Redox Control of a Dendritic Ferrocenyl‐Based Homogeneous Catalyst

The application of a dendrimer in a redox‐switchable catalytic process is reported. A monomeric and the corresponding dendritic ferrocenylphosphane ligand were used to develop well‐defined controllable catalysts with distinct redox states. The corresponding ruthenium(II) complexes catalyze the isomerization of the allylic alcohol 1‐octen‐3‐ol. By adding a chemical oxidant or reductant, it was possible to reversibly switch the catalytic activity of the complexes. On oxidation, the ferrocenium moiety withdraws electron density from the phosphane, thereby lowering its basicity. The resulting electron‐poor ruthenium center shows much lower activity for the redox isomerization and the reaction rate is markedly reduced.     

Ruthenium Complexes with Dendritic Ferrocenyl Phosphanes: Synthesis,Characterization, and Application in the Catalytic Redox Isomerization of Allylic Alcohols

An efficient system for the catalytic redox isomerization of the allylic alcohol 1‐octen‐3‐ol to 3‐octanone is presented. The homogeneous ruthenium(II) catalyst contains a monodentate phosphane ligand with a ferrocene moiety in the backbone and provides 3‐octanone in quantitative yields. The activity is increased by nearly 90 % with respect to the corresponding triphenyl phosphane ruthenium(II) complex. By grafting the catalyst at the surface of a dendrimer, the catalytic activity is further increased. By introducing different spacers between ferrocene and phosphorus, the influence on the electronic properties of the complexes is shown by evaluating the electrochemical behavior of the compounds.     

Electrocatalytic Carbon Dioxide Reduction by Using Cationic Pentamethylcyclopentadienyl–Iridium Complexes with Unsymmetrically Substituted Bipyridine Ligands

Eight [Ir(bpy)Cp*Cl]⁺‐type complexes (bpy= bipyridine, Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar‐saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO₂‐saturated MeCN/H₂O (9:1, v/v) solutions showed catalytic currents for CO₂ reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), E_app=?1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO₂ with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.     

Cationic Copper(I) Complexes as Highly Efficient Catalysts for Single and Double A3‐Coupling Mannich Reactions of Terminal Alkynes: Mechanistic Insights and Comparative Studies with Analogous Gold(I) Complexes

Cationic Cu?L complexes (L=Buchwald‐type phosphane) with N co‐ligands have been characterised by structural and spectroscopic properties. These copper(I) complexes are extremely active catalysts, far more active than analogous gold(I) complexes, to promote the single and double A³ coupling of terminal alkynes, pyrrolidine and formaldehyde. The activity data show the possible ways in which the solvent can influence the catalytic performance by limiting complex solubility, by solvent decomposition or instability of the copper(I) redox state. Isolation of copper(I) complexes that are likely to be the key catalytic species has allowed light to be shed on the reaction mechanism.     

Recent Advances in Photocatalytic CO2 Reduction Using Earth‐Abundant Metal Complexes‐Derived Photocatalysts

Photochemical reduction of CO₂ with H₂O into energy‐rich chemicals using inexhaustible solar energy is an appealing strategy to simultaneously address the global energy and environmental issues. Earth‐abundant metal complexes show promising application in this field due to their easy availability, rich redox valence and tunable property. Great progress has been seen on catalytic reduction of CO₂ under visible light illumination employing earth‐abundant metal complexes and their hybrids as key contributors, especially for producing CO and HCOOH via the two‐electron reduction process. In this minireview, we will summarize and update advances on earth‐abundant metal complex‐derived photocatalytic system for visible‐light driven CO₂ photoreduction over the last 5 years. Homogeneous earth‐abundant metal complex photocatalysts and earth‐abundant metal complex derived hybrid photocatalysts were both presented with focus on efficient improvement strategy.     

Catalytic Enantioselective CH Functionalization of Alcohols by Redox‐Triggered Carbonyl Addition: Borrowing Hydrogen,Returning Carbon

The use of alcohols and unsaturated reactants for the redox‐triggered generation of nucleophile–electrophile pairs represents a broad, new approach to carbonyl addition chemistry. Discrete redox manipulations that are often required for the generation of carbonyl electrophiles and premetalated carbon‐centered nucleophiles are thus avoided. Based on this concept, a broad, new family of enantioselective C? C coupling reactions that are catalyzed by iridium or ruthenium complexes have been developed, which are summarized in this Minireview.     

