Redox-induced Electron Transfer Research Articles

Electrochemical water oxidation reaction by dinuclear Re(V) oxo complexes with a 1,4-benzoquinone core via the redox induced electron transfer (RIET) process.

We report two dinuclear rhenium(V) oxo complexes 1 and 2 types, [ReV(O)(Cl)3(L2-)ReV(O)(Cl)3][NBu4]2 (1, L2- = dianionic 2,5-dihydroxy 1,4-benzoquinone (DBQ2-)) and (2, L2- = dianionic chloranilic acid (CA2-) ligands), as a homogeneous electrocatalyst for water oxidation reactions in the acetonitrile-water mixture. The evolution of dioxygen gas at the anode was confirmed by a GC-TCD study. In controlled potential electrolysis (CPE), oxidation at 1.30 V (vs. Ag/AgCl) at neutral pH, 1 and 2 afforded 1+ [ReVI(O)(Cl)3(DBQ˙3-)ReVI(O)(Cl)3]- and 2+ [ReVI(O)(Cl)3(CA˙3-)ReVI(O)(Cl)3]- ions, respectively, via the redox induced electron transfer (RIET) process. Electrochemically generated species of 1+ and 2+ could be isolated in dry acetonitrile. 1+ and 2+ ions give strong EPR signals in fluid solution as well as under frozen glass conditions due to the [ReVI(O)(Cl)3(L˙3-)ReVI(O)(Cl)3]- ↔ [ReVI(O)(Cl)3(L2-)ReV(O)(Cl)3]- (where L2- = DBQ2- and CA2-) equilibrium. However, the continuation of the CPE study (1.30 V vs. Ag/AgCl) in the presence of acetonitrile-water mixture oxidised the in situ generated species of 1+ and 2+ to higher valent ReVIO species. These species (1+ and 2+) bound water through the water nucleophilic attack (WNA) to produce peroxide intermediate species of [ReV(OOH)(Cl)3(DBQ2-)ReV(OOH)(Cl)3] (A1) and [ReV(OOH)(Cl)3(CA2-)ReV(OOH)(Cl)3] (A2) for catalysts 1 and 2, respectively. Interestingly, A1 and A2 were authenticated and analysed by ESI mass spectrometry and infrared spectroscopy and were the active precursors of this water oxidation process. The extent of current generation under similar conditions suggested that complex 1 is superior to complex 2 for the water oxidation reaction. Notably, the maximum turnover frequency (TOFmax) of catalysts 1 and 2 were 2.1 and 1.6 s-1 at 0.27 V and 0.24 V over potential, respectively, which is very significant in WOR.

Two-Electron Redox Tuning of Cyclopentadienyl Cobalt Complexes Enabled by the Phenylenediamide Ligand.

Achieving multielectron activity at first-row transition-metal complexes has important implications for homogeneous catalysis using earth-abundant metals. Here, we report a family of cobalt-phenylenediamide complexes that undergo reversible 2e- oxidation regardless of the ligand substituents, enabling unprecedented multielectron redox tuning over 0.5 V and, in each case, affording the dicationic Co(III)-benzoquinonediimine species. The neutral complexes are best described as delocalized systems with π-bonding in the metallocycle, consistent with a closed-shell singlet ground state predicted by density functional theory (DFT) calculations. Our DFT results also predict an ECE pathway for 2e- oxidation (ECE = electrochemical step, chemical step, electrochemical step), where the first 1e- step involves redox-induced electron transfer to yield a Co(II) intermediate. Disruption of the metallocycle bonding in this state enables a change in the coordination geometry through association of an addition ligand, which is critical for accessing the potential inversion. The electronic properties of the phenylenediamide ligand govern whether the second electron is lost from the ligand or metal, providing a remarkable example of tunable 2e- behavior at first-row systems.

Redox induced electron transfer in lithium polysulfide – A DFT study

This study reports the existence of redox-induced electron transfer in lithium polysulfides (RIET). The presence of redox-induced electron transfer in lithium polysulfides is confirmed using a DFT study. The sulfur gets oxidized, and lithium gets reduced during the removal of an electron from lithium polysulfides. The condensed Fukui function from Natural Population Analysis (NPA) charge and electron density augment RIET. The higher number of sulfur contributes to the total nucleophilic character of lithium polysulfide and holds lithium tightly. The presence of an electric field enhances the nucleophilicity of sulfur atoms and, hence, the reactivity of lithium polysulfides.

Valence Tautomerism of p-Block Element Compounds - An Eligible Phenomenon for Main Group Catalysis?

