Redox‐Active Ligand‐Induced Homolytic Bond Activation

Coordination of the novel redox‐active phosphine‐appended aminophenol pincer ligand (PNO^H2) to Pd^II generates a paramagnetic complex with a persistent ligand‐centered radical. The complex undergoes fully reversible single‐electron oxidation and reduction. Homolytic bond activation of diphenyldisulfide by the single‐electron reduced species leads to a ligand‐based mixed‐valent dinuclear palladium complex with a single bridging thiolate ligand. Mechanistic investigations support an unprecedented intramolecular ligand‐to‐disulfide single‐electron transfer process to induce homolytic S? S cleavage, thereby releasing a thiyl (sulfanyl) radical. This could be a new strategy for small‐molecule bond activation.     

Electrocatalytic Hydrogen Production by an Aluminum(III) Complex: Ligand‐Based Proton and Electron Transfer

Environmentally sustainable hydrogen‐evolving electrocatalysts are key in a renewable fuel economy, and ligand‐based proton and electron transfer could circumvent the need for precious metal ions in electrocatalytic H₂ production. Herein, we show that electrocatalytic generation of H₂ by a redox‐active ligand complex of Al³⁺ occurs at ?1.16 V vs. SCE (500 mV overpotential).     

Electron Transfer in Metal‐Organic Molecules. A Time Resolved EXAFS and Optical Spectroscopy Study

We have measured, by means of ultrafast x‐ray absorption and optical spectroscopy, the M‐O (M=Fe, Co) and Co‐N metal to ligand bond length change as a function of time and the formation and decay of the excited states and intermediate species, after excitation with a 267 nm femtosecond pulse. These experimental data combined with DFT calculations allowed us to determine the mechanism of electron transfer operating in the redox reaction of two metal‐ligand complexes, [M(III)(C₂O₄)₃]^3‐ and [Co(III)(NH₃)₆ ]³⁺. Based on the data we find that, even though both molecules are excited into their charge transfer band, the redox reaction of [M(III)(C₂O₄)₃]^3‐ proceeds via intermolecular electron transfer while [Co(III)(NH₃)₆ ]³⁺ electron transfer mechanism is intramolecular.     

An In‐Depth Look at the Reactivity of Non‐Redox‐Metal Alkylperoxides

Over the past 150 years, a certain mythology has arisen around the mechanistic pathways of the oxygenation of organometallics with non‐redox‐active metal centers as well as the character of products formed. Notably, there is a widespread perception that the formation of commonly encountered metal alkoxide species results from the auto‐oxidation reaction, in which a parent metal alkyl compound is oxidized by the metal alkylperoxide via oxygen transfer reaction. Now, harnessing a well‐defined zinc ethylperoxide incorporating a β‐diketiminate ligand, the investigated alkylperoxide compounds do not react with the parent metal alkyl complex as well as Et₂Zn to form a zinc alkoxide. Upon treatment of the zinc ethylperoxide with Et₂Zn, a previously unobserved ligand exchange process is favored. Isolation of a zinc hydroxide carboxylate as a product of decomposition of the parent zinc ethylperoxide demonstrates the susceptibility of the latter to O?O bond homolysis.     

Synthesis,Structure, and Highly Efficient Copper‐Catalyzed Aziridination with a Tetraaza‐Bispidine Ligand

The distorted trigonal‐bipyramidal Cu^II complex [Cu(L¹)(NCCH₃)]²⁺ of the novel tetradentate bispidine‐derived ligand L¹ with four tertiary amine donors (L¹=1,5‐diphenyl‐3‐methyl‐7‐(1,4,6‐trimethyl‐1,4‐diazacycloheptane‐6‐yl)diazabicyclo[3.3.1]nonane‐9‐one) is a very efficient catalyst for the aziridination of olefins in the presence of a nitrene source. In agreement with the experimental data (in situ spectroscopy, product distribution, and its dependence on the geometry of the substrate and of the nitrene source), a theoretical analysis based on DFT calculations indicates that the active catalyst has the Cu center in its +II oxidation state, that electron transfer is not involved, and that the conversion of the olefin to an aziridine is a stepwise process involving a radical intermediate. The striking change of efficiency and reaction mechanism between classical copper–bispidine complexes and the novel L¹‐based catalyst is primarily attributed to the structural variation, enforced by the ligand architecture.     

