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.     

Redox‐Active Ligands in Catalysis

Nature’s use of redox‐active moieties combined with 3d transition‐metal ions is a powerful strategy to promote multi‐electron catalytic reactions. The ability of these moieties to store redox equivalents aids metalloenzymes in promoting multi‐electron reactions, avoiding high‐energy intermediates. In a biomimetic spirit, chemists have recently developed approaches relying on redox‐active moieties in the vicinity of metal centers to catalyze challenging transformations. This approach enables chemists to impart noble‐metal character to less toxic, and cost effective 3d transitional metals, such as Fe or Cu, in multi‐electron catalytic reactions.     

Redox Materials by the Covalent Entrapment of Redox‐Active Dirhodium(II,II) Species in a Siloxane Network

Summary: Hydrolysis and polycondensation of the coupling agent (aminopropyl)triethoxysilane (APS), axially coordinated to the redox‐active complex [Rh₂(form)₂(CH₃COO)₂(APS)₂], lead to the insertion of redox‐active inorganic microdomains into a siloxane network; the new polymers undergo cyclic redox reactions indicating that dirhodium(II ,II ) centres retain their redox activity even when incorporated into siloxane networks.

The redox‐active complex [Rh₂(form)₂(CH₃COO)₂(APS)₂] (form = N,N′‐di‐p‐tolylformamidinate) incorporated into a siloxane network here.     

The Formazanate Ligand as an Electron Reservoir: Bis(Formazanate) Zinc Complexes Isolated in Three Redox States

The synthesis of bis(formazanate) zinc complexes is described. These complexes have well‐behaved redox‐chemistry, with the ligands functioning as a reversible electron reservoir. This allows the synthesis of bis(formazanate) zinc compounds in three redox states in which the formazanate ligands are reduced to “metallaverdazyl” radicals. The stability of these ligand‐based radicals is a result of the delocalization of the unpaired electron over four nitrogen atoms in the ligand backbone. The neutral, anionic, and dianionic compounds (L₂Zn^0/?1/?2) were fully characterized by single‐crystal X‐ray crystallography, spectroscopic methods, and DFT calculations. In these complexes, the structural features of the formazanate ligands are very similar to well‐known β‐diketiminates, but the nitrogen‐rich (NNCNN) backbone of formazanates opens the door to redox‐chemistry that is otherwise not easily accessible.     

A Novel Pentadentate Redox‐Active Ligand and Its Iron(III) Complexes: Electronic Structures and O2 Reactivity

A novel redox‐active ligand, H₄^Ph2SL^AP ( 1 ) which was designed to be potentially pentadentate with an O,N,S,N,O donor set is described. Treatment of 1 with two equivalents of potassium hydride gave access to octametallic precursor complex [H₂^Ph2SL^APK₂(thf)]₄ ( 2 ), which reacted with FeCl₃ to yield iron(III) complex [H₂^Ph2SL^APFeCl] ( 3 ). Employing Fe[N(SiMe₃)₂]₃ for a direct reaction with 1 led to ligand rearrangement through C?S bond cleavage and thiolate formation, finally yielding [HL^APFe] ( 5 ). Upon exposure to O₂, 3 and 5 are oxidized through formal hydrogen‐atom abstraction from the ligand NH units to form [^Ph2SL^SQFeCl] ( 4 ) and [L^SQFe] ( 6 ) featuring two or one coordinated iminosemiquinone moieties, respectively. Mössbauer measurements demonstrated that the iron centers remain in their +III oxidation states. Compounds 3 and 5 were tested with respect to their potential as models for the catechol dioxygenase. Thus, they were treated with 3,5‐di‐tert‐butyl‐catechol, triethylamine and O₂. It turned out that the iron–catecholate complexes react with O₂ in dichloromethane at ambient conditions through C?C bond cleavage mainly forming extradiol cleavage products. Intradiol products are only side products and quinone formation becomes negligible. This observation has been rationalized by a dissociation of two donor functions upon coordination of the catecholate.     

