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.

Graphical abstractA novel zinc phthalocyanine and its precursor with bis(2-hydroxyethyl)aminoethoxy groups were synthesized. Treatment of these compounds with phenylboronic acid afforded phenylboronic azaesters. The boron atom in the phthalonitrile derivative is tetra-coordinated 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 due to steric hindrance of the planar macrocycles.Figure optionsDownload full-size imageDownload as PowerPoint slide