Valence tautomerism has had a remarkable impact on several branches of transition metal chemistry. By switching between different valence tautomeric states, physicochemical properties and reactivities can be triggered reversibly. Is this phenomenon transferrable into the p‐block – or is it already happening there? This Perspective collects observations of p‐block element‐ligand systems that might be assignable to valence tautomerism. Further, it discusses occurrences in p‐block element compounds that exhibit the related effect of redox‐induced electron transfer. As disclosed, the concept of valence tautomerism with p‐block elements is at a very early stage. However, given the substantial disparity in the properties of those elements in different redox states, it might offer a valid extension for future developments in main group catalysis.

A Copper(I) Complex with Two Unpaired Electrons, Synthesised by Oxidation of a Copper(II) Complex with Two Redox-Active Ligands.

Two homoleptic copper(II) complexes [Cu(L1)2] and [Cu(L2)2] with anionic redox‐active ligands were synthesised, one with urea azine (L1) and the other with thio‐urea azine (L2) ligands. One‐electron oxidation of the complexes initiates an unprecedented redox‐induced electron transfer process, leading to monocationic copper(I) complexes [Cu(L1)2]+ and [Cu(L2)2]+ with two oxidised ligands. While [Cu(L1)2]+ is best described as a CuI complex with two neutral radical ligands that couple antiferromagnetically, [Cu(L2)2]+ is a CuI complex with two clearly different ligand units in the solid state and with a magnetic susceptibility close to a diamagnetic compound. Further one‐electron oxidation of the complex with L1 ligands results in a dication [Cu(L1)2]2+, best described as a CuI complex with a twofold oxidised, monocationic ligand and a neutral radical ligand. The stability in at least three redox states, the accumulation of spin density at the ligands and the facile ligand‐metal electron transfer make these complexes highly attractive for a variety of applications; here the catalytic aerobic oxidation of alcohols to aldehydes is tested.

Stimulation of Redox-Induced Electron Transfer by Interligand Hydrogen Bonding in a Cobalt Complex with Redox-Active Guanidine Ligand.

Octahedrally coordinated cobalt(II) complexes with a redox‐active bisguanidine ligand and acac co‐ligands were synthesized and their redox chemistry analysed in detail. The N−H functions in a bisguanidine ligand with partially alkylated guanidino groups form N−H⋅⋅⋅O hydrogen bonds with the acac co‐ligands, thereby massively influencing the redox chemistry. For all complexes, the first one‐electron oxidation is metal‐centred, leading to CoIII complexes with neutral bisguanidine ligand units. Further one‐electron oxidation is ligand‐centred in the case of Co–bisguanidine complexes with fully alkylated guanidino groups, giving CoIII complexes with radical monocationic bisguanidine ligands. On the other hand, the hydrogen‐bond strengthening upon oxidation of the Co–bisguanidine complex with partially alkylated guanidino groups initiates metal reduction (CoIII→CoII) and two‐electron oxidation of the guanidine ligand, providing the first example for the stimulation of redox‐induced electron transfer by interligand hydrogen bonding.

Evidence for a Single Electron Shift in a Lewis Acid-Base Reaction.

The Lewis acid-base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B(C6F5)3) to give the conventional B-Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV-vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid-base product. The implications of this observation are briefly discussed in light of the recent finding that radical pairs are formed in frustrated Lewis pairs.

Cobalt complexes with hemilabile o-iminobenzoquinonate ligands: a novel example of redox-induced electron transfer.

The tetracoordinated square-planar CoIII complex (imSQC(O)Ph)CoIII(APC(O)Ph) (1) bearing a radical anion and the closed-shell o-amidophenolate forms of the functionalized o-aminophenol H2LC(O)Ph were synthesized. The intermediate spin state (SCo = 1) CoIII center was found for compound 1. The cyclic voltammogram of derivative 1 contains two oxidative processes and one reductive redox process as well as an additional multi-electron wave at high negative potentials above -2 V, which can involve both the ligand and metal center. One-electron oxidation of 1 by silver triflate produces the [(imSQC(O)Ph)CoII(imQC(O)Ph)]OTf·2toluene (2) derivative with the trigonal prismatic coordination environment of the metal arising from the additional coordination of -C(O)Ph hemilabile groups. This is a first example of a trigonal prismatic coordination polyhedron in cobalt-based complexes featuring o-iminobenzoquinone ligands. The trigonal prismatic geometry achieved by the unique flexibility of the ligand allows metal-to-ligand redox-induced electron transfer (RIET). Chemical oxidation of complex 1 promotes the reduction of CoIII to CoII in compound 2 due to the redox-active nature of o-iminobenzoquinonate ligands. Remarkably, this is the first example of RIET in cobalt-based derivatives with this type of ligand. The oxidative states of the ligands and cobalt ion in both complexes were unequivocally established according to the X-ray data collection by using the utility of "metric oxidation state" (MOS). The spin states of the metal centers were unambiguously determined by density functional theory. The strong antiferromagnetic exchange via metal-ligand interactions is dominant in compounds 1 and 2, giving the doublet (S = 1/2) and triplet (S = 1) ground spin state, respectively.

