Transition metal dioxygen complex
Dioxygen complexes are
Mononuclear complexes of O2
O2 binds to a single metal center either "end-on" (η1-) or "side-on" (η2-). The bonding and structures of these compounds are usually evaluated by single-crystal X-ray crystallography, focusing both on the overall geometry as well as the O–O distances, which reveals the bond order of the O2 ligand.
Complexes of η1-O2 ligands
O2
Complexes of η2-O2 ligands
η2-bonding is the most common motif seen in coordination chemistry of dioxygen. Such complexes can be generated by treating low-valent metal complexes with oxygen. For example, Vaska's complex reversibly binds O2 (Ph = C6H5):
- IrCl(CO)(PPh3)2 + O2 ⇌ IrCl(CO)(PPh3)2O2
The conversion is described as a 2 e− redox process: Ir(I) converts to Ir(III) as dioxygen converts to peroxide. Since O2 has a triplet ground state and Vaska's complex is a singlet, the reaction is slower than when singlet oxygen is used.[7] The magnetic properties of some η2-O2 complexes show that the ligand, in fact, is superoxide, not peroxide.[8]
Most complexes of η2-O2 are generated using
Binuclear complexes of O2
These binding modes include μ2-η2,η2-, μ2-η1,η1-, and μ2-η1,η2-. Depending on the degree of electron-transfer from the dimetal unit, these O2 ligands can again be described as peroxo or superoxo. Hemocyanin is an O2-carrier that utilizes a bridging O2 binding motif. It features a pair of copper centers.[10]
.
Salcomine, the cobalt(II) complex of salen ligand is the first synthetic O2 carrier.[12] Solvated derivatives of the solid complex bind 0.5 equivalent of O2:
- 2 Co(salen) + O2 → [Co(salen)]2O2
Reversible electron transfer reactions are observed in some dinuclear O2 complexes.[13]
Relationship to other oxygenic ligands and applications
Dioxygen complexes are the precursors to other families of oxygenic ligands. Metal oxo compounds arise from the cleavage of the O–O bond after complexation. Hydroperoxo complexes are generated in the course of the reduction of dioxygen by metals. The reduction of O2 by metal catalysts is a key half-reaction in fuel cells.
Metal-catalyzed oxidations with O2 proceed via the intermediacy of dioxygen complexes, although the actual oxidants are often oxo derivatives. The reversible binding of O2 to metal complexes has been used as a means to purify oxygen from air, but cryogenic distillation of liquid air remains the dominant technology.
References
- PMID 25707468.
- ISBN 0-12-352651-5.
- ISBN 0-935702-73-3.
- ISBN 978-0-08-043748-4.
- .
- ISBN 0-935702-73-3.
- .
- .
- ISBN 0-7506-3365-4.
- PMID 28103018.
- .
- .
- .