Hydrogen atom transfer from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)] to TEMPO: a negative enthalpy of activation predicted by the Marcus equation

Characteristic Activity Parameters of Electron Donors and Electron Acceptors

It is well-known that for an electron transfer reaction, the electron-donating ability of electron donors and the electron-accepting ability of electron acceptors can be quantitatively described by the oxidation potential of electron donors and the reduction potential of electron acceptors. However, for an electron transfer reaction, the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors cannot be quantitatively described by a characteristic parameter of electron donors and a characteristic parameter of electron acceptors till now. In this paper, a characteristic activity parameter of electron donors and electron acceptors named as their thermo-kinetic parameter is proposed to quantify the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors in electron transfer reactions. At the same time, the thermo-kinetic parameter values of 70 well-known electron donors and the corresponding 70 conjugated electron acceptors in acetonitrile at 298 K are determined. The activation free energies of 4900 typical electron transfer reactions in acetonitrile at 298 K are estimated according to the thermo-kinetic parameter values of 70 electron donors and 70 conjugated electron acceptors, and the estimated results have received good verification of the corresponding independent experimental measurements. The physical meaning of the thermo-kinetic parameter is examined. The relationship of the thermo-kinetic parameter with the corresponding redox potential as well as the relationship of the activation free energy with the corresponding thermodynamic driving force of electron transfer reactions is examined. The results show that the observed relationships between the thermo-kinetic parameters and the redox potentials as well as the observed relationships between the activation free energy and the thermodynamic driving force depend on the choice of electron donors and electron acceptors as well as the electron transfer reactions. The greatest contribution of this paper is to realize the symmetry and unification of kinetic equations and the corresponding thermodynamic equations of electron transfer reactions.

Hydrogen Atom Transfer Thermodynamics of Homologous Co(III)- and Mn(III)-Superoxo Complexes: The Effect of the Metal Spin State

Systematic investigations on H atom transfer (HAT) thermodynamics of metal O₂ adducts is of fundamental importance for the design of transition metal catalysts for substrate oxidation and/or oxygenation directly using O₂. Such work should help elucidate underlying electronic-structure features that govern the OO-H bond dissociation free energies (BDFEs) of metal-hydroperoxo species, which can be used to quantitatively appraise the HAT activity of the corresponding metal-superoxo complexes. Herein, the BDFEs of two homologous Co^III- and Mn^III-hydroperoxo complexes, 3-Co and 3-Mn, were calculated to be 79.3 and 81.5 kcal/mol, respectively, employing the Bordwell relationship based on experimentally determined pK _a values and redox potentials of the one-electron-oxidized forms, 4-Co and 4-Mn. To further verify these values, we tested the HAT capability of their superoxo congeners, 2-Co and 2-Mn, toward three different substrates possessing varying O-H BDFEs. Specifically, both metal-superoxo species are capable of activating the O-H bond of 4-oxo-TEMPOH with an O-H BDFE of 68.9 kcal/mol, only 2-Mn is able to abstract a H atom from 2,4-di-tert-butylphenol with an O-H BDFE of 80.9 kcal/mol, and neither of them can react with 3,5-dimethylphenol with an O-H BDFE of 85.6 kcal/mol. Further computational investigations suggested that it is the high spin state of the Mn^III center in 3-Mn that renders its OO-H BDFE higher than that of 3-Co, which features a low-spin Co^III center. The present work underscores the role of the metal spin state being as crucial as the oxidation state in modulating BDFEs.

Coordination-Induced Bond Weakening

Coordination-induced bond weakening is a phenomenon wherein ligand X-H bond homolysis occurs in concert with the energetically favorable oxidation of a coordinating metal complex. The coupling of these two processes enables thermodynamically favorable proton-coupled electron transfer reductions to form weak bonds upon formal hydrogen atom transfer to substrates. Moreover, systems utilizing coordination-induced bond weakening have been shown to facilitate the dehydrogenation of feedstock molecules including water, ammonia, and primary alcohols under mild conditions. The formation of exceptionally weak substrate X-H bonds via small molecule homolysis is a powerful strategy in synthesis and has been shown to enable nitrogen fixation under mild conditions. Coordination-induced bond weakening has also been identified as an integral process in biophotosynthesis and has promising applications in renewable chemical fuel storage systems. This review presents a discussion of the advances made in the study of coordination-induced bond weakening to date. Because of the broad range of metal and ligand species implicated in coordination-induced bond weakening, each literature report is discussed individually and ordered by the identity of the low-valent metal. We then offer mechanistic insights into the basis of coordination-induced bond weakening and conclude with a discussion of opportunities for further research into the development and applications of coordination-induced bond weakening systems.

Catalytic redox-neutral C-H functionalisation with TEMPO in water to access aminomethyl-substituted pyrroles

A redox-neutral C-H functionalisation in water employing catalytic TEMPO to synthesize aminomethyl-substituted pyrroles is reported. Starting from cheap and commercial chemical feedstocks (ketoesters and anilines), our approach delivered targeted products in good yields and represents an endeavour to address redox economy in radical transformations.

Transformation of Formazanate at Nickel(II) Centers to Give a Singly Reduced Nickel Complex with Azoiminate Radical Ligands and Its Reactivity toward Dioxygen

The heteroleptic (formazanato)nickel bromide complex LNi(μ-Br)₂NiL [LH = Mes-NH-N═C(p-tol)-N═N-Mes] has been prepared by deprotonation of LH with NaH followed by reaction with NiBr₂(dme). Treatment of this complex with KC₈ led to transformation of the formazanate into azoiminate ligands via N-N bond cleavage and the simultaneous release of aniline. At the same time, the potentially resulting intermediate complex L'₂Ni [L' = HN═C(p-tol)-N═N-Mes] was reduced by one additional electron, which is delocalized across the π system and the metal center. The resulting reduced complex [L'₂Ni]K(18-c-6) has a S = ¹/₂ ground state and a square-planar structure. It reacts with dioxygen via one-electron oxidation to give the complex L'₂Ni, and the formation of superoxide was detected spectroscopically. If oxidizable substrates are present during this process, these are oxygenated/oxidized. Triphenylphosphine is converted to phosphine oxide, and hydrogen atoms are abstracted from TEMPO-H and phenols. In the case of cyclohexene, autoxidations are triggered, leading to the typical radical-chain-derived products of cyclohexene.