Copper Complexes of New Redox-Active 4,5-Bisguanidino-Substituted Benzodioxole Ligands: Control of the Electronic Structure by Counter-Ligands, Solvent, and Temperature.

Herein, we analyze the possibility of controlling the electronic structure of mononuclear copper complexes featuring new redox-active 4,5-bisguanidino-substituted benzodioxole ligands. The nature of the guanidino groups, the anionic counter-ligands, the applied solvent (polarity), and the temperature are the parameters that decide if a CuII complex with neutral ligand unit or a CuI complex with radical monocationic ligand unit is the adequate description. Under special conditions, a temperature-dependent equilibrium of the two valence tautomeric forms (CuII /neutral ligand and CuI /radical monocationic ligand) is achieved. Removal of a ligand-centered electron from a paramagnetic CuII complex with a neutral ligand unit leads to a diamagnetic CuI complex with a dicationic ligand unit through a redox-induced electron-transfer (RIET) process.

Electronic structures of ruthenium complexes encircling non-innocent ligand assembly

Electronic structural forms of selected mononuclear and dinuclear ruthenium complexes encompassing redox non-innocent terminal as well as bridging ligands have been addressed. The sensitive valence and spin situations of the complexes have been established in the native and accessible redox states via detailed analysis of their crystal structures, electrochemistry, UV/VIS/NIR spectroelectrochemistry, EPR signatures at the paramagnetic states and DFT calculations. Mononuclear complexes exhibit significant variations in valence and spin distribution processes based on the simple modification of the non-innocent ligand frameworks as well as electronic nature of the co-ligands, σ-donating or π-accepting. Dinuclear complexes with modified pyrazine, p-quinone and azo-derived redox-active bridging ligands show complex features including redox-induced electron-transfer (RIET), remote metal to metal spin-interaction in a three-spin metal-bridge-metal arrangement as well as electron-transfer driven chemical transformation (EC). The sensitivity concerning the electronic structural aspects of the ruthenium complexes with non-innocent ligands (L) in accessible redox states could be convincingly addressed via the critical and collective analysis of crystal structures, magnetism, spectroelectrochemistry in combination with DFT calculations.

Recent advances on the chemistry of transition metal complexes of 2-(arylazo)pyridines and its arylamino derivatives

Recent advancement on the redox properties of a selection of transition metal complexes of the azoaromatic ligands: bidentate L(1) [2-(arylazo)pyridine] and tridentate HL(2) [2-(aminoarylphenylazo)pyridine] are described and compared. Due to the presence of a low lying azo-centered π*-orbital, these azoaromatic ligands may exist in multiple valent states. The coordination chemistry of the L(1) ligands was thoroughly studied during the 1980s. These complexes undergo facile reduction in solution at low accessible potentials. One electron reduced azo-complexes, though known for a long time to occur in solution, have only recently been isolated in a crystalline state. New synthetic protocols for the synthesis of metal-bound azo-radical complexes have been developed. Low-valent metal complexes such as metal carbonyls have been found to be excellent starting materials for this purpose. In a few selected cases, syntheses of these complexes were also achieved from very high valent metal oxides using triphenylphosphine as both a reducing and oxo-abstracting agent. Issues related to the ambiguities of the electronic structures in the azo-metal complexes have been discussed considering bond parameters, redox and spectral properties. Unusual redox events such as RIET (Redox-Induced Electron Transfer) phenomena in a few systems have been elaborated and compared with the known example. Novel examples of N=N bond cleavage reactions via four-electron reduction and subsequent C-N bond formation in metal-bound coordinated ligands have been noted.

