni, and NC164 Ni 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.