Outer-sphere Electron-transfer Pathways Research Articles

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.

Singly Unified Driving Force Dependence of Outer-Sphere Electron-Transfer Pathways of Nonheme Manganese(IV)-Oxo Complexes in the Absence and Presence of Lewis Acids.

Epoxidation of styrene derivatives, sulfoxidation of thioanisole derivatives, and hydroxylation of toluene derivatives by a nonheme manganese(IV)-oxo complex binding triflic acid, [(N4Py)MnIV(O)]2+-(HOTf)2 [1-(H+)2], and scandium triflate, [(N4Py)MnIV(O)]2+-(Sc(OTf)3)2 [1-(Sc3+)2], occur via outer-sphere electron-transfer (OSET) pathways, exhibiting singly unified driving force dependence, enabling one to predict absolute values of the second-order rate constants of these three types of substrate oxidations by the manganese(IV)-oxo complex, using the Marcus theory of electron transfer. When [(N4Py)MnIV(O)]2+ (1) was replaced by [(N4Py)FeIV(O)]2+ (2), OSET pathways were changed to inner-sphere electron-transfer (ISET) pathways. The difference in the OSET versus ISET pathways is clarified based on the difference in the Lewis basicity of the oxo moieties in 1 and 2.

Cerium Photosensitizers: Structure-Function Relationships and Applications in Photocatalytic Aryl Coupling Reactions.

Two complete mixed-ligand series of luminescent Ce(III) complexes with the general formulas [(Me3Si)2NC(N(i)Pr)2]xCe(III)[N(SiMe3)2]3-x (x = 0, 1-N; x = 1, 2-N, x = 2, 3-N; x = 3, 4) and [(Me3Si)2NC(N(i)Pr)2]xCe(III)(OAr)3-x (x = 0, 1-OAr; x = 1, 2-OAr, x = 2, 3-OAr; x = 3, 4) were developed, featuring photoluminescence quantum yields up to 0.81(2) and lifetimes to 117(1) ns. Although the 4f → 5d absorptive transitions for these complexes were all found at ca. 420 nm, their emission bands exhibited large Stokes shifts with maxima occurring at 553 nm for 1-N, 518 nm for 2-N, 508 nm for 3-N, and 459 nm for 4, featuring yellow, lime-green, green, and blue light, respectively. Combined time-dependent density functional theory (TD-DFT) calculations and spectroscopic studies suggested that the long-lived (2)D excited states of these complexes corresponded to singly occupied 5dz(2) orbitals. The observed difference in the Stokes shifts was attributed to the relaxation of excited states through vibrational processes facilitated by the ligands. The photochemistry of the sterically congested complex 4 was demonstrated by C-C bond forming reaction between 4-fluoroiodobenzene and benzene through an outer sphere electron transfer pathway, which expands the capabilities of cerium photosensitizers beyond our previous results that demonstrated inner sphere halogen atom abstraction reactivity by 1-N.

Kinetics of the oxidation of hydroxyurea with vanadium(V) ions in acidic aqueous solution

Hydroxyurea (HU) effectively reduces vanadate (VO2+) into vanadyl (VO2+) species in acidic aqueous solution acting as a two-electron donor. The reaction starts by the formation of a transient complex followed by an electron transfer process that includes the formation and subsequent fading out of a free radical, U• (U•≡H2N–C(=O)N(H)O•). The rate determining step of the redox reaction is the formation of a free radical either by the inner-sphere one-electron transfer within the formed VO2+-complexes, or by an outer sphere one-electron transfer from VO2+-complex to the second VO2+ ion. Assuming a rapid pre-equilibrium for the proton-transfer and complexation reactions, an inner-sphere electron transfer pathway has been proposed (VO2-U2+\({\mathop{\longrightarrow}\limits^{k_{1}}}\) VO2+-U•2+, k1 = 1.4 s−1) when HU is in excess. When VO2+ in excess, an additional reaction possibly proceeds through two outer-sphere electron-transfer pathways: VO2-U2+ + VO2+ + 2H2O \({\mathop{\longrightarrow}\limits^{k_{4}}}\) VO2+ + VO2+ + U•, and VO2-U2+ + VO(OH)2+ + 2H2O \({\mathop{\longrightarrow}\limits^{k_{5}}}\) VO(OH)2+ + VO2+ + U•, characterized by k4 = 22 s−1 mol−1 dm3, and k5 = 3.8 × 103 s−1 mol−1 dm3.