Spontaneous Formation of an Fe/Mn Diamond Core: Models for the Fe/Mn Sites in Class 1c Ribonucleotide Reductases

A handful of oxygen-activating enzymes has recently been found to contain Fe/Mn active sites, like Class 1c ribonucleotide reductases and R2-like ligand-binding oxidase, which are closely related to their better characterized diiron cousins. These enzymes are proposed to form high-valent intermediates with Fe-O-Mn cores. Herein, we report the first examples of synthetic Fe/Mn complexes that mimic doubly bridged intermediates proposed for enzymatic oxygen activation. Fe K-edge extended X-ray absorption fine structure (EXAFS) analysis has been used to characterize the structures of each of these compounds. Linear compounds accurately model the Fe···Mn distances found in Fe/Mn proteins in their resting states, and doubly bridged diamond core compounds accurately model the distances found in high-valent biological intermediates. Unlike their diiron analogues, the paramagnetic nature of Fe/Mn compounds can be analyzed by EPR, revealing S = 1/2 signals that reflect antiferromagnetic coupling between the high-spin Fe(III) and Mn(III) units of heterobimetallic centers. These compounds undergo electron transfer with various ferrocenes, linear compounds being capable of oxidizing diacetyl ferrocene, a weak reductant, and diamond core compounds being capable of oxidizing acetyl ferrocene. Diamond core compounds can also perform HAT reactions from substrates with X-H bonds with bond dissociation free energies (BDFEs) up to 75 kcal/mol and are capable of oxidizing TEMPO-H at rates of 0.32-0.37 M^-1 s^-1, which are comparable to those reported for some mononuclear Fe^III-OH and Mn^III-OH compounds. However, such reactivity is not observed for the corresponding diiron compounds, a difference that Nature may have taken advantage of in evolving enzymes with heterobimetallic active sites.

Metal-Ion Influence on Ligand-Centered Hydrogen-Atom Transfer

The ligand-centered hydrogen-atom-transfer (HAT) reactivity was examined for a family of group 10 metal complexes containing a tridentate pincer ligand derived from bis(2-mercapto-p-tolyl)amine, [SNS]H₃. Six new metal complexes of palladium and platinum were synthesized with the [SNS] ligand platform in different redox and protonation states to complete the group 10 series previously reported with nickel. The HAT reactivity was examined for this family of nickel, palladium, and platinum complexes to determine the impact of a metal ion on the ligand-centered reactivity. Thermodynamic measurements revealed that N-H bond dissociation free energies increased by approximately 10 kcal mol^-1 along the series Ni < Pd < Pt driven by changes to both the redox potential and pK_a of the ligand. Kinetic analyses for all three metal complexes suggest that the barrier to the HAT reactivity is primarily entropic rather than enthalpic for this system.

Redox-Neutral TEMPO Catalysis: Direct Radical (Hetero)Aryl C-H Di- and Trifluoromethoxylation

Applications of TEMPO. catalysis for the development of redox-neutral transformations are rare. Reported here is the first TEMPO. -catalyzed, redox-neutral C-H di- and trifluoromethoxylation of (hetero)arenes. The reaction exhibits a broad substrate scope, has high functional-group tolerance, and can be employed for the late-stage functionalization of complex druglike molecules. Kinetic measurements, isolation and resubjection of catalytic intermediates, UV/Vis studies, and DFT calculations support the proposed oxidative TEMPO. /TEMPO+ redox catalytic cycle. Mechanistic studies also suggest that Li2 CO3 plays an important role in preventing catalyst deactivation. These findings will provide new insights into the design and development of novel reactions through redox-neutral TEMPO. catalysis.

Mononuclear Manganese(III) Superoxo Complexes: Synthesis, Characterization, and Reactivity

Metal-superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe-, Co-, and Ni-superoxo complexes have been reported to date, well-defined Mn-superoxo complexes remain rather rare. Herein, we report two mononuclear Mn^III-superoxo species, Mn(BDPP)(O₂^•-) (2, H₂BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDP^BrP)(O₂^•-) (2', H₂BDP^BrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O₂ into solutions of their Mn^II precursors, Mn(BDPP) (1) and Mn(BDP^BrP) (1'), at -80 °C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin Mn^III center (S_Mn = 2) antiferromagnetically coupled to a superoxo radical ligand (S_OO• = 1/2), yielding an overall S = 3/2 ground state. Complexes 2 and 2' were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form Mn^III-hydroperoxo species, Mn(BDPP)(OOH) (5) and Mn(BDP^BrP)(OOH) (5'). Complexes 5 and 5' can be independently prepared by the reactions of the isolated Mn^III-aqua complexes, [Mn(BDPP)(H₂O)]OTf (6) and [Mn(BDP^BrP)(H₂O)]OTf (6'), with H₂O₂ in the presence of NEt₃. The parallel-mode EPR measurements established a high-spin S = 2 ground state for 5 and 5'.

