On the Basicity of Conjugated Nitrogen Heterocycles in Different Media

In this work we explored the relationship between the structure and solvent effects on the basicity of a large selection of conjugated N-heterocyclic nitrogen bases in different media: the polar aprotic solvent acetonitrile, the polar protic solvent water and the gas phase. Altogether, 58 previously unpublished basicity values in different media for 39 compounds are presented, including 30 experimentally determined pK(a) values in acetonitrile. We present the pK(a) and gas-phase basicity values for quino[7,8-h]quinoline, which is one of the most basic conjugated nitrogen heterocyclic compounds without basicity-enhancing substituents. The trends in basicity are rationalized by comparing the basicity data of related compounds in different solvents, as well as by using isodesmic reactions. The gas-phase basicity is predominantly determined by the ability of a molecule to disperse the excess positive charge over a large number of atoms. In solution the situation is less clear and smaller systems with localized charge often lead to higher basicities because of solvent effects. In particular, it was found that the fusion of an additional benzene ring does not always lead to an increase in basicity in solution: its effect can be either basicity-increasing or -decreasing, depending on the ring size, number and position of nitrogen atoms and medium. A correlation between the measured pK(a) values in MeCN and in water suggests that these two different solvents exert a similar effect on the basicity of the studied heterocycles.