Redox-Active Ligands Facilitate Bimetallic O2 Homolysis at Five-Coordinate Oxorhenium(V) Centers

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

Metal-ligand synergy driven functionalisation of alkylene linked bis(aldimine) on a diruthenium(II) platform. Cyclisation versus oxygenation

This article addresses the impact of metal-ligand redox cooperativity on the functionalisation of coordinated ligands. It demonstrates the structure-reactivity correlation of bis(aldimine) derived bis-bidentate L (Py-CHN-(CH2)n-NCH-Py, with n = 2 (L1), 3 (L2), 4 (L3)) as a function of the conformation (syn/anti) of its alkylene linker as well as the overall structural form (cis/trans) of (acac)2RuII(μ-L)RuII(acac)2 complex moieties (1-5) possessing an electron-rich acetylacetonate (acac) co-ligand. A systematic variation of the bridging alkylene unit of L in RuII/RuII-derived 1-5 led to the following reactivity/redox events, which were validated through structural, spectroscopic, electrochemical and theoretical evaluations: (i) Cyclisation of the ethylene linked (syn conformation) bis-aldimine unit of L1 via C-C coupling yielded pyrazine bridged (acac)2RuII(μ-L1')RuII(acac)2, 1a, while the corresponding anti-form (ethylene linker) of the metal-bound L1 in 2 ((acac)2RuII(μ-L1)RuII(acac)2) led to oxygenation at the ligand backbone (bis-aldimine (L) → bis(carboxamido) (L'')) via O2 activation to generate RuIIIRuIII-derived (acac)2RuIII(μ-L1''2-)RuIII(acac)2 (2a). (ii) Consequently, propylene and butylene linked L2 and L3 bridged between two {Ru(acac)2} units in 3 and 4/5 underwent oxygenation of L to L'' to yield diruthenium(III) complexes 3a and 4a/5a, respectively. (iii) In contrast, analogous L bridged oxidised [(acac)2RuIII(μ-L)RuIII(acac)2](ClO4)2 ([2](ClO4)2-[5](ClO4)2) and [{(PPh3)2(CO)(H)RuII}2(μ-L)](ClO4)2 ([6](ClO4)2-[8](ClO4)2) involving electron poor co-ligands failed to undergo the oxygenation of L irrespective of its n value, reemphasising the effective role of redox interplay between RuII and L particularly in the presence of an electron-rich acac co-ligand in the functionalisation of the latter in 1a-5a.

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand

The reaction of TaMe3 Cl2 with the rigid acridane-derived trisamine H3 NNN yields the tantalum(V) complex [TaCl2 (NNNcat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O2 yielding the oxido-bridged bimetallic complex [{TaCl2 (NNNsq )}2 O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O2 activation was investigated by DFT calculations revealing initial binding of O2 to the tantalum(V) center followed by complete O2 scission to produce a terminal oxido-complex.

Ligands modification strategies for mononuclear water splitting catalysts

Artificial photosynthesis (AP) has been proved to be a promising way of alleviating global climate change and energy crisis. Among various materials for AP, molecular complexes play an important role due to their favorable efficiency, stability, and activity. As a result of its importance, the topic has been extensively reviewed, however, most of them paid attention to the designs and preparations of complexes and their water splitting mechanisms. In fact, ligands design and preparation also play an important role in metal complexes’ properties and catalysis performance. In this review, we focus on the ligands that are suitable for designing mononuclear catalysts for water splitting, providing a coherent discussion at the strategic level because of the availability of various activity studies for the selected complexes. Two main designing strategies for ligands in molecular catalysts, substituents modification and backbone construction, are discussed in detail in terms of their potentials for water splitting catalysts.

