Integrating proton coupled electron transfer (PCET) and excited states

In many of the chemical steps in photosynthesis and artificial photosynthesis, proton coupled electron transfer (PCET) plays an essential role. An important issue is how excited state reactivity can be integrated with PCET to carry out solar fuel reactions such as water splitting into hydrogen and oxygen or water reduction of CO₂ to methanol or hydrocarbons. The principles behind PCET and concerted electron–proton transfer (EPT) pathways are reasonably well understood. In Photosystem II antenna light absorption is followed by sensitization of chlorophyll P₆₈₀ and electron transfer quenching to give P₆₈₀⁺. The oxidized chlorophyll activates the oxygen evolving complex (OEC), a CaMn₄ cluster, through an intervening tyrosine–histidine pair, Y_Z. EPT plays a major role in a series of four activation steps that ultimately result in loss of 4e^?/4H⁺ from the OEC with oxygen evolution. The key elements in photosynthesis and artificial photosynthesis – light absorption, excited state energy and electron transfer, electron transfer activation of multiple-electron, multiple-proton catalysis – can also be assembled in dye sensitized photoelectrochemical synthesis cells (DS-PEC). In this approach, molecular or nanoscale assemblies are incorporated at separate electrodes for coupled, light driven oxidation and reduction. Separate excited state electron transfer followed by proton transfer can be combined in single semi-concerted steps (photo-EPT) by photolysis of organic charge transfer excited states with H-bonded bases or in metal-to-ligand charge transfer (MLCT) excited states in pre-associated assemblies with H-bonded electron transfer donors or acceptors. In these assemblies, photochemically induced electron and proton transfer occur in a single, semi-concerted event to give high-energy, redox active intermediates.     

Pulse radiolysis of (RhI(CO)2Cl)2 and (RhII(CH3COO)2)2 solutions under CO or N2 atmosphere

Ethanolic solutions of (Rh^I(CO)₂Cl)₂ and aqueous solutions of (Rh^IICH₃COO)₂)₂ have been investigated by pulse radiolysis under CO or N₂ atmosphere. In the first case the reduction of the Rh^I complex is shown to proceed via CO^- formation. In the second case, several steps have been evidenced, one of them extremely fast, indicating an exceptional reactivity of such a binuclear rhodium structure towards the electron. Spectra of transient species at different times are presented. A species absorbing at 520 nm, already present at 10 ns, is assigned to a Rh^IRh^II complex resulting itself from a reaction of the initial salt with pre-solvated electrons. A mechanism is proposed to account for the decay kinetics of e^-_aq and the spectral changes. The rate constants are evaluated for each of the five steps occuring within the first microsecond.     

Electrochemical behaviour of tris (2,2′-bipyridine) osmium(II) complex in dimethylformamide

The electrochemical behaviour of Os(bpy)₃²⁺ (bpy=2,2′-bipyridine) has been investigated in N,N-dimethylformamide by utilizing predominantly the techniques of polarography and cyclic voltammetry. The study has been carried out at different temperatures in the range ?20 to +30° C. The number of reduction waves observed depends markedly on temperature. For intermediate temperatures, the complex exhibits six reduction waves, the maximum number of waves observed as a function of temperature.The first three reduction processes, corresponding to the first three reduction waves, are one-electron, diffusion-controlled reversible processes in all conditions. Conversely, process four is consistent with one-electron reversible transfer only at the lowest temperature. In fact, for higher temperatures the liberation of bpy, preferentially as a radical anion rather than a neutral molecule, occurs. In the latter case, the liberated neutral bpy molecule can be reduced by one-electron transfer. Process five is due to the reduction of species formed by chemical reaction in the preceding electrode process, i.e. the bpy radical anion and Os(bpy)₂^1?. Process six is consistent with the addition of five electrons to the starting complex, followed by the liberation and successive reduction of the bpy radical anion.     

