A redetermination of 2-(6-diethylamino-3-diethyliminio-3H-xanthen-9-yl)benzoate–ethyl gallate (1/1) at room temperature

The title compound, C28H30N2O3·C9H10O5, is a well known red leuco complex of 2-(6-diethylamino-3-diethyliminio-3H-xanthene-9-yl)benzoate (rhodamine B base abbreviated to RBB: leuco dye) with ethyl gallate (EG: developer). The structure of the complex at room temperature has recently been reported by Sekiguchi, Takayama, Gotanda & Sano [(2007) Chem. Lett. 36, 1010–1011]. The RBB–EG complex forms a dimer (RBB⋯EG⋯EG⋯RBB) through intermolecular O—H⋯O hydrogen bonds. In a subsequent re-examination of the structure at room temperature, we found the RBB molecule to be disordered with a methyl group of one ethyl substituent of a diethylamino group at one extremity of the xanthene unit disordered over two positions [occupancies: 0.735 (5)/0.265 (5)]. Furthermore, at the other end of the xanthene residue, the entire diethylamino substituent (i.e. the N atom and the associated C and H atoms) was also disordered over two sites with occupancies 0.653 (7)/0.347 (7). This leads to four kinds of RBB conformations, which, in turn, results in the formation of 16 discrete RBB⋯EG⋯EG⋯RBB dimers in the crystal.

The title compound, C 28 H 30 N 2 O 3 ÁC 9 H 10 O 5 , is a well known red leuco complex of 2-(6-diethylamino-3-diethyliminio-3Hxanthene-9-yl)benzoate (rhodamine B base abbreviated to RBB: leuco dye) with ethyl gallate (EG: developer). The structure of the complex at room temperature has recently been reported by Sekiguchi, Takayama, Gotanda & Sano [(2007) Chem. Lett. 36, 1010Lett. 36, -1011. The RBB-EG complex forms a dimer (RBBÁ Á ÁEGÁ Á ÁEGÁ Á ÁRBB) through intermolecular O-HÁ Á ÁO hydrogen bonds. In a subsequent reexamination of the structure at room temperature, we found the RBB molecule to be disordered with a methyl group of one ethyl substituent of a diethylamino group at one extremity of the xanthene unit disordered over two positions [occupancies: 0.735 (5)/0.265 (5)]. Furthermore, at the other end of the xanthene residue, the entire diethylamino substituent (i.e. the N atom and the associated C and H atoms) was also disordered over two sites with occupancies 0.653 (7)/ 0.347 (7). This leads to four kinds of RBB conformations, which, in turn, results in the formation of 16 discrete RBBÁ Á ÁEGÁ Á ÁEGÁ Á ÁRBB dimers in the crystal.

Related literature
For the previous determination of the structure of the 1:1 RBB/EG complex at room temperature, see: Sekiguchi et al. (2007) and for the structure of a second triclinic form of the same complex at 93 K, see: Mizuguchi (2008). For the related structure of n-propyl gallate, see: Iwata et al. (2005); .

Experimental
Crystal data
there was a residual electron density peak of about 1.35 e Å -3 . For this reason, a redetermination of the structure has been carried out at room temperature in the present investigation. This revealed that the RBB molecule is disordered as shown in Fig. 1b with the C28 methyl group of one ethyl substituent of the N1 diethylamino group at one extremity of the xanthene moiety as well as the entire diethylamino-substituent (i.e. N2 atom with the associated C and H atoms) at the other end of the xanthene unit disordered over two positions.
As shown in Fig. 1b, the RBB molecule is disordered at C28A/C28B together with their associated H atoms at one extremity of the xanthene moiety. The disordered structure is separated into the major (0.735 (5)) and minor (0.265 (5)) components which correspond to the RBB-A and RBB-B forms respectively, as shown in Fig. 1a. These are similar to those found at 93K, where the diethylamino-substituents lie either on the same side, or on opposite sides of the xanthene plane.
Similarly, the disorder at the entire N2 diethylamino-substituents at the other end of the xanthene plane also leads to the presence of the two conformations described above: N2A group (diethylamino-substituents on the same side; occupancy 0.653 (7)) and N2B group (on opposite sides; occupancy 0.347 (7)). Figs. 2-5 show plots of the four possible structures of (I). Of these, the previous report (Sekiguchi et al., 2007) only identified the conformation shown in Fig. 2.
The lactone ring is opened to form a zwitterionic structure and the benzene ring with the carboxylate is twisted to be nearly perpendicular to the xanthene plane with a dihedral angle of 98.9 (1)° between the O1/C1-C13 plane of the xanthene and the C14-C19 plane of the benzene ring. The xanthene moiety is nearly flat (mean deviation from the least-squares plane, 0.0300 Å).
There are intra and intermolecular O-H···O hydrogen bonds leading to the formation of the RBB/EG complexes as shown in Fig. 6. For example, two major RBB/EG complexes are further connected by intermolecular O-H···O hydrogen bonds between two EGs to form a dimer as shown in Fig. 7. The existence of the four possible RBB conformations leads to the formation of 16 kinds of RBB···EG···EG···RBB dimers in the crystal. The formation of the EG dimer is similar to that found in n-propyl gallate Hitachi et al., 2005).
supplementary materials sup-2 Experimental Rhodamine B base and 4-hydroxybenzophenone were purchased from Sigma-Aldrich Corp. and Wako Pure Chemical Industries, Ltd., respectively. Single crystals of (I) were grown by recrystallization from a toluene solution which includes an equimolar quantity of both chemicals. After 24 h, a number of red crystals were obtained in the form of blocks.