Redox-Active Ligands: Concepts and Catalysis

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Redox-Active Ligands: Concepts and Catalysis
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Contents
Preface
Part I. Introduction and Concepts
	1. Anatomy of a Redox-Active Ligand
		1.1 Introduction
		1.2 Biological Inspiration: From the Enzyme to the Flask, a Continued Journey
			1.2.1 Radicals in Biological Systems
			1.2.2 Redox Cofactors: Electrons and Protons in Metalloenzymatic Systems
		1.3 Chemical History: Puzzle‐Solving for Coordination Chemists
			1.3.1 The Curious Case of Metal Complexes with Dithiolene Ligands
			1.3.2 What are the Basic/Minimal Features?
			1.3.3 Classification According to Modes of Action
		1.4 Combining Spectroscopy and Theory: How to Spot a Redox‐Active Ligand?
		1.5 Non‐innocent, Cooperative, Electro‐Active, or Redox‐Active?
			1.5.1 Defining Terms
			1.5.2 Related Notions
		1.6 Unusual Ligands and Unusual Reactivities with a Redox‐Active Ligand
			1.6.1 Reactivity at Ligand
			1.6.2 Open‐Shell Reactivity: Radical Formation
			1.6.3 Two‐Electron Chemistry: CC Bond Formation
		1.7 Perspectives and Concluding Remarks
		References
	2. Mechanistic Studies of Catalytic Nitrene‐Transfer Reactions Involving Redox‐Active Ligands and Substrates
		2.1 Introduction
		2.2 Characterization of Radical‐type Intermediates
			2.2.1 Single‐crystal X‐ray Diffraction
			2.2.2 X‐ray Absorption Spectroscopy
			2.2.3 Electron Paramagnetic Resonance
			2.2.4 Nuclear Magnetic Resonance Spectroscopy
			2.2.5 Effective Magnetic Moment and Spin State
			2.2.6 UV‐vis and IR Absorption Spectroscopy and (Spectro‐)Electrochemistry
			2.2.7 Computational Studies
			2.2.8 Concluding Remarks
		2.3 Mechanistic Studies
			2.3.1 Kinetic Analysis
			2.3.2 Kinetic Isotope Effect
			2.3.3 Hammett Analysis
			2.3.4 Trapping and Poisoning
			2.3.5 Computational Methods
			2.3.6 Concluding Remarks
		2.4 Case Studies for Nitrene Transfer Aided by Redox‐Active Ligands and Substrates
			2.4.1 Electronic Structures of Nitrene Complexes
			2.4.2 Metal‐to‐Substrate Single‐electron Transfer
			2.4.3 Ligand‐to‐Substrate Single‐electron Transfer
			2.4.4 Spin‐flip‐assisted Reactions
			2.4.5 Electron Transfer Coupled to Spin‐flips
				2.4.5.1 Nitrene Radical Formation and Transfer with a [Cu(NOisq)2] Complex
				2.4.5.2 Nitrene Radical Formation and Transfer with Cobalt‐TAML Complexes
			2.4.6 Concluding Remarks and Outlook
		References
	3. Redox‐Active Ligands From a Computational Perspective
		3.1 Introduction
		3.2 Electronic Structure Determination Through DFT and Spectroscopy
		3.3 Redox‐Active Ligands as Electron Reservoirs
			3.3.1 Energy Conversion
				3.3.1.1 Noble Metal Reactivity
				3.3.1.2 Group Transfer and Radical Reactivity
			3.3.2 Photoredox Catalysis – Solar to Chemical Energy Conversion
			3.3.3 Energy Storage: Redox‐flow Batteries
		3.4 In Silico Description and Engineering of Redox‐Active Ligands
			3.4.1 Computing Reduction Potentials
			3.4.2 Rationalizing the Redox‐Active Behavior of Quinoid and Bipyridine Ligands
			3.4.3 Computational Techniques for Characterizing Ligand Redox Activity
			3.4.4 Ligand Redox Activity in the Excited State – Photoredox Catalysis
		3.5 Conclusions
		Acknowledgments
		References
Part II. Applications
	4. Complexes of Stable N-aryl Radicals and Their Catalytic Applications
		4.1 Introduction and General Considerations on Exocyclic N-aryl Radicals
		4.2 Complexes Featuring Anilinyl Radicals
			4.2.1 Simple Anilines
			4.2.2 TACN‐Anilines
			4.2.3 Tripods Anilines
			4.2.4 Anilinosalens
