A Ruthenium(III)–Oxyl Complex Bearing Strong Radical Character

Proton‐coupled electron‐transfer oxidation of a Ru^II?OH₂ complex, having an N‐heterocyclic carbene ligand, gives a Ru^III?O^. species, which has an electronically equivalent structure of the Ru^IV=O species, in an acidic aqueous solution. The Ru^III?O^. complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru?O bond showed a lower‐energy Raman scattering at 732 cm^?1 and the Ru?O bond length was estimated to be 1.77(1) Å. The Ru^III?O^. complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C?H abstraction from the formyl group by the Ru^III?O^. complex bearing a strong radical character as the active species.     

Evaluation of the Oxo‐bridged Dinuclear Ruthenium Ammine Complex as Redox Mediator in an Electrochemical Biosensor

The mediation of electron‐transfer by oxo‐bridged dinuclear ruthenium ammine [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ for the oxidation of glucose was investigated by cyclic voltammetry. These ruthenium (III) complexes exhibit appropriate redox potentials of 0.131–0.09 V vs. SCE to act as electron‐transfer mediators. The plot of anodic current vs. the glucose concentration was linear in the concentration range between 2.52×10^?5 and 1.00×10^?4 mol L^?1. Moreover, the apparent Michaelis‐Menten kinetic (K_M^app) and the catalytic (K_cat) constants were 8.757×10^?6 mol L^?1 and 1,956 s^?1, respectively, demonstrating the efficiency of the ruthenium dinuclear oxo‐complex [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ as mediator of redox electron‐transfer.     

Kinetics of the reaction of Di-μ-acetatodi-μ-oxalatodiaquodiruthenium(II,III) anion with N-(2-hydroxyethyl)-ethylenediaminetriacetatoaquotitanium(III)

Reduction of [Ru₂(CH₃COO)₂(C₂O₄)₂(H₂O)₂]^? by N-(2-hydroxyethyl)-ethylenediaminetriacetatoaquotitanium(III) [Ti(HEDTA)] involves several distinct stages. The first stage has a half-time of less than 1 ms, and is interpreted as a substitution reaction leading to a multinuclear intermediate. The second stage has a second-order rate constant of 5 x 10₃M^?1s^?1 [25°C, μ = 0.1 m (LiCF₃SO₃)]. The rate-limiting process for the second stage is electron transfer within the assembled multinuclear complex. Subsequent slower stages correspond to breakup of the multinuclear Ru(II)₂-Ti(IV) complex formed by electron transfer. The overall rate of reduction of this oxidant by Ti(HEDTA) is less than the corresponding rate for the reaction in which Ti³⁺ acts as reductant, mainly because the stability of the binuclear complex is reduced by the presence of the aminoacid ligand. The data are consistent with the conclusion that the ligand increases the rate of intramolecular ET, probably by reducing geometric change associated with oxidation of Ti(III) to Ti(IV).     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.     

Structural-dynamic approach and path analysis of the intramolecular electron transfer in the bridged complexes [(NH3)5Ru-py-(C2H2) n -py-Ru(NH3)5]5+ withn = 0,1,2,3

The electron interaction transfer paths in the mixed-valence complexes[(NH₃)₅Ru-py-(C₂H₂)_n-py-Ru(NH₃)₅]⁵⁺ are analyzed in the framework of perturbation theory. A structural-dynamic approach to treatment of the intramolecular electron transfer is proposed. This approach is based on the solution of the nonstationary Schrödinger equation in a basis of many-electron functions describing separate structural sites of the electron-transport chain Ry^II→pyridyl→ C₂H₂)_n→pyridyl→Ru^III.     