A Series of Diamagnetic Pyridine Monoimine Rhenium Complexes with Different Degrees of Metal‐to‐Ligand Charge Transfer: Correlating 13C NMR Chemical Shifts with Bond Lengths in Redox‐Active Ligands

A set of pyridine monoimine (PMI) rhenium(I) tricarbonyl chlorido complexes with substituents of different steric and electronic properties was synthesized and fully characterized. Spectroscopic (NMR and IR) and single‐crystal X‐ray diffraction analyses of these complexes showed that the redox‐active PMI ligands are neutral and that the overall electronic structure is little affected by the choices of the substituent at the ligand backbone. One‐ and two‐electron reduction products were prepared from selected starting compounds and could also be characterized by multiple spectroscopic methods and X‐ray diffraction. The final product of a one‐electron reduction in THF is a diamagnetic metal–metal‐bonded dimer after loss of the chlorido ligand. Bond lengths in and NMR chemical shifts of the PMI ligand backbone indicate partial electron transfer to the ligand. Two‐electron reduction in THF also leads to the loss of the chlorido ligand and a pentacoordinate complex is obtained. The comparison with reported bond lengths and ¹³C NMR chemical shifts of doubly reduced free pyridine monoaldimine ligands indicates that both redox equivalents in the doubly reduced rhenium complex investigated here are located in the PMI ligand. With diamagnetic complexes varying over three formal reduction stages at the PMI ligand we were, for the first time, able to establish correlations of the ¹³C NMR chemical shifts with the relevant bond lengths in redox‐active ligands over a full redox series.     

On the Ligand‐to‐Metal Charge‐Transfer Photochemistry of the Copper(II) Complexes of Quercetin and Rutin

In contrast to the UV‐photoinduced ligand photoionization of the flavonoid complexes of Fe^III, redox reactions initiated in ligand‐to‐metal charge‐transfer excited states were observed on irradiation of the quercetin ( 1 ) and rutin ( 2 ) complexes of Cu^II. Solutions of complexes with stoichiometries [Cu^IIL₂] (L=quercetin, rutin) and [Cu^II₂L_n] (n=1, L=quercetin; n=3, L=rutin) were flash‐irradiated at 351 nm. Transient spectra observed in these experiments showed the formation of radical ligands corresponding to the one‐electron oxidation of L and the reduction of Cu^II to Cu^I. The radical ligands remained coordinated to the Cu^I centers, and the substitution reactions replacing them by solvent occurred with lifetimes τ<350 ns. These are lifetimes shorter than the known lifetimes (τ>1 ms) of the quercetin and rutin radical's decay.     

Ultrafast Photoinduced Energy and Electron Transfer in Multi‐Modular Donor–Acceptor Conjugates

New multi‐modular donor–acceptor conjugates featuring zinc porphyrin (ZnP), catechol‐chelated boron dipyrrin (BDP), triphenylamine (TPA) and fullerene (C₆₀), or naphthalenediimide (NDI) have been newly designed and synthesized as photosynthetic antenna and reaction‐center mimics. The X‐ray structure of triphenylamine‐BDP is also reported. The wide‐band capturing polyad revealed ultrafast energy‐transfer (k_ENT=1.0×10¹² s^?1) from the singlet excited BDP to the covalently linked ZnP owing to close proximity and favorable orientation of the entities. Introducing either fullerene or naphthalenediimide electron acceptors to the TPA‐BDP‐ZnP triad through metal–ligand axial coordination resulted in electron donor–acceptor polyads whose structures were revealed by spectroscopic, electrochemical and computational studies. Excitation of the electron donor, zinc porphyrin resulted in rapid electron‐transfer to coordinated fullerene or naphthalenediimide yielding charge separated ion‐pair species. The measured electron transfer rate constants from femtosecond transient spectral technique in non‐polar toluene were in the range of 5.0×10⁹–3.5×10¹⁰ s^?1. Stabilization of the charge‐separated state in these multi‐modular donor–acceptor polyads is also observed to certain level.     