Solid‐State Thermolysis of a fac‐Rhenium(I) Carbonyl Complex with a Redox Non‐Innocent Pincer Ligand

The development of rhenium(I) chemistry has been restricted by the limited structural and electronic variability of the common pseudo‐octahedral products fac‐[ReX(CO)₃L₂] (L₂=α‐diimine). We address this constraint by first preparing the bidentate bis(imino)pyridine complexes [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₃X] (X=Cl 2 , Br 3 ), which were characterized by spectroscopic and X‐ray crystallographic means, and then converting these species into tridentate pincer ligand compounds, [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₂X] (X=Cl 4 , Br 5 ). This transformation was performed in the solid‐state by controlled heating of 2 or 3 above 200 °C in a tube furnace under a flow of nitrogen gas, giving excellent yields (≥95 %). Compounds 4 and 5 define a new coordination environment for rhenium(I) carbonyl chemistry where the metal center is supported by a planar, tridentate pincer‐coordinated bis(imino)pyridine ligand. The basic photophysical features of these compounds show significant elaboration in both number and intensity of the d–π* transitions observed in the UV/Vis spec tra relative to the bidentate starting materials, and these spectra were analyzed using time‐dependent DFT computations. The redox nature of the bis(imino)pyridine ligand in compounds 2 and 4 was examined by electrochemical analysis, which showed two ligand reduction events and demonstrated that the ligand reduction shifts to a more positive potential when going from bidentate 2 to tridentate 4 (+160 mV for the first reduction step and +90 mV for the second). These observations indicate an increase in electrostatic stabilization of the reduced ligand in the tridentate conformation. Elaboration on this synthetic methodology documented its generality through the preparation of the pseudo‐octahedral rhenium(I) triflate complex [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₂OTf] ( 7 , 93 % yield).     

How a Redox‐Innocent Metal Promotes the Formal Reductive Elimination of Biphenyl Using Redox‐Active Ligands

One of the most compelling strategies for utilizing redox‐active ligands is to perform redox events at the ligands to avoid accessing prohibitively high energy oxidation states at the metal center. This has been demonstrated experimentally in many systems, yet there is little understanding of the fundamental electronic structures involved with these transformations or how to control them. Here, the reductive elimination of biphenyl from [M(isq)₂Ph₂] (M=Ti, Zr, and Hf and isq=2,4‐di‐tert‐butyl‐6‐tert‐butyliminosemiquinone) was studied computationally. It was found that the metal remains in the +IV oxidation state and all redox chemistry was mediated by the redox‐active ligands. Two types of electron‐transfer mechanisms were identified, an asymmetric unpaired electron transfer (UET) and a symmetric pairwise electron transfer (PET), the former always being lower in energy. The energetic differences between these two mechanisms were explained through simple molecular orbital theory arguments. Despite the metal’s redox‐inactivity, it still has a marked influence on the calculated energetics of the reaction, with the Ti systems being much more reactive than the Zr/Hf systems. This primarily originates from the shorter Ti?Ph bond, which leads to a stronger filled‐filled interaction between these ligands at the reactant state. This greater reactant destabilization leads to the lower activation energies.     

Diferrocenophosphaborin: A Planar‐Chiral,Redox‐Active and Anion‐Responsive Ambiphilic Ligand

A new class of Janus‐like ambiphilic ligands is introduced. The rigid diferrocene backbone in heterocycles 4‐SnP and 4‐BP creates an unprecedented chiral environment as demonstrated by multinuclear NMR and single‐crystal X‐ray studies. In addition, the ligands are redox‐responsive and the Lewis acidic borane moiety in 4‐BP can be exploited to further tune the properties: a clear decrease in the CO stretching frequency of a Vaska‐type Rh^I complex 5‐BP is observed upon addition of fluoride ions. Thus, the Lewis acid and Lewis base sites influence each other and their strength can be modulated by redox chemistry and anion binding.     

