Redox-inactive Metal Ions Research Articles

Metal ions-mediated concerted electron-proton transfer enables catalytic oxidation of phenolic contaminants by permanganate

Permanganate has been extensively applied in water treatment due to its ease of handling and high stability. However, the impact of common water constituents, especially metal ions, on permanganate oxidation is poorly understood. Here, we report that many redox-inactive metal ions, such as Ca2+, Mg2+, Zn2+, Cu2+, and Al3+, can enhance the reactivity of permanganate with phenolic compounds. Moreover, the enhancing effects of metal ions are highly pH-dependent with the largest promotion effect obtained at the pH close to phenols’ pKa. Experimental and computational analysis revealed that the oxidation of protonated phenols by permanganate underwent proton-coupled electron transfer (PCET) pathways, regardless of the presence of metal ions. Nonetheless, metal ions could catalyze the concerted electron-proton transfer (CEPT) but exhibited negligible effect on ETPT (electron transfer followed by proton transfer) and PTET (proton transfer followed by electron transfer) reactions, accounting for the pH-dependent effects of metal ions. Correlation between CEPT rate constants and the complexing capability of metal ions with phenols suggested that the co-existing metal ions may coordinate to phenolic O–H group and thus facilitate the CEPT reaction of phenols. This study could shed light on the application of permanganate in real practice and the modulation of CEPT reactions.

Metal Ion-/Proton-Coupled Electron Transfer (MPCET) on ortho-Quinone.

Quinol/quinone equilibria are ubiquitous in nature and find multiple technological applications, most recently in electrical charge storage. Much research has been devoted to proton-coupled electron transfer (PCET) in such systems and to bidentate complexation of ortho-quinol (catechol) ligands with multivalent metal ions but rarely to the interplay of these two reactions. Here, we investigate the impact of a redox-inactive metal ion, as a complexing and charge-compensating agent, on redox processes of catechol in aqueous solutions, that is, in the presence of proton equilibria. We pay separate attention to their thermodynamics and kinetics, which can be regulated by the pH and buffer capacity. As the proton buffer concentration decreases, proton equilibria during catechol PCET are slower to establish, thus kinetically prioritizing the participation of the metal ion rather than the proton in the redox charge compensation. Making use of this kinetic interplay can be a general strategy to conceive organic battery cathodes for proton-free metal-ion aqueous batteries.

Fresh Impetus in the Chemistry of Calcium Peroxides.

Redox-inactive metal ions are essential in modulating the reactivity of various oxygen-containing metal complexes and metalloenzymes, including photosystem II (PSII). The heart of this unique membrane-protein complex comprises the Mn4CaO5 cluster, in which the Ca2+ ion acts as a critical cofactor in the splitting of water in PSII. However, there is still a lack of studies involving Ca-based reactive oxygen species (ROS) systems, and the exact nature of the interaction between the Ca2+ center and ROS in PSII still generates intense debate. Here, harnessing a novel Ca-TEMPO complex supported by the β-diketiminate ligand to control the activation of O2, we report the isolation and structural characterization of hitherto elusive Ca peroxides, a homometallic Ca hydroperoxide and a heterometallic Ca/K peroxide. Our studies indicate that the presence of K+ cations is a key factor controlling the outcome of the oxygenation reaction of the model Ca-TEMPO complex. Combining experimental observations with computational investigations, we also propose a mechanistic rationalization for the reaction outcomes. The designed approach demonstrates metal-TEMPO complexes as a versatile platform for O2 activation and advances the understanding of Ca/ROS systems.

Scandium Ion-Promoted Electron-Transfer Disproportionation of 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-Oxide (PTIO•) in Acetonitrile and Its Regeneration Induced by Water.

