Hydrogen bonding in two solid phases of phenazine-chloranilic acid (1/1) determined at 170 and 93 K

Department of Chemistry, Faculty of Science, Okayama University* Department of Chemistry, College of Humanities and Sciences, Nihon University**
○Kazuma Gotoh* Tetsuo Asaji** Hiroyuki Ishida*

Horiuchi et al. [1, 2] have revealed that the title compound, C12H8N2・C6H2O4Cl2, has a ferroelectric phase (space group P21) below 253 K, in which the phenazine and chloranilic acid molecules are arranged alternately through O-H...N hydrogen bonds to form a one-dimensional supramolecular structure. Recently, Saito et al. [3] reported two phase transitions at 136 and 146 K by heat capacity measurements. The low temperature phases were also detected by NQR measurements [4, 5]. In the present study, we have determined the crystal structure in the low-temperature phase stable below 136 K and re-determined the structure in the ferroelectric phase in order to clarify the hydrogen bonding scheme in each phase. In both phases the compound crystallizes in the space group P21. These structures are essentially same, except for the H atoms involved in the hydrogen bonds formed between the base and the acid. In the ferroelectric phase, H atoms in the hydrogen bonds are attached to O of the acid as reported by Horiuchi et al., but one H atom (H4) was found to have a considerably large Uiso value compared to that of the other H atom (H2), suggesting that atom H4 is disordered in the hydrogen bond. On the other hand, in the low temperature phase the H atom corresponding to atom H4 was found to be attached to N, which implies that proton transfer of one of the two H atoms of the acid to the base occurs in this phase.
[1] Horiuchi et al. (2005). Nat. Mater. 4, 163-166. [2] Horiuchi et al. (2005). J. Am. Chem. Soc. 127, 5010-5011. [3] Saito et al. (2006). J. Phys. Soc. Jpn., 75, 033601-1-3. [4] Asaji et al. (2006). J. Mol. Struct. 791, 89-92. [5] Watanabe et al. (2006). Annual Meeting of Chem. Soc. Jpn. , Tokyo, Japan, 2PC-021.