Catalytic hydrogen atom transfer from hydrosilanes to vinylarenes for hydrosilylation and polymerization

Because of the importance of hydrogen atom transfer (HAT) in biology and chemistry, there is increased interest in new strategies to perform HAT in a sustainable manner. Here, we describe a sustainable, net redox-neutral HAT process involving hydrosilanes and alkali metal Lewis base catalysts - eliminating the use of transition metal catalysts - and report an associated mechanism concerning Lewis base-catalysed, complexation-induced HAT (LBCI-HAT). The catalytic LBCI-HAT is capable of accessing both branch-specific hydrosilylation and polymerization of vinylarenes in a highly selective fashion, depending on the Lewis base catalyst used. In this process, earth abundant, alkali metal Lewis base catalyst plays a dual role. It first serves as a HAT initiator and subsequently functions as a silyl radical stabilizing group, which is critical to highly selective cross-radical coupling. EPR study identified a potassiated paramagnetic species and multistate density function theory revealed a high HAT character, yet multiconfigurational nature in the transition state of the reaction.

Model of the MitoNEET [2Fe-2S] Cluster Shows Proton Coupled Electron Transfer

MitoNEET is an outer membrane protein whose exact function remains unclear, though a role of this protein in redox and iron sensing as well as in controlling maximum mitochondrial respiratory rates has been discussed. It was shown to contain a redox active and acid labile [2Fe-2S] cluster which is ligated by one histidine and three cysteine residues. Herein we present the first synthetic analogue with biomimetic {SN/S2} ligation which could be structurally characterized in its diferric form, 52-. In addition to being a high fidelity structural model for the biological cofactor, the complex is shown to mediate proton coupled electron transfer (PCET) at the {SN} ligated site, pointing at a potential functional role of the enzyme's unique His ligand. Full PCET thermodynamic square schemes for the mitoNEET model 52- and a related homoleptic {SN/SN} capped [2Fe-2S] cluster 42- are established, and kinetics of PCET reactivity are investigated by double-mixing stopped-flow experiments for both complexes. While the N-H bond dissociation free energy (BDFE) of 5H2- (230 ± 4 kJ mol-1) and the free energy ΔG°PCET for the reaction with TEMPO (-48.4 kJ mol-1) are very similar to values for the homoleptic cluster 4H2- (232 ± 4 kJ mol-1, -46.3 kJ mol-1) the latter is found to react significantly faster than the mitoNEET model (data for 5H2-: k = 135 ± 27 M-1 s-1, ΔH‡ = 17.6 ± 3.0 kJ mol-1, ΔS‡ = -143 ± 11 J mol-1 K-1, and ΔG‡ = 59.8 kJ mol-1 at 293 K). Comparison of the PCET efficiency of these clusters emphasizes the relevance of reorganization energy in this process.

Moving protons and electrons in biomimetic systems

An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates and/or products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H(+)) and one electron (e(-)), which is equivalent to transfer of a hydrogen atom (H(•)). In this Current Topic, we review key concepts in PCET reactivity and describe important advances in biomimetic PCET chemistry, with a special emphasis on research that has enhanced efforts to understand biological PCET reactions.

Electron transfer and proton-coupled electron transfer reactivity and self-exchange of synthetic [2Fe-2S] complexes: models for Rieske and mitoNEET clusters

This report describes the thermochemistry, proton-coupled electron transfer (PCET) reactions and self-exchange rate constants for a set of bis-benzimidazolate-ligated [2Fe–2S] clusters. These clusters serve as a model for the chemistry of biological Rieske and mitoNEET clusters. PCET from [Fe₂S₂(^Prbbim)(^PrbbimH)]^2– (4) and [Fe₂S₂(^Prbbim)(^PrbbimH₂)]^1– (5) to TEMPO occurs via concerted proton–electron transfer (CPET) mechanisms (^PrbbimH₂ = 4,4-bis-(benzimidazol-2-yl)heptane). Intermolecular electron transfer (ET) self-exchange between [Fe₂S₂(^Prbbim)₂]^2– (1) and [Fe₂S₂(^Prbbim)₂]^3– (2) occurs with a rate constant of (1.20 ± 0.06) × 10⁵ M^–1 s^–1 at 26 °C. A similar self-exchange rate constant is found for the related [2Fe–2S] cluster [Fe₂S₂(SArO)₂]^2–/3–, SArO^2– = thiosalicylate. These are roughly an order of magnitude slower than that reported for larger [4Fe–4S] clusters and 1 order of magnitude faster than that reported for N-ligated high-spin iron complexes. These results suggest that the rate of intermolecular ET to/from [Fe–S] clusters is modulated by cluster size. The measured PCET self-exchange rate constant for 1 and 4 at −30 °C is (3.8 ± 0.7) × 10⁴ M^–1 s^–1. Analysis of rate constants using the Marcus cross-relation suggests that this process likely occurs via a concerted proton–electron transfer (CPET) mechanism. The implications of these findings to biological systems are also discussed, including the conclusion that histidine-ligated [2Fe–2S] clusters should not have a strong bias to undergo concerted e^–/H⁺ transfers.

[Fe₂S₂(^Prbbim)(^PrbbimH_x)]^y- clusters have been generated in multiple redox and protonation states. Their PCET and ET thermochemistry and reactivity are described. The PCET self-exchange reaction occurs by concerted e⁻/H⁺ exchange, and the ET self-exchange barriers for different clusters are shown to scale with [Fe−S] cluster size. The implications of these results for the reactivity of biochemical imidazole-ligated clusters is discussed.

