Outer-sphere Electron Transfer Research Articles

Monovalent Nickel-Mediated Radical Formation: A Concerted Halogen-Atom Dissociation Pathway Determined by Electroanalytical Studies.

The recent success of nickel catalysts in stereoconvergent cross-coupling and cross-electrophile coupling reactions partly stems from the ability of monovalent nickel species to activate C(sp3) electrophiles and generate radical intermediates. This electroanalytical study of the commonly applied (bpy)Ni catalyst elucidates the mechanism of this critical step. Data rule out outer-sphere electron transfer and two-electron oxidative addition pathways. The linear free energy relationship between rates and the bond-dissociation free energies, the electronic and steric effects of the nickel complexes and the electrophiles, and DFT calculations support a variant of the halogen-atom abstraction pathway, the inner-sphere electron transfer concerted with halogen-atom dissociation. This mechanism accounts for the observed reactivity of different electrophiles in cross-coupling reactions and provides a mechanistic rationale for the chemoselectivity obtained in cross-electrophile coupling over homocoupling.

Reorganization Energies, Entropies, and Free Energy Surfaces for Electron Transfer.

Reorganization energies for an intramolecular self-exchange electron-transfer reaction are calculated by quantum-classical molecular dynamics simulations in four solvents with varying polarity and at temperatures ranging from 250 to 350 K. The reorganization free energies for polar solvents decrease systematically with increasing temperature, indicating that they include substantial contributions from entropy changes. The variances of the energy gap between the reactant and product states have a major component that is relatively insensitive to temperature. Explanations are suggested for these observations, which appear to necessitate rethinking the free energy functions of a distributed coordinate that frequently are used in discussions of reaction dynamics.

Semimicroscopic Theory for Underscreening-Induced Anomalous Migration–Diffusion Coupling

We develop a novel theory for the impedance response of reversible electron transfer reactions under the influence of migration–diffusion coupling. Theory accounts for the influence of the supporting electrolyte, electroactive species, solvent, metal, and the interfacial electric field accounting underscreening. This semimicroscopic formalism is based on a modified electrode surface boundary constraint, which circumvents the essential nonlinearity in the Nernst–Planck equation. The interfacial electric field determines the kinetics of formation of compact electric double layer (C-EDL) embedded with electroactive species and influence the mass transport. This kinetics is governed by an intrinsic C-EDL formation resistance (RH), solution resistance (RΩ), and the extent of ion packing dependent on electrolyte composition. Theory unravels a characteristic ohmic-migration frequency, ω* = D/LMΩ2, a prerequisite for the outer sphere electron transfer step. A dimensionless migration–diffusion coupling number δM is unriddled, which quantifies the extent of migration contribution dependent on the potentials at IHP (ϕ1), OHP (ϕ2), diffuse layer (ϕDL), ion size-corrected screening length, charge on electroactive species, interfacial diffusivity, and RH. The magnitude of impedance (in low-frequency regime) evinces an anomalous nonmonotonic variation with ionic strength. Finally, theory accredits us to understand the underscreening-induced anomalies in migration–diffusion-controlled impedance response.

Controlled Single-Electron Transfer via Metal-Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways.

Electrocatalysis enables the construction of C-C bonds under mild conditions via controlled formation of carbon-centered radicals. For sequences initiated by alkyl halide reduction, coordinatively unsaturated Ni complexes commonly serve as single-electron transfer agents, giving rise to the foundational question of whether outer- or inner-sphere electron transfer oxidative addition prevails in redox mediation. Indeed, rational design of electrochemical processes requires the discrimination of these two electron transfer pathways, as they can have outsized effects on the rate of substrate bond activation and thus impact radical generation rates and downstream product selectivities. We present results from combined synthetic, electroanalytical, and computational studies that examine the mechanistic differences of single electron transfer to alkyl halides imparted by Ni metal-ligand cooperativity. Electrogenerated reduced Ni species, stabilized by delocalized spin density onto a redox-active tpyPY2Me polypyridyl ligand, activates alkyl iodides via outer-sphere electron transfer, allowing for the selective activation of alkyl iodide substrates over halogen atom donors and the controlled generation and sequestration of electrogenerated radicals. In contrast, the Ni complex possessing a redox-innocent pentapyridine congener activates the substrates in an inner-sphere fashion owning to a purely metal-localized spin, thereby activating both substrates and halogen atom donors in an indiscriminate fashion, generating a high concentration of radicals and leading to unproductive dimerization. Our data establish that controlled electron transfer via Ni-ligand cooperativity can be used to limit undesired radical recombination products and promote selective radical processes in electrochemical environments, providing a generalizable framework for designing redox mediators with distinct rate and potential requirements.