Cluster self-assembly and anion binding by metal complexes of non-innocent thiazolidinyl-thiolate ligands

Zn^II and Fe^II chloride complexes of a di(methylthiazolidinyl)pyridine ligand were deprotonated to form the corresponding thiolate complexes supported by redox-active iminopyridine moieties. The thiolate donor groups are nucleophilic and reactive toward oxidants, electrophiles, and protons, while the pendant thiazolidine rings are available for hydrogen bonding. Anion exchange with the weakly-coordinating triflate anion resulted in self-assembly of the iminopyridine complexes to form a trimeric [M₃S₃] cluster. Hydrogen bonding closely associates anions with this trimetallic core.

Metal-ligand cooperative approaches in homogeneous catalysis using transition metal complex catalysts of redox noninnocent ligands

Catalysis offers a straightforward route to prepare various value-added molecules starting from readily available raw materials. The catalytic reactions mostly involve multi-electron transformations. Hence, compared to the inexpensive and readily available 3d-metals, the 4d and 5d-transition metals get an extra advantage for performing multi-electron catalytic reactions as the heavier transition metals prefer two-electron redox events. However, for sustainable development, these expensive and scarce heavy metal-based catalysts need to be replaced by inexpensive, environmentally benign, and economically affordable 3d-metal catalysts. In this regard, a metal-ligand cooperative approach involving transition metal complexes of redox noninnocent ligands offers an attractive alternative. The synergistic participation of redox-active ligands during electron transfer events allows multi-electron transformations using 3d-metal catalysts and allows interesting chemical transformations using 4d and 5d-metals as well. Herein we summarize an up-to-date literature report on the metal-ligand cooperative approaches using transition metal complexes of redox noninnocent ligands as catalysts for a few selected types of catalytic reactions.

Extensive Redox Non-Innocence in Iron Bipyridine-Diimine Complexes: a Combined Spectroscopic and Computational Study

Metal-ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)]3+ to the monoanion [Fe(BDI]-1. Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report.

Assigning Ligand Redox Levels in Complexes of 2-Aminophenolates: Structural Signatures

The purpose of this Viewpoint is to provide a broad-ranging update of advances in the coordination chemistry of redox-active (noninnocent) 2-aminophenolates, with emphasis on two ligand backbone structural parameters, the average of C-O and C-N (C-O/N) bond distances and Δa values, signifying the degree of bond-length alternation in the six-membered ring, in order to identify the redox level of the coordinated ligands. In the absence of magnetic, spectroscopic, and redox results, it has been established that it is possible to assign the electronic ground state of a coordination complex of 2-aminophenolates with consideration of charge, metal-ligand bond distances, and two very promising ligand backbone structural parameters. From a closer look at the sensitive ligand backbone metrical parameters of a diversified group of about 120 transition-metal complexes, a few very useful generalizations have been made.

Mobilization and partitioning of rare earth elements in the presence of humic acids and siderophores

Developing rare earth elements (plus yttrium, REY) as a group of environmental tracer requires comprehensive understandings in their geochemical behaviors associated with natural organic matter. Recent work highlighted the promotions on REY mobilization and cerium oxidation by siderophores during silicate dissolution, but the mechanism remained ambiguous. Here, we performed batch fluid-rock interaction experiments to explore the functions of siderophore desferrioxamine B (DFOB) and humic acids (HA) towards REY mobility and partitioning during REY-bearing ferrihydrite dissolution. To acquire in-depth knowledge of organic controls on REY, we used multiple strategies, including elemental, multispectral, and electrochemical analyses, to investigate the organic regulation on REY geochemistry. This study sheds light on the function of ligand-specific selectivity and solid-fluid organic molecular fractionation, primarily dependent on hydrochemical settings (pH, organic compounds, ionic strength, and oxicity). Our results confirm the catalytic oxidation ability of ligand, which forms DFOB-Ce(IV) (K = 10⁴², electrochemistry), producing positive Ce anomalies in solutions by ligand-driven redox shifting. Both HA and DFOB showed high affinities to HREY, and facilitated LREY/HREY partitioning. The mobilization of REY and the development of Ce anomalies were limited by HA coatings that modified surface properties and disturbed the approach of DFOB. Excess siderophores attack inert HA coatings, facilitating REY liberation and Ce redox activities. The release of REY and catalytic oxidation of Ce can be inhibited at high ionic strength or under oxygen deficiency. Our study reveals that natural organic matter significantly influences the fate of REY in iron oxides, and crucial for the biogeochemical cycles of REY in nature.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