Light‐Triggered Release of Trapped Charges in Molecular Assemblies

We demonstrate the mediation of charge transport and release in thin films and devices by shifting the redox properties of layers of metal complexes by light. The nanoscale surface arrangement of both photo‐ and electrochemically‐active components is essential for the function of the thin films. Layers of well‐defined ruthenium complexes on indium‐tin‐oxide electrodes provide electron‐transport channels that allow the electrochemical addressing of layers of isostructural cobalt complexes. These cobalt complexes are electrochemically inactive when assembled directly on transparent metal‐oxide electrodes. The interlayer of ruthenium complexes on such electrodes allows irreversible oxidation of the cobalt complexes. However, shifting the redox properties of the ruthenium complexes by excitation with light opens up an electron‐transport channel to reduce the cobalt complexes; hence releasing the trapped positive charges.     

Ruthenium(II) complexes incorporating tridentate Schiff base ligands: synthesis,spectroscopic, redox,catalytic and biological properties

A series of new diamagnetic ruthenium(II) complexes of the type [RuCl(CO)(B)(L)] (where B = PPh₃, AsPh₃ or Py; L = monobasic tridentate Schiff base ligands derived from o‐aminophenol or o‐aminothiophenol with ethylacetoacetate or ethylbenzoylacetate) have been synthesized and these complexes were characterized by physico‐chemical and spectroscopic methods. Cyclic voltammograms of all the complexes show quasi‐reversible oxidation in the range 0.24–1.05 V and the quasi‐reversible reduction in the range ? 0.14 to ? 0.51 V. The observed redox potentials show little variation with respect to the replacement of triphenyl phosphine/arsine by pyridine. The complexes were tested as catalysts in the oxidation of primary and secondary alcohols using molecular oxygen at room temperature and also in C? C coupling reactions. Further, the antibacterial properties of the free ligands and their metal complexes were evaluated against certain bacteria such as Escherichia coli and Staphylococcus aureus. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Facile Tuning of Luminescent Platinum(II) Schiff Base Complexes from Yellow to Near‐Infrared: Photophysics,Electrochemistry, Electrochemiluminescence and Theoretical Calculations

The photophysical and related properties of platinum(II) Schiff base complexes can be finely and predictably tuned over a wide range of wavelengths by small and easily implemented changes to ligand structure. A series of such complexes, differing only in the number and positioning of methoxy substituents on the phenoxy ring, were synthesised and their photophysical, electrochemical and electrochemiluminescent (ECL) properties investigated. Theoretical calculations were performed in order to gain further insight into the relationship between structure and properties in these materials. By positioning methoxy groups para and/or ortho to either the imine or the oxygen group on the ligand, electron density could be directed selectively toward the LUMO or HOMO as required. This allowed the emission colour (both photoluminescent and electrochemiluminescent) to be tuned over a wide range between 587 and 739 nm. The variation in orbital energies was also manifested in the positions of the absorption bands and the redox properties of the complexes, as well as in the NMR shifts for the uncoordinated ligands. All reported complexes displayed intense electrochemiluminescence (ECL), which could be initiated either by annihilation or co‐reactant pathways. The relationship between the electrochemical and photophysical properties and the efficiency of the ECL is discussed. For two of the complexes solid‐state ECL could be generated from electrodeposited layers of the complex.     

Redox Multifunctionality in a Series of PtII Dithiolene Complexes of a Tetrathiafulvalene‐Based Diphosphine Ligand

The redox‐active and chelating diphosphine, 3,4‐dimethyl‐3′,4′‐bis(diphenylphosphino)‐tetrathiafulvalene, denoted as P2 , is engaged in a series of platinum complexes, [(P2)Pt(dithiolene)], with different dithiolate ligands, such as 1,2‐benzenedithiolate (bdt), 1,3‐dithiole‐2‐thione‐4,5‐dithiolate (dmit), and 5,6‐dihydro‐1,4‐dithiin‐2,3‐dithiolate (dddt). The complexes are structurally characterized by X‐ray diffraction, together with a model compound derived from bis(diphenylphosphino)ethane, namely, [(dppe)Pt(dddt)] . Four successive reversible electron‐transfer processes are found for the [(P2)Pt(dddt)] complex, associated with the two covalently linked but electronically uncoupled electrophores, that is, the TTF core and the platinum dithiolene moiety. The assignments of the different redox processes to either one or the other electrophore is made thanks to the electrochemical properties of the model compound [(dppe)Pt(dddt)] lacking the TTF redox core, and with the help of theoretical calculations (DFT) to understand the nature and energy of the frontier orbitals of the [(P2)Pt(dithiolene)] complexes in their different oxidation states. The first oxidation of the highly electron‐rich [(P2)Pt(dddt)] complex can be unambiguously assigned to the redox process affecting the Pt(dddt) moiety rather than the TTF core, a rare example in the coordination chemistry of tetrathiafulvalenes acting as ligands.     