2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO•), a persistent nitronyl nitroxide radical, has been used for the detection and trapping of nitric oxide, as a redox mediator for batteries, for the activity estimation of antioxidants, and so on. However, there is no report on the reactivity of PTIO• in the presence of redox-inactive metal ions. In this study, it is demonstrated that the addition of scandium triflate, Sc(OTf)3 (OTf = OSO2CF3), to an acetonitrile (MeCN) solution of PTIO• resulted in an electron-transfer disproportionation to generate the corresponding cation (PTIO+) and anion (PTIO-), the latter of which is suggested to be stabilized by Sc3+ to form [(PTIO)Sc]2+. The decay of the absorption band at 361 nm due to PTIO•, monitored using a stopped-flow technique, obeyed second-order kinetics. The second-order rate constant for the disproportionation, thus determined, increased with increasing the Sc(OTf)3 concentration to reach a constant value. A drastic change in the cyclic voltammogram recorded for PTIO• in deaerated MeCN containing 0.10 M Bu4NClO4 was also observed upon addition of Sc(OTf)3, suggesting that the large positive shift of the one-electron reduction potential of PTIO• (equivalent to the one-electron oxidation potential of PTIO-) in the presence of Sc(OTf)3 may result in the disproportionation. When H2O was added to the PTIO•-Sc(OTf)3 system in deaerated MeCN, PTIO• was completely regenerated. It is suggested that the complex formation of Sc3+ with H2O may weaken the interaction between PTIO- and Sc3+, leading to electron-transfer comproportionation to regenerate PTIO•. The reversible disproportionation of PTIO• was also confirmed by electron paramagnetic resonance (EPR) spectroscopy.

Shift of the reduction potential of nickel(II) Schiff base complexes in the presence of redox innocent metal ions.

With the objective of gaining insight into the modulation of the reduction potential of the Ni(II/I) couple, we have synthesized two mononuclear nickel(II) complexes, NiLen (H2Len = N,N'-bis(3-methoxysalicylidene)-1,2-diamino-2-methylpropane) and NiLpn (H2Lpn = N,N'-bis(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane) of two N2O4 donor ligands and recorded their cyclic voltammograms. Both the nickel complexes show reversible reduction processes for the Ni(II/I) couple in acetonitrile solution but the reduction potential of NiLpn (E1/2 = -1.883 V) is 188 mV more positive than that of NiLen (E1/2 = -2.071 V). In the presence of redox inactive metal ions (Li+, Na+, K+, Mg2+, Ca2+ and Ba2+), the reduction potentials are shifted by 49-331 mV and 99-435 mV towards positive values compared to NiLen and NiLpn, respectively. The shift increases with the decrease of the pKa of the respective aqua-complexes of the metal ion but is poorly co-linear; however, better linearity is found when the shift of the mono- and bi-positive metal ion aqua complexes is plotted separately. Spectrophotometric titrations of these two nickel complexes with the guest metal ions in acetonitrile showed a well-anchored isosbestic point in all cases, confirming the adduct formation of NiLen and NiLpn with the metal ions. Structural analysis of single crystals, [(NiLen)Li(H2O)2]·ClO4 (1), [(NiLpn)Li(H2O)]·ClO4 (2), [(NiLpn)2Na]·BF4 (3) and [(NiLpn)2Ba(H2O)(ClO4)]·ClO4 (4), also corroborates the heterometallic adduct formation. The orbital energies of the optimised heterometallic adducts from which electron transfers originated were calculated in order to explain the observed reduction process. A strong linear connection between the calculated orbital energies and the experimental E1/2 values was observed. According to MEP and 2D vector field plots, the largest shift for divalent metal ions is most likely caused by the local electric field that they impose in addition to Lewis acidity.

Chemistry of a series of heterobimetallic complexes MnIII2(CaII/SrII)X2 (X = Cl-, Br-).

This manuscript describes the syntheses, structures and magnetism of MnIII-CaII/SrII complexes which are compositionally relevant in the context of the oxygen evolving complex (OEC) of photosystem II (PS II). A series of trimetallic tetraoxo complexes containing redox-inactive metal ions CaII or SrII were synthesized using a tetranucleating ligand framework. The structural characteristics of these complexes, with the oxido ligands bridging the redox-inactive metals and the manganese centres, make them particularly relevant to biological and heterogeneous metal-oxido clusters. Electrochemical studies of these compounds show that the reduction potentials are highly dependent upon the Lewis acidity of the redox-inactive metal, identifying the chemical basis for the observed differences in electrochemistry. This correlation provides insights into the role of the CaII/SrII ion in modulating the redox potential of the OEC and of other redox-inactive ions in tuning the redox potentials of other metal-oxide electrocatalysts. Temperature dependent magnetic measurements have also been performed for the complexes.