Cation-Dependent Interfacial Structures and Kinetics for Outer-Sphere Electron-Transfer Reactions

The kinetics of aqueous outer-sphere electron-transfer (ET) reactions are determined in large part by noncovalent electrostatic interactions that originate from the surrounding electrolyte solution. In this work, we examine the role of spectator cations in modifying the rate of heterogeneous ET for an [Fe(CN)6]3–/[Fe(CN)6]4– redox pair. We combine the results of electrochemical measurement, in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), classical molecular dynamics simulation, and theoretical modeling to demonstrate how changing the identity of the spectator cation species over a series that includes Li+, Na+, K+, Rb+ to Cs+ influences the solvation properties and ET kinetics of the redox species. By analyzing the results in the context of the Marcus–Hush–Chidsey (MHC) theory, we find that the solvent reorganization energy increases systematically as the cationic radius decreases. The trend can be attributed to cation-dependent coordination environments of the redox species, whereby more cations of less charge density such as Cs+ than Li+ are present in the redox solvation shell in bulk and at the electrified interface, promoting weaker hydrogen bonds and lowering the effective interfacial static dielectric constant. We discuss the implications of these findings for enabling the tunability of reaction thermodynamics and rates in electrochemical processes.

Redox Properties of CeIVDOTA in Carbonated Aqueous Solutions. A Radiolytic and an Electrochemical Study.

The redox chemistry of CeIIIDOTA in cage in carbonate solutions was studied using electrochemistry and radiolysis techniques (continuous radiolysis and pulse radiolysis). Spectroscopic measurements point out that the species present in the solutions at high bicarbonate concentrations are [CeIIIDOTA(CO3)]3- (or less plausible [CeIIIDOTA(HCO3)]2-) with the carbonate (bicarbonate) anion as the ninth ligand versus [CeIIIDOTA(H2O)]- present in the absence of bicarbonate. Electrochemical results show a relatively low increase in the thermodynamic stabilization of the redox couple CeIV/III in the presence of carbonate versus its aqueous analogue. [CeIVDOTA(CO3)]2- and [CeIVDOTA(H2O)], prepared electrochemically, decompose photolytically. However, kept in the dark, both are relatively long lived; [CeIVDOTA(H2O)], though, is orders of magnitude kinetically more stable (a considerably longer half-life). Thus, one concludes that the carbonate species have a different mechanism of decomposition depending also on the presence of dioxygen after its preparation (in deaerated/aerated solutions). The [CeIVDOTA(CO3)]2- species is produced radiolytically by oxidation of the trivalent species by CO3•- with a rate constant, measured using pulse radiolysis, of 3.3 × 105 M-1 s-1. This rate constant is at least 1 order of magnitude smaller than most of the rate constants so far reported for the reaction of CO3•- with transition metal/lanthanide (cerium)/actinide complexes. This result together with the bulkiness of the reactants might suggest an outer-sphere electron transfer rather than the inner-sphere one so far proposed. The lifetime of the tetravalent cerium species obtained radiolytically in the presence of carbonate is shorter than the electrochemical one, suggesting a different conformer involved.

A Mononuclear Non-Heme Manganese(III)–Aqua Complex in Oxygen Atom Transfer Reactions via Electron Transfer

