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 (beta-series) and non-peripherally substituted Pcs (alpha-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 H-1 NMR spectroscopies in chloroform upon titration with trifluoroacetic acid.
Equilibrium constants (log K) indicated significant difference within the series. Basicity decreased as follows: alpha-alkyloxy >> alpha-alkylsulfanyl > beta-alkyloxy > beta-alkyl > beta-alkylsulfanyl similar to alpha-alkyl with log K higher than 7 down to 2.6 M-1.
Increased basicity of alpha-alkyloxy and alpha-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 beta-series clearly correlated with the electronic effects of substituents.
H-1 NMR studies confirmed the possibility of the weak bonding interactions in alpha-alkyloxy and alpha-alkylsulfanyl Pcs, however, the position of the H-1 NMR signal of azomethine-NH proton was even more influenced by the electronic effects of present substituents than by the weak interactions.