Nucleophilicity of heteroaromatic N-oxides in coordination with Zn(II) tetraphenylporphyrinate and in the substitution reactions

Crystal structure of molecular complexes of zinc(II) tetraphenylporphyrin with pyridine and quinoline N-oxides

“…Unlike observations found for pyridine and its derivatives, the complex formation of Zn-TPP with heteroaromatic N-oxides and aniline with substituents in the 2, 3, and 4 positions and at the nitrogen atom (except for 4-halo derivatives) is accompanied by a low heat release and can be considered, to a first approximation, as isenthalpic processes [3,4].…”

“…At the coordination of Zn-TPP with heteroaromatic N-oxides in chloroform [4], the ΔH 0 values for pyridine N-oxides with substituents in the 3 and 4 positions and the styryl substituent in the 2 position, as well as for 4-substituted quinolone N-oxides are close to each other (ΔH 0 av -13.78±0.15 kJ/mol, n 22, series А), whereas the ΔH 0 values for 2-methylpyridine, 2methylquinoline, and 2-styrylquinoline N-oxides, too, vary in a narrow range but are much higher (ΔH 0 av -12.04±0.18 kJ/mol, n 9, series B). Thus, in each of the two series of heteroaromatic N-oxides (for aniline with substituents in the 3 and 4 positions, ΔH 0 av -14.76±0.14 kJ/mol [3]), the coordination with Zn-TPP is a quasi-isenthalpic process.…”

“…As shown in [4], the ΔS 0 of Zn-TPP complex formation with 3-and 4-substituted pyridine and quinoline N-oxides in chloroform is proportional to the basicity of the ligands [Eqs. (1) and (2), respectively].…”

“…(4)]. This can be ascribed [7] to the gradual increase in the p-contribution to the lone pair orbital of the nitrogen atom due to electron density displacement to the benzene ring and, as a result, sp 3 →sp 2 rehybridization of the nitrogen atom.…”