Iron(III) citrate

Iron(III) citrate is not a single, specific molecule. Instead, it is a family of various coordination complexes. These complexes form when ferric ions bind with citrate. Citrate is a conjugate base derived from citric acid. Most of these compounds appear in shades of orange or red-brown. Because they can form small groups or long chains, they are often called oligomers or polymers. Many different forms can exist at the same time in a state called equilibrium.

These chemical structures change depending on their environment. At a physiological pH, which is the level of acidity in living things, ferric citrate forms an insoluble red polymer. This means it becomes a solid that does not dissolve easily. However, under different conditions, it can form anionic complexes. These are molecules that carry a negative electrical charge. For example, in the presence of excess citrate anions, the iron forms specific negatively charged complexes. These include structures like [Fe(C6H5O7)2]5- and [Fe(C6H5O7)3]7-.

In nature, these compounds play a vital role in how organisms get nutrients. Plant roots and certain microorganisms release citrates into the soil. These citrates can solubilize iron compounds. Solubilization means turning a solid into something that can dissolve in liquid. For instance, ferric hydroxide reacts with citrates to create soluble complexes. This process provides a pathway for various organisms to absorb ferric ions from the earth.

Light also triggers a significant chemical change called photoreduction. This happens when the iron(III) ions are exposed to blue or ultraviolet light. During this process, the iron(III) is reduced to a ferrous ion, which is iron(II). At the same time, the carboxyl group next to the hydroxyl group undergoes oxidation. This chemical reaction yields carbon dioxide and acetonedicarboxylate.

Plants use this photoreduction to move nutrients through their bodies. Iron is carried from the roots up to the leaves in the sap as dissolved ferric citrate. Once it reaches the leaves, light triggers the change into iron(II). This specific form of iron can then be transported into the plant cells. This cycle is an essential part of plant metabolism.

Scientists have worked to understand these complex structures for decades. In 2001, researchers Xiang Hao, Yongge Wei, and Shiwei Zhang studied a specific type. They focused on a binuclear iron(III) citrate complex. Their research investigated the crystal structure and the magnetic properties of this complex. Earlier, in 2000, Wu Feng and Deng Nansheng published a review on the photochemistry of iron species. Their work helped explain how light causes the degradation of organic compounds.

Medicine also uses these complexes to treat human health conditions. Ferric citrate is used to regulate blood iron levels in certain patients. Specifically, it helps adults with chronic kidney disease who are undergoing dialysis. The complex works by forming an insoluble compound with phosphate found in the diet. This prevents the body from taking up too much phosphate through the digestive system.

A recent medical advancement involves a product called Xoanacyl. This medicine uses a ferric citrate coordination complex as its active substance. It is intended to treat iron deficiency and hyperphosphatemia together. In March 2025, the Committee for Medicinal Products for Human Use recommended its authorization. The applicant for this medicine is Averoa SAS. Xoanacyl was officially authorized for medical use in the European Union in June 2025.