On the mechanism of interaction between copper (I) and O2

The mechanism of the interaction of Cu⁺-α,α-dipyridyl complex (Cu⁺L₂) with O₂ in both neutral and acid media was studied by the stopped-flow method. The dependence of the mechanism on the acidity of the medium was established. In an acid medium H⁺ participated in a direct O₂ reduction to HO₂ by interaction with an oxygen adduct L₂Cu⁺O₂ formed without displacement of ligand molecules. In a neutral medium the reaction rate was limited by inner sphere charge transfer from Cu⁺ to O₂ to form an oxygen “charge transfer” complex L₂CuO⁺₂. The latter interacted either with the second ion Cu⁺L₂ or with the free ligand, or else it dissociated, reversibly or irreversibly, to form a radical anion O^?₂. The bimolecular rate constants of the oxygen “adduct” and “charge transfer” complex formation appeared to be k_bi = (1.0 ± 0.1) × 10⁵ and (1.5 ± 0.2) × 10⁴M^?1?sec^?1, respectively. The effective termolecular rate constants of O₂ reduction to HO₂ in an acid medium (with contribution from H⁺) and to O^?₂ in a neutral medium (with contribution from α,α-dipyridyl) were k_ter = 2.7 × 10⁸ and 10⁷M^?2?sec^?1. The rate constants of the elementary steps were estimated. The auto-oxidation mechanism of the aquoion and complexes of Cu⁺ is discussed in terms of the results obtained.     

In situ monitoring of potential oscillations in the reduction of IO3− by electrochemical quartz crystal microbalance

Electrochemical quartz crystal microbalance (EQCM) has been employed to study the potential oscillatory mechanism for the IO₃⁻ reduction accompanying periodic hydrogen evolution. The new experimental results that were obtained by in situ EQCM monitoring clearly demonstrate the all key steps involved in the oscillation: the diffusion-limited depletion of IO₃⁻ by reduction, the formation, growth and departure of hydrogen bubbles on the surface, and the convection-induced replenishment of IO₃⁻ by the hydrogen evolution. In addition to the frequency response to the surface mass change as reported in the literature, our study first shows that simultaneous frequency responses to the changes of density and viscosity in the diffusion layer during the oscillation can also provide meaningful and even decisive information on the oscillatory mechanism for the oscillators involving the coupling of electrochemical reactions with diffusion and convection mass transfer.     

Synthesis,photophysical and electrochemical studies of di-2-pyridyl ketone complexes of rhodium(III)

Complexes of rhodium(III) with di-2-pyridyl ketone (dpk), Rh(dpk)(MeCN)Cl₃ (1) and cis-[Rh(dpk)₂Cl₂]⁺ (2), have been successfully prepared and characterized. At low temperature (77 K), complex (2) in EtOH/MeOH (4:1, v/v) shows a broad, symmetric and structureless red emission with a microsecond lifetime and, hence, is assigned as the dd* phosphorescence. Electrochemical data, including cyclic voltammetry, normal pulse voltammetry, triple pulse voltammetry and controlled potential electrolysis, have been obtained for the two dpk complexes of rhodium(III) in MeCN. On the basis of analysis of the electrochemical (1,2) and luminescence data (2), electron transfer mechanisms are proposed. For complex (1), two reduction processes occur at the metal-localized orbitals with elimination of chlorides during the first reduction step. This is followed by a one-electron reduction at the metal. For complex (2), three electrons are transferred to the metal in two successive reduction steps accompanied by elimination of two chlorides. After these two reduction steps another one-electron reduction occurs at the dpk ligand.     

Oxygen reduction on the Au (311) electrode surface in alkaline electrolyte

Oxygen reduction was studied for the first time using a single crystal electrode in a rotating disc-ring arrangement. The Au (311) surface shows a complex behaviour, with a very high activity in certain potential regions. The first electron transfer is rate determining in the region of 4-electron reduction. As with Au (100), a 4 e⁻ reduction changes into a 2 e⁻ process, which reverts back to a 4 e⁻reaction at very negative potentials. Based on a general reaction scheme of O₂ reduction, a map of the operating potential dependent reaction pathways was constructed. Nearly 60% of the mass flux of O₂ undergoes a direct reduction to OH⁻ in the region of mixed control. The high activity of Au (311) was ascribed to a high step density and AuOH present on its surface.     

Influence of the Mixed Adsorption Layer of 1‐Butanol/Toluidine Isomers on the Two Step Electroreduction of Zinc(II) Ions

Studies of mixed adsorption layers with respect to their influence on kinetics of Zn²⁺ ions reduction indicate dynamics of this process in the presence of inhibitor and accelerating substances. This effect can be seen as a much greater increase of standard constant rates compared with a similar increase without the inhibitor. 1‐Butanol was used as an inhibitor in the studies and p‐toluidine and m‐toluidine as accelerating substances. The adsorption measurements show that in the range of Zn²⁺ ion reduction potentials in the solutions containing 1‐butanol and the definite isomer of toluidine, toluidine plays a dominant role in establishing equilibrium in the mixed adsorption layer. The obtained values of the true standard constant rates of the transfer of the first electron k_s1^t and the second electron k_s2^t indicate that in each of the studied systems the stage of the first electron transfer is more strongly accelerated compared with the stage of the second electron transfer. The linear character of the dependence k_s1^t and k_s2^t in the function of the surface excess and in the function of 1‐butanol concentration indicates that active complexes are formed for each case of the mixed adsorption layer.     