Fast proton-coupled electron transfer observed for a high-fidelity structural and functional [2Fe-2S] Rieske model

Rieske cofactors have a [2Fe–2S] cluster with unique {His₂Cys₂} ligation and distinct Fe subsites. The histidine ligands are functionally relevant, since they allow for coupling of electron and proton transfer (PCET) during quinol oxidation in respiratory and photosynthetic ET chains. Here we present the highest fidelity synthetic analogue for the Rieske [2Fe–2S] cluster reported so far. This synthetic analogue 5^x– emulates the heteroleptic {His₂Cys₂} ligation of the [2Fe–2S] core, and it also serves as a functional model that undergoes fast concerted proton and electron transfer (CPET) upon reaction of the mixed-valent (ferrous/ferric) protonated 5H^2– with TEMPO. The thermodynamics of the PCET square scheme for 5^x– have been determined, and three species (diferric 5^2–, protonated diferric 5H^–, and mixed-valent 5^3–) have been characterized by X-ray diffraction. pK_a values for 5H^– and 5H^2– differ by about 4 units, and the reduction potential of 5H^– is shifted anodically by about +230 mV compared to that of 5^2–. While the N–H bond dissociation free energy of 5H^2– (60.2 ± 0.5 kcal mol^–1) and the free energy, ΔG°_CPET, of its reaction with TEMPO (−6.3 kcal mol^–1) are similar to values recently reported for a homoleptic {N₂/N₂}-coordinated [2Fe–2S] cluster, CPET is significantly faster for 5H^2– with biomimetic {N₂/S₂} ligation (k = (9.5 ± 1.2) × 10⁴ M^–1 s^–1, ΔH^‡ = 8.7 ± 1.0 kJ mol^–1, ΔS^‡ = −120 ± 40 J mol^–1 K^–1, and ΔG^‡ = 43.8 ± 0.3 kJ mol^–1 at 293 K). These parameters, and the comparison with homoleptic analogues, provide important information and new perspectives for the mechanistic understanding of the biological Rieske cofactor.

Inverse solvent effects in the heterogeneous and homogeneous epoxidation of cis-2-heptene with [2-percarboxyethyl]-functionalized silica and meta-chloroperbenzoic acid

The organized solvent layer on the solid surface determines the reaction rate in the heterogeneous epoxidation of cis-2-heptene.

Understanding hydrogen atom transfer: from bond strengths to Marcus theory

Hydrogen atom transfer (HAT), a key step in many chemical, environmental, and biological processes, is one of the fundamental chemical reactions: A-H + B → A + H-B. Traditional HAT involves p-block radicals such as tert-BuO(•) abstracting H(•) from organic molecules. More recently, the recognition that transition metal species undergo HAT has led to a broader perspective, with HAT viewed as a type of proton-coupled electron transfer (PCET). When transition metal complexes oxidize substrates by removing H(•) (e(-) + H(+)), typically the electron transfers to the metal and the proton to a ligand. Examples with iron-imidazolinate, vanadium-oxo, and many other complexes are discussed. Although these complexes may not "look like" main group radicals, they have the same pattern of reactivity. For instance, their HAT rate constants parallel the A-H bond strengths within a series of similar reactions. Like main group radicals, they abstract H(•) much faster from O-H bonds than from C-H bonds of the same strength, showing that driving force is not the only determinant of reactivity. This Account describes our development of a conceptual framework for HAT with a Marcus theory approach. In the simplest model, the cross relation uses the self-exchange rate constants (k(AH/A) for AH + A) and the equilibrium constant to predict the rate constant for AH + B: k(AH/B) = (k(AH/A)k(BH/B)K(eq)f)(1/2). For a variety of transition metal oxidants, k(AH/B) is predicted within one or two orders of magnitude with only a few exceptions. For 36 organic reactions of oxyl radicals, k(AH/B) is predicted with an average deviation of a factor of 3.8, and within a factor of 5 for all but six of the reactions. These reactions involve both O-H or C-H bonds, occur in either water or organic solvents, and occur over a range of 10(28) in K(eq) and 10(13) in k(AH/B). The treatment of organic reactions includes the well-established kinetic solvent effect on HAT reactions. This is one of a number of secondary effects that the simple cross relation does not include, such as hydrogen tunneling and the involvement of precursor and successor complexes. This Account includes a number of case studies to illustrate these and various other issues. The success of the cross relation, despite its simplicity, shows that the Marcus approach based on free energies and intrinsic barriers captures much of the essential chemistry of HAT reactions. Among the insights derived from the analysis is that reactions correlate with free energies, not with bond enthalpies. Moreover, the radical character or spin state of an oxidant is not a primary determinant of HAT abstracting ability. The intrinsic barriers for HAT reactions can be understood, at least in part, as Marcus-type inner-sphere reorganization energies. The intrinsic barriers for diverse cross reactions are accurately obtained from the HAT self-exchange rate constants, a remarkable and unprecedented result for any type of chemical reaction other than electron transfer. The Marcus cross relation thus provides a valuable new framework for understanding and predicting HAT reactivity.

A Simple Marcus-Theory Type Model for Hydrogen Atom Transfer/Proton-Coupled Electron Transfer

Hydrogen atom transfer reactions are the simplest class of proton-coupled electron transfer (PCET) processes. These reactions involve transfer of one electron and one proton from one reagent to another, in the same kinetic step: XH + Y → X + HY. A predictive model for these reactions based on the Marcus cross relation is described. The model predicts rate constants within one or two orders of magnitude in most cases, over a very wide range of reactants and solvents. This remarkable result implies a surprising generality of the additivity postulate for the reaction intrinsic barriers, and a smaller role for the quantum mechanical details of the proton and electron transfers.