Manifestations of Noninnocent Ligand Behavior

The potential of redox-active ligands to behave "noninnocently" in transition-metal coordination compounds is reflected with respect to various aspects and situations. These include the question of establishing "correct" oxidation states, the identification and characterization of differently charged radical ligands, the listing of structural and other consequences of ligand redox reactions, and the distinction between barrierless delocalized "resonance" cases M(n)/L(n) ↔ M(n+1)L(n-1) versus separated valence tautomer equilibrium situations M(n)/L(n) ⇌ M(n+1)L(n-1). Further ambivalence arises for dinuclear systems with radical bridge M(n)(μ-L(•))M(n) versus mixed-valent alternatives M(n+1)(μ-L(-))M(n), for noninnocent ligand-bridged coordination compounds of higher nuclearity such as (μ(3)-L)M(3), (μ(4)-L)M(4), (μ-L)(4)M(4), or coordination polymers. Conversely, the presence of more than one noninnocently behaving ligand at a single transition-metal site in situations such as L(n)-M-L(n-1) or L(•)-M-L(•) may give rise to corresponding ligand-to-ligand interaction phenomena (charge transfer, electron hopping, and spin-spin coupling) and to redox-induced electron transfer with counterintuitive oxidation-state changes. The relationships of noninnocent ligand behavior with excited-state descriptions and perspectives regarding material properties and single-electron or multielectron reactivity are also illustrated briefly.

Redox Induced Electron Transfer in Doublet Azo-Anion Diradical Rhenium(II) Complexes. Characterization of Complete Electron Transfer Series

Reactions of dirhenium decacarbonyl with the two azoaromatic ligands, L(a) = (2-phenylazo)pyridine and L(b) = (4-chloro-2-phenylazo)pyridine (general abbreviation of the ligands is L) afford paramagnetic rhenium(II) complexes, [Re(II)(L(*-))(2)(CO)(2)] (1) (S = 1/2 ground state) with two one-electron reduced azo-anion radical ligands in an octahedral geometrical arrangement. At room temperature (300 K) the complexes 1a-b, showed magnetic moments (mu(eff)) close to 1.94 mu(B), which is suggestive of the existence of strong antiferromagnetic interactions in the complexes. The results of magnetic measurements on one of the complexes, 1b, in the temperature range 2-300 K are reported. The above complexes showed two cathodic and two anodic responses in cyclic voltammetry where one-electron oxidation leads to an unusual redox event involving simultaneous reduction of the rhenium(II) and oxidation of the second ligand via intramolecular electron transfer. The oxidized complexes 1a(+) and 1b(+) are air stable and were isolated as crystalline solids as their tri-iodide (I(3)(-)) salts. The structures of the two representative complexes, 1b and [1b]I(3), as determined by X-ray crystallography, are compared. The anionic complexes, [1](-) and [1](2-) were characterized in solution by their spectral properties.

A Five-Center Redox System: Molecular Coupling of Two Noninnocent Imino-o-benzoquinonato-Ruthenium Functions through a π Acceptor Bridge

Combining the concepts of noninnocent behavior of metal/ligand entities and the coupling of redox-active moieties via an electronically mediating bridge led to the synthesis and the structural, electrochemical, and spectroscopic characterization of [Cl(Q)Ru(mu-tppz)Ru(Q)Cl](n) where Q(o) is 4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone and tppz(o) is 2,3,5,6-tetrakis(2-pyridyl)pyrazine, the available oxidation states being Ru(II,III,IV), Q(o,*-,2-), and tppz(o,*-,2-). One-electron transfer steps between the n = (2-) and (4+) states were studied by cyclic voltammetry and by EPR, UV-vis-NIR spectroelectrochemistry for the structurally characterized anti isomer of [Cl(Q)Ru(mu-tppz)Ru(Q)Cl](PF(6))(2), 2(PF(6))(2), the only configuration obtained. The combined investigations reveal that 2(2+) is best described as [Cl(Q(*-))Ru(III)(mu-tppz(o))Ru(III)(Q(*-))Cl](2+) with antiferromagnetic coupling between the ruthenium(III) and the iminosemiquinone components at each end. A metal-based spin as evident from large g factor anisotropy (EPR) and an intense intervalence absorption band at 1850 nm in the near-infrared (NIR) suggest that oxidation occurs at both iminosemiquinones to yield two Ru(II,III)-bonded quinones, implying redox-induced electron transfer. Reduction takes place stepwise at the metal centers yielding iminosemiquinone complexes of Ru(III,II) as evident from radical complex EPR spectra with small (99,101)Ru hyperfine contributions. After complete metal reduction to ruthenium(II) the bridging ligand tppz is being reduced stepwise as apparent from typical NIR absorption bands around 1000 nm and from small g anisotropy of the monoanion [Cl(Q(*-))Ru(II)(mu-tppz(*-))Ru(II)(Q(*-))Cl](-). A structure-based DFT calculation confirms the Ru-Cl character of the HOMO and the iminoquinone-dominated LUMO and illustrates the orbital interaction pattern of the five electron transfer active components in this new system.