Electrochemical studies of mercury(II)

The reduction of Hg²⁺ is studied by linear sweep voltammetry, rotated disk electrode voltammetry, and chronoamperometry in a non-complexing medium at a vitreous carbon electrode (VCE). At the VCE which is completely free of any mercury deposit, the reduction of Hg²⁺ is found to be of first order, involving two electrons and reversible at slow sweep rates. When the VCE is partially covered with mercury droplets, the reduction mechanism is different and occurs in two steps. At the most active sites on the VCE where mercury droplets are formed during a previous cathodic sweep, Hg²⁺ undergoes disproportionation to Hg₂²⁺ which is subsequently reduced to Hg. The second step involves the simple two-electron, diffusion-controlled reduction of Hg²⁺ to Hg at the bare electrode surface.     

On the polarographic reduction of nitrate ion in the presence of zirconium(IV)

The polarographic reduction of nitrate ion in the presence of zirconium(IV) is studied by dc and phase-selective ac polarography. The total reduction process was proved by means of controlled-potential electrolysis and chemical analysis to conform to NO⁻₃+8H⁺6e⁻→NH₂OHH⁺=2 H₂O. Using a measurement of differential capacity, a large part of the difference between the reduction transfer to the vicinity of the electrode but to a charge transfer step and/or a chemical reaction step. The zirconium(IV) is concluded to act as an intermediary for the charge transfer from the electrode to the nitrate ion.     

An electrochemical study of (η-C5H5)2Ti(NCS)2

The reduction of titanocene dithiocyanate in various solvents has been examined using the techniques of polarography, voltammetry, controlled potential electrolysis and cyclic voltammetry. In THF and CH₂Cl₂, Cp₂Ti(NCS)₂ undergoes successive one-electron transfer reactions, and by using a combination of electrochemical and EPR techniques it has been possible to confirm the stability of [Cp₂Ti(NCS)₂]^? in these solvents. In DMF, however, an irreversible dimerization follows the first electron transfer.     

Stufenweise und chemisch reversible Reduktion von Zweikernkomplexen {(μ‐bmtz)[MCl(η6‐Cym)]2}[PF6]2 (M = Ru,Os; bmtz = 3,6‐Bis(2′‐pyrimidyl)1,2,4,5‐tetrazin) mit bis zu sechs Elektronen

Stepwise and Chemically Reversible Reduction of {(μ‐bmtz)[MCl(η⁶‐Cym)]₂}[PF₆]₂ (M = Ru, Os; bmtz = 3,6‐Bis(2′‐pyrimidyl)1,2,4,5‐tetrazine) with up to Six Electrons The title compounds were synthesized and characterized spectroscopically. Analysis of cyclic voltammetry in conjunction with spectroelectrochemistry (UV/VIS/NIR, ESR) allowed us to identify the products of sequential, chemically reversible reduction with up to six electrons. It is the first time that the molecular coupling of electron transfer steps (E) and bond breaking/bond forming chemical processes (C) has been studied for a diruthenium system. In that case, the chemically reversible sequence E, EC, EEC, E and E was observed whereas the diosmium analogue showed the unusual sequence E, EC, E, EEC and E.     

On the coupled oxidation-reduction mechanism of molecular nitrogen fixation

A new mechanism for the catalytic reduction of N₂ was proposed. According to the mechanism, reduction is preceded by the oxidation step with the formation of N₂O. The mechanism allows the participation of weaker reducing agents than those in purely reductive processes. Probable individual steps are considered, in particular, the oxygen atom transfer from the superoxide radical anion O₂ ^–· in a cyclic complex containing the N₂ molecule in the coordination sphere of a metal. The proposed mechanism can explain N₂ reduction involving recently discovered nitrogenase in which O₂ ^–· acts as an electron donor and N₂ reduction in purely chemical systems including the air nitrogen and relatively weak reducing agents.     

