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Reversible O2 binding by small-molecule analogues

المؤلف:  Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong

المصدر:  Shriver and Atkins Inorganic Chemistry ,5th E

الجزء والصفحة:  ص740-741

2025-10-23

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Reversible O₂ binding by small-molecule analogues

Key point: Proteins binding O₂ reversibly do so by preventing its reduction and eventual O-O bond cleavage. This protection is difficult to achieve with small molecules. Certain elaborate macrocyclic Fe (II) complexes exhibit reversible O₂ binding by providing steric hindrance to attack on the coordinated O₂. Much effort has been spent on synthesizing simple complexes that coordinate O₂ reversibly and could be used as blood substitutes in special circumstances, such as emergency surgery. The problem is that although O₂ reacts with d-block metal ions to form complexes in which the O−O bond is retained (as in superoxo and peroxo species), these products tend to undergo irreversible decomposition involving rapid O−O bond cleavage and formation of water or oxides. Overcoming this problem requires complexes designed to protect the coordinated O−O ligand, preventing it from reacting further. Sterically hindered Fe(II) complexes such as the ‘basket’ porphyrin (22) achieve this protection by preventing a second Fe(II) complex from attacking the distal O atom of the superoxo species to form a bridged peroxo intermediate. As we shall see in Section 27.10, peroxo complexes of Fe (III) tend to undergo rapid O−O bond proteolysis, resulting in formation of H₂O and Fe(IV) O.

Simple Cu complexes that can coordinate O₂ reversibly are also rare, but studies have revealed interesting chemistry that is particularly relevant for developing catalysts for oxygenation reactions. Analogues of the dinuclear Cu(I) centre of haemocyanin react with O₂ but have a strong tendency to undergo further reactions that involve cleavage of the O-O bond, an example being the rapid equilibrium between µ-η2: η2 peroxo-dicopper (II) and bis(µ-oxo) copper (III) complexes shown in Fig. 27.20.