The reaction of methoxy radicals with nitric oxide and nitrogen dioxide

By allowing dimethyl peroxide (10^?4M) to decompose in the presence of nitric oxide (4.5 × 10^?5M), nitrogen dioxide (6.5 × 10^?5M) and carbon tetrafluoride (500 Torr), it has been shown that the ratio k₂/k_2′ = 2.03 ± 0.47: CH₃O + NO → CH₃ONO (reaction 2) and CH₃O + NO₂ → CH₃ONO₂ (reaction 2′). Deviations from this value in this and previous work is ascribed to the pressure dependence of both these reactions and heterogeneity in reaction (2). In contrast no heterogeneous effects were found for reaction (2′) making it an ideal reference reaction for studying other reactions of the methoxy radical. We conclude that the ratio k₂/k_2′ is independent of temperature and from k₁ = 10^10.2±0.4M^?1 sec^?1 we calculate that k_2′ = 10^9.9±0.4M^?1 sec^?1. Both k₂ and k_2′ are pressure dependent but have reached their limiting high-pressure values in the presence of 500 Torr of carbon tetrafluoride. Preliminary results show that k₄ = 10.^9.0±0.6 10^?4.5±1.1/ΘM^?1 sec^?1 (Θ = 2.303RT kcal mole^?1) and by k₄ = 10^8.6±0.6 10^?2.4±1.1/ΘM^?1 sec^?1: CH₃O + O₂ → CH₂O + HO₂ (reaction 4) and CH₃O + t-BuH → CH₃OH + (t-Bu) (reaction 4′).     

Kinetics of the reactions OH (v = O) + NH3 →; H2O + NH2 and OH(v = O) + O3 → HO2 + O2 AT 298°K

The rate constants for the reactions OH(X₂Π, ν = O) + NH₃ → ^k1 H₂O + NH₂ and OH(X²Π, ν = O) + O₃^k2 → HO₂ + O₂ were measured at 298°K by the flash photolysis resonance fluorescence technique. The values of the rate constants thus obtained are K₁ = (4.1 ± 0.6) × 10^?14 and k₂ = (6.5 ± 1.0) × 10^?14 in units of cm³ molecule ^?1 sec¹. The results are discussed in terms of understanding the dynamics of the perturbed stratosphere.     

The reaction of the hydroxyl radical with acetylene

The reaction of OH with acetylene was studied in a discharge flow system at room temperature. OH was generated by the reaction of atomic hydrogen with NO₂ and was monitored throughout the reaction using ESR spectroscopy. Mass-spectrometric analysis of the reaction products yielded the following results: (1) less than 3 molecules of OH were consumed, and less than 2 molecules of H₂O were formed for every molecule of acetylene that reacted; (2) CO was identified as the major carbon-containing product; (3) NO, formed in the generation of OH, reacted with a reaction intermediate to give among other products N₂O. These observations placed severe limitations on the choice of a reaction mechanism. A mechanism containing the reaction OH + C₂H₂ → HC₂O + H₂ better accounted for the experimental results than one involving the abstraction reaction OH + C₂H₂ → C₂H + H₂O. The rate constant for the initial reaction was measured as 1.9 ± 0.6 × 10^?13 cm³ molecule^?1 sec^?1.     

Tuning Excited State Isomerization Dynamics through Ground State Structural Changes in Analogous Ruthenium and Osmium Sulfoxide Complexes

The complexes [Ru(bpy)₂(pyESO)](PF₆)₂ and [Os(bpy)₂(pyESO)](PF₆)₂, in which bpy is 2,2′‐bipyridine and pyESO is 2‐((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by ¹H NMR, UV–visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X‐ray structural analysis were grown for [Ru(bpy)₂(pyESO)](PF₆)₂. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (electrochemical‐chemical electrochemical‐chemical) mechanism in which isomerization follows Ru²⁺ oxidation and Ru³⁺ reduction. The S‐ and O‐bonded Ru^3+/2+ couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)₂(pyESO)](PF₆)₂, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge‐transfer excitation of [Ru(bpy)₂(pyESO)](PF₆)₂ results in a significant change in the absorption spectrum. The S‐bonded isomer of [Ru(bpy)₂(pyESO)]²⁺ features a lowest energy absorption maximum at 390 nm and the O‐bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)₂(pyESO)]²⁺ was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)₂(pyESO)]²⁺. These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.     

REDOX AND pH-INDUCED SWITCHING OF THE COORDINATION SITES IN THE 3-HYDROXYPICOLINATE RUTHENIUM(III)-edta COMPLEX

Abstract

Ruthenium(III)-edta reacts with the 3-hydroxypicolinate ligand (Hhpic^?) at pH 5, yielding the practically colorless [Ru^III(edta)(κN, κO-Hhpic)]^2? complex (H₂hpic = 3-hydroxypicolinic acid; H₄edta = ethylenedinitrilotetraacetic acid). Above pH 9, deprotonation of the phenolic group promotes an intramolecular linkage isomerization process, generating the faint red [Ru^III(edta) (κO, κO-hpic)]^3? complex. Both isomers can be electrochemically reduced, converting into a single deep red [Ru^II(edta)(κN, κO-Hhpic)]^3? complex strongly stabilized by ruthenium-to-pyridinecarboxylate d _π → p*_π charge-transfer interactions. The observed distinct binding properties as a function of the oxidation states and pH have been rationalized based on semiempirical theoretical calculations for the complexes.     

