Dioxygen/transition-metal complexes

Chemistry of transition metal complexes supported by hydrotris(pyrazolyl) borates and chemistry of dioxygen complexes based on these ligands 99YGK619. 

The dioxygen ligand in mononuclear group VIII transition metal complexes. J. S. Valentine, Chem. Rev., 1973, 73, 235-245 (101). 

Dioxygen activation in transition metal complexes in the light of molecular orbital calculations. R. Boca, Coord. Chem. Rev., 1983, 50,1-72 (245). 

Vasudevan P, Santosh, Mann N, Tyagi S. 1990. Transition metal complexes of porphyrins and phthalocyanines as electiocatalysts for dioxygen reduction. Transition Metal Chemistry, 15, 81-90. 

Dioxygen Activation by Transition Metal Complexes. Atom Transfer and Free Radical Chemistry in Aqueous Media... 

Reviews on the activation of dioxygen by transition-metal complexes have appeared recently 9497 ). Details of the underlying reaction mechanisms could in some cases be resolved from kinetic studies employing rapid-scan and low-temperature kinetic techniques in order to detect possible reaction intermediates and to analyze complex reaction sequences. In many cases, however, detailed mechanistic insight was not available, and high-pressure experiments coupled to the construction of volume profiles were performed in efforts to fulfill this need. 

One of the most fundamental questions when dealing with the activation of dioxygen by transition metal complexes is whether the process is controlled kinetically by ligand substitution or by electron transfer. A model system that involved the binding of dioxygen to a macrocyclic hexamethylcyclam Co(II) complex to form the correspond-... 

Despite the above aspects of the interaction of molecular oxygen with metal fragments, we will limit our electrochemical discussion to the reactivity of transition metal complexes with molecular oxygen in the context of modelling the dioxygen transport. 

The reaction of dioxygen with transition metal complexes has stimulated the curiosity of scientists for decades. Seminal studies by Vaska in the 1960s revealed that dioxygen coordinates reversibly to the the Ir center in (Ph3P)2lr(CO)Cl, 14 (Eq. 15) [101-104], Following this report, numerous groups reported dioxygen adducts of other late transition metals [105],... 

DIOXYGEN ACTIVATION BY TRANSITION METAL COMPLEXES. ATOM TRANSFER AND FREE RADICAL CHEMISTRY IN AQUEOUS MEDIA... 

The present volume is a non-thematic issue and includes seven contributions. The first chapter byAndreja Bakac presents a detailed account of the activation of dioxygen by transition metal complexes and the important role of atom transfer and free radical chemistry in aqueous solution. The second contribution comes from Jose Olabe, an expert in the field of pentacyanoferrate complexes, in which he describes the redox reactivity of coordinated ligands in such complexes. The third chapter deals with the activation of carbon dioxide and carbonato complexes as models for carbonic anhydrase, and comes from Anadi Dash and collaborators. This is followed by a contribution from Sasha Ryabov on the transition metal chemistry of glucose oxidase, horseradish peroxidase and related enzymes. In chapter five Alexandra Masarwa and Dan Meyerstein present a detailed report on the properties of transition metal complexes containing metal-carbon bonds in aqueous solution. Ivana Ivanovic and Katarina Andjelkovic describe the importance of hepta-coordination in complexes of 3d transition metals in the subsequent contribution. The final chapter by Sally Brooker and co-workers is devoted to the application of lanthanide complexes as luminescent biolabels, an exciting new area of development. 

In Fig. 1 some of the most common metal-dioxygen binding modes found for mononuclear and dinuclear transition metal complexes are diagrammed. Note that the terms superoxo and peroxo ultimately... 

As research in the area of dioxygen transition-metal complexes progresses, the relative novelty of each structure may change. The structures that seem common now may be overtaken by newer structures. Thus, Fig. 1 should be used as an outline rather than as the definitive word on possible structures for metal-dioxygen complexes. 

Although a wide variety of nitrogen donor ligands exist in transition-metal complexes, relatively few general types have been employed in those complexes that have been found to activate dioxygen. Generally, the ligands are at least bidentate, and often are multidentate. 

Such important studies may be relevant to the activation of dioxygen by other transition-metal complexes and to the investigation of the mechanism of the evolution of dioxygen by photosynthetic man-ganese-oxo complexes. 

A wide range of transition metal complexes will carry dioxygen reversibly.1 In all cases, electron transfer takes place from the metal centre to the antibonding ir-orbitals of the bound dioxygen. In some cases the bound dioxygen appears to be present as superoxide and in others as peroxide [as shown by (02) values around 1100 and 800 cm 1 respectively]. 

Oxygen combines with almost all other elements, usually, however, only on heating. Dioxygen will sometimes react reversibly11 with certain transition metal complexes and the ligand behavior of 02 and its ions is discussed in Section 11-10. The potentials in aqueous solution are... 

Reduced transition metal complexes are traditionally implicated as the initiators for the autoxidation of fats, lipids, and foodstuffs. However, whether this involves direct activation of O2 or of reduced dioxygen (02 -, H00% and HOOH) is unclear. Reduced metal plus HOOH yields HO via Fenton chemistry and probably is the pathway for initiation of autoxidation in many systems. Although there has been an expectation that one or more of the intermediates from the autoxidation of reduced transition metals (equation 112) can act as the initiator for the autoxidation of organic substrates, direct experimental evidence has not been presented. 

---