Synthesis and applications of high-spin iron tetra-azamacrocyclic complexes

Abstract

Herein we describe the synthesis, characterization, and role several iron complexes in the coupling of pyrrole and phenylboronic acid to form 2-phenylpyrrole. The oxidation state and spin state of the iron complexes were characterized using X-ray crystallography, UV-vis absorbance spectroscopy, cyclic voltammetry, and in some cases electron paramagnetic reasonance spectroscopy. Furthermore, the results indicate that the iron(III) complexes are essential for catalytic and regioselective production of the 2-phenylpyrrole product. The complexes were compared to evaluate the effect of five properties on catalyst reaction yields: the coordination requirements of the catalyst, half-potential, topological constraint/rigidity, N-atom modification(s), increasing oxidative stability of the complex, and geometric parameters.^The need for two labile cis-coordination sites was confirmed based on a 42% decrease in catalytic reaction yield observed when complexes containing penta-dentate ligands were used in place of complexes with tetra-dentate ligands. A strong correlation between iron(III/II) redox potential and catalytic reaction yields was also observed, with [Fe2+L6(Cl)2] providing the highest yield (81%, -405 mV). A Lorentzian fitting of redox potential versus yields predicts that these catalysts can undergo more fine tuning to further increase yields. Interestingly, the remaining properties explored did not show a direct, strong relationship to catalytic reaction yields. Furthermore, the role of the sacrificial oxidant, the lack of radical participation, and the formation of æ-oxodiiron species was established leading to a more detailed mechanistic cycle. Finally, a library of five pyclen-based ligands have been developed for use as iron and europium chelators.^A preliminary investigation into iron PCTA complexes indicate that the iron center can bind to all seven of the ligand donors, however, the iron complex does not afford a CEST signal. Establishing that heptadentate ligands containing carboxylic acid pendent arms are not ideal for the developments of iron PARACEST agents.