Copper(II)‐Mediated Transformation of a Tridentate Non‐Innocent Ligand into a Tetradentate Salen‐Type Innocent Ligand

A non‐innocent ligand, H₄L, was synthesized by introducing a ? CH₂NH₂ group at the ortho carbon atom to the aniline moiety of 2‐anilino‐4,6‐di‐tert‐butylphenol. The new ligand was characterized by IR and NMR spectroscopy and mass spectrometry techniques. Upon treatment with CuCl₂ ? 2 H₂O, this non‐innocent ligand provided a mononuclear four‐coordinate salen‐type Cu^II complex by complete modification of the ligand backbone. The complex was characterized by IR spectroscopy, mass spectrometry, X‐ray single‐crystal diffraction, electron paramagnetic resonance (EPR) spectroscopy, and UV/Vis/near‐IR spectroscopy techniques. X‐ray crystallographic analysis showed an asymmetric environment around the Cu^II center with a small (≈12°) twist between the two biting planes. Analysis of the X‐band EPR spectrum also supported the asymmetric environment and also indicated the presence of an unpaired electron on the d orbital. The UV/Vis/near‐IR spectrum showed strong absorption bands for metal‐to‐ligand charge transfer and ligand‐to‐metal charge transfer along with a Cu^II‐centered d–d transition. Mechanistic investigation of the formation of complex 1 indicated that modification of the ligand backbone proceeded through ligand‐centered amine to imine oxidation as well as through C? N bond‐breaking processes. During these processes, 3,5‐di‐tert‐butyl‐1,2‐benzoquinone and 2‐aminobenzylidene were produced. Ammonia, generated in situ through hydrolysis of the imine to the aldehyde, reacted with 3,5‐di‐tert‐butyl‐1,2‐benzoquinone to form the corresponding 3,5‐di‐tert‐butyl‐1,2‐iminobenzoquinone moiety, which upon two‐electron reduction in the reaction medium formed 3,5‐di‐tert‐butyl‐1,2‐aminophenol. This aminophenol underwent condensation with the H₂L5 ligand that was formed by self‐condensation of two molecules of 2‐aminobenzaldehyde and provided the modified ligand backbone.     

An Organic Mixed‐Valence Ligand for Multistate Redox‐Active Coordination Networks

The multistate redox‐active/multi‐interactive ligand 5,5′,8,8′‐tetra(4‐pyridyl)‐2,2′‐(1,4‐phenylene)bis‐1H‐perimidine (H₂TPP) was designed and synthesized. H₂TPP undergoes four one‐electron oxidation steps, and was used for the preparation of a multistate redox‐active coordination network in a solid–liquid interface reaction using molten Cd²⁺ salts. The multiple redox states of H₂TPP were confirmed spectroscopically by stepwise four‐electron oxidation. Spectroscopic analysis indicated that the mixed‐valence states of the ligand are class II on the UV/Vis/NIR timescale and borderline class II/class III on the ESR timescale.     

Correlated Coordination and Redox Activity of a Hemilabile Noninnocent Ligand in Nickel Complexes