Energy transfer processes in reactions of He(23S) with triatomic molecules. II. H2O and H2S

The energy transfer reactions He(2³S) + H₂O and He(2³S) + H₂S were studied spectroscopically in the visible and ultraviolet ranges in a flowing afterglow apparatus. No primary triatomic ion emission was observed in this study. Only dissociative fragments were found to emit. In the He(2³S)/H₂O system intense OH(A²Σ⁺ → X²Π_i) emission bands and hydrogen Balmer series were observed while in the He(2³S)/H₂S system intense HS⁺(A³Π_i → X³ Σ^?), weak hydrogen Balmer series and some atomic sulfur lines were found. It is concluded that dissociative processes are competitive with Penning ionization in these energy transfer reactions with other possible reaction channels playing inferior roles. The post-ionization process of ion—electron recombination in the flowing afterglow dominates the emission results in the He(2³S)/H₂O system.     

Kinetics and mechanism of the reduction of diaquotetrakis (2,2′-bipyridine)-µ-oxodiruthenium(III) ion by hypophosphorous acid in acidic medium

The kinetics and mechanism of the reduction of diaquotetrakis(2,2′-bipyridine)-µ-oxodiruthenium(III), [(H₂O)₂(bipy)₄Ru₂O]⁴⁺, by H₃PO₂ has been studied in aqueous acid at ionic strength = 0.5 mol dm^?3 (NaClO₄), [H⁺] = 5.0 × 10^?2 mol dm^?3 and temperature = 31 ± 1 °C. Measurement of the stoichiometry showed that 1 mole of [(H₂O)₂(bipy)₄Ru₂O]⁴⁺ was reduced by 1 mole of H₃PO₂. The reaction was found to be first order with respect to both [(H₂O)₂(bipy)₄Ru₂O⁴⁺] and [H₃PO₂], hence second order overall. Variations in the ionic strength and dielectric constant of the reaction medium had no effect on the rate. Also, addition of various ions to the reaction medium did not significantly alter the rate. Free radicals were identified during the course of the reaction by a polymerisation test. Spectroscopic information and Michaelis–Menten plots suggested the absence of an intermediate complex prior to electron transfer. [(H₂O)₂(bipy)₂Ru]²⁺, the reduction product of [(H₂O)₂(bipy)₄Ru₂O]⁴⁺, plus H₃PO₃, the oxidation product of H₃PO₂, were identified in the product solutions. It is suggested that the reaction proceeds through the outer sphere pathway. A mechanism for the reaction is proposed.     

Kinetics and Mechanism of the Reaction of a Ruthenium(VI) Nitrido Complex with HSO3− and SO32− in Aqueous Solution

The kinetics and mechanism of the reaction of S^IV (SO₃^2?+HSO₃^?) with a ruthenium(VI) nitrido complex, [(L)Ru^VI(N)(OH₂)]⁺ (Ru^VIN, L=N,N′‐bis(salicylidene)‐o‐cyclohexyldiamine dianion), in aqueous acidic solutions are reported. The kinetic results are consistent with parallel pathways involving oxidation of HSO₃^? and SO₃^2? by Ru^VIN. A deuterium isotope effect of 4.7 is observed in the HSO₃^? pathway. Based on experimental results and DFT calculations the proposed mechanism involves concerted N?S bond formation (partial N‐atom transfer) between Ru^VIN and HSO₃^? and H⁺ transfer from HSO₃^? to a H₂O molecule.     