Stereoselectivity in electron-transfer reactions in chiral media

The oxidation of [Co(edta)](2-) by [IrCl(6)](2-) proceeds by both inner-sphere and outer-sphere electron-transfer pathways. In the presence of added [Co(en)(3)](3+), the outer-sphere pathway is enhanced. When optically active [Co(en)(3)](3+) is used, the [Co(edta)](-) formed is optically active, reflecting a 1.5% DeltaLambda selectivity. It is proposed that the selectivity arises from preferential formation and reactivity of the DeltaLambda ion pair, {[Co(edta)](2-),[Co(en)(3)](3+)}. Direct reaction of [Co(edta)](-) with [Co(en)(3)](2+) has also been investigated in the optically active solvent, (S)-(-)-1,2-propylene carbonate. The induction is small, forming 0.75% Delta-[Co(en)(3)](3+), consistent with the important role for hydrogen bonding in determining the precursor stereoselectivity to the exclusion of solvent.

Reduction of Vanadium(V) by l-Ascorbic Acid at Low and Neutral pH: Kinetic, Mechanistic, and Spectroscopic Characterization

L-Ascorbic acid interacts with vanadium(V) over the pH range of 0.4-7.0 to form three different coordination complexes. Both inner- and outer-sphere electron-transfer pathways are proposed to form vanadium(IV) complexes with L-ascorbate or dehydroascorbate, respectively. Effects of the pH on the coordination of L-ascorbic acid to the vanadium(V) center were observed and are presumably related to the speciation of the vanadium(V) ion. Three vanadium(IV) complexes were observed using ambient-temperature electron paramagnetic resonance spectroscopy. Two of these complexes are proposed to be vanadium(IV) L-ascorbate complexes, and one is consistent with a vanadium(IV) dehydroascorbic acid complex proposed earlier. These reduction reactions will occur under physiological conditions and could be important to the reduction of vanadium(V)-containing coordination complexes used as insulin-enhancing agents for treatment of diabetes.

Further evidence supporting an inner sphere mechanism in the NO reduction of the copper(II) complex Cu(dmp)22+ (dmp=2,9-dimethyl-1,10-phenanthroline)

Described are further studies directed towards elucidating the mechanism of the nitric oxide reduction of the copper(II) model system, Cu(dmp)2(2+) (I, dmp=2,9-dimethyl-1,10-phenanthroline). The reaction of I with NO in methanol results in the formation of Cu(dmp)2+ (II) and methyl nitrite (CH3ONO), with a second order rate constant kNO=38.1 M-1 s-1 (298K). The activation parameters for this reaction in buffered aqueous medium were measured to be DeltaH(double dagger)=41.6 kJ/mol and DeltaS(double dagger)=-82.7 kJ/mol deg. The addition of azide ion (N3-) as a competing nucleophile results in a marked acceleration in the rate of the copper(II) reduction. Analysis of the kinetics for the NO reduction of the bulkier Cu(dpp)(2)2+ (IV, dpp=2,9-diphenyl-1,10-phenanthroline) and the stronger oxidant, Cu(NO2-dmp)2(2+) (V, NO2-dmp=5-nitro-2,9-dimethyl-1,10-phenanthroline), gave the second order rate constants kNO=21.2 and 29.3 M-1 s-1, respectively. These results argue against an outer sphere electron transfer pathway and support a mechanism where the first step involves the formation of a copper-nitrosyl (Cu(II)-NO or Cu(I)-NO+) adduct. This would be followed by the nucleophilic attack on the bound NO and the labilization of RONO to form the nitrite products and the cuprous complex.

Revelation of the nature of the reducing species in titanocene halide-promoted reductions.

