Predominant effect of connecting atom and position of substituents on azomethine nitrogens’ basicity in phthalocyanines
Abstract
The basicity of azomethine nitrogens was studied on a series of phthalocyanines (Pcs) that differed in a position of substituent, i.e. peripherally (β-series) and non-peripherally substituted Pcs (α-series), and in a type of substituent (alkylsulfanyl, alkyloxy or alkyl). Appropriate 3,6- or 4,5-disubstituted phthalonitriles were prepared either by nucleophilic substitution or by Negishi coupling. Target zinc Pcs were synthesized by Linstead method. The basicity was studied by the mean of absorption and 1H NMR spectroscopies in chloroform upon titration with trifluoroacetic acid. Equilibrium constants (log K) indicated significant difference within the series. Basicity decreased as follows: α-alkyloxy ≫ α-alkylsulfanyl > β-alkyloxy > β-alkyl > β-alkylsulfanyl ∼ α-alkyl with log K higher than 7 down to 2.6 M−1. Increased basicity of α-alkyloxy and α-alkylsulfanyl Pcs is caused by the stabilization of trapped hydrogen at azomethine nitrogen via hydrogen bonding and van der Waals interactions, respectively. The basicity of β-series clearly correlated with the electronic effects of substituents. 1H NMR studies confirmed the possibility of the weak bonding interactions in α-alkyloxy and α-alkylsulfanyl Pcs, however, the position of the 1H NMR signal of azomethine-NH proton was even more influenced by the electronic effects of present substituents than by the weak interactions.
Dedicated to Professor Tomás Torres on the occasion of his 65th birthday
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