Redox Self‐Adaptation of a Nitrene Transfer Catalyst to the Substrate Needs

The development of iron catalysts for carbon–heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an Fe^IIIFe^IV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a Fe^IIIFe^IVL^. active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self‐adaptation of the catalyst to the substrate needs.     

Covalency‐Driven Preservation of Local Charge Densities in a Metal‐to‐Ligand Charge‐Transfer Excited Iron Photosensitizer

Covalency is found to even out charge separation after photo‐oxidation of the metal center in the metal‐to‐ligand charge‐transfer state of an iron photosensitizer. The σ‐donation ability of the ligands compensates for the loss of iron 3d electronic charge, thereby upholding the initial metal charge density and preserving the local noble‐gas configuration. These findings are enabled through element‐specific and orbital‐selective time‐resolved X‐ray absorption spectroscopy at the iron L‐edge. Thus, valence orbital populations around the central metal are directly accessible. In conjunction with density functional theory we conclude that the picture of a localized charge‐separation is inadequate. However, the unpaired spin density provides a suitable representation of the electron–hole pair associated with the electron‐transfer process.     

Visible‐Light‐Promoted Dearomative Fluoroalkylation of β‐Naphthols through Intermolecular Charge Transfer

The first visible‐light‐promoted dearomative fluoroalkylation of β‐naphthols was realized without the assistance of any transition‐metal catalysts or external photosensitizers. Inexpensive fluoroalkyl iodides were directly used as efficient fluoroalkylation reagents under very mild reaction conditions. The scope of this process was found to be general and broad, and both trifluoromethyl and perfluoroalkyl groups (‐C₄F₉, ‐C₆F₁₃, and ‐C₈F₁₇) were installed in excellent yields. Preliminary mechanistic studies suggest that visible‐light‐promoted intermolecular charge transfer within the naphtholate–fluoroalkyl iodide electron donor–acceptor (EDA) complex induces a single electron transfer in the absence of photocatalysts.     

Tandem CH Activation/Arylation Catalyzed by Low‐Valent Iron Complexes with Bisiminopyridine Ligands

Tandem C?H activation/arylation between unactivated arenes and aryl halides catalyzed by iron complexes that bear redox‐active non‐innocent bisiminopyridine ligands is reported. Similar reactions catalyzed by first‐row transition metals have been shown to involve substrate‐based aryl radicals, whereas our catalytic system likely involves ligand‐centered radicals. Preliminary mechanistic investigations based on spectroscopic and reactivity studies, in conjunction with DFT calculations, led us to propose that the reaction could proceed through an inner‐sphere C?H activation pathway, which is rarely observed in the case of iron complexes. This bielectronic noble‐metal‐like behavior could be sustained by the redox‐active non‐innocent bisiminopyridine ligands.     

A Nickel Dithiolate Water Reduction Catalyst Providing Ligand‐Based Proton‐Coupled Electron‐Transfer Pathways

A nickel pyrazinedithiolate ([Ni(dcpdt)₂]²⁻; dcpdt=5,6‐dicyanopyrazine‐2,3‐dithiolate), bearing a NiS₄ core similar to the active center of [NiFe] hydrogenase, is shown to serve as an efficient molecular catalyst for the hydrogen evolution reaction (HER). This catalyst shows effectively low overpotentials for HER (330–400 mV at pH 4–6). Moreover, the turnover number of catalysis reaches 20 000 over the 24 h electrolysis with a high Faradaic efficiency, 92–100 %. The electrochemical and DFT studies reveal that diprotonated one‐electron‐reduced species (i.e., [Ni^II(dcpdt)(dcpdtH₂)]⁻ or [Ni^II(dcpdtH)₂]⁻) forms at pH<6.4 via ligand‐based proton‐coupled electron‐transfer (PCET) pathways, leading to electrocatalytic HER without applying the highly negative potential required to generate low‐valent nickel intermediates. This is the first example of catalysts exhibiting such behavior.     