Understanding the Effect of Internal Electrostatic Fields Created by Alkaline Earth Metal Ions Poised over Secondary Coordination Sphere of Molecular Iron Complexes.

Understanding the effect of the local electrical field around the reaction center in enzymes and molecular catalysis is an important topic of research. Herein, we explored the electrostatic field exerted by the alkaline earth metal ions (M2+ = Mg2+, Ca2+, Sr2+, and Ba2+) around Fe in FeIII(Cl) complexes by experimental and computational investigations. M2+ coordinated dinuclear FeIII(Cl) complexes (12M) were synthesized and characterized by X-ray crystallography and different spectroscopic techniques. EPR and magnetic moment measurements exhibited the presence of high-spin FeIII centers in the 12M complexes. Electrochemical investigations revealed FeIII/FeII reduction potential values shifted anodically in 12M complexes compared to 1. Likewise, 2p3/2 and 2p1/2 peaks in the XPS data were found to shift positively in the 12M complexes, demonstrating that redox-inactive metal ions make FeIII more electropositive. However, nearly similar λmax values in the UV-vis spectra were observed in 1 and 12M complexes. The first-principles-based computational simulations further revealed the impact of M2+ on stabilizing 3d-orbitals of Fe. The distortion in Laplacian distribution (∇2ρ(r)) of electron density around M2+ also indicates the possibility of having Fe-M interactions in these complexes. The absence of a bond critical point between FeIII and M2+ ions in the 12M complexes indicates dominant through-space interaction between these metal centers. Experimental and computational studies collectively imply that the installation of internal electrostatic fields exerted by M2+ ions in 12M complexes alters the electronic structure of FeIII.

In vitro radical-scavenging mechanism of melatonin and its in vivo protective effect against radiation-induced lipid peroxidation

Melatonin (N-acetyl-5-methoxytryptamine, MLT), an evolutionarily conserved indoleamine, is known to act as an antioxidant. However, the evidence indicating the role of MLT as a powerful chain-breaking antioxidant by scavenging peroxyl radical remains controversial. The radical-scavenging rate of MLT determined in this study in methanol using galvinoxyl radical (GO•) was much lower than that of an α-tocopherol model compound. The acceleration of the GO•-scavenging reaction by MLT was observed in the presence of magnesium ion (Mg2+), a bio-related redox-inactive metal ion, suggesting that this reaction may proceed via a rate-determining electron transfer followed by proton transfer. The coordination of Mg2+ to the carbonyl oxygen in the one-electron reduced species of GO• (GO–) may stabilize the product, resulting in the acceleration of the electron-transfer process. We also demonstrated that prophylactically administrated MLT efficiently inhibited the lipid peroxide-derived protein modification, which can be detected by a sensitive marker, Nε-(hexanoyl)lysine adduct, in the plasma of X-irradiated mice. The relatively weak GO•-scavenging activity of MLT suggests that the ameliorative effect of MLT against in vivo lipid peroxidation does not result from the direct scavenging of lipid peroxyl radicals by MLT. Therefore, the observed superior protective efficiency of MLT against in vivo lipid peroxidation may partly support the earlier studies, which reported the synergistic antioxidative effect of the metabolites of MLT.

Electrochemical generation of high-valent oxo-manganese complexes featuring an anionic N5 ligand and their role in O-O bond formation.

