Cerium bis(tetradiazepinoporphyrazinate): synthesis and peculiarities of spectral and electrochemical behavior

Abstract

Bis{tetrakis(5,7-bis(4-tert-butylphenyl)-6H-1,4-diazepino)[2,3-b,g,l,q]porphyrazinato} cerium has been prepared for the first time via two alternative synthetic pathways: template macrocyclization of 5,7-bis(4-tert-butylphenyl)-6H-1,4-diazepine-2,3-dicarbonitrile and direct complexation employing its cyclic tetramer, a free-base porphyrazine ligand. A combination of HRMS, UV-Vis, FT-IR and NMR spectroscopic methods allowed the identification of the tetravalent oxidation state of cerium ion in the double-decker obtained. Unlike Pc₂Ce^IV (Pc²⁻ = phthalocyaninate anion), the compound revealed a complicated UV-Vis spectrum that demonstrated a strong solvent dependence. A comparative electrochemical study of the cerium complex and the corresponding free-base ligand was carried out. Ligand-centered processes are diffusion controlled, whereas Ce^4+/3+ transformation is characterized by slow electron transfer kinetics. This is as a result of poor accessibility of the metal center to an electrode surface due to the presence of bulky ligands, as well as a considerable structural rearrangement of the sandwich molecule.