Synthesis, structure, and properties of helical bis-Zn(ii) complexes of hexapyrrolic ligands

Abstract

Three new examples of covalently linked bis(3-pyrrolyl BODIPY)s were prepared as key precursors for the synthesis of rare hexapyrrolic ligands by treating readily available 3-pyrrolyl BODIPY with different ketones under acid-catalyzed conditions. The new bis(3-pyrrolyl BODIPY) complexes were thoroughly characterized using HR-MS, NMR spectroscopy, absorption, fluorescence, electrochemistry, and theoretical methods. The bis(3-pyrrolyl BODIPY) complexes were treated with AlCl₃ in CH₃OH at reflux for 2 h to demask BF₂ units to afford novel hexapyrrolic ligands. The hexapyrrolic ligands without isolating were treated with Zn(CH₃COO)₂ in CH₃CN at room temperature for 2 h followed by repeated recrystallization to afford crystalline bis-Zn(II) complexes. The X-ray crystallographic structure obtained for one of the bis-Zn(II) complexes revealed that it was a double-stranded bimetallic helicate containing two non-superimposable M and P enantiomers in equal amounts, which were nonseparable. Each Zn(II) ion in the bis-Zn helicate complexes was coordinated to four nitrogen atoms of two dipyrrin units from two hexapyrrolic ligands in a distorted tetrahedral geometry. The distance between two Zn(II) ions (Zn1–Zn2) in bis-Zn(II) helicate was 8.211 Å, which was longer than the distance between two Cu(II) ions (Cu1–Cu2) in our previously reported bis-Cu(II) helicate complex (7.330 Å). NMR studies indicated that the bis-Zn(II) complexes were highly symmetric. The bis-Zn(II) helicates showed broad absorption bands in the region of 400–700 nm and were nonfluorescent. DFT/TD-DFT studies were in agreement with the experimental observations.