On the Basicity of Conjugated Nitrogen Heterocycles in Different Media

Abstract

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 Altogether, 58 previously unpublished basicity values in different media for 39 compounds are presented, including 30 experimentally determined pKa values in acetonitrile. We present the pKa 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 pKa values in MeCN and in water suggests that these two different solvents exert a similar effect on the basicity of the studied heterocycles.

On the Basicity of Organic Bases in Different Media

“…The newly obtained values are generally in good agreement with those published earlier , , . For 78 % of bases the differences are less than 0.03 p K a units and for 98 % of bases less than 0.1 p K a units.…”

“…The revised basicity scale (Table ) is composed of previously published data, , and unpublished results for 9‐chloroacridine and 2‐aminoacridine (experimental conditions as in ref …”

Crystallographic Evidence for a Sterically Induced Ferryl Tilt in a Non‐Heme Oxoiron(IV) Complex that Makes it a Better Oxidant

“…[15] The stronger binding of benzimidazole induces a trans effect that increases the equatorial pyridine Fe À Nb ond lengths.T his, along with the longer axial Fe-N bond length of 2.115 (5) , increases the average FeÀNbond length for complex 2.Steric effects in complex 3 are responsible for its longer FeÀNbond lengths and outweigh the electronic effects of quinolines. [15] The stronger binding of benzimidazole induces a trans effect that increases the equatorial pyridine Fe À Nb ond lengths.T his, along with the longer axial Fe-N bond length of 2.115 (5) , increases the average FeÀNbond length for complex 2.Steric effects in complex 3 are responsible for its longer FeÀNbond lengths and outweigh the electronic effects of quinolines.…”

Weak Intramolecular Interactions to Stabilize Supramolecular Fullerene‐Porphyrin Conjugates and to Control the Conformation of Multiporphyrinic Arrays

“…This is in particular the case when such ligands are interacting with metalloporphyrins as confirmed by X‐ray crystal structure analysis . With a p K A value of 1.17 in H 2 O, the 1,2,3‐triazole ring is however significantly less basic than pyridine (p K A = 5.23) . As a result, 1,2,3‐triazoles should be weaker ligands for metalloporphyrins in solution when compared to pyridines.…”