			4.2.5 Conjugated Anilines
			4.2.6 Dipyrrin‐Anilines
		4.3 Bidentate o‐diaminobenzenes and Their Radicals
			4.3.1 Homoleptic Complexes
			4.3.2 Heteroleptic Complexes
			4.3.3 9,10‐Phenanthrenediimine
		4.4 Pincer Ligands and Their Radicals
		4.5 Branched Tetradentate o‐diaminobenzene and Associated Radicals
			4.5.1 Pivotal/Tripodal N4 Ligands
			4.5.2 “Planar” N4 Systems
			4.5.3 Macrocyclic Ligands
		4.6 Polydentate Ligands Featuring One Bidentate Diiminosemiquinone Radical
			4.6.1 Salphen
			4.6.2 “Diamido” Platform
			4.6.3 Diaminobenzene Platform
		4.7 Representative Catalytic Applications
			4.7.1 Alcohols Oxidation
			4.7.2 Small Molecules Activation: O2/H2O2‐ and H2‐ Activation, H2 Production
			4.7.3 Intra‐ and Intermolecular Nitrene Transfers: C–H Bond Amination, Aziridination
			4.7.4 Miscellaneous Transformations
		4.8 Conclusion
		References
	5. Redox‐Active Ligands in Coordination Chemistry and Organic Synthesis
		5.1 Introduction
		5.2 Controlled Formation of Conjugated Complexes with Redox‐Active Polyanilines or 1,4‐Benzoquinonediimines
		5.3 Catalytic Application of Hybrid Systems Consisting of Redox‐Active Polyanilines and Transition Metals
		5.4 Conclusion
		References
	6. Metal Complexes Bearing Redox‐Active Supporting Ligands that Promote Chemical Transformations Involving Protons and Electrons
		6.1 Introduction
		6.2 Dioxygen Reduction to Water
			6.2.1 Cytochrome c Oxidase
			6.2.2 Synthetic Metal Complexes that Utilize Redox‐Active Ligands to Reduce O2 to Water
		6.3 Dioxygen Reduction Coupled with Substrate Dehydrogenation
			6.3.1 Copper‐Radical Oxidases
			6.3.2 Synthetic Metal Complexes that Utilize Redox‐Active Ligands to Couple O2 Reduction with Substrate Dehydrogenation
			6.3.3 Synthetic Metal Complexes that Utilize Redox‐Active Ligands to Couple Substrate Dehydrogenation with H2 Generation
		6.4 Dioxygen Reduction Coupled with Substrate Hydroxylation
			6.4.1 Cytochrome P450
			6.4.2 Synthetic Metal Complexes that Utilize Redox‐Active Ligands to Couple O2 Reduction with Substrate Hydroxylation
		6.5 Conclusions and Future Perspectives
		References
Part III. Case Studies
	7. Redox-Active Guanidine Ligands
		7.1 Introduction
		7.2 Properties and Reactivity of Uncoordinated Redox‐Active Guanidines
		7.3 Redox‐Active Guanidines as Ligands in Coordination Chemistry
			7.3.1 Survey of Different Redox States of the Guanidine Ligand (Neutral, Dicationic, or Monocationic) in Coordination Compounds
			7.3.2 Directed Stimulation of Intramolecular Electron Transfer (IET) in Copper Complexes with Redox‐Active Guanidine Ligands
				7.3.2.1 Valence Tautomerism (VT)
				7.3.2.2 IET Triggered by Consecutive Reactions
				7.3.2.3 Redox‐Induced Electron Transfer (RIET)
				7.3.2.4 IET Triggered by Counterligand Addition
				7.3.2.5 IET Triggered by Counterligand Substitution
				7.3.2.6 IET Triggered by Coordination of Metals to a Secondary Coordination Sphere
			7.3.3 Catalytic Application in the Cross‐Coupling of Phenols
		7.4 Perspectives
		References
	8. Coordination Chemistry with Lanthanides and Redox‐Active Ligands
		8.1 Introduction
		8.2 Quinone, Iminoquinone, and O‐phenylenediamine‐Based Complexes
		8.3 Diazadienes
		8.4 Iminopyridines and Bis(imino)pyridines
		8.5 Nitroxides
		8.6 N‐heterocycles
		8.7 Conclusion and Outlook
		References
	9. Actinide Complexes of Redox Non‐innocent Ligands
		9.1 Bipyridyl Ligands
		9.2 Pyrrole Ligands
		9.3 Tmtaa Ligand
		9.4 Schiff‐Base Ligands
		9.5 Pyridine(di‐imine) Ligands
		9.6 Phosphite Ligands
		9.7 Quinone Ligands
		9.8 Aryloxide Ligands
		9.9 Conclusion
		References
Index