Active Intermediates in Copper Nitrite Reductase Reactions Probed by a Cryotrapping‐Electron Paramagnetic Resonance Approach

Redox active metalloenzymes catalyse a range of biochemical processes essential for life. However, due to their complex reaction mechanisms, and often, their poor optical signals, detailed mechanistic understandings of them are limited. Here, we develop a cryoreduction approach coupled to electron paramagnetic resonance measurements to study electron transfer between the copper centers in the copper nitrite reductase (CuNiR) family of enzymes. Unlike alternative methods used to study electron transfer reactions, the cryoreduction approach presented here allows observation of the redox state of both metal centers, a direct read‐out of electron transfer, determines the presence of the substrate/product in the active site and shows the importance of protein motion in inter‐copper electron transfer catalyzed by CuNiRs. Cryoreduction‐EPR is broadly applicable for the study of electron transfer in other redox enzymes and paves the way to explore transient states in multiple redox‐center containing proteins (homo and hetero metal ions).     

Three‐Way Cooperativity in d8 Metal Complexes with Ligands Displaying Chemical and Redox Non‐Innocence

Reversible proton‐ and electron‐transfer steps are crucial for various chemical transformations. The electron‐reservoir behavior of redox non‐innocent ligands and the proton‐reservoir behavior of chemically non‐innocent ligands can be cooperatively utilized for substrate bond activation. Although site‐decoupled proton‐ and electron‐transfer steps are often found in enzymatic systems, generating model metal complexes with these properties remains challenging. To tackle this issue, we present herein complexes [(cod_?H)M(μ‐L^2?) M (cod_?H)] (M=Pt^II, [ 1 ] or Pd^II, [ 2 ], cod=1,5‐cyclooctadiene, H₂L=2,5‐di‐[2,6‐(diisopropyl)anilino]‐1,4‐benzoquinone), in which cod acts as a proton reservoir, and L^2? as an electron reservoir. Protonation of [ 2 ] leads to an unusual tetranuclear complex. However, [ 1 ] can be stepwise reversibly protonated with up to two protons on the cod_?H ligands, and the protonated forms can be stepwise reversibly reduced with up to two electrons on the L^2? ligand. The doubly protonated form of [ 1 ] is also shown to react with OMe^? leading to an activation of the cod ligands. The site‐decoupled proton and electron reservoir sources work in tandem in a three‐way cooperative process that results in the transfer of two electrons and two protons to a substrate leading to its double reduction and protonation. These results will possibly provide new insights into developing catalysts for multiple proton‐ and electron‐transfer reactions by using metal complexes of non‐innocent ligands.     

Cobalt‐Catalyzed Hydrogen‐Atom Transfer Induces Bicyclizations that Tolerate Electron‐Rich and Electron‐Deficient Intermediate Alkenes

A novel Co^II‐catalyzed polyene cyclization was developed that is uniquely effective when performed in hexafluoroisopropanol as the solvent. The process is presumably initiated by metal‐catalyzed hydrogen‐atom transfer (MHAT) to 1,1‐disubstituted or monosubstituted alkenes, and the reaction is remarkable for its tolerance of internal alkenes bearing either electron‐rich methyl or electron‐deficient nitrile substituents. Electron‐rich aromatic terminators are required in both cases. Terpenoid scaffolds with different substitution patterns are obtained with excellent diastereoselectivities, and the bioactive C20‐oxidized abietane diterpenoid carnosaldehyde was made to showcase the utility of the nitrile‐bearing products. Also provided are the results of several mechanistic experiments that suggest the process features an MHAT‐induced radical bicyclization with late‐stage oxidation to regenerate the aromatic terminator.     

