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Intramolecular Hydrogen-Bond Interactions Tune Reactivity in Biomimetic Bis(μ-hydroxo)dicobalt Complexes
Alyssa A DeLucia1, Kimberly A Kelly1, Kevin A Herrera1
1Department of Chemistry, University of Illinois, Urbana-Champaign, 600 S. Mathews Ave. Urbana, Illinois 61801, United States.
Inorganic Chemistry
|October 4, 2021
Summary
This study details cobalt complexes mimicking enzymes, showing how hydrogen bonds tune reactivity. Modulating these bonds controls the formation of metal-oxo intermediates, crucial for biological processes.
Area of Science:
- Bioinorganic Chemistry
- Organometallic Chemistry
- Enzyme Mimicry
Background:
- Metalloenzymes utilize active site hydrogen-bond (H-bond) networks to regulate high-valent transition metal-oxo intermediates.
- Nonheme diiron enzymes are a key class of metalloenzymes where such intermediates are critical.
Purpose of the Study:
- To synthesize and characterize dinuclear cobalt complexes as structural models for nonheme diiron enzymes.
- To investigate the influence of intramolecular H-bond interactions on the electronic properties and reactivity of cobalt-oxo intermediates.
- To establish a relationship between ligand substitution, H-bond strength, and reaction kinetics.
Main Methods:
- Kinetically controlled synthesis of dinuclear cobalt complexes with a Co2(μ-OH)2 core.
- Structural analysis using X-ray diffraction (XRD).
- Solution state characterization via 1H NMR spectroscopy and linear free energy relationship (LFER) studies.
- Rapid-mixing experiments to probe reaction kinetics.
Main Results:
- A series of dinuclear cobalt complexes [Co2(μ-OH)2(μ-OAc)(κ1-OAc)2(pyR)4][PF6] were synthesized and characterized.
- NMR data revealed a LFER indicating that electron-withdrawing pyridine substituents strengthen the intramolecular H-bond, making the μ-OH bridges more oxo-like.
- Deprotonation led to cubane complexes [Co4(μ-O)4(μ3-OAc)4(pyR)4], with reaction rates showing a 6000-fold acceleration upon increasing electron-withdrawing character of the pyridine substituent.
- The study demonstrates tunable reactivity by modulating the μ-OH pKa through H-bond interactions.
Conclusions:
- Intramolecular H-bond interactions in cobalt complexes can effectively tune the pKa of bridging hydroxyl groups, influencing the stability and reactivity of metal-oxo intermediates.
- These findings provide insights into how nature might control metalloenzyme reactivity through similar dynamic H-bond networks.
- The synthesized cobalt complexes serve as valuable models for understanding the mechanisms of nonheme diiron enzymes.
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