Synthesis and reactivity of oxo-peroxo-vanadium(V) bipyridine compounds

The vanadium(IV) compound [V(IV)O(OH)((t)Bu(2)bpy)(2)]BF(4) (V(IV)O(OH)) ((t)Bu(2)bpy = 4,4'-di-tert-butylbipyridine) is slowly oxidized by O(2) in ethereal solvents to give the oxo-peroxo compound [V(V)O(O(2))((t)Bu(2)bpy)(2)]BF(4) (V(V)O(O(2))) in excellent yield. This and related compounds were fully characterized by NMR, IR, and optical spectroscopies; mass spectrometry; elemental analyses; and an X-ray crystal structure of the 4,4'-dimethylbipyridine analog, [V(V)O(O(2))(Me(2)bpy)(2)]BF(4). Monitoring the reaction of V(IV)O(OH) with O(2) in THF/acetonitrile mixtures by (1)H NMR and optical spectroscopies surprisingly shows that the initial product is the cis-dioxo compound [V(V)(O)(2)((t)Bu(2)bpy)(2)]BF(4) (V(V)O(2)), which then converts to V(V)O(O(2)). Reaction of V(IV)O(OH) with (18)O(2) gives ca. 60% triply (18)O labeled V(V)O(O(2)). The mechanism of formation of V(V)O(O(2)) is complex and may occur via initial reduction of O(2) at vanadium(IV) to give a superoxo-vanadium(V) intermediate, autoxidation of the THF solvent, or both. That V(V)O(2) is generated first appears to be due to the ability of V(IV)O(OH) to act as a hydrogen atom donor. For instance, V(IV)O(OH) reacts with V(V)O(O(2)) to give V(V)O(2). V(V)O(O(2)) is also slowly reduced to V(IV)O(OH) by the organic hydrogen atom donors hydroquinone and TEMPOH (2,2,6,6-tetramethylpiperidin-1-ol) as well as by triphenylphosphine. Notably, the peroxo complex V(V)O(O(2)) is much less reactive with these substrates than the analogous dioxo compound V(V)O(2).

Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation

Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO(*)) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments are shown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2'-6,6'-tetramethyl-piperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and alpha-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.

Proton-coupled electron transfer of ruthenium(III)-pterin complexes: a mechanistic insight

Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters including pK(a) and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru(II)(Hdmp)(TPA)](ClO(4))(2) (1), [Ru(II)(Hdmdmp)(TPA)](ClO(4))(2) (2), [Ru(II)(dmp(-))(TPA)]ClO(4) (3), and [Ru(II)(dmdmp(-))(TPA)]ClO(4) (4) (Hdmp = 6,7-dimethylpterin, Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, TPA = tris(2-pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between 1 and one-electron oxidized species, [Ru(III)(dmp(-))(TPA)](2+), was determined to be 89 kcal mol(-1), which was large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from phenol derivatives to [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants (k) were determined to exhibit a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP; BDE = 79.15 kcal mol(-1)) to [Ru(III)(dmp(-))(TPA)](2+), the activation parameters were determined to be DeltaH(double dagger) = 1.6 +/- 0.2 kcal mol(-1) and DeltaS(double dagger) = -36 +/- 2 cal K(-1) mol(-1). This small activation enthalpy suggests a hydrogen-bonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate, and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.

Nitroxyl radical plus hydroxylamine pseudo self-exchange reactions: tunneling in hydrogen atom transfer

Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO(*) (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ((t)Bu(2)NO(*)), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and (t)Bu(2)NOH. The reactions have been monitored by UV-vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |DeltaG (o)| < or = 1.4 kcal mol(-1) and therefore are close to self-exchange reactions. The reaction of 4-oxo-TEMPO(*) + TEMPO-H --> 4-oxo-TEMPO-H + TEMPO(*) occurs with k(2H,MeCN) = 10 +/- 1 M(-1) s(-1) in MeCN at 298 K (K(2H,MeCN) = 4.5 +/- 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k(2D,MeCN) = 0.44 +/- 0.05 M(-1) s(-1) with k(2H,MeCN)/k(2D,MeCN) = 23 +/- 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH(2)Cl(2), 23 +/- 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO(*) with 4-MeO-TEMPO-H(D) also has a large KIE, k(3H)/k(3D) = 21 +/- 3 in MeCN. For these three reactions, the E(aD) - E(aH) values, between 0.3 +/- 0.6 and 1.3 +/- 0.6 kcal mol(-1), and the log(A(H)/A(D)) values, between 0.5 +/- 0.7 and 1.1 +/- 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of (t)Bu(2)NO(*) + TEMPO-H(D) in MeCN has a large KIE, 16 +/- 3 in MeCN, and very unusual isotopic activation parameters, E(aD) - E(aH) = -2.6 +/- 0.4 and log(A(H)/A(D)) = 3.1 +/- 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH(3))(2)NO(*) + (CH(3))(2)NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO((*)/H), (t)Bu(2)NO((*)/H), and Ph(2)NO((*)/H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines.

Slow hydrogen atom transfer reactions of oxo- and hydroxo-vanadium compounds: the importance of intrinsic barriers

