Crystal Engineering: A Salt of Mellitate Ion, MA-2, with Hydronium, H3O+, Containing π-π Stacks of Neutral Phenazine, PZ, in the Crystal

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 pi-pi interactions and sandwiched between MA ribbons suggested a design of models with more significant pi-pi interactions to produce conducting materials. Phenazine (9,10-diazaanthracene) was considered as a promising choice. The X-ray structure of the complex (MA(1)-PZ(4)) 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 similar to 3.39-3.60 A, well suited for pi-pi overlap. The proximal placement of such threaded modules enables their transformation to layered stacks thus possibly enhancing their pi-pi interactions. Crystal structure parameters are: sp. gr. C2/c, a = 13.2819(23) angstrom, b = 11.3669(20) angstrom, c = 33.3757(57) angstrom, b = 96.515(3), R-1 = 0.0521 for 4,198 data (I > 2 sigma(I)) and wR(2) = 0.0828 for all 6,194 data.