Kinetics of the heterogeneous electron transfer reaction of triplet pyrene in micelles to Br2− radicals in aqueous solution

A combined flash photolysis and pulse radiolysis experiment was carried out to produce triplet pyrene (P) molecules in micelles of cetyltrimethylammonium bromide and Br₂^? in the surrounding aqueous medium. The reaction ³P_mic + Br → P + 2 Br^? was followed by optical absorption measurements in the 10^?8?10^?4–sec range. This reaction possesses a “fast” and a “slow” component with respective rate constants of 2.3 × 10⁶ sec^?1 and 1 × 10⁹M^?1 · sec^?1. The fast component is related to the probability of a Br₂^? radical meeting a triplet pyrene containing micelle on the first encounter (only 16% of the micelles contained a triplet molecule). Reactions involving more than one Br₂^? radical–micelle encounter are ascribed to the slow component. The presence of two components reflects the fact that the residence time of a Br₂^? radical in the vicinity of a cationic micelle is substantially longer than the diffusion time of the radical between micelles. Thus the conditions met in micellar chemistry differ dramatically from those in ordinary solution kinetics where the encounter time is generally much shorter than the time between encounters. Some considerations on the energetics of this electron transfer reaction are also presented.     

Vibrational deactivation of HF(υ = 1) in the H2O + HF dimer: HF(υ = 1) + H2O(000) → HF(υ = 0) + H2O(001) + ΔE

The vibration-vibration energy transfer in the near-resonant collision HF(υ = 1) + H₂O(000) → HF(υ = 0) + H₂O(001) + ΔE = 205 cm^?1 has been investigated on the basis of the model of the nonrigid H₂O-HF dimer formation for temperatures not greatly higher than room temperature. The energy mismatch ΔE is considered to be removed by the slow translational motion of two molecules in the complex about their equilibrium separation. A strong negative temperature dependence of the energy exchange rate is shown between 300 and 500 K.     

A Study of picosecond dehalogenation of chlorobenzene anions in liquids by positronium inhibition measurements

Radiation chemistry and results of Ps yields indicate that the following processes occur in the positron spur in solution of halogen-substituted hydrocarbons, RX_n: e⁺ + e^? → Ps, e^? + RX _n → (RX_n)^? → RX_n?1 + X^?, e⁺ + (RX_n)^? → Ps + RX_n, e⁺ + X^? → [X^?, e⁺]. Hence the trapped electron can form Ps only if (RX _n)^? is stable or has a lifetime that is longer than o comparable to the Ps formation time. Previous studies have shown that some of the strongly chlorinated benzenes (n = 4.5 give reasonable inhibition in benzene but not in linear hydrocarbons. The reason is very probably that the dechlorination time is much shorter in benzene than in saturated hydrocarbons because Cl^? is more strongly solvated in benzene than in non-aromatic hydrocarbons. To test those ideas further we have begun detailed studies of solutions of the possible “intermediate” inhibitors, viz. 1,2,3,5- and 1,2,4,5-C₆H₂Cl₄, in mixtures of C₆H₆/C₆H₁₄ different methyl-substituted benzene aniline, anisole, dioxane and ethylbenzene. The results are discussed and interpreted in terms of the spur model. The Ps inhibition efficiency of the two isomeric forms of tetrachlorobenzene studied, appears most probably to depend on intramolecular electron transfer with subsequent dehalogenation of the molecular anion on a picosecond timescale. The divergence in inhibitor efficiency obtained for the chlorobenzenes when dissolved in aromatic solvents compared to the same solutes when dissolved in a saturated alkane appears most probably to be caused by complex formation between the initially formed chlorobenzene anion and benzene molecules, which permits a rapid relaxation of the molecular anion with subsequent bond stretching and expulsion of the chloride anion.     

Reversible intersystem crossing,fluorescence lifetime lengthening and collisionally induced phosphorescence in biacetyl

The emissions of biacetyl excited at 4200 Å were studied at pressures down to 10^?3 torr. Apart from the well-known nanosecond fluorescence, a new emission of the same spectral composition was found with a non-exponential decay in the microsecond range. Furthermore the phosphorescence, as defined by its spectral composition, was found to be collisionally induced.The results imply that after excitation, the molecule rapidly transfers (rate constant k_S→T) to the triplet state, giving rise to the nanosecond decay time; and can then transfer back to the singlet state (rate constant k_T→S), giving rise to the microsecond emission. At the same time internal conversion can occur (k_S→S0). From an analysis of the data we find for k_S→S0 = 2.4 × 10⁷ sec^?1, k_S→T = 7.6 × 10⁷ sec^?1, k_T→S = 1.9 × 10⁵ sec^?1. The kinetic treatment can be transformed to a quantum mechanical one, yielding values for the triplet level density (?_T), the coupling element V_ST and the number of triplet states (N) coupled to the singlet excited. At 4200 Å we find ?_T = 6.3 × 10⁵cm, V_ST = 1.0 × 10^?5 cm^?1, N = 400.Phosphorescence occurs only when the molecule is deactivated by collisions to a vibronic triplet state below the vibrationless excited singlet state. The efficiency of biacetyl collisions is 0.54.     