Reactions are described that interconvert vanadium(IV) oxo-hydroxo complexes [V(IV)O(OH)(R(2)bpy)(2)]BF(4) (1a-c) and vanadium(V) dioxo complexes [V(V)O(2)(R(2)bpy)(2)]BF(4) (2a-c) [R(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridine ((t)Bu(2)bpy), a; 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy), b; 2,2'-bipyridine (bpy), c]. These are rare examples of pairs of isolated, sterically unencumbered, first-row metal-oxo/hydroxo complexes that differ by a hydrogen atom (H(+) + e(-)). The V(IV)-(t)Bu(2)bpy derivative 1a has a useful (1)H NMR spectrum, despite being paramagnetic. Complex 2a abstracts H(*) from organic substrates with weak O-H and C-H bonds, converting 2,6-(t)Bu(2)-4-MeO-C(6)H(2)OH (ArOH) and 2,2,6,6-tetramethyl-N-hydroxypiperidine (TEMPOH) to their corresponding radicals ArO(*) and TEMPO, hydroquinone to benzoquinone, and dihydroanthracene to anthracene. The equilibrium constant for 2a + ArOH <==> 1a + ArO(*) is (4 +/- 2) x 10(-3), implying that the VO-H bond dissociation free energy (BDFE) is 70.6 +/- 1.2 kcal mol(-1). Consistent with this value, 1a is oxidized by 2,4,6-(t)Bu(3)C(6)H(2)O(*). All of these reactions are surprisingly slow, typically occurring over hours at ambient temperatures. The net hydrogen-atom pseudo-self-exchange 1a + 2b <==> 2a + 1b, using the (t)Bu- and Me-bpy substituents as labels, also occurs slowly, with k(se) = 1.3 x 10(-2) M(-1) s(-1) at 298 K, DeltaH(double dagger) = 15 +/- 2 kcal mol(-1), and DeltaS(double dagger) = 16 +/- 5 cal mol(-1) K. Using this k(se) and the BDFE, the vanadium reactions are shown to follow the Marcus cross relation moderately well, with calculated rate constants within 10(2) of the observed values. The vanadium self-exchange reaction is ca. 10(6) slower than that for the related Ru(IV)O(py)(bpy)(2)(2+)/Ru(III)OH(py)(bpy)(2)(2+) self-exchange. The origin of this dramatic difference has been probed with DFT calculations on the self-exchange reactions of 1c + 2c and on monocationic ruthenium complexes with pyrrolate or fluoride in place of the py ligands. The calculations reproduce the difference in barrier heights and show that transfer of a hydrogen atom involves more structural reorganization for vanadium than the Ru analogues. The vanadium complexes have larger changes in the metal-oxo and metal-hydroxo bond lengths, which is traced to the difference in d-orbital occupancy in the two systems. This study thus highlights the importance of intrinsic barriers in the transfer of a hydrogen atom, in addition to the thermochemical (bond strength) factors that have been previously emphasized.

Hydrogen atom transfer reactions of a ruthenium imidazole complex: hydrogen tunneling and the applicability of the Marcus cross relation

The reaction of Ru(II)(acac)2(py-imH) (Ru(II)imH) with TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives Ru(III)(acac)2(py-im) (Ru(III)im) and the hydroxylamine TEMPO-H by transfer of H(*) (H(+) + e(-)) (acac = 2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k(3H) = 1400 +/- 100 M(-1) s(-1) at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (Delta G(o)). Deuterium transfer from Ru(II)(acac)2(py-imD) (Ru(II)imD) to TEMPO(*) is surprisingly much slower at k(3D) = 60 +/- 7 M(-1) s(-1), with k(3H)/k(3D) = 23 +/- 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E(a3D) - E(a3H) = 1.9 +/- 0.8 kcal mol(-1). The large k(3H)/k(3D) and DeltaE(a) values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru(II)imH and Ru(III)im, measured by (1)H NMR line broadening, occurs with k(4H) = (3.2 +/- 0.3) x 10(5) M(-1) s(-1) at 298 K and k(4H)/k(4D) = 1.5 +/- 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A(4H)/A(4D)) = -0.5 +/- 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for Ru(II)imH(D) + TEMPO(*) that are greater than those observed: k(3H,calc)/k(3H) = 31 +/- 4 and k(3D,calc)/k(3D) = 140 +/- 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.

Facile concerted proton-electron transfers in a ruthenium terpyridine-4'-carboxylate complex with a long distance between the redox and basic sites

We have designed and prepared ruthenium complexes with terpyridine-4'-carboxylate (tpyCOO) ligands, in which there are six bonds between the redox-active Ru and the basic carboxylate. The protonated Ru(II) complex, RuII(dipic)(tpyCOOH) (Ru(II)COOH), is prepared in one-pot from [(p-cymene)RuCl2]2, tpyCOONa, and then sodium pyridine-2,6-dicarboxylate [Na(dipic)]. A crystal structure of the deprotonated Ru(II) complex, Ru(II)COO-, shows a distance of 6.9 A between the metal and basic sites. The Ru(III) complex (Ru(III)COO) has been isolated by one-electron oxidation of Ru(II)COO- with triarylaminium radical cations (NAr3*+). Ru(III)COO has a bond dissociation free energy (BDFE) of 81 +/- 1 kcal mol(-1), from pKa and E1/2 measurements. It oxidizes 2,4,6-tri-tert-butylphenol (BDFE = 77 +/- 1 kcal mol(-1)) by removal of e- and H+ (triple bond H*) to form 2,4,6-tri-tert-butylphenoxyl radical and Ru(II)COOH, with a second-order rate constant of (2.3 0.2) x 10(4) M(-1) s(-1) and a kH/kD of 7.7 1.2. Thermochemical analysis suggests a concerted proton-electron transfer (CPET) mechanism for this reaction, despite the 6.9 A distance between the redox-active Ru and the H+-accepting oxygen. Ru(III)COO also oxidizes the hydroxylamine TEMPOH to the stable free radical TEMPO and xanthene to bixanthyl. These reactions appear to be similar to processes that have been previously termed hydrogen atom transfer.

Synthesis and characterization of ruthenium bis(beta-diketonato) pyridine-imidazole complexes for hydrogen atom transfer