Metal-oxygen complexes, such as metal-oxo [M(O2-)], -hydroxo [M(OH-)], -peroxo [M(O22-)], -hydroperoxo [M(OOH-)], and -superoxo [M(O2•-)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenylphosphine (Ph3P). However, OAT of metal-aqua complexes, [M(OH2)]n+, has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(III)-aqua complex, [(dpaq)MnIII(OH2)]2+ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), to PhSMe and Ph3P derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(III)-hydroxo complex, [(dpaq)MnIII(OH)]+ (2), to the substrates. Mechanistic studies reveal that OAT reaction of 1 occurs via electron transfer from 4-methoxythioanisole to 1 to produce the 4-methoxythioanisole radical cation and [(dpaq)MnII(OH2)]+, followed by nucleophilic attack of H2O in [(dpaq)MnII(OH2)]+ to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)2C6H3S•(OH)Me, which disproportionates or undergoes electron transfer to 1 to yield methyl 4-methoxyphenyl sulfoxide. Formation of the thioanisole radical cation derivatives is detected by the stopped-flow transient absorption measurements in OAT from 1 to 2,4-dimethoxythioanisole and 3,4-dimethoxythioanisole, being compared with that in the photoinduced electron transfer oxidation of PhSMe derivatives, which are detected by laser-induced transient absorption measurements. Similarly, OAT from 1 to Ph3P occurs via electron transfer from Ph3P to 1, and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and Ph3P derivatives, to 1 are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.

Effect of a Au underlayer on outer-sphere electron transfer across a Au/graphene/electrolyte interface.

The effect of a gold underlayer on the outer-sphere non-adiabatic electron transfer on a graphene surface is investigated theoretically using both periodic and cluster DFT calculations. We propose a model that describes the alignment of energy levels and charge redistribution at the metal/graphene/redox electrolyte interface. Model calculations were performed for the [Fe(CN)6]3-/4- and [Ru(NH3)6]3+/2+ redox couples. It is shown that the gold support increases the rate constant of electron transfer. Gold electronic states hybridize with graphene wave functions, which provides an effective overlap with reactant orbitals outside the graphene layer and favors an increasing reaction rate. Although the Fermi level shift relative to the Dirac point in graphene depends significantly on the redox couple, this weakly affects the electron transfer kinetics at the Au(111)/graphene/electrolyte interface due to a small contribution of graphene states to the rate constant as compared to gold ones.

Outer-sphere electron transfer does not underpin B12-dependent olefinic reductive dehalogenation in anaerobes.

Anaerobic microbial B12-dependent reductive dehalogenation may pave a way to remediate soil, sediment, and underground water contaminated with halogenated olefins. The chemical reaction is initiated by electron transfer (ET) from supernucleophilic cob(I)alamin (B12s). However, the inherent mechanism as outer-sphere or inner-sphere route is still under debate. To clarify the possibility of an outer-sphere pathway, we calculated free energy barriers of the initial steps of all outer-sphere ET routes by Marcus theory employing density functional theory (DFT). For 18 fluorinated, chlorinated, and brominated ethenes as representative olefins, 164 of 165 reactions with free energy barriers larger than 20 kcal mol-1 are not feasible under physiological dehalogenase conditions. Moreover, electronic structure analysis of perbromoethene with an outer-sphere free energy barrier of 18.2 kcal mol-1 reveals no ET initiation down to Co⋯Br and Co⋯C distances of 3.15 Å. The results demonstrate that the B12-catalyzed reductive dechlorination of olefins in microbes should proceed through an inner-sphere ET pathway.

Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis.

Homogeneous electrochemical catalysis of N2O reduction to N2 is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes. An activation-driving force correlation is revealed with the organic species characteristic of a redox catalysis involving an outer-sphere electron transfer from the radical anions or dianions of the reduced catalyst to N2O. Taking into account the previously estimated reorganization energy required to form the N2O radical anions leads to an estimation of the N2O/N2O˙− standard potential in acetonitrile electrolyte. The direct reduction of N2O at a glassy carbon electrode follows the same quadratic activation driving force relationship. Our analysis reveals that the catalytic effect of the mediators is due to a smaller reorganization energy of the homogeneous electron transfer than that of the heterogeneous one. The physical effect of “spreading” electrons in the electrolyte is shown to be unfavorable for the homogeneous reduction. Importantly, we show that the reduction of N2O by low valent rhenium and manganese bipyridyl carbonyl complexes is of a chemical nature, with an initial one-electron reduction process associated with a chemical reaction more efficient than the simple outer-sphere electron transfer process. This points to an inner-sphere mechanism possibly involving partial charge transfer from the low valent metal to the binding N2O and emphasizes the differences between chemical and redox catalytic processes.

