Different phenylboronic acid azaester formation modes in a substituted zinc phthalocyanine and its precursor

With the aim of preparing a novel Zn phthalocyanine bearing four phenylboronic azaester substituents at peripheral positions, 4-[2-(bis(2-hydroxyethyl)amino)ethoxy]phthalonitrile (2) was synthesized by the aromatic nucleophilic substitution reaction of 4-nitrophthalonitrile with triethanolamine. 2,9(10),16(17),23(24)-Tetrakis-[2-(bis(hydroxyethyl)amino) ethoxy]-phthalocyaninatozinc (II) (4) was prepared from the cyclotetramerization of dinitrile 2 in the presence of anhydrous zinc(II) acetate by microwave irradiation. Alternatively, treatment of 2 with phenylboronic acid yielded the phenylboronic azaester (3), namely 4-(2-(2-phenyl-1,3,6,2-dioxazaborocan-6-yl) ethoxy)phthalonitrile. Treatment of phthalocyanine 4 with phenylboronic acid in a mixture of DMSO/toluene (20/1, v/v) afforded the desired phthalocyanine 5. The novel compounds were characterized by elemental analyses, IR, UV-Vis, ¹H, ¹³C and ¹¹B NMR spectra. The ¹¹B NMR data indicated that the boron atom of phthalonitrile 3 is in the tetracoordinated state with the formation of a coordinative NB bond (closed form), while in the case of phthalocyanine 5, the open conformer possessing a tricoordinated B atom is favoured.