Ruthenium bis(beta-diketonato) complexes have been prepared at both the RuII and RuIII oxidation levels and with protonated and deprotonated pyridine-imidazole ligands. RuII(acac)2(py-imH) (1), [RuIII(acac)2(py-imH)]OTf (2), RuIII(acac)2(py-im) (3), RuII(hfac)2(py-imH) (4), and [DBU-H][RuII(hfac)2(py-im)] (5) have been fully characterized, including X-ray crystal structures (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). For the acac-imidazole complexes 1 and 2, cyclic voltammetry in MeCN shows the RuIII/II reduction potential (E1/2) to be -0.64 V versus Cp2Fe+/0. E1/2 for the deprotonated imidazolate complex 3 (-1.00 V) is 0.36 V more negative. The RuII bis-hfac analogues 4 and 5 show the same DeltaE1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 and -0.07 V). The difference in acidity between the acac and hfac derivatives is much smaller, with pKa values of 22.1 and 19.3 in MeCN for 1 and 4, respectively. From the E1/2 and pKa values, the bond dissociation free energies (BDFEs) of the N-H bonds in 1 and 4 are calculated to be 62.0 and 79.6 kcal mol(-1) in MeCN - a remarkable difference of 17.6 kcal mol(-1) for such structurally similar compounds. Consistent with these values, there is a facile net hydrogen atom transfer from 1 to TEMPO* (2,2,6,6-tetramethylpiperidine-1-oxyl radical) to give 3 and TEMPO-H. The DeltaG degrees for this reaction is -4.5 kcal mol(-1). 4 is not oxidized by TEMPO* (DeltaG degrees = +13.1 kcal mol(-1)), but in the reverse direction TEMPO-H readily reduces in situ generated RuIII(hfac)2(py-im) (6). A RuII-imidazoline analogue of 1, RuII(acac)2(py-imnH) (7), reacts with 3 equiv of TEMPO* to give the imidazolate 3 and TEMPO-H, with dehydrogenation of the imidazoline ring.

Negative kinetic temperature effect on the hydride transfer from NADH analogue BNAH to the radical cation of N-benzylphenothiazine in acetonitrile

The reaction rates of 1-(p-substituted benzyl)-1,4-dihydronicotinamide (G-BNAH) with N-benzylphenothiazine radical cation (PTZ(*+)) in acetonitrile were determined. The results show that the reaction rates (k(obs)) decreased from 2.80 x 10(7) to 2.16 x 10(7) M(-1) s(-1) for G = H as the reaction temperature increased from 298 to 318 K. The activation enthalpies of the reactions were estimated according to Eyring equation to give negative values (-3.4 to -2.9 kcal/mol). Investigation of the reaction intermediate shows that the charge-transfer complex (CT-complex) between G-BNAH and PTZ(*+) was formed in front of the hydride transfer from G-BNAH to PTZ(*+). The formation enthalpy of the CT-complex was estimated by using the Benesi-Hildebrand equation to give the values from -6.4 to -6.0 kcal/mol when the substituent G in G-BNAH changes from CH(3)O to Br. Detailed thermodynamic analyses on each elementary step in the possible reaction pathways suggest that the hydride transfer from G-BNAH to PTZ(*+) occurs by a concerted hydride transfer via a CT-complex. The effective charge distribution on the pyridine ring in G-BNAH at the various stages-the reactant G-BNAH, the charge-transfer complex, the transition-state, and the product G-BNA(+)-was estimated by using the method of Hammett-type linear free energy analysis, and the results show that the pyridine ring carries relative effective positive charges of 0.35 in the CT-complex and 0.45 in the transition state, respectively, which indicates that the concerted hydride transfer from G-BNAH to PTZ(*+) was practically performed by the initial charge (-0.35) transfer from G-BNAH to PTZ(*+) and then followed by the transfer of hydrogen atom with partial negative charge (-0.65). It is evident that the present work would be helpful in understanding the nature of the negative temperature effect, especially on the reaction of NADH coenzyme with the drug phenothiazine in vivo.

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.

Models for proton-coupled electron transfer in photosystem II

The coupling of proton and electron transfers is a key part of the chemistry of photosynthesis. The oxidative side of photosystem II (PS II) in particular seems to involve a number of proton-coupled electron transfer (PCET) steps in the S-state transitions. This mini-review presents an overview of recent studies of PCET model systems in the authors' laboratory. PCET is defined as a chemical reaction involving concerted transfer of one electron and one proton. These are thus distinguished from stepwise pathways involving initial electron transfer (ET) or initial proton transfer (PT). Hydrogen atom transfer (HAT) reactions are one class of PCET, in which H(+) and e (-) are transferred from one reagent to another: AH + B --> A + BH, roughly along the same path. Rate constants for many HAT reactions are found to be well predicted by the thermochemistry of hydrogen transfer and by Marcus Theory. This includes organic HAT reactions and reactions of iron-tris(alpha-diimine) and manganese-(mu-oxo) complexes. In PS II, HAT has been proposed as the mechanism by which the tyrosine Z radical (Y(Z)*) oxidizes the manganese cluster (the oxygen evolving complex, OEC). Another class of PCET reactions involves transfer of H(+) and e (-) in different directions, for instance when the proton and electron acceptors are different reagents, as in AH-B + C(+) --> A-HB(+) + C. The oxidation of Y(Z) by the chlorophyll P680 + has been suggested to occur by this mechanism. Models for this process - the oxidation of phenols with a pendent base - are described. The oxidation of the OEC by Y(Z)* could also occur by this second class of PCET reactions, involving an Mn-O-H fragment of the OEC. Initial attempts to model such a process using ruthenium-aquo complexes are described.

Substrate oxidation by copper-dioxygen adducts: mechanistic considerations

A series of copper-dioxygen adducts [{Cu(II)(MePY2)(R)}(2)(O(2))](B(C(6)F(5))(4))(2) (1(R)()), systematically varying in their electronic properties via ligand pyridyl donor substituents (R = H, MeO, and Me(2)N), oxidize a variety of substrates with varying C-H or O-H bond dissociation enthalpies. Detailed mechanistic studies have been carried out, including investigation of 1(R)() thermodynamic redox properties, 1(R)() tetrahydrofuran (THF) and N,N'-dimethylaniline (DMA) oxidation kinetics (including analyses of substrate dicopper binding equilibria), and application of mechanistic probes (N-cyclopropyl-N-methylaniline (CMA) and (p-methoxyphenyl)-2,2-dimethylpropanol (MDP)), which can distinguish if proton-coupled electron-transfer (PCET) processes proceed through concerted electron-transfer proton-transfer (ETPT) or consecutive electron-transfer proton-transfer (ET/PT) pathways. The results are consistent with those of previous complementary studies; at low thermodynamic driving force for substrate oxidation, an ET/PT is operable, but once ET (i.e., substrate one-electron oxidation) becomes prohibitively uphill, the ETPT pathway occurs. Possible differences in coordination structures about 1(Me)()()2(N)()/1(MeO)() compared to those of 1(H)() are also used to rationalize some of the observations.