The compound [Ni(Q_M)₂], Q_M=4,6‐di‐tert‐butyl‐N‐(2‐methylthiomethylphenyl)‐o‐iminobenzoquinone, is a singlet diradical species with approximately planar configuration at the tetracoordinate metal atom and without any Ni?S bonding interaction. One‐electron oxidation results in additional twofold Ni?S coordination (d_Ni?S≈2.38 Å) to produce a complex cation of [Ni(Q_M)₂](PF₆) with hexacoordinate Ni^II and two distinctly different mer‐configurated tridentate ligands. The O,O′‐trans arrangement in the neutral precursor is changed to an O,O′‐cis configuration in the cation. The EPR signal of [Ni(Q_M)₂](PF₆) has a very large g anisotropy and the magnetic measurements indicate an S=3/2 state. The dication was structurally characterized as [Ni(Q_M)₂](ClO₄)₂ to exhibit a similar NiN₂O₂S₂ framework as the monocation. However, the two tridentate (O,N,S) ligands are now equivalent according to the formulation [Ni^II(Q_M⁰)₂]²⁺. Cyclic voltammetry reflects the qualitative structure change on the first, but not on the second oxidation of [Ni(Q_M)₂], and spectroelectrochemistry reveals a pronounced dependence of the 800–900 nm absorption on the solvent and counterion. Reduction of the neutral form occurs in an electrochemically reversible step to yield an anion with an intense near‐infrared absorption at 1345 nm (ε=10400 M ^?1 cm^?1) and a conventional g factor splitting for a largely metal‐based spin (S=1/2), suggesting a [(Q_M ^{. ?})Ni^II(Q_M^2?)]^? configuration with a tetracoordinate metal atom with antiferromagnetic Ni^II–(Q_M ^{. ?}) interactions and symmetry‐allowed ligand‐to‐ligand intervalence charge transfer (LLIVCT). Calculations are used to understand the Ni?S binding activity as induced by remote electron transfer at the iminobenzoquinone redox system.     

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.     

Non‐Innocent Ligand Effects on Low‐Oxidation‐State Indium Complexes

Attempts to coordinate neutral ligands to low oxidation state indium centers are often hindered by disproportionation pathways that produce elemental indium and higher oxidation state species. In contrast, we find that reactions of the salt, InOTf (OTf=trifluoromethanesulfonate), with α‐diimine ligands yielded intensely colored compounds with no evidence of decomposition. X‐ray structural analysis of InOTf ? ^MesDAB^Me (^MesDAB^Me=N,N‐dimesityl‐2,3‐dimethyl‐diazabutadiene; 1 ) reveals a discrete molecular compound with a pyramidal coordination environment at the indium center, consistent with the presence of a stereochemically active lone pair of electrons on indium and a neutral diazabutadiene chelate ligand. The use of the less‐electron‐rich ^MesDAB^H ligand (^MesDAB^H=N,N‐dimesityl‐diazabutadiene) engenders dramatically different reactivity and produces a metallopolymer (InOTf ? ^MesDAB^H)_∞ ( 2 ) linked via C? C and In? In bonds. The difference in reactivity is rationalized by cyclic voltammetry and DFT studies that suggest more facile electron transfer from In^I to the ^MesDAB^H and bis(aryl)acenaphthenequinonediimine (BIAN) ligands. Solution EPR spectroscopy indicates the presence of non‐interacting ligand‐based radicals in solution, whereas solid‐state EPR studies reflect the presence of a thermally accessible spin triplet consistent with reversible C? C bond cleavage.     

Thermochromism of CuI Tetrakisguanidine Complexes: Reversible Activation of Metal‐to‐Ligand Charge‐Transfer Bands

Tetranuclear, intensely blue‐coloured Cu^I complexes were synthesised in which two Cu₂X₃^? units (X=Br or I) are bridged by a dicationic GFA (guanidino‐functionalised aromatic) ligand. The UV/Vis spectra show a large metal‐to‐ligand charge‐transfer (MLCT) band around 638 nm. The tetranuclear “low‐temperature” complexes are in a temperature‐dependent equilibrium with dinuclear Cu^I “high‐temperature” complexes, which result from the reversible elimination of two CuX groups. A massive thermochromism effect results from the extinction of the strong MLCT band upon CuX elimination with increasing temperature. For all complexes, quantum chemical calculations predict a small and method‐dependent energy difference between the possible electronic structures, namely Cu^I and dicationic GFA ligand (closed‐shell singlet) versus Cu^II and neutral GFA ligand (triplet or broken‐symmetry state). The closed‐shell singlet state is disfavoured by hybrid‐DFT functionals, which mix in exact Hartree–Fock exchange, and is favoured by larger basis sets and consideration of a polar medium.