The different kinetic and mechanistic behaviors of molybdenum and tungsten in the reduction of tris(benzene‐1,2‐dithiolato)Mo(VI) and W(VI) complexes by ascorbic acid in aqueous media

The mono‐electronic reduction of tris(benzene‐1,2‐dithiolato)Mo(VI) and W(VI) complexes (ML₃: M = Mo, W; L = S₂C₆H^2?₄, S₂C₆H₃CH^2?₃) to their anionic forms ML^?₃ by L (+)‐ascorbic acid (H₂A) has been studied in tetrahydrofurane (THF):water and THF:methanol by means of diode‐array, stopped‐flow, and mass spectrometry–electrospray ionization (MS‐ESI) spectroscopy. The kinetic study in methanol demonstrates that the reaction is first order in each reactant, the electron transfer being rate limiting. This fact was assessed by the absence of a primary saline effect and by the correlation observed between the activation free enthalpy (ΔG^≠) and the reduction potentials measured by cyclic voltamperometry. In aqueous media, Mo(VI)‐tris(dithiolenes) also reduce to their Mo(V) anionic forms. The reaction obeys the rate law ? d[ML₃]/dt = (k_S+k_A[H₂A]_T)[ML₃] (M = Mo), in agreement with a parallel kinetic scheme involving the reduction of complexes by ascorbic acid (k_A) and by interaction with the solvent (k_S). Unexpectedly, the W(VI) complexes were not reduced by excess hydrogen ascorbate in the presence of water. These compounds underwent an extremely rapid autoreduction, which initially yielded an oxo W(VI)‐dithiolene and [W(S₂C₆H₄)₃]^?, as assessed by the MS‐ESI spectra. This observation suggests that tungsten tris(dithiolenes) are capable of coordinating water efficiently, undergoing further reduction after ligand displacement. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 279–291, 2011     

Thermal decomposition of the [Pt(NH3)4]2+ complex in nax zeolite: effect of calcination procedure

The effect of the calcination procedure on the decomposition of the [Pt(NH₃)₄]²⁺ complex in a NaX zeolite was studied by mass spectrometry (MS-TPDE) and diffuse reflectance spectroscopy (DRS). The decomposition of the complex took place in two steps. In the first step, under oxygen, the [Pt(NH₃)₄]²⁺ complex was first converted to [Pt(NH₃)₂]²⁺ complex, accompanied by nitrogen release. In the second step, corresponding to the decomposition of the remaining two amine ligands, NO formation was also observed. Under He, the decomposition also occurred in two steps with H₂ liberation. A reaction scheme was proposed for these results.     

The catalytic reduction of nitrobenzene at the [MoVIO2(O2CC(S)(C6H5)2)2]2− complex intercalated in a Zn(II)–Al(III) layered double hydroxide host: A kinetic model for the molybdenum–pterin binding site in nitrate reductase

The heterogeneous reduction of nitrobenzene by thiophenol catalyzed by the dianionic bis(2‐sulfanyl‐2,2‐diphenylethanoxycarbonyl) dioxomolybdate(VI) complex, [Mo^VIO₂(O₂CC(S)(C₆H₅)₂)₂]²⁻, intercalated into a Zn(II)–Al(III) layered double hydroxide host [Zn_3−xAl_x(OH)₆]^x+, has been investigated under anaerobic conditions. Aniline was found to be the only product formed through a reaction consuming six moles of thiophenol for each mol of aniline produced. The kinetics of the system have been analyzed in detail. In excess of thiophenol, all reactions follow first‐order kinetics (ln([PhNO₂]/[PhNO₂]₀) = −k_appt) with the apparent rate constant k_app being a complex function of both initial nitrobenzene and thiophenol concentrations, as well as linearly dependent on the amount of solid catalyst used. A mechanism for this catalytic reaction consistent with the kinetic experiments as well as the observed properties of the intercalated molybdenum complex has thiophenol inducing the initial coupled proton–electron transfer steps to form an intercalated Mo^IV species, which is oxidized back to the parent Mo^VI complex by nitrobenzene via a two‐electron oxygen atom transfer reaction that yields nitrosobenzene. This mechanism is widespread in enzymatic catalysis and in model chemical reactions. The intermediate nitrosobenzene thus formed is reduced directly by excess thiophenol to aniline. The values of rate coefficients indicate that reduction of nitrobenzene proceeds much faster than proton‐assisted oxidation of thiophenol. This may account for the observation that the presence of protonic amberlite IR‐120(H) increases considerably the rate of the overall reaction catalyzed. Activation parameters in excess of the protonic resin and PhSH were ΔH^≠ = 80 kJ mol⁻¹ and ΔS^≠ = −70 J mol⁻¹ K⁻¹. The large negative activation entropy is consistent with an associative transition state. The present system is characterized by a well‐defined catalytic cycle with multiple‐turnovers reductions of nitrobenzene to aniline without appreciable deactivation. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 212–224, 2001     