Observation of Redox-Induced Electron Transfer and Spin Crossover for Dinuclear Cobalt and Iron Complexes with the 2,5-Di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate Bridging Ligand

Dinuclear [(TPyA)M(II)(DBQ(2-))M(II)(TPyA)](BF(4))(2) [TPyA = tris(2-pyridylmethyl)amine; DBQ(2-) = 2,5-di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate; M = Co (1(2+)), Fe (2(2+)), Ni (3(2+))] complexes have been prepared by the reaction of M(2+), TPyA, H(2)DBQ, and triethylamine in MeOH solution. Their monooxidized form [(TPyA)M(III)(DBQ(*3-))M(III)(TPyA)](3+) [Co = (1(3+)), Fe (2(3+))] has been synthesized by using ferrocenium tetrafluoroborate, and the dioxidized form of 1(2+), [(TPyA)Co(III)(DBQ(2-))Co(III)(TPyA)](4+) (1(4+)), has been obtained by using thianthrinium tetrafluoroborate. These dinuclear compounds were characterized by X-ray crystallography, electrochemistry, magnetism, and EPR spectroscopy. Valence ambiguous 1(3+) forms via redox-induced electron transfer, whereby the one-electron oxidation of the [Co(II)(DBQ(2-))Co(II)](2+) core forms [Co(III)(DBQ(*3-))Co(III)](3+), and it also exhibits spin crossover behavior to the core [Co(III)(DBQ(2-))Co(II)](3+) above room temperature. The M ions in 1 and 2 have a distorted octahedral geometry by coordination with four nitrogens of a TPyA, two oxygens of a DBQ(2-/*3-). Due to the interdimer offset face-to-face pi-pi and/or herringbone interactions, 1(2+), 1(3+), and 2(2+) show extended 1-D and/or 2-D supramolecular structures. The existence of DBQ(*3-) in 1(3+) is confirmed from both solid-state magnetic and solution EPR data. Co- and Ni-based 1(2+) and 3(2+) show weak antiferromagnetic interactions [1(2+): g = 2.44, J/k(B) = -3.20 K (-2.22 cm(-1)); 3(2+): g = 2.13, J/k(B) = -3.22 K (-2.24 cm(-1)), H = -2JS(1)*S(2) for 1(2+) and 3(2+)], while Fe-based 2(2+) exhibits strong spin crossover behavior above room temperature. 1(2+) has three reversible one-electron transfer waves at E(1/2) (vs SCE in MeCN) = -1.121, 0.007, and 0.329 V, and a fourth wave at -1.741 V that exhibits a slight chemical irreversibility. The first three correspond to [Co(II)DBQ(2-)Co(II)](2+) reduction to [Co(II)DBQ(*3-)Co(II)](+), and oxidation to [Co(III)DBQ(*3-)Co(III)](3+) and [Co(III)DBQ(2-)Co(III)](4+), respectively. The mechanism of the multielectron transfer oxidation from [Co(II)DBQ(2-)Co(II)](2+) to [Co(III)DBQ(*3-)Co(III)](3+) is unknown; the energy of stabilization for oxidizing the Co(II) centers in the presence of DBQ(*3-), relative to oxidizing the Co(II) centers in the presence of DBQ(2-) is computed to be 1.45 eV. 2(2+) also has three reversible one-electron transfer waves at 0.802, 0.281, and -1.007 V that correspond to two successive one-electron oxidations (2(2+)/2(3+) and 2(3+)/2(4+)), and a one-electron reduction (2(2+)/2(+)). 2(2+) has the [Fe(hs)(II)(DBQ(2-))Fe(hs)(II)](2+) electronic structure that becomes [Fe(hs)(III)(DBQ(*3-))Fe(hs)(III)](3+) upon oxidation. The latter undergoes spin crossover above room temperature to populate the [Fe(hs)(III)(DBQ(2-))Fe(hs)(II)](3+) excited state.

ChemInform Abstract: Oxidation Leading to Reduction: Redox‐Induced Electron Transfer (RIET)

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Oxidation Leading to Reduction: Redox‐Induced Electron Transfer (RIET)

Complex electron transfer reactions have been characterized whereby in addition to electron transfer, subsequent electrochemical, chemical and even in some cases biological consequences occur. These include a secondary electron transfer that leads to a major rearrangement of the electronic structure, such that an initial oxidation leads to a reduction (or an initial reduction leads to an oxidation) for these valence ambiguous compounds. Mixed valency and valence-tautomeric behaviors can additionally result from these complex electron-transfer-induced reactions.