RuIII(edta) complexes as molecular redox catalysts in chemical and electrochemical reduction of dioxygen and hydrogen peroxide: inner-sphere versus outer-sphere mechanism

The reduction of molecular oxygen (O2) and hydrogen peroxide (H2O2) by [RuII(edta)(pz)]2− (edta4− = ethylenediaminetetraacetate; pz = pyrazine) has been studied spectrophotometrically and kinetically in aqueous solution. Exposure of the aqua-analogue [RuII(edta)(H2O)]2− to O2 and H2O2 resulted in the formation of [RuIII(edta)(H2O)]− species, with subsequent formation of the corresponding RuV Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O complex. A working mechanism for the O2 and H2O2 reduction reactions mediated by the RuII(edta) complexes is proposed. The role of the coordinated water molecule (by its absence or presence in the primary coordination sphere) in controlling the mechanistic pathways, outer-sphere or inner-sphere, is discussed.

Micellar and Polymer Catalysis in the Kinetics of Oxidation of L-lysine by Permanganate Ion in Perchloric Acid Medium

ABSTRACT Kinetics of oxidation of L-lysine by permanganate ion in a perchloric acid medium was investigated to explore the order of the reaction with respect to oxidant and substrate and to study the catalytic behaviour of sodium lauryl sulphate (SLS) and polyethylene glycol (PEG). The reaction was found to be first-order with respect to the oxidant and the substrate and zero-order with respect to hydrogen ion. Changes in the sodium sulphate concentration produce a non-significant variation in the rate of the reaction. SLS and PEG were found to catalyze the reaction. Surfactant catalysis was modelled by Piszkiewicz's cooperativity model, while polymer catalysis was explained with the help of the Benesi-Hildebrand equation. The temperature dependence of the rate of the reaction was elucidated, and activation parameters were obtained. Interestingly, the reaction was found to possess positive activation entropy indicating the dissociative nature of the transition state and outer-sphere electron transfer mechanism. A mechanism of the reaction that is supported by the experimental findings was suggested. Keywords: L-lysine, permanganate ion, micellar catalysis, polymer catalysis, outer sphere electron transfer mechanism.

Kinetic study of surfactant cobalt (III) complexes by [Fe(CN)6 4-]: Outer-Sphere Electron-Transfer in Ionic liquids and Liposome Vesicle

UV-Vis., absorption spectroscopy are used to monitor the electron transfer reaction between the surfactant cobalt(III) complexes, cis-[Co(ip)2(C14H29NH2)2]3+, cis-[Co(dpq)2(C14H29NH2)2]3+ and cis-[Co(dpqc)2(C14H29NH2)2]3+ (ip = imidazo[4,5-f][1,10]phenanthroline, dpq = dipyrido[3,2-d:2’-3’-f]quinoxaline, dpqc = dipyrido[3,2-a:2’,4’-c](6,7,8,9-tetrahydro)phenazine, C14H29NH2=Tetradecylamine) and [Fe(CN)6]4- ion in liposome vesicles (DPPC) and ionic liquids ((BMIM)Br) were investigated at different temperatures under pseudo first order conditions using an excess of the reductant. The reactions were found to be second order and the electron transfer is postulated as outer-sphere. The rate constant for the electron transfer reactions were found to increase with increasing concentrations of ionic liquids. The effects of hydrophobicity of the long aliphatic double chains of these surfactant complex ions into liposome vesicles on these reactions have also been studied. Below the phase transition temperature of DPPC, the rate decreased with increasing concentration of DPPC, while above the phase transition temperature the rate increased with increasing concentration of DPPC. Kinetic data and activation parameters are interpreted in terms of an outer-sphere electron transfer mechanism. In all these media the S# values are found to be negative in direction in all the concentrations of complexes used indicative of more ordered structure of the transition state. This is consistent with a model in which the surfactant cobalt(III) complexes and Fe(CN)64- ions bind to the DPPC in the transition state. Thus, the results have been explained based on the self-aggregation, hydrophobic effect, and the reactants with opposite charge.

Dehalococcoides-Mediated B12-Dependent Reductive Dehalogenation of Aromatics Does Not Proceed through Outer-Sphere Electron Transfer.