Isotope effects, dynamics, and the mechanism of solvolysis of aryldiazonium cations in water

The mechanism of the heterolytic solvolysis of p-tolyldiazonium cation in water was studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. Significant (13)C kinetic isotope effects were observed at the ipso (k(12)C/k(13)C = 1.024), ortho (1.017), and meta (1.013) carbons, indicative of substantial weakening of the C(2)-C(3) and C(5)-C(6) bonds at the transition state. This is qualitatively consistent with a transition state forming an aryl cation, but on a quantitative basis, simple S(N)1 heterolysis does not account best for the isotope effects. Theoretical S(N)2Ar transition structures for concerted displacement of N(2) by a single water molecule lead to poor predictions of the experimental isotope effects. The best predictions of the (13)C isotope effects arose from transition structures for the heterolytic process solvated by clusters of water molecules. These structures, formally saddle points for concerted displacements on the potential energy surface, may be described as transition structures for solvent reorganization around the aryl cation. Quasiclassical dynamics trajectories starting from these transition structures afforded products very slowly, compared to a similar S(N)2 displacement, and the trajectories often afforded long-lived aryl cation intermediates. Critical prior evidence for aryl cation intermediates is reconsidered with the aid of DFT calculations. Overall, the nucleophilic displacement process for aryldiazonium ions in water is at the boundary between S(N)2Ar and S(N)1 mechanisms, and an accurate view of the reaction mechanism requires consideration of dynamic effects.

A Theoretical Study of the Iminoxyl/Oxime Self-Exchange Reaction. A Five-Center, Cyclic Proton-Coupled Electron Transfer

In solution, the self-exchange reactions for oxygen-centered pi-radicals, e.g., PhO. + PhOH <==>PhOH + PhO., are known to occur with low activation enthalpies (E(a) approximately equal to 2 kcal/mol). For the PhO./PhOH couple and, we conclude, for other O-centered pi-radicals, exchange occurs by proton-coupled electron transfer (PCET) with the proton transferred between oxygen electron pairs while the electron migrates between oxygen orbitals orthogonal to the -O- - -H- - -O- transition state plane (Mayer et al. J. Am. Chem. Soc. 2002, 123, 11142). Iminoxyls, R(2)C=NO., are sigma-radicals with substantial spin density on the nitrogen. The R(2)C=NO./R(2)C=NOH self-exchange has a significant E(a) (Mendenhall et al. J. Am. Chem. Soc. 1973, 95, 627). For this exchange, DFT calculations have revealed a counterintuitive cisoid transition state in which the seven atoms, >C=NO- - -H- - -ON=C<, lie in a plane (R = H, Me) or, for steric reasons, two planes twisted at 45.2 degrees (R = Me(3)C). The planar transition state has the two N-O dipoles close to each other and pointing in the same direction and an O- - -H- - -O angle of 165.4 degrees . A transoid transition state for R = H lies 3.4 kcal/mol higher in energy than the cisoid despite a more favorable arrangement of the dipoles and a near linear O- - -H- - -O. It is concluded that iminoxyl/oxime self-exchange reactions occur by a five-center, cyclic PCET mechanism with the proton being transferred between electron pairs on the oxygens and the electron migrating between in-plane orbitals on the two nitrogens (R(N-N) = 2.65 A). The calculated E(a) values (8.8-9.9 kcal/mol) are in satisfactory agreement with the limited experimental data.

Ligand-Assisted Reduction of Osmium Tetroxide with Molecular Hydrogen via a [3+2] Mechanism

Osmium tetroxide is reduced by molecular hydrogen in the presence of ligands in both polar and nonpolar solvents. In CHCl3 containing pyridine (py) or 1,10-phenanthroline (phen), OsO4 is reduced by H2 to the known Os(VI) dimers L2Os(O)2(mu-O)2Os(O)2L2 (L2 = py2, phen). However, in the absence of ligands in CHCl3 and other nonpolar solvents, OsO4 is unreactive toward H2 over a week at ambient temperatures. In basic aqueous media, H2 reduces OsO4(OH)n(n-) (n = 0, 1, 2) to the isolable Os(VI) complex, OsO2(OH)4(2-), at rates close to that found in py/CHCl3. Depending on the pH, the aqueous reactions are exergonic by deltaG = -20 to -27 kcal mol(-1), based on electrochemical data. The second-order rate constants for the aqueous reactions are larger as the number of coordinated hydroxide ligands increases, k(OsO4) = 1.6(2) x 10(-2) M(-1) s(-1) < k(OsO4(OH)-) = 3.8(4) x 10(-2) M(-1) s(-1) < k(OsO4(OH)2(2-)) = 3.8(4) x 10(-1) M(-1) s(-1). The observation of primary deuterium kinetic isotope effects, k(H2)/k(D2) = 3.1(3) for OsO4 and 3.6(4) for OsO4(OH)-, indicates that the rate-determining step in each case involves H-H bond cleavage. Density functional calculations and thermochemical arguments favor a concerted [3+2] addition of H2 across two oxo groups of OsO4(L)n and argue against H* or H- abstraction from H2 or [2+2] addition of H2 across one Os=O bond. The [3+2] mechanism is analogous to that of alkene addition to OsO4(L)n to form diolates, for which acceleration by added ligands has been extensively documented. The observation that ligands also accelerate H2 addition to OsO4(L)n highlights the analogy between these two reactions.