Generation of high-valent oxomanganese complexes through controlled removal of protons and electrons from low-valent congeners is a crucial step toward the synthesis of functional analogues of the native oxygen evolving complex (OEC). In-depth studies of the water oxidation activity of such biomimetic compounds help in understanding the mechanism of O-O bond formation presumably occurring in the last step of the photosynthetic cycle. Scarce reports of reactive high-valent oxomanganese complexes underscore the impetus for the present work, wherein we report the electrochemical generation of the non-heme oxomanganese(IV) species [(dpaq)MnIV(O)]+ (2) through a proton-coupled electron transfer (PCET) process from the hydroxomanganese complex [(dpaq)MnIII(OH)]ClO4 (1). Controlled potential spectroelectrochemical studies of 1 in wet acetonitrile at 1.45 V vs. NHE revealed quantitative formation of 2 within 10 min. The high-valent oxomanganese(IV) transient exhibited remarkable stability and could be reverted to the starting complex (1) by switching the potential to 0.25 V vs. NHE. The formation of 2via PCET oxidation of 1 demonstrates an alternate pathway for the generation of the oxomanganese(IV) transient (2) without the requirement of redox-inactive metal ions or acid additives as proposed earlier. Theoretical studies predict that one-electron oxidation of [(dpaq)MnIV(O)]+ (2) forms a manganese(V)-oxo (3) species, which can be oxidized further by one electron to a formal manganese(VI)-oxo transient (4). Theoretical analyses suggest that the first oxidation event (2 to 3) takes place at the metal-based d-orbital, whereas, in the second oxidation process (3 to 4), the electron eliminates from an orbital composed of equitable contribution from the metal and the ligand, leaving a single electron in the quinoline-dominant orbital in the doublet ground spin state of the manganese(VI)-oxo species (4). This mixed metal-ligand (quinoline)-based oxidation is proposed to generate a formal Mn(VI) species (4), a non-heme analogue of the species 'compound I', formed in the catalytic cycle of cytochrome P-450. We propose that the highly electrophilic species 4 catches water during cyclic voltammetry experiments and results in O-O bond formation leading to electrocatalytic oxidation of water to hydrogen peroxide.

Effects of metal ions on cyanocobalamin stability in heated milk protein‐based matrices

SummaryLosses of vitamin B12 during the processing and/or storage of foods and supplements have been associated with both reducing and oxidising agents. We evaluated the effects of six redox active (Cu2+, Fe2+, Fe3+, Mn2+, Ce3+, and Cr6+) and one redox inactive (Zn2+) metal ions on cyanocobalamin stability in heat‐treated, milk protein‐based matrices. The metal ion could either decrease (by ≤76%) or increase (by ≤56%) cyanocobalamin loss, depending on metal ion concentration and redox potential, as well as on matrix properties (metal ion binding, redox activity, and reactive oxygen species generation) and on protein properties (cysteine content, conformation, and charge density). The largest losses of cyanocobalamin appeared to be caused by redox cycling mechanisms (for Cu2+ and Fe2+); however, indications were that such losses could be mitigated by metal ion modification of the matrix. Only the redox inactive metal ion Zn2+ inhibited cyanocobalamin loss in all experimentally evaluated matrices.

Peroxide-Selective Reduction of O2 at Redox-Inactive Rare-Earth(III) Triflates Generates an Ambiphilic Peroxide.

Metal peroxides are key species involved in a range of critical biological and synthetic processes. Rare-earth (group III and the lanthanides; Sc, Y, La-Lu) peroxides have been implicated as reactive intermediates in catalysis; however, reactivity studies of isolated, structurally characterized rare-earth peroxides have been limited. Herein, we report the peroxide-selective (93-99% O22-) reduction of dioxygen (O2) at redox-inactive rare-earth triflates in methanol using a mild metallocene reductant, decamethylferrocene (Fc*). The first molecular praseodymium peroxide ([PrIII2(O22-)(18C6)2(EG)2][OTf]4; 18C6 = 18-crown-6, EG = ethylene glycol, -OTf = -O3SCF3; 2-Pr) was isolated and characterized by single-crystal X-ray diffraction, Raman spectroscopy, and NMR spectroscopy. 2-Pr displays high thermal stability (120 °C, 50 mTorr), is protonated by mild organic acids [pKa1(MeOH) = 5.09 ± 0.23], and engages in electrophilic (e.g., oxygen atom transfer) and nucleophilic (e.g., phosphate-ester cleavage) reactivity. Our mechanistic studies reveal that the rate of oxygen reduction is dictated by metal-ion accessibility, rather than Lewis acidity, and suggest new opportunities for differentiated reactivity of redox-inactive metal ions by leveraging weak metal-ligand binding events preceding electron transfer.