Several anaerobic bacteria can couple the reduction of aromatic halides to energy conservation. This organohalide respiration is catalyzed by enzymes containing cob(I)alamin, an activated supernucleophilic form of the coenzyme vitamin B12. However, the mechanism underlying the electron transfer (inner-sphere vs outer-sphere ET) still remains elusive. To clarify this issue, we selected 36 fluoro-, chloro-, and bromobenzenes as representative substrates and calculated their free-energy barriers at the quantum chemical density functional theory level, considering a wide range of theoretically possible outer-sphere ET mechanisms. Across all 336 reaction routes addressed, 334 routes involve free-energy barriers larger than 20 kcal/mol. For two reaction routes with highly brominated benzenes, free-energy barriers below 20 kcal/mol imply abiotic reduction as observed in experiments. Thus, microbial B12-dependent aromatic reductive dehalogenation does not proceed through an outer-sphere ET mechanism. Instead, the present study strongly suggests that microbe-catalyzed reductive dehalogenation of aromatic halides is governed by inner-sphere ET.

Structures and Free Energies of Cerium Ions in Acidic Electrolytes and Structures of Electrolyte-Electrode Interfaces to Inform Kinetic Performance in Redox Flow Battery Applications

Despite the interest in renewable energy as a substitute for fossil fuels, renewables make up less than 20%1 of total electrical energy in the United States in part because their intermittency prevents them from being a dominant fraction without storage.2 The redox flow battery (RFB) is a promising energy storage technology because of its high-power density and long lifetime,2 which can be used to address intermittency by decoupling customer demand and renewable electricity generation. Currently, however, RFBs incur substantial energy losses during operation at high current densities and are therefore too costly for market deployment. A promising chemistry for the positive electrode of a RFB which would improve efficiency by increasing the total voltage is the cerium redox couple, which has a potential up to 0.74 V greater than the positive vanadium electrode.3,4 The Ce3+/Ce4+ electron transfer redox potential changes depending on the electrolyte,3 which we hypothesize is due to the anion complexation thermodynamics. It is not currently clear whether only one or both of the Ce ions are complexed by anions in solution. Knowledge of the structures and free energies of cerium ions in acidic electrolytes would enable control over the Ce3+/Ce4+ redox potential for RFB applications. Additionally, understanding whether the Ce3+/Ce4+ redox reaction involves a change in the inner sphere is important for discerning the charge transfer mechanism and kinetics.In addition to a lack of information on the structure of Ce3+ and Ce4+ anions in acids, previous work in zinc-cerium5,6 and hydrogen-cerium7 batteries show that the cerium electrode kinetics are a limiting factor on battery performance. Our preliminary kinetic studies of the redox couple in sulfuric acid indicate that the kinetics are influenced more by the electrolyte than by the electrode material, which is consistent with an outer-sphere electron transfer mechanism. If the Ce3+/Ce4+ electron transfer does proceed via an apparent outer-sphere mechanism, the reorganization energy,8 which is dependent on the electrolyte, will control the rate of reaction. To determine whether the reaction follows an outer-sphere mechanism, we must isolate the behavior of the cerium ions at the electrode interface. In this study, we therefore explore the structure of the electrode-electrolyte interface as well as study the structure of cerium anion complexes.To probe the electrode-electrolyte interface, we use in-situ Extended X-ray Absorption Fine Structure (EXAFS) of the platinum (Pt) L3-edge of a Pt electrode during Ce3+ oxidation in two of the most relevant electrolytes for battery applications, H2SO4 and CH3SO3H. At the high positive potentials applied, the Pt is oxidized, and therefore the relevant electrode is a Pt-oxide. To study the ionic structure in different aqueous acidic electrolytes (i.e., HCl, H2SO4, H3NSO3, CH3SO3H, HNO3, CF3SO3H, and HClO4) and extract free energies of Ce3+ and Ce4+ complexes, we use experimental UV-Vis spectroscopy and EXAFS, and Density Functional Theory (DFT) calculations. Based on the combination of spectroscopy, DFT calculations, and the direction of the redox potential shift from non-complexing media, we hypothesize that Ce4+ is complexed by anions in electrolyte, while Ce3+ is hydrated by water. We find from the UV-Vis spectra that the dominant Ce3+ structure is the same in all acids studied, and that the dominant Ce4+ complex changes with electrolyte. EXAFS of Ce3+ allows us to determine a coordination number of nine water molecules. Our DFT calculations enable us to extract bond lengths of the first coordination shell of the cerium ions. The DFT-predicted free energies of Ce4+ anion complexation agree with free energies we calculated from redox potentials reported in literature, supporting our hypothesis that the shift in redox potential can be attributed to the anion complexation of Ce4+. Additionally, this agreement shows that we can predict standard redox potentials for cerium in simple aqueous electrolytes. Ultimately, these characterization findings clarify the fundamental interactions of cerium ions with their electrolyte environment and a relevant electrode as well as the Ce3+/Ce4+ thermodynamics and will guide kinetics studies of the Ce3+/Ce4+ redox couple for use in redox flow battery applications. U.S. EIA, Electric Power Annual (2019).Weber, A. et al. J. Appl. Electrochem. 41, 1137–1164 (2011).Piro, N. et al. Coord. Chem. Rev. 260, 21–36 (2014).Smith, G. & Getz, C. Ind. Eng. Chem. Res. 10, 191–195 (1938).Walsh, F. et al. Chempluschem 80, 288–311 (2015).Nikiforidis, G. et al. Electrochim. Acta 140, 139–144 (2014).Tucker, M. et al. J. Power Sources 327, 591–598 (2016).Bard, A. & Faulkner, L. (John Wiley & Sons, Inc., 2001).