The fundamental nature of Ti(III) complexes generated in tetrahydrofuran by reduction of Cp(2)TiCl(2) has been clarified by means of cyclic voltammetry and kinetic measurements. While the electrochemical reduction of Cp(2)TiCl(2) leads to the formation of Cp(2)TiCl(2)(-), the use of metals such as Zn, Al, or Mn as reductants affords Cp(2)TiCl and (Cp(2)TiCl)(2) in a mixture having a dimerization equilibrium constant of 3 x 10(3) M(-)(1), independent of the metal used. Thus, we find it unlikely that the trinuclear complexes or ionic clusters known from the solid phase should be present in solution as previously suggested. The standard potentials determined for the redox couples Cp(2)TiCl(2)/Cp(2)TiCl(2)(-), (Cp(2)TiCl)(2)(+)/(Cp(2)TiCl)(2), Cp(2)TiCl(+)/Cp(2)TiCl, and Cp(2)Ti(2+)/Cp(2)Ti(+) increase in the order listed. However, the reactivity of the different Ti(III) complexes is assessed as (Cp(2)TiCl)(2) greater, similar Cp(2)TiCl approximately Cp(2)Ti(+) >> Cp(2)TiCl(2)(-) in their reactions with benzyl chloride and benzaldehyde. None of the reactions proceed by an outer-sphere electron transfer pathway, and clearly the inner-sphere character is much higher in the case of Cp(2)Ti(+) than for (Cp(2)TiCl)(2), Cp(2)TiCl, and in particular Cp(2)TiCl(2)(-). As to the electron acceptor, the inner-sphere character increases, going from benzyl chloride to benzaldehyde, and it is suggested that the chlorine atom in benzyl chloride and the oxygen atom in benzaldehyde may function as bridges between the reactants in the transition state.

Mechanistic studies on the interaction of reduced cobalamin (vitamin B12r) with nitroprusside.

The electron-transfer reaction between reduced cobalamin (Cbl(II)) and sodium pentacyanonitrosylferrate(II) (sodium nitroprusside, NP), as well as the subsequent processes following the electron-transfer step, were investigated by spectroscopic (UV-vis, (1)H NMR, EPR), electrochemical (CV, DPV) and kinetic (stopped-flow) techniques. In an effort to clarify the complex reaction pattern observed at physiological pH, systematic spectroscopic and kinetic studies were undertaken as a function of pH (1.8-9) and NP concentration (0.0001 - 0.09 M). The kinetics of the electron-transfer reaction was studied under pseudo-first-order conditions with respect to NP. The reaction occurs in two parallel paths of different order, viz. pseudo-first and pseudo-second order with respect to the NP concentration, respectively. The contribution of each path depends on pH and the [NP]/[Cbl(II)] ratio. At low pH and total NP concentration (pH < 3, [NP]/[Cbl(II)] approximately 1), the cyano-bridged successor complex [Cbl(III)-(mu-NC)-Fe(I)(CN)(3)(NO(+))](-) (1(s)()) is the final reaction product formed in an inner-sphere electron transfer reaction that is coupled to the release of cyanide from coordinated nitroprusside. At higher pH, subsequent reactions were observed which involve the attack of cyanide released in the electron transfer step on the initially formed cyano-bridged species, and lead to the formation of Cbl(III)CN and [Fe(I)(CN)(4)(NO(+))](2)(-). The strong dependence of the rate and mechanism of the subsequent reactions on pH is attributed to the large variation in the effective nucleophilicity of the cyanide ligand in the studied pH range. An alternative electron-transfer pathway observed in the presence of excess NP involves the reaction of the precursor complex [Cbl(II)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](2)(-) (1(p)()) with NP to give [Cbl(III)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](-) (2) and reduced nitroprusside, [Fe(CN)(5)NO](3)(-), as the initial reaction products. Analysis of the kinetic data allowed elucidation of the rate constants for the inner- and outer-sphere electron-transfer pathways. The main factors which influence the kinetics and thermodynamics of the observed electron-transfer steps are discussed on the basis of the spectroscopic, kinetic and electrochemical results. A general picture of the reaction pathways that occur on a short (s) and long (min to h) time scale as a function of pH and relative reactant concentrations is derived from the experimental data. In addition, the release of NO resulting from the one-electron reduction of NP by Cbl(II) was monitored with the use of a sensitive NO electrode. The results obtained in the present study are discussed in reference to the possible influence of cobalamin on the pharmacological action of nitroprusside.

Oxidation of hexachloroiridate(III) by OH. Evidence for both inner and outer sphere electron transfer pathways

Two rate processes have been observed by optical pulse radiolysis studies for the oxidation of hexachloroiridate(III) by OH in the pH range 3·0–4·5 and the rate constant for the faster process, k1= 1·3 × 1010 l mol–1 s–1, which comprises both inner and outer sphere electron transfer pathways was found to be independent of pH, whereas the rate constant for the slower process, k2= 1·2 × 105[H+] l mol–1 s–1, showed a first-order dependence on the hydrogen ion concentration.