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A Four-Coordinate End-On Superoxocopper(II) Complex: Probing the Link between Coordination Number and Reactivity
Suman Debnath1, Shoba Laxmi1, Olivia McCubbin Stepanic2
1Division of Chemistry and Biological Chemistry, School of Chemistry, Chemical Engineering and Biotechnology, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore.
Journal of the American Chemical Society
|August 28, 2024
Summary
A new four-coordinate superoxocopper(II) complex exhibits enhanced reactivity compared to its five-coordinate counterpart. This difference, attributed to higher electrophilicity, is crucial for understanding copper-containing enzymes and their oxidation mechanisms.
Area of Science:
- Inorganic Chemistry
- Bioinorganic Chemistry
- Coordination Chemistry
Background:
- Five-coordinate end-on superoxocopper(II) complexes primarily react via hydrogen atom transfer (HAT).
- Most reported four-coordinate superoxocopper(II) complexes display nucleophilic reactivity, contrasting with their five-coordinate counterparts.
- Understanding the factors governing the reactivity of superoxocopper(II) complexes is essential for elucidating biological oxidation processes.
Purpose of the Study:
- To investigate the origin of differing reactivity between four- and five-coordinate end-on superoxocopper(II) complexes.
- To synthesize and characterize a novel four-coordinate superoxocopper(II) complex with a sterically encumbered ligand.
- To compare the substrate reactivity of the new four-coordinate complex with a known five-coordinate analogue.
Main Methods:
- Synthesis of a four-coordinate end-on superoxocopper(II) complex, [CuII(η1-O2•-)(dpb2-MeBPA)]+ (1).
- Kinetic isotope effect (KIE) measurements to probe reaction mechanisms.
- Correlation of second-order rate constants (k2) with oxidation potentials (Eox) for various substrates.
Main Results:
- Complex 1 reacts with phenols via a hydrogen atom transfer (HAT) mechanism, similar to the five-coordinate complex [CuII(η1-O2•-)(dpb3-TMPA)]+ (2).
- Complex 1 exhibits significantly enhanced HAT reactivity, with rates >100 times faster for certain phenols compared to complex 2.
- Complex 1 demonstrates superior ability to oxidize C-H bonds in substrates like N-methyl-9,10-dihydroacridine, reacting ~200 times faster than complex 2.
Conclusions:
- The enhanced substrate oxidation facility of complex 1 is attributed to its higher inherent electrophilicity, a direct result of its lower coordination number.
- These findings highlight the critical role of coordination number in dictating the reactivity of superoxocopper(II) intermediates.
- The results provide valuable insights into the mechanisms of copper-containing enzymes, where four-coordinate superoxocopper(II) intermediates are often implicated as active oxidants.
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