Elimination of 50% Iodine and Excellent Performance of Dye-Sensitized Solar Cell Enabled by TEMPO Radical Dendrimer–Iodide Dual Redox Systems

Detrimental effect of counter electrode corrosion, electrolyte leakage problem, and sublimation of iodine ions affect the performance and stability of dye-sensitized solar cells (DSSCs) based on a liquid redox electrolyte system. TEMPO/TEMPO+ is the most desirable one to replace the existing redox couples in liquid electrolyte owing to its rapid and reversible one electron kinetics, high diffusion coefficient, and electrochemical stability. In the present study, a fifth generation polyurethane dendrimer end-capped with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) was prepared and employed as an active redox mediator to replace up to 75% of iodine ions in the standard liquid iodide electrolyte system to elucidate the effect of a quick self-exchange reaction in DSSCs. In this concern, quasi-gel type electrolytes were prepared by incorporating different amounts of a newly prepared TEMPO radical dendrimer to a liquid I-/I3- redox system and were used as a dual redox mediator for DSSCs. Blending of 50 wt % of I-/I3- electrolyte with 50 wt % of the radical dendrimer improved the device performance (η) unprecedentedly from 6.56 to 9.54% upon illumination with 1 sun intensity. Thorough electrochemical characterization of cells was carried out and all such parameters authenticated this remarkable enhancement. Stability of the device was also found to be good, which retained its >80% efficiency after 30 days. (Less)

Samarium Iodide Showcase: Unraveling the Mechanistic Puzzle

ConspectusSmI2 was introduced to organic chemistry as a single electron transfer agent in 1977. After ca. 15 years of latency, the scientific community has realized the high potential of this reagent, and its chemistry has started blooming. This versatile reagent has mediated a myriad of new bond formations, cyclizations, and other reactions. Its popularity stems largely from the fact that three different intermediates, radical anions, radicals, and anions, depending on the ligand or additive used, could be obtained. Each of these intermediates could in principle lead to a different product. While these options vastly enrich the repertoire of SmI2, they necessitate a thorough mechanistic understanding, especially concerning how appropriate ligands direct the SmI2 to the desired intermediate. Our first paper on this subject dealt with the reduction of an activated double bond. The results were puzzling, especially the H/D isotope effect, which depended on the order of the reagents addition. This seminal paper was fundamental to an understanding of how the SmI2 works and enabled us to later explain various phenomena. For example, it was found that in a given reaction, when MeOH is used as a proton source, a spiro compound is obtained, while a bicyclic product is obtained when t-BuOH is used. Our contribution culminated in formulating guidelines for the rational use of proton donors in SmI2 reactions.The need to understand the complexity of the effect of additives on various processes is nicely demonstrated in photoinduced reactions. For example, hexamethylphosphoramide (HMPA) enhances the reduction of anthracene while hampering the reaction of benzyl chloride. The mechanistic understanding gained enabled us also to broaden the scope of photostimulated reactions from substrates reacting by a dissociative electron transfer mechanism to normal reductions, which are difficult to accomplish at the ground state. Harnessing the classical knowledge of proton transfer mechanisms to our SmI2 research enabled us to decipher an old conundrum: why does the combination of water and amine have such an enhancing effect on the reactivity of SmI2, which is not typical of these two when used separately. In our studies on the affinity of ligands to SmI2, we discovered that, in contradistinction to the accepted dogma, SmI2 is much more azaphilic than it is oxophilic. On the basis of the size difference between Sm3+ and Sm2+, we developed a simple diagnostic tool for the nature of the steps following the electron transfer. The reduction of imines showed that substrate affinity to SmI2 plays also a crucial role. In these reactions, new features such as autocatalysis and catalysis by quantum dots were discovered. Several studies of the ligand effect lead to a clear formulation of when an inner sphere or outer sphere electron transfer should be expected. In addition, several reactions where proton-coupled electron transfer (PCET) is the dominant mechanism were identified. Finally, the surprisingly old tool of NMR “shift reagents” was rediscovered and used to directly derive essential information on the binding constants of ligands and substrates to SmI2.