Intercage Electron Transfer Driven by Electric Field in Robin–Day‐Type Molecules

A new class of isomers, namely, intercage electron‐transfer isomers, is reported for fluorinated double‐cage molecular anion e^?@C₂₀F₁₈(NH)₂C₂₀F₁₈ with C₂₀F₁₈ cages: 1 with the excess electron inside the left cage, 2 with the excess electron inside both cages, and 3 with the excess electron inside the right cage. Interestingly, the C₂₀F₁₈ cages may be considered as two redox sites existing in a rare nonmetal mixed‐valent (0 and ?1) molecular anion. The three isomers with two redox sites may be the founding members of a new class of mixed‐valent compounds, namely, nonmetal Robin–Day Class II with localized redox centers for 1 and 3 , and Class III with delocalized redox centers for 2 . Two intercage electron‐transfers pathways involving transfer of one or half an excess electron from one cage to the other are found: 1) Manipulating the external electric field (?0.001 a.u. for 1 → 3 and ?0.0005 a.u. for 1 → 2 ) and 2) Exciting the transition from ground to first excited state and subsequent radiationless transition from the excited state to another ground state for 1 and 3 . For the exhibited microscopic electron‐transfer process 1 → 3 , 2 may be the transition state, and the electron‐transfer barrier of 6.021 kcal mol^?1 is close to the electric field work of 8.04 kcal mol^?1.     

The Reactions of H Atoms with H2S and COS

The kinetics of the reactions of H atoms with H₂S and with COS were measured at 298deg;K in a flow system using mass-spectrometric detection. The rate constants were found to be 3.8 × 10^?13 and 2.2 × 10^?14 cm₃ part^?1 sec^?1, respectively, with an estimated accuracy of 25%. At high flow rates of H₂S, 0.5 molecules of H₂S are consumed per H atom originally present. At all flow rates of COS, H₂S is a major product, CO production equals COS consumption, and 0.5 molecules of COS are consumed per H atom. The results are consistent with the reaction HS + HS → H₂S + S being the dominant process for thiyl radicals, and evidence is presented to indicate that its reaction rate is close to collisional frequency.     

IR,Raman, and Surface‐enhanced Raman Spectroscopic Study on Triruthenium Dipyridylamide Diruthenium Nickel Dipyridylamide Family: Metal‐metal Bonding and Structures

We report the infrared, Raman, and surface‐enhanced Raman scattering (SERS) spectra of triruthenium dipyridylamido complexes and of diruthenium mixed nickel metal‐string complexes. From the results of analysis on the vibrational modes, we assigned their vibrational frequencies and structures. The infrared band at 323–326 cm^?1 is assigned to the Ru₃ asymmetric stretching mode for [Ru₃(dpa)₄Cl₂]^0–2+. In these complexes we observed no Raman band corresponding to the Ru₃ symmetric stretching mode although this mode is expected to have substantial Raman intensity. There is no frequency shift in the Ru₃ asymmetric stretching modes for the complexes with varied oxidational states. No splitting in Raman spectra for the pyridyl breathing line indicates similar bonding environment for both pyridyls in dpa^– , thus a delocalized structure in the [Ru₃]^6–8+ unit is proposed. For Ru₃(dpa)₄(CN)₂ complex series, we assign the infrared band at 302 cm^?1 to the Ru₃ asymmetric stretching mode and the weak Raman line at 285 cm^?1 to the Ru₃ symmetric stretching. Coordination to the strong axial ligand CN^– weakens the Ru‐Ru bonding. For the diruthenium nickel complex [Ru₂Ni(dpa)₄Cl₂]^0–1+, the diruthenium stretching mode ν_Ru‐Ru is assigned to the intense band at 327 and 333 cm^?1 in the Raman spectra for the neutral and oxidized forms, respectively. This implies a strong Ru‐Ru metal‐metal bonding.