Crystal engineering: a salt of mellitate ion, MA⁻², with hydronium, H₃O⁺, containing π-π stacks of neutral phenazine, PZ, in the crystal

Abstract

The structure of the benznenehexacarboxylic acid (mellitic acid, MA)-phenanthroline (PL) complex (MA⁻² 2PL⁺¹) consisting of infinite stacks of phenanthroline separated for optimum π-π interactions and sandwiched between MA ribbons suggested a design of models with more significant π-π interactions to produce conducting materials. Phenazine (9,10-diazaanthracene) was considered as a promising choice. The X-ray structure of the complex (MA₁-PZ₄) from MA⁻² and neutral PZ was a major surprise since the complex was not only radically different from MA-PL but also the MA exhibited the rare direct interaction with the base PZ. In a novel arrangement, a continuous chain formed by the linking of the peri carboxylic groups of MA⁻² mediated with hydronium ions, H₃O⁺, supports a 4⁻fold arrangement of four phenazine molecules where four carboxylic groups are engaged in hydrogen bond formation with one of the nitrogens of each PZ. The running chain of MA⁻² supports the formation of stacks of phenazine molecules which are separated by ~3.39-3.60 Å, well suited for π-πoverlap. The proximal placement of such threaded modules enables their transformation to layered stacks thus possibly enhancing their π-πinteractions. Crystal structure parameters are: sp. gr. C2/c, α= 13.2819(23) Å, β= 11.3669(20) Å, γ = 33.3757(57) Å, β= 96.515(3), R₁ = 0.0521 for 4,198 data (I > 2 σ(I)) and wR₂ = 0.0828 for all 6,194 data.