9-[(2,6-Dimethoxyphenoxy)carbonyl]-10-methylacridinium trifluoromethanesulfonate

In the crystal structure of the title compound, C23H20NO4 +·CF3SO3 −, the cations are linked through C—H⋯O, C—H⋯π and π–π interactions [centroid-centroid distances = 3.641 (2) and 3.885 (2) Å]. The cation and the anion are held together by C—H⋯O and S—O⋯π interactions. The acridine ring system and the benzene ring in the cation are oriented at a dihedral angle of 8.7 (1)°. The carboxy group is twisted at an angle of 83.2 (1)° relative to the acridine skeleton.


Comment
Phenyl 10-alkylacridinium-9-carboxylates have long been known as chemiluminescent indicators or the chemiluminogenic fragments of chemiluminescent labels (Zomer & Jacquemijns, 2001). These compounds are widely applied in assays of biologically and environmentally important entities such as antigens, antibodies, enzymes or DNA fragments (Becker et al., 1999;Adamczyk et al., 2004). The reaction of the cations of these salts with hydrogen peroxide in alkaline media produces light. Our own investigations (Rak et al., 1999) and those of others (Zomer & Jacquemijns, 2001) have revealed that oxidation of acridinium chemiluminogens is accompanied by the removal of the phenoxycarbonyl fragment and the convertion of the rest of molecules to electronically excited, light-emitting 10-alkyl-9-acridinones. It has been found that the efficiency of chemiluminescence is affected by the constitution of the phenyl fragment (Zomer & Jacquemijns, 2001).
Continuing our investigations onto the above mentioned effect, we synthesized the compound containing two methoxy groups in the phenyl fragment. Here, we present its structure. Methoxy groups, which possess electron-attractive features, may influence the stability and chemiluminogenic ability of the compound investigated.
In the cation of the title compound ( Fig. 1), the bond lengths and angles characterizing the geometry of the acridinium moiety are typical of acridine-based derivatives (Sikorski et al., 2008). With respective average deviations from planarity of 0.037 (3) Å and 0.010 (3) Å, the acridine and benzene ring systems in the cation are oriented at 8.7 (1)°. The carboxy group is twisted at an angle of 83.2 (1)° relative to the acridine skeleton. The mean planes of the adjacent acridine moieties are either parallel or inclined at an angle of 10.9 (1)° in the lattice.
Experimental 2,6-Dimethoxyphenylacridine-9-carboxylate was prepared by heating anhydrous acridine-9-carboxylic acid with thionyl chloride, followed by esterification of the resulting acid chloride with an equimolar quantity of 2,6-dimethoxyphenol (Sato, 1996). The reaction was carried out in anhydrous dichloromethane in the presence of N,N-diethylethanamine (1.5 molar excess) and a catalytic amount of N,N-dimethyl-4-pyridinamine (room temperature, 15 -25 h). The crude product was purified chromatographically (SiO 2 , cyclohexane/ethyl acetate, 3/2 v/v). The 2,6-dimethoxyphenylacridine-9-carboxylate thus obtained was subsequently dissolved in anhydrous dichloromethane and treated with a fivefold molar excess of methyl triluoromethanesulfonate dissolved in the same solvent (under an Ar atmosphere at room temperature for 4 h). The crude salt was dissolved in a small amount of ethanol, filtered and precipitated with a 25 v/v excess of diethyl ether (yield 42%).
Yellow crystals suitable for X-ray investigations were grown from absolute ethanol solution (m.p. 243-245 K).