Hexakis(urea-O)iron Complex Salts as a Versatile Material Family: Overview of Their Properties and Applications

Due to their Fe- and N-containing reactive urea ligand content, the hexakis(urea-O)iron(II) and hexakis(urea-O)iron(III) complexes were found to be versatile materials in various application fields of industry and environmental protection. In our present work, we have comprehensively reviewed the synthesis, structural and spectroscopic details, and thermal properties of hexakis(urea-O)iron(II) and hexakis(urea-O)iron(III) salts with different anions (NO₃^-, Cl^-, Br^- I^-, I₃^-, ClO₄^-, MnO₄^-, SO₄^2-, Cr₂O₇^2-, and S₂O₈^2-). We compared and evaluated the structural, spectroscopic (IR, Raman, UV-vis, Mössbauer, EPR, and X-ray), and thermogravimetric data. Based on the thermal behavior of these complexes, we evaluated the solid-phase quasi-intramolecular redox reactions of anions and urea ligands in these complexes and summarized the available information on the properties of the resulting simple and mixed iron-containing oxides. Furthermore, we give a complete overview of the application of these complexes as catalysts, reagents, absorbers, or agricultural raw materials.