A coordinatively unsaturated iridium complex with an unsymmetrical redox-active ligand: (spectro)electrochemical and reactivity studies

Metal–ligand cooperativity can be used in iridium complexes with an unsymmetrically substituted redox-active diamidobenzene ligand for bond activation reactions.

Reactions of cerium complexes with transition metal nitrides: synthesis and structure of heterometallic cerium complexes containing bridging catecholate ligands

Heterometallic cerium complexes containing bridging catecholate ligands have been synthesized from cerium complexes with Kläui's tripodal ligand and metal catecholates.

The structure of a one-electron oxidized Mn(iii)-bis(phenolate)dipyrrin radical complex and oxidation catalysis control via ligand-centered redox activity

The tetradentate ligand dppH3, which features a half-porphyrin and two electron-rich phenol moieties, was prepared and chelated to manganese. The mononuclear Mn(iii)-dipyrrophenolate complex 1 was structurally characterized. The metal ion lies in a square pyramidal environment, the apical position being occupied by a methanol molecule. Complex 1 displays two reversible oxidation waves at 0.00 V and 0.47 V vs. Fc⁺/Fc, which are assigned to ligand-centered processes. The one-electron oxidized species 1+ SbF6- was crystallized, showing an octahedral Mn(iii) center with two water molecules coordinated at both apical positions. The bond distance analysis and DFT calculations disclose that the radical is delocalized over the whole aromatic framework. Complex 1+ SbF6- exhibits an S_tot = 3/2 spin state due to the antiferromagnetic coupling between Mn(iii) and the ligand radical. The zero field splitting parameters are D = 1.6 cm^-1, E/D = 0.18(1), g_⊥ = 1.99 and g_∥ = 1.98. The dication 12+ is an integer spin system, which is assigned to a doubly oxidized ligand coordinated to a Mn(iii) metal center. Both 1 and 1+ SbF6- catalyze styrene oxidation in the presence of PhIO, but the nature of the main reaction product is different. Styrene oxide is the main reaction product when using 1, but phenylacetaldehyde is formed predominantly when using 1+ SbF6-. We examined the ability of complex 1+ SbF6- to catalyze the isomerization of styrene oxide and found that it is an efficient catalyst for the anti-Markovnikov opening of styrene oxide. The formation of phenylacetaldehyde from styrene therefore proceeds in a tandem E-I (epoxidation-isomerization) mechanism in the case of 1+ SbF6-. This is the first evidence of control of the reactivity for styrene oxidation by changing the oxidation state of a catalyst based on a redox-active ligand.

