10-Methyl-9-(2-nitrophenoxycarbonyl)acridinium trifluoromethanesulfonate

The crystal structure of the title compound, C21H15N2O4 +·CF3O3S−, is stabilized by C—H⋯O and C—H⋯F hydrogen bonds, by C—F⋯π, N—O⋯π and S—O⋯π interactions, and by O⋯O [2.70 (4) Å] and O⋯F [2.85 (1) or 2.92 (1) Å] contacts; π–π interactions are also present. In the packing of the molecules, acridine units are either parallel or inclined at an angle of 12.5 (1)°. The nitrophenoxycarbonyl unit is disordered over two position; the site occupancy factors are 0.89 and 0.11.


Comment
Phenyl 10-alkylacridinium-9-carboxylates are known to be chemiluminescent indicators or chemiluminogenic fragments of chemiluminescent labels, which have found numerous applications in assays of biologically and environmentally important entities (Becker et al., 1999;Adamczyk et al., 2004). The reaction of the above-mentioned cations with hydrogen peroxide in alkaline media produces light, and the determination of its intensity enables labeled entities or entities present in the medium to be assayed quantitatively, even at the attomole level (Roda et al., 2003). Investigations have revealed that oxidation of these cations is accompanied by the removal of the phenoxycarbonyl fragment and conversion of the rest of the molecule to electronically excited, light-emitting 10-alkyl-9-acridinones (Rak et al., 1999;Razavi & McCapra, 2000a,b;Zomer & Jacquemijns, 2001). It may thus be expected that the efficiency of chemiluminescence is affected by changes in the structure of the phenyl fragment. In order to find out whether this actually takes place, investigations were undertaken on phenyl 10-methylacridinium-9-carboxylates differently substituted in the phenyl fragment. Here, the crystal structure of the NO 2 -phenyl-substituted derivative is presented. The compound was synthesized and investigated since the strongly electron attracting NO 2 group present in the phenyl fragment may be expected to substantially influence its stability and chemiluminogenic ability.
Parameters characterizing the geometry of the acridine ring are typical of acridine-based derivatives (Sikorski et al., 2007).
The crystal structure is stabilized by a network of the aforementioned short-range interactions, as well as by long-range electrostatic interactions between ions.
Experimental 9-(2-Nitrophenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate was synthesized by treating 2-nitrophenyl acridine-9-carboxylate [obtained in the same way as described elsewhere (Sato, 1996;Sikorski et al., 2007)], dissolved in anhydrous dichloromethane, with a fivefold molar excess of methyl trifluoromethanesulfonate, dissolved in the same solvent, under an Ar atmosphere at room temperature for 4 h. The crude salt was dissolved in small amount of ethanol, filtered and precipitated with 25 v/v excess of diethyl ether (yield 63%). Yellow crystals suitable for X-ray investigations were grown from absolute ethanol solution.

Refinement
All H atoms were positioned geometrically and refined using a riding model, with C-H distances of 0.95 Å and with U iso (H) = 1.2U eq (C), or C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for the methyl group. The geometries of the disordered nitrophenoxycarbonyl fragment were refined anisotropically assuming C-C distances in the C18A-C23A benzene ring equal to 1.39 Å (Sridhar et al., 2006). Fig. 1. The molecular structure of the title compound showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. Cg1, Cg2, Cg3, Cg4 and Cg4 A denote the ring centroids.  10-Methyl-9-(2-nitrophenoxycarbonyl)acridinium trifluoromethanesulfonate