Thermal decomposition of the [Pt(NH3)4]2+ complex in NaX zeolite

The effect of the calcination procedure on the decomposition of the [Pt(NH₃)₄]²⁺ complex in a cesium-containing NaX zeolite was studied by thermal decomposition accompanied by mass spectrometry and diffuse reflectance spectroscopy, as well as electron paramagnetic resonance and infrared spectroscopy. The decomposition of the complex took place in two steps. Under oxygen, the [Pt(NH₃)₄]²⁺ complex was first converted into the [Pt(NH₃)₂]²⁺ complex in the first step, with predominant nitrogen release. In the second step, corresponding to the decomposition of the remaining two amine ligands, NO was also formed and adsorbed. Oxygen paramagnetic species were also observed. Under He, the decomposition also occurred in two steps with H₂ release.     

Electrochemical reduction of cobalt and nickel complexes with ligands stabilizing metal in low oxidation state

Electrochemical reduction of cobalt(ii) complexes containing -acceptor ligands (L = bpy, Ph₂Ppy) proceeds through three consecutive reversible steps: one-electron transfer to form a more stable Co^IL complex, transfer of two electrons at more negative potentials to form an anionic [NiL]^– complex, and reduction of the ligand to the radical anion. The stability of the cobalt complexes with different ligands decreases in the series Ph₂Ppy > Ph₃P > bpy.     

Spectroscopic Investigation of the Europium(3+) Ion in a New ZnY4W3O16 Matrix

A new Zn and Eu tungstate was characterized by spectroscopic techniques. This tungstate, of the formula ZnEu₄W₃O₁₆, crystallized in the orthorhombic system and was synthesized by a solid‐state reaction. It melts incongruently at 1330°. The luminescent properties, including excitation and emission processes, luminescent dynamics, and local environments of the Eu³⁺ ions in ZnEu₄W₃O₁₆ and ZnY₄W₃O₁₆ : Eu³⁺ diluted phases (1, 5, and 10 mol‐% of Eu³⁺ ion) were studied basing on the f⁶‐intraconfigurational transitions in the 250–720 nm spectral range. The excitation spectra of this system (λ_em 615 and 470 nm) show broad bands with maxima at 265 and 315 nm related to the ligand‐to‐metal charge‐transfer (LMCT) states. The emission spectra under excitation at the O→W (265 nm) and O→Eu³⁺ (315 nm) LMCT states present the blue‐green emission bands. The emission of tungstate groups mainly originate from the charge‐transfer state of excited 2p orbitals of O^2? to the empty orbitals of the central W⁶⁺ ions. On the other hand, in the emission of the Eu³⁺ ions, both the charge transfer from O^2? to Eu³⁺ and the energy transfer from W⁶⁺ ions to Eu³⁺ are involved. The emission spectra under excitation at the ⁷F₀→⁵L₆ transition of the Eu³⁺ ion (394 nm) of ZnY₄W₃O₁₆ : Eu³⁺ diluted samples show narrow emission lines from the ⁵D₃, ⁵D₂, and ⁵D₁ emitting states. The effect of the active‐ion (Eu³⁺) concentration on the colorimetric characteristic of the emissions of the compound under investigation are presented.     

Retro-Bingel reaction in the electrochemical reduction of bis(dialkoxyphosphoryl)methanofullerenes

Four steps of reduction were detected for bis(diethoxyphosphoryl)- and bis(diisopropoxyphosphoryl)methano[60]fullerenes (1, 2) and bis(diethoxyphosphoryl)methano[70]fullerene (3) by cyclic voltammetry in the o-dichlorobenzene—DMF (3 : 1, v/v)/Bu₄NBF₄ (0.1 mol L^–1) system on a glass-carbon electrode. At the first step the reversible transfer of one electron affords stable radical anions 1 and 2 (g = 1.9999, H = 1.9 G). When two electrons per molecule are transferred, the methano fragment is rapidly eliminated (retro-Bingel reaction). This process involves the step-by-step cleavage of two C—C bonds of exo-carbon with the fullerene shell in combination with the stepwise transfer of other two electrons and a proton to form finally the carbanion of the methano fragment and fullerene dianion. For all studied compounds, the elimination rate is much higher than that for bis(alkoxycarbonyl)- and dialkoxyphosphoryl(alkoxycarbonyl)methano[60]fullerenes, which makes it possible to propose bisphosphorylmethane groups as protective in synthesis of new fullerene derivatives.