Strain‐Promoted Azide–Alkyne Cycloaddition with Ruthenium(II)–Azido Complexes

The reactivity of an exemplary ruthenium(II)–azido complex towards non‐activated, electron‐deficient, and towards strain‐activated alkynes at room temperature and low millimolar azide and alkyne concentrations has been investigated. Non‐activated terminal and internal alkynes failed to react under such conditions, even under copper(I) catalysis conditions. In contrast, as expected, rapid cycloaddition was observed with electron‐deficient dimethyl acetylenedicarboxylate (DMAD) as the dipolarophile. Since DMAD and related propargylic esters are excellent Michael acceptors and thus unsuitable for biological applications, we investigated the reactivity of the azido complex towards cycloaddition with derivatives of cyclooctyne (OCT), bicyclo[6.1.0]non‐4‐yne (BCN), and azadibenzocyclooctyne (ADIBO). While no reaction could be observed in the case of the less strained cyclooctyne OCT, the highly strained cyclooctynes BCN and ADIBO readily reacted with the azido complex, providing the corresponding stable triazolato complexes, which were amenable to purification by conventional silica gel column chromatography. An X‐ray crystal structure of an ADIBO cycloadduct was obtained and verified that the formed 1,2,3‐triazolato ligand coordinates the metal center through the central N2 atom. Importantly, the determined second‐order rate constant for the ADIBO cycloaddition with the azido complex (k₂=6.9 × 10^?2 M ^?1 s^?1) is comparable to the rate determined for the ADIBO cycloaddition with organic benzyl azide (k₂=4.0 × 10^?1 M ^?1 s^?1). Our results demonstrate that it is possible to transfer the concept of strain‐promoted azide–alkyne cycloaddition (SPAAC) from purely organic azides to metal‐coordinated azido ligands. The favorable reaction kinetics for the ADIBO‐azido‐ligand cycloaddition and the well‐proven bioorthogonality of strain‐activated alkynes should pave the way for applications in living biological systems.     

Localized Mixed‐Valence and Redox Activity within a Triazole‐Bridged Dinucleating Ligand upon Coordination to Palladium

The new dinucleating redox‐active ligand ( L^H4 ), bearing two redox‐active NNO‐binding pockets linked by a 1,2,3‐triazole unit, is synthetically readily accessible. Coordination to two equivalents of Pd^II resulted in the formation of paramagnetic (S= ) dinuclear Pd complexes with a κ²‐N,N′‐bridging triazole and a single bridging chlorido or azido ligand. A combined spectroscopic, spectroelectrochemical, and computational study confirmed Robin–Day Class II mixed‐valence within the redox‐active ligand, with little influence of the secondary bridging anionic ligand. Intervalence charge transfer was observed between the two ligand binding pockets. Selective one‐electron oxidation allowed for isolation of the corresponding cationic ligand‐based diradical species. SQUID (super‐conducting quantum interference device) measurements of these compounds revealed weak anti‐ferromagnetic spin coupling between the two ligand‐centered radicals and an overall singlet ground state in the solid state, which is supported by DFT calculations. The rigid and conjugated dinucleating redox‐active ligand framework thus allows for efficient electronic communication between the two binding pockets.     

Development of a Metal‐Ion‐Mediated Base Pair for Electron Transfer in DNA

A new C‐nucleoside structurally based on the hydroxyquinoline ligand was synthesized that is able to form stable pairs in DNA in both the absence and the presence of metal ions. The interactions between the metal centers in adjacent Cu^II‐mediated base pairs in DNA were probed by electron paramagnetic resonance (EPR) spectroscopy. The metal–metal distance falls into the range of previously reported values. Fluorescence studies with a donor–DNA–acceptor system indicate that photoinduced charge‐transfer processes across these metal‐ion‐mediated base pairs in DNA occur more efficiently than over natural base pairs.