Synthesis and redox reactions of bis(verdazyl)palladium complexes

The synthesis and ligand-centered redox chemistry of palladium complexes bearing two potentially bidentate verdazyl ligands is explored. Reaction of 1,5-diisopropyl-3-pyridin-2-yl-6-oxoverdazyl radical 1 with Pd(NCMe)₄·2BF₄ gives a complex containing two coordinated verdazyl radicals. The structure of this complex consists of one verdazyl bound to Pd in a bidentate mode and the second verdazyl bound in a monodentate fashion through the pyridine substituent; the fourth coordination site is occupied by a solvent molecule (acetonitrile (3) or dimethyl sulfoxide (4)). Two-electron reduction of this complex with decamethylferrocene affords a bis(verdazyl) palladium complex (5) in which both verdazyls have been reduced to their anionic state and are both bound to Pd in bidentate manner. Complex 5 can be independently synthesized by a redox reaction between 1 and Pd₂(dba)₃. Reduced complex 5 can be re-oxidized to 3 or 4 with AgBF₄; in contrast, oxidation with PhICl₂ leads to ligand dissociation, ultimately giving radical 1 and a mono(verdazyl)dichloropalladium complex 2. One-electron oxidation using PhICl₂ produces a formally "mixed valent" (in ligand) bis(verdazyl)chloropalladium complex (6) with one bidentate verdazyl anion ligand and one monodentate (pyridine-bound) verdazyl radical. Attempted protonation of the verdazyl ligands in complex 5 leads to complete ligand dissociation and protonation of both the tetrazine and pyridine moieties; deprotonation regenerates 5. Subsequent air oxidation of the tetrazane/pyridinium cation (formed as a tetrachloropalladate salt) leads to re-coordination of the verdazyl ligands to give 6 initially, but ultimately produces a combination of free radical 1 and 2.

Redox non-innocence permits catalytic nitrene carbonylation by (dadi)Ti[double bond, length as m-dash]NAd (Ad = adamantyl)

Application of the diamide, diimine {-CH[double bond, length as m-dash]N(1,2-C6H4)N(2,6-iPr2-C6H3)}2 m ((dadi) m ) ligand to titanium provided adducts (dadi)TiL x (1-L x ; L x = THF, PMe2Ph, (CNMe)2), which possess the redox formulation [(dadi)4-]Ti(iv)L x , and 22 πe- (4n + 2). Related complexes containing titanium-ligand multiple bonds, (dadi)Ti[double bond, length as m-dash]X (2 [double bond, length as m-dash]X; X = O, NAd), exhibit a different dadi redox state, [(dadi)2-]Ti(iv)X, consistent with 20 πe- (4n). The Redox Non-Innocence (RNI) displayed by dadi m impedes binding by CO, and permits catalytic conversion of AdN3 + CO to AdNCO + N2. Kinetics measurements support carbonylation of 2 [double bond, length as m-dash]NAd as the rate determining step. Structural and computational evidence for the observed RNI is provided.

Mechanism of Me-Re Bond Addition to Platinum(II) and Dioxygen Activation by the Resulting Pt-Re Bimetallic Center

Unusual cis-oxidative addition of methyltrioxorhenium (MTO) to [PtMe₂(bpy)], (bpy = 2,2'-bipyridine) (1) is described. Addition of MTO to 1 first gives the Lewis acid-base adduct [(bpy)Me₂Pt-Re(Me)(O)₃] (2) and subsequently affords the oxidative addition product [(bpy)Me₃PtReO₃] (3). All complexes 1, MTO, 2, and 3 are in equilibrium in solution. The structure of 2 was confirmed by X-ray crystallography, and its dissociation constant in solution is 0.87 M. The structure of 3 was confirmed by extended X-ray absorption fine structure and X-ray absorption near-edge structure in tandem with one- and two-dimensional NMR spectroscopy augmented by deuterium and ¹³C isotope-labeling studies. Kinetics of formation of compound 3 revealed saturation kinetics dependence on [MTO] and first-order in [Pt], complying with prior equilibrium formation of 2 with oxidative addition of Me-Re being the rate-determining step. Exposure of 3 to molecular oxygen or air resulted in the insertion of an oxygen atom into the platinum-rhenium bond forming [(bpy)Me₃PtOReO₃] (4) as final product. Density functional theory analysis on oxygen insertion pathways leading to complex 4, merited on the basis of Russell oxidation pathway, revealed the involvement of rhenium peroxo species.