Strongly Coupled Cyclometalated Ruthenium–Triarylamine Hybrids: Tuning Electrochemical Properties,Intervalence Charge Transfer,and Spin Distribution by Substituent Effects

Nine cyclometalated ruthenium complexes with a redox‐active diphenylamine unit in the para position to the Ru?C bond were prepared. MeO, Me, and Cl substituents on the diphenylamine unit and three types of auxiliary ligands—bis(N‐methylbenzimidazolyl)pyridine (Mebip), 2,2′:6′,2′′‐terpyridine (tpy), and trimethyl‐4,4′,4′′‐tricarboxylate‐2,2′:6′,2′′‐terpyridine (Me₃tctpy)—were used to vary the electronic properties of these complexes. The derivative with an MeO‐substituted amine unit and Me₃tctpy ligand was studied by single‐crystal X‐ray analysis. All complexes display two well‐separated redox waves in the potential region of +0.1 to +1.0 V versus Ag/AgCl, and the potential splitting ranges from 360 to 510 mV. Spectroelectrochemical measurements show that these complexes display electrochromism at low potentials and intense near‐infrared (NIR) absorptions. In the one‐electron oxidized form, the complex with the Cl‐substituted amine unit and Mebip ligand shows a moderate ligand‐to‐metal charge transfer at 800 nm. The other eight complexes show asymmetric, narrow, and intense intervalence charge‐transfer transitions in the NIR region, which are independent of the polarity of the solvent. The Mebip‐containing complexes display rhombic or broad isotropic EPR signals, whereas the other seven complexes show relatively narrow isotropic EPR signals. In addition, DFT and time‐dependent DFT studies were performed to gain insights into the spin distributions and NIR absorptions.     

(Cyclopentadienone)iron Complexes: Synthesis,Mechanism and Applications in Organic Synthesis

(Cyclopentadienone)iron tricarbonyl complexes are catalytically active, inexpensive, easily accessible and air-stable that are extensively studied as an active pre-catalyst in homogeneous catalysis. Its versatile catalytic activity arises exclusively due to the presence of a non-innocent ligand, which can trigger its unique redox properties effectively. These complexes have been employed widely in (transfer)hydrogenation (e. g., reduction of polar multiple bonds, Oppenauer-type oxidation of alcohols), C−C and C−N bond formation (e. g., reductive aminations, α-alkylation of ketones) and other synthetic transformations. In this review, we discuss the remarkable advancement of its various synthetic applications along with synthesis and mechanistic studies, until February 2021.     

A Switchable Gold Catalyst by Encapsulation in a Self‐Assembled Cage

Dinuclear gold complexes have the ability to interact with one or more substrates in a dual‐activation mode, leading to different reactivity and selectivity than their mononuclear relatives. In this contribution, this difference was used to control the catalytic properties of a gold‐based catalytic system by site‐isolation of mononuclear gold complexes by selective encapsulation. The typical dual‐activation mode is prohibited by this catalyst encapsulation, leading to typical behavior as a result of mononuclear activation. This strategy can be used as a switch (on/off) for a catalytic reaction and also permits reversible control over the product distribution during the course of a reaction.     

Preparation and Redox Properties of N,N,N‐1,3,5‐Trialkylated Flavin Derivatives and Their Activity as Redox Catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H₂O₂ oxidation of methyl p‐tolyl sulfide. Introduction of electron‐withdrawing groups increased the stability of the reduced catalyst precursor.     

Transition Metal Complexes of a Salen–Fullerene Diad: Redox and Catalytically Active Nanostructures for Delivery of Metals in Nanotubes

A covalently‐linked salen–C₆₀ (H₂L) assembly binds a range of transition metal cations in close proximity to the fullerene cage to give complexes [M(L)] (M=Mn, Co, Ni, Cu, Zn, Pd), [MCl(L)] (M=Cr, Fe) and [V(O)L]. Attaching salen covalently to the C₆₀ cage only marginally slows down metal binding at the salen functionality compared to metal binding to free salen. Coordination of metal cations to salen–C₆₀ introduces to these fullerene derivatives strong absorption bands across the visible spectrum from 400 to 630 nm, the optical features of which are controlled by the nature of the transition metal. The redox properties of the metal–salen–C₆₀ complexes are determined both by the fullerene and by the nature of the transition metal, enabling the generation of a wide range of fullerene‐containing charged species, some of which possess two or more unpaired electrons. The presence of the fullerene cage enhances the affinity of these complexes for carbon nanostructures, such as single‐, double‐ and multiwalled carbon nanotubes and graphitised carbon nanofibres, without detrimental effects on the catalytic activity of the metal centre, as demonstrated in styrene oxidation catalysed by [Cu(L)]. This approach shows promise for applications of salen–C₆₀ complexes in heterogeneous catalysis.     

