Molecular structure and protonation trends in 6-methoxy- and 8-methoxy-2,4,5-tris(dimethylamino)quinolines

•Strong basicity interplay between aza- and peri-NMe2 groups in quinolines was studied.•6- and 8-Methoxy-2,4,5-tris(dimethylamino)quinolines display different protonation mode.•6-Methoxy-2,4,5-tris(dimethylamino)quinoline is the most basic of quinolines ever known.

Molecular structure and protonation trends of 6-methoxy- 7 and 8-methoxy- 8 derivatives of 2,4,5-tris(dimethylamino)quinoline were studied using X-ray measurements, NMR spectra and theoretical calculations. It has been found that while 8 in the solid state forms protic salts of a quinolinium type, its isomer 7 behaves as a typical proton sponge giving the anilinium cation with a proton chelated by peri-NMe2 groups. In solution, both compounds are simultaneously monoprotonated at two possible centers but again a tendency of 7 to form anilinium cation is much higher if compared to 8. It has been also shown that basicity of 7 is the largest among all known derivatives of 4,5-bis(dimethylamino)quinoline and even slightly exceeds that of 1,8-bis(dimethylamino)naphthalene (parent proton sponge). This was attributed to the “buttressing effect” exhibited by the 6-MeO group. Stable double salts of 7 with picrate and perchlorate anions unprecedented for azines with conjugated aza and NMe2 groups were obtained and thoroughly studied.

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