New avenues for ligand-mediated processes--expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands

Redox-active ligands have evolved from being considered spectroscopic curiosities - creating ambiguity about formal oxidation states in metal complexes - to versatile and useful tools to expand on the reactivity of (transition) metals or to even go beyond what is generally perceived possible. This review focusses on metal complexes containing either catechol, o-aminophenol or o-phenylenediamine type ligands. These ligands have opened up a new area of chemistry for metals across the periodic table. The portfolio of ligand-based reactivity invoked by these redox-active entities will be discussed. This ranges from facilitating oxidative additions upon d(0) metals or cross coupling reactions with cobalt(iii) without metal oxidation state changes - by functioning as an electron reservoir - to intramolecular ligand-to-substrate single-electron transfer to create a reactive substrate-centered radical on a Pd(ii) platform. Although the current state-of-art research primarily consists of stoichiometric and exploratory reactions, several notable reports of catalysis facilitated by the redox-activity of the ligand will also be discussed. In conclusion, redox-active ligands containing catechol, o-aminophenol or o-phenylenediamine moieties show great potential to be exploited as reversible electron reservoirs, donating or accepting electrons to activate substrates and metal centers and to enable new reactivity with both early and late transition as well as main group metals.

Reverse Catalase Reaction: Dioxygen Activation via Two-Electron Transfer from Hydroxide to Dioxygen Mediated By a Manganese(III) Salen Complex

Although atmospheric dioxygen is regarded as the most ideal oxidant, O2 activation for use in oxygenation reactions intrinsically requires a costly sacrificial reductant. The present study investigated the use of aqueous alkaline solution for O2 activation. A manganese(III) salen complex, Mn(III)(salen)(Cl), in toluene reacts with aqueous KOH solution under aerobic conditions, which yields a di-μ-oxo dimanganese(IV) salen complex, [Mn(IV)(salen)]2(μ-O)2. The (18)O isotope experiments show that (18)O2 is indeed activated to give [Mn(IV)(salen)]2(μ-(18)O)2 via a peroxide intermediate. Interestingly, the (18)OH(-) ion in H2(18)O was also incorporated to yield [Mn(IV)(salen)]2(μ-(18)O)2, which implies that a peroxide species is also generated from (18)OH(-). The addition of benzyl alcohol as a stoichiometric reductant selectively inhibits the (18)O incorporation from (18)OH(-), indicating that the reaction of Mn(III)(salen)(Cl) with OH(-) supplies the electrons for O2 reduction. The conversion of both O2 and OH(-) to a peroxide species is exactly the reverse of a catalase-like reaction, which has a great potential as the most efficient O2 activation. Mechanistic investigations revealed that the reaction of Mn(III)(salen)(Cl) with OH(-) generates a transient species with strong reducing ability, which effects the reduction of O2 by means of a manganese(II) intermediate.

Combination of redox-active ligand and lewis acid for dioxygen reduction with π-bound molybdenum-quinonoid complexes

A series of π-bound Mo-quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal-arene interactions between Mo and the π system of the quinonoid fragment. The Mo-catechol complex (2a) was found to react within minutes with 0.5 equiv of O(2) to yield a Mo-quinone complex (3), H(2)O, and CO. Si- and B-protected Mo-catecholate complexes also react with O(2) to yield 3 along with (R(2)SiO)n and (ArBO)(3) byproducts, respectively. Formally, the Mo-catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O(2) oxygens. Unreactive by itself, the Mo-dimethyl catecholate analogue reduces O(2) in the presence of added Lewis acid, B(C(6)F(5))(3), to generate a Mo(I) species and a bis(borane)-supported peroxide dianion, [[(F(5)C(6))(3)B](2)O(2)(2-)], demonstrating single-electron-transfer chemistry from Mo to the O(2) moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O(2) with pendant acids weaker than B(C(6)F(5))(3). Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O(2) is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O(2) species.

Nickel(ii) radical complexes of thiosemicarbazone ligands appended by salicylidene, aminophenol and aminothiophenol moieties

The neutral nickel(ii) complexes are chameleon pro-radical compounds: under their one-electron oxidized form they feature an iminosemiquinonate (or iminothiosemiquinonate) radical, while under their reduced form they are α-diimine π-radicals.