Redox Non‐Innocence of Coordinated 2‐(Arylazo) Pyridines in Iridium Complexes: Characterization of Redox Series and an Insight into Voltage‐Induced Current Characteristics

Two examples of a rare class of di‐radical azo‐anion complexes of 2‐(arylazo) pyridine with Ir^III carrier are introduced. Their electronic structures have been elucidated using a host of physical methods that include X‐ray crystallography, cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory. Room temperature magnetic moments of these are consistent with two nearly non‐interacting azo‐anion radicals. These displayed rich electrochemical properties consisting of six numbers of reversible and successive one electron CV‐waves. Redox processes occur entirely at the coordinated ligands without affecting metal redox state. Apart from reporting their chemical characterization, I‐V characteristics of these complexes in film state are investigated using sandwich‐type devices comprising of a thin film of 100–125 nm thickness placed between two gold‐plated ITO electrodes. These showed memory switching properties covering a useful voltage range with a reasonable ON/OFF ratio and also are suitable for RAM/ROM applications. I‐V characteristics of two similar complexes of Rh and Cr with identical ligand environment and electronic structure are also referred for developing an insight into the memory switching ability of Ir‐ and Rh‐ complexes on the basis of comparative analysis of responses of the respective systems. In a nutshell, thorough analysis of voltage driven redox dynamics and corresponding solid and solution state current responses of all the systems are attempted and there from an unexplored class of switching devices are systematically introduced.     

Binuclear Ruthenium Complexes with Benzo[1,2‐b;4,5‐b′]dithiophene Analogues as Bridge Ligands: Syntheses,Characterization and Notable Difference on Electronic Coupling

Diruthenium ethynyl complexes 1 – 3 ( 1 : 1,5‐dithia‐s‐indacene‐4,8‐dione; 2 : 4,8‐diethoxybenzo[1,2‐b:4,5‐ b']dithiophene; 3 : 4,8‐didodecyloxybenzo[1,2‐b:4,5‐b']dithiophene) have been synthesized by incorporating the respective conjugated heterocyclic spacer and characterized by NMR and elemental analysis. The effects of bridge ligands’ properties on electronic coupling between redox‐active ruthenium terminal groups were investigated by electrochemistry, UV /vis/near‐IR and IR spectroelectrochemistry combined with density functional theory (DFT ) and time‐dependent DFT calculations. Electrochemistry results indicated that complexes 1 – 3 exhibit two fully reversible oxidation waves, and complexes 2 and 3 with electron‐rich and π‐conjuagted bridge ligands are characterized by excellent electrochemical properties. Furthermore, the larger ν(C ≡ C) separation from the IR spectroelectrochemical results of 2 and 3 and the intense NIR absorption features of singly oxidized species 2 ⁺ and 3 ⁺ revealed that their molecular skeletons have superior abilities to delocalize the positive charge. The spin density distribution from DFT calculations proved the conclusions of this study.     

Stereoconvergent,Redox‐Neutral Access to Tetrahydroquinoxalines through Relay Epoxide Opening/Amination of Alcohols

We present an economical catalytic procedure to convert readily available 1,2‐diaminobenzenes and terminal epoxides into valuable 1,2,3,4‐tetrahydroquinoxalines in a highly enantioselective fashion. This procedure operates through relay zinc and iridium catalysis, and achieves redox‐neutral and stereoconvergent production of valuable chiral heterocycles from racemic starting materials with water as the only side product. The use of commercially available reagents and catalysts and a convenient procedure also make this catalytic method attractive for practical application.     

Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands

We describe herein the synthesis and characterization of ruthenium complexes with multifunctional bipyridyl diphosphonate ligands as well as initial water oxidation studies. In these complexes, the phosphonate groups provide redox‐potential leveling through charge compensation and σ donation to allow facile access to high oxidation states. These complexes display unique pH‐dependent electrochemistry associated with deprotonation of the phosphonic acid groups. The position of these groups allows them to shuttle protons in and out of the catalytic site and reduce activation barriers. A mechanism for water oxidation by these catalysts is proposed on the basis of experimental results and DFT calculations. The unprecedented attack of water at a neutral six‐coordinate [Ru^IV] center to yield an anionic seven‐coordinate [Ru^IV?OH]^? intermediate is one of the key steps of a single‐site mechanism in which all species are anionic or neutral. These complexes are among the fastest single‐site catalysts reported to date.