Effect of Redox-Inactive Metal Ion-Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity.

Mononuclear nickel(II) and nickel(III) complexes of a bisamidate-bisalkoxide ligand, (NMe4)2[NiII(HMPAB)] (1) and (NMe4)[NiIII(HMPAB)] (2), respectively, have been synthesized and characterized by various spectroscopic techniques including X-ray crystallography. The reaction of redox-inactive metal ions (Mn+ = Ca2+, Mg2+, Zn2+, Y3+, and Sc3+) with 2 resulted in 2-Mn+ adducts, which was assessed by an array of spectroscopic techniques including X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and reactivity studies. The X-ray structure of Ca2+ coordinated to Ni(III) complexes, 2-Ca2+T, was determined and exhibited an average Ni-Ca distance of 3.1253 Å, close to the metal ions' covalent radius. XAS analysis of 2-Ca2+ and 2-Y3+ in solution further revealed an additional coordination to Ca and Y in the 2-Mn+ adducts with shortened Ni-M distances of 2.15 and 2.11 Å, respectively, implying direct bonding interactions between Ni and Lewis acids (LAs). Such a short interatomic distance between Ni(III) and M is unprecedented and was not observed before. EPR analysis of 2 and 2-Mn+ species, moreover, displayed rhombic signals with gav > 2.12 for all complexes, supporting the +III oxidation state of Ni. The NiIII/NiII redox potential of 2 and 2-Mn+ species was determined, and a plot of E1/2 of 2-Mn+ versus pKa of [M(H2O)n]m+ exhibited a linear relationship, implying that the NiIII/NiII potential of 2 can be tuned with different redox-inactive metal ions. Reactivity studies of 2 and 2-Mn+ with different 4-X-2,6-ditert-butylphenol (4-X-DTBP) and other phenol derivatives were performed, and based on kinetic studies, we propose the involvement of a proton-coupled electron transfer (PCET) pathway. Analysis of the reaction products after the reaction of 2 with 4-OMe-DTBP showed the formation of a Ni(II) complex (1a) where one of the alkoxide arms of the ligand is protonated. A pKa value of 24.2 was estimated for 1a. The reaction of 2-Mn+ species was examined with 4-OMe-DTBP, and it was observed that the k2 values of 2-Mn+ species increase by increasing the Lewis acidity of redox-inactive metal ions. However, the obtained k2 values for 2-Mn+ species are much lower compared to the k2 value for 2. Such a variation of PCET reactivity between 2 and 2-Mn+ species may be attributed to the interactions between Ni(III) and LAs. Our findings show the significance of the secondary coordination sphere effect on the PCET reactivity of Ni(III) complexes and furnish important insights into the reaction mechanism involving high-valent nickel species, which are frequently invoked as key intermediates in Ni-mediated enzymatic reactions, solar-fuel catalysis, and biomimetic/synthetic transformation reactions.

Revealing the Influence of Diverse Secondary Metal Cations on Redox-Active Palladium Complexes.

Incorporation of redox-inactive metals into redox-active complexes and catalysts attracts attention for engendering new reactivity modes, but this strategy has not been extensively investigated beyond the first-row of the transition metals. Here, the isolation and characterization of the first series of heterobimetallic complexes of palladium with mono-, di-, and tri-valent redox-inactive metal ions are reported. A Reinhoudt-type heteroditopic ligand with a salen-derived [N2 ,O2 ] binding site for Pd and a crown-ether-derived [O6 ] site has been used to prepare isolable adducts of the Lewis acidic redox-inactive metal ions (Mn+ ). Comprehensive data from single-crystal X-ray diffraction analysis reveal distinctive trends in the structural properties of the heterobimetallic species, including an uncommon dependence of the Pd⋅⋅⋅M distance on Lewis acidity. The reorganization energy associated with reduction of the heterobimetallic species is strongly modulated by Lewis acidity, with the slowest heterogeneous electron transfer kinetics associated with the strongest incorporated Lewis acids. This hitherto unexplored reorganization energy penalty for electron transfer contrasts with prior thermodynamic studies, revealing that kinetic parameters should be considered in studies of reactivity involving heterobimetallic species.