Mechanistic elucidation of the role of metal oxidation states in nickel mediated electrocatalytic coupling of benzyl halides

We present the mechanistic understanding of an electrochemically-driven nickel-catalyzed coupling reaction. Computational analysis reveals that the spin density is mostly residing on the nickel (Ni) center when NiII is reduced to NiI. Ni-mediated halogen atom abstraction through outer-sphere electron-transfer pathway to yield coupling products under mild conditions is demonstrated. Importantly, we have elucidated the role of NiI and Ni0 for successive coupling of benzyl bromide and benzyl chloride derivatives, respectively, to corresponding bibenzyl products. The Ni-catalyst bearing a PN3P-ligand is an effective catalyst, producing a strong ligand effect on the reactivity and selectivity for the homocoupling reactions.

Ligand Substitution and Electronic Structure Studies of Bis(phosphine)Cobalt Cyclooctadiene Precatalysts for Alkene Hydrogenation.

Diene self-exchange reactions of the 17-electron, formally cobalt(0) cyclooctadienyl precatalyst, (R,R)-(iPrDuPhos)Co(COD) (P 2 CoCOD, (R,R)-iPrDuPhos = 1,2-bis((2R,5R)-2,5-diisopropylphospholano)benzene, COD = 1,5-cyclooctadiene) were studied using natural abundance and deuterated 1,5-cyclooctadiene. Exchange of free and coordinated diene was observed at ambient temperature in benzene-d 6 solution and kinetic studies support a dissociative process. Both neutral P 2 CoCOD and the 16-electron, cationic cobalt(I) complex, [(R,R)-(iPrDuPhos)Co(COD)][BArF 4] (BArF 4 = B[(3,5-(CF3)2)C6H3]4) underwent instantaneous displacement of the 1,5-cyclooctadiene ligand by carbon monoxide and generated the corresponding carbonyl derivatives. The solid-state parameters, DFT-computed Mulliken spin density and analysis of molecular orbitals suggest an alternative description of P 2 CoCOD as low-spin cobalt(II) with the 1,5-cyclooctadiene acting as a LX2-type ligand. This view of the electronic structure provides insight into the nature of the ligand substitution processes and the remarkable stability of the neutral cobalt complexes toward protic solvents observed during catalytic alkene hydrogenation.

Driving force dependence of inner-sphere electron transfer for the reduction of CO2 on a gold electrode.

The kinetics of the inner-sphere electron transfer reaction between a gold electrode and CO2 was measured as a function of the applied potential in an aqueous environment. Extraction of the electron transfer rate constant requires deconvolution of the current associated with CO2 reduction from the competing hydrogen evolution reaction and mass transport. Analysis of the inner-sphere electron transfer reaction reveals a driving force dependence of the rate constant that has similar characteristics to that of a Marcus-Hush-Levich outer-sphere electron transfer model. Consideration of simple assumptions for CO2 adsorption on the electrode surface allows for the evaluation of a CO2,ads/CO2 •- ads standard potential of ∼-0.75 ± 0.05 V vs Standard Hydrogen Electrode (SHE) and a reorganization energy on the order of 0.75 ± 0.10 eV. This standard potential is considerably lower than that observed for CO2 reduction on planar metal electrodes (∼>-1.4 V vs SHE for >10 mA/cm2), thus indicating that CO2 reduction occurs at a significant overpotential and thus provides an imperative for the design of better CO2 reduction electrocatalysts.