Studies in negative ion mass spectrometry. X—Electron capture by a series of tris(hexafluoroacetylacetonato) metal chelates

Results of electron capture and negative ion mass spectrometric studies are reported for a series of tris-chelates of the type Metal. L₃, where L refers to the ligand or enolate ion of the β-diketone 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoracetylacetone), and the metals are: Sc(III), Ti(III), V(III), Cr(III), Mn(III), Fe(III), Co(III), A1(III), Ga(III), In(III). The negative ion mass spectra were all relatively simple; the most abundant ions being the molecular and ligand ions for all the metals studied. Reaction schemes have been established to account for the appearance of all significant fragment ions, many of which have been formed as a result of fluorine atom transfer processes. For the transition metal complexes, evidence for elimination of neutral divalent metal fluorides comes from the ion decomposition reactions [Metal.L.F₂]^?→[L]^?, and for the Group III metal complexes, [Metal.L₃]^?→[Metal.L₂]^? as well as [Metal.L₂]^?→[L]^? processes indicate that the metals have been reduced as a consequence of the initial electron capture and subsequent fragmentations of metal-containing ions. The influence of the metal atom and its 3d-electron configuration has been shown not to affect significantly the electron capture processes. However, the relative instabilities of molecular anions of the transition metal tris-complexes show an approximately linear dependence on the increasing 3d-electron populations of the metal ions from Ti(III) to Co(III).     

Kinetics of the Reductions of (Ethylenediaminetetraacetato)cobaltate(III) by 4,4-Dipyridylamine Complexes of Pentacyanoferrate(II) and Pentaammineruthenium(II)

The reactions of Fe(CN)₅dpa^3? and Ru(NH₃)₅dpa²⁺ (dpa = 4,4′-dipyridylamine) with Co(edta)^? have been investigated kinetically. For Fe(CN)₅dpa^3? complex, a linear relationship was observed between the pseudo-First-order rate constants and the concentrations of Co(edta) which leads to a specific rate 0.876 ± 0.006 M^?1S^?1 at T = 25°C., μ = 0.10 M and pH = 8.0. For the Ru(NH₃)₅dpa²⁺ system, the plots k_obs vs [Co(edta)^?] become nonlinear at concentrations of Co(edta) greater than 0.01 M and the reaction is interpreted on the basis of a mechanism involving the formation of an ion pair between Ru(NH₃)₅dpa²⁺ and Co(edta)^? followed by electron transfer from Ru(II) to Co(III). The nonlinear least squares fit of the kinetic results shows that Q_ip = 10.6 ± 0.7 M^?1 and k_et = 93.9 ± 0.7 s^?1 at pH = 8.0,μ = 0.10 M and T = 25°C.     

Tuning excited-state electron transfer from an adiabatic to nonadiabatic type in donor-bridge-acceptor systems and the associated energy-transfer process

Through design and synthesis of a new series of dyads I-III composed of 2,3-dimethoxynaphthalene as an electron donor (D) and 2,3-dicyanonaphthalene as an acceptor (A) bridged by n-norbornadiene (n = 1-3) we demonstrate an excellent prototype to switch the excited-state electron-transfer dynamics from an adiabatic to a nonadiabatic process. I reveals a remarkable excitonic effect and undergoes an adiabatic type of electron transfer (ET), resulting in a unique charge-transfer emission, of which the peak wavelength exhibits strong solvatochromism. Conversely, upon exciting the donor moiety, a fast D --> A energy transfer takes place for II (approximately 3 ps) and III (< or =30 ps), followed by a nonadiabatic type, weak coupled electron transfer with a relatively slow ET rate, giving rise to dual emission in polar solvents. Further detailed temperature-dependent studies of the ET rate deduced reaction barriers of 2.7 kcal/mol (for II) and 1.3 kcal/mol (for III) in diethyl ether and CH2Cl2, respectively. The results lead to a deduction of the reaction free energy and reorganization energy for both II (in diethyl ether) and III (in CH2Cl2). Theoretical (for I) and experimental (for II and III) approaches estimate the electronic coupling to be 860, 21.9, and 3.2 cm(-1) for I, II, and III, respectively, supporting the adiabatic versus nonadiabatic switching mechanism.     

Photoinduced intramolecular electron transfer reactions within donor-acceptor linked compounds in solution and within donor-acceptor ion-pairs in crystal

Intramolecular electron transfer (ET) processes within donor-acceptor linked compounds in solution and donor-acceptor ion-pairs in crystal have been investigated by means of laser photolysis kinetic spectroscopy. An excited Ru(II)-moiety of donor-acceptor compounds undergoes intramolecular electron-transfer to either ruthenium(III) ion, rhodium(III) ion or a cobalt(III) ion, followed by back ET to regenerate