Role of Redox-Inactive Metal Ions in Modulating the Reduction Potential of Uranyl Schiff Base Complexes: Detailed Experimental and Theoretical Studies.

A mononuclear uranyl complex, [UO2L] (1), has been synthesized with the ligand N,N'-bis(3-methoxy-2-hydroxybenzylidene)-1,6-diamino-3-azahexane (H2L). The complex showed a reversible U(VI)/U(V) redox couple in cyclic voltammetric measurements. The reduction potential of this couple showed a positive shift upon the addition of redox-inactive alkali- and alkaline-earth Lewis acidic metal ions (Li+, Na+, K+, Ca2+, Sr2+, and Ba2+) to an acetonitrile solution of complex 1. The positive shift of the reduction potential has been explained on the basis of the Lewis acidity and internal electric-field effect of the respective metal ions. The bimetallic complexes [UO2LLi(NO3)] (2), [UO2LNa(BF4)]2 (3), [UO2LK(PF6)]2 (4), [(UO2L)2Ca]·(ClO4)2·CH3CN (5), [(UO2L)2Sr(H2O)2]·(ClO4)2·CH3CN (6), and [(UO2L)2Ba(ClO4)]·(ClO4) (7) have also been isolated in the solid state by reacting complex 1 with the corresponding metal ions and characterized by single-crystal X-ray diffraction. Density functional theory calculations of the optimized [UO2LM]n+ complexes have been used to rationalize the experimental reduction and electric-field potentials imposed by the non-redox-active cations.

Electrochemical Properties and Reactivity Study of [MnV(O)(μ-OR-Lewis Acid)] Cores.

A mononuclear manganese(V) oxo complex of a bis(amidate)bis(alkoxide) ligand, (NMe4)[MnV(HMPAB)(O)] [2; H4HMPAB = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene], was synthesized and structurally characterized. A Mn-Oterm distance of 1.566(4) Å was observed in the solid-state structure of 2, consistent with the Mn≡O formulation. The reaction of redox-inactive metal ions (Mn+ = Li+, Ca2+, Mg2+, Y3+, and Sc3+) with 2 resulted in the formation of 2-Mn+ species, which were characterized by UV-vis, 1H NMR, cyclic voltammetry, and in situ IR spectroscopy. Theoretical calculations suggested that the alkoxide oxygen atoms of the ligand scaffold are energetically most favorable for coordinating the Mn+ ions in 2. Complex 2 revealed one-electron-reduction potential at -0.01 V versus ferrocenium/ferrocene, which shifted anodically upon coordination of Mn+ ions to 2, and such a shift became more prominent with stronger Lewis acids. The oxygen-atom transfer (OAT) reactivities of 2 and 2-Mn+ species with triphenylphosphine were compared, which exhibited a systematic increase of the reaction rate with increasing Lewis acidity of Mn+ ions, and a plot of log k2 versus Lewis acidity of Mn+ ions (ΔE) followed a linear relationship. It was observed that 2-Sc3+ was ca. 3200 times more reactive toward the OAT reaction compared to 2. Hammett analysis of 2 exhibited a V-shaped plot, indicating a change of the reaction mechanism upon going from electron-rich to electron-deficient Ar3P substrates. In contrast, 2-Ca2+ and 2-Sc3+ showed an electrophilic nature toward the OAT reaction, thus demonstrating the role of the Lewis acid in controlling the OAT mechanism. The hydrogen-atom abstraction reaction of 2 and 2-Mn+ adducts with 1-benzyl-1,4-dihydronicotinamide was investigated, and it was observed that the rate of reaction did not vary considerably with the Lewis acidity of Mn+ ions. On the basis of Eyring analysis of 2 and 2-Mn+ adducts, we hypothesized an entropy-controlled hydrogen-atom-transfer reaction for 2-Sc3+, which is different from the reaction mechanism of 2 and 2-Ca2+.

Rare-Earth Elements Can Structurally and Energetically Replace the Calcium in a Synthetic Mn4CaO4-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis.

The oxygen-evolving center (OEC) in photosynthesis is a unique biological Mn4CaO5 cluster catalyzing the water-splitting reaction. A great current challenge is to achieve a robust and precise mimic of the OEC in the laboratory. Herein, we report synthetic Mn4XO4 clusters (X = calcium, yttrium, gadolinium) that closely resemble the OEC with regard to the main metal-oxide core and peripheral ligands, as well as the oxidation states of the four Mn ions and the redox potential of the cluster. We demonstrate that rare-earth elements can structurally replace the calcium in neutral Mn4XO4 clusters. All three Mn4XO4 clusters with different redox-inactive metal ions display essentially the same redox properties, challenging the conventional view that the Lewis acidity of the redox-inactive metal ions could modulate the redox potential of the heteronuclear-oxide clusters. The new synthetic rare-earth element-containing Mn4XO4 clusters reported here provide robust and structurally well-defined chemical models and shed new light on the design of new water-splitting catalysts in artificial photosynthesis.

Controlled Regulation of the Nitrile Activation of a Peroxocobalt(III) Complex with Redox-Inactive Lewis Acidic Metals.

Redox-inactive metal ions play vital roles in biological O2 activation and oxidation reactions of various substrates. Recently, we showed a distinct reactivity of a peroxocobalt(III) complex bearing a tetradentate macrocyclic ligand, [CoIII(TBDAP)(O2)]+ (1) (TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane), toward nitriles that afforded a series of hydroximatocobalt(III) complexes, [CoIII(TBDAP)(R-C(═NO)O)]+ (R = Me (3), Et, and Ph). In this study, we report the effects of redox-inactive metal ions on nitrile activation of 1. In the presence of redox-inactive metal ions such as Zn2+, La3+, Lu3+, and Y3+, the reaction does not form the hydroximatocobalt(III) complex but instead gives peroxyimidatocobalt(III) complexes, [CoIII(TBDAP)(R-C(═NH)O2)]2+ (R = Me (2) and Ph (2Ph)). These new intermediates were characterized by various physicochemical methods including X-ray diffraction analysis. The rates of the formation of 2 are found to correlate with the Lewis acidity of the additive metal ions. Moreover, complex 2 was readily converted to 3 by the addition of a base. In the presence of Al3+, Sc3+, or H+, 1 is converted to [CoIII(TBDAP)(O2H)(MeCN)]2+ (4), and further reaction with nitriles did not occur. These results reveal that the reactivity of the peroxocobalt(III) complex 1 in nitrile activation can be regulated by the redox-inactive metal ions and their Lewis acidity. DFT calculations show that the redox-inactive metal ions stabilize the peroxo character of end-on Co-η1-O2 intermediate through the charge reorganization from a CoII-superoxo to a CoIII-peroxo intermediate. A complete mechanistic model explaining the role of the Lewis acid is presented.

Redox-Inactive Metal Ions That Enhance the Nucleophilic Reactivity of an Alkylperoxocopper(II) Complex.

The importance of redox-inactive metal ions in modulating the reactivity of redox-active biological systems is a subject of great current interest. In this work, the effect of redox-inactive metal ions (M3+ = Sc3+, Y3+, Yb3+, La3+) on the nucleophilic reactivity of a mononuclear ligand-based alkylperoxocopper(II) complex, [Cu(iPr2-tren-C(CH3)2O2)]+ (1), was examined. 1 was prepared by the addition of hydrogen peroxide and triethylamine to the solution of [Cu(iPr3-tren)(CH3CN)]+ (iPr3-tren = tris[2-(isopropylamino)ethyl]amine) via the formation of [Cu(iPr3-tren)(O2H)]+ (2) in methanol (CH3OH) at 30 °C. 1 was characterized using density functional theory (DFT) calculations and spectroscopic methods such as UV-vis, resonance Raman (rR), and electron paramagnetic resonance (EPR). DFT calculations support the electronic structure of 1 with an intermediate geometry between the trigonal-bipyramidal and square-pyramidal geometries, which is consistent with the observed EPR signal exhibiting a signal with g⊥ = 2.03 (A⊥ = 16 G) and g|| = 2.19 (A|| = 158 G). The Cu-O bond stretching frequency of 1 was observed at 507 cm-1 for 16O2 species (486 cm-1 for 18O2 species), and its O-O vibrational energy was determined to be 799 cm-1 for 16O2 species (759 cm-1 for 18O2 species) by rR spectroscopy. The reactivity of 1 was investigated in oxidative nucleophilic reactions. The positive slope of the Hammett plot (ρ = 2.3(1)) with para-substituted benzaldehydes and the reactivity order with 1°-, 2°-, and 3°-CHO demonstrate well the nucleophilic character of this copper(II) ligand-based alkylperoxo complex. The Lewis acidity of M3+ improves the oxidizing ability of 1. The modulated reactivity of 1 with M3+ was revealed to be an opposite trend of the Lewis acidity of M3+ in aldehyde deformylation.

Effects of Covalency on Anionic Redox Chemistry in Semiquinoid-Based Metal-Organic Frameworks.

Two iron-semiquinoid framework materials, (H2NMe2)2Fe2(Cl2 dhbq)3 (1) and (H2NMe2)4Fe3(Cl2 dhbq)3(SO4)2 (Cl2 dhbqn- = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) (2-SO4), are shown to possess electrochemical capacities of up to 195 mAh/g. Employing a variety of spectroscopic methods, we demonstrate that these exceptional capacities arise from a combination of metal- and ligand-centered redox processes, a result supported by electronic structure calculations. Importantly, similar capacities are not observed in isostructural frameworks containing redox-inactive metal ions, highlighting the importance of energy alignment between metal and ligand orbitals to achieve high capacities at high potentials in these materials. Prototype lithium-ion devices constructed using 1 as a cathode demonstrate reasonable capacity retention over 50 cycles, with a peak specific energy of 533 Wh/kg, representing the highest value yet reported for a metal-organic framework. In contrast, the capacities of devices using 2-SO4 as a cathode rapidly diminish over several cycles due to the low electronic conductivity of the material, illustrating the nonviability of insulating frameworks as cathode materials. Finally, 1 is further demonstrated to access similar capacities as a sodium-ion or potassium-ion cathode. Together, these results demonstrate the feasibility and versatility of metal-organic frameworks as energy storage materials for a wide range of battery chemistries.

Tuning Electron-Transfer Reactivity of a Chromium(III)–Superoxo Complex Enabled by Calcium Ion and Other Redox-Inactive Metal Ions

Calcium ion plays an indispensable role for water oxidation by oxygen-evolving complex (OEC) composed of a manganese-oxo cluster (Mn4CaO5) in Photosystem II. In this context, the effects of Ca2+ ion and other redox-inactive metal ions on the redox reactivity of high-valent metal-oxo and metal-peroxo complexes have been studied extensively. Among metal-oxygen intermediates involved in interconversion between H2O and O2, however, the effects of Ca2+ ion and other redox-inactive metal ions (Mn+) on the redox reactivity of metal-superoxo complexes have yet to be reported. Herein, we report that electron transfer (ET) from octamethylferrocene (Me8Fc) to a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)CrIII(O2)]+ (1), occurs in the presence of redox-inactive metal ions (Mn+ = Ca2+, Mg2+, Y3+, Al3+, and Sc3+); in the absence of the redox-inactive metal ions, ET from Me8Fc to 1 does not occur. The second-order rate constants (ket) of ET from Me8Fc to 1 in the presence of a redox-inactive metal ion increased with increasing concentration of Mn+ ([Mn+]), exhibiting a second-order dependence on [Mn+]: ket = kMCET[Mn+]2, where kMCET is the fourth-order rate constant of metal ion-coupled electron transfer (MCET). This means that two Mn+ ions are bound to the one-electron reduced species of 1. Such a binding of two Mn+ ions associated with the ET reduction of 1 resulted in a 92 mV positive shift of the one-electron reduction potential of 1 (Ered) with increasing log([Mn+]). The log kMCET values increased linearly with the increasing Lewis acidity of Mn+ (ΔE), which was determined from the g values of O2•--Mn+ complexes. The driving force dependence of log ket of MCET from ferrocene derivatives to 1 in the presence of Mn+ has been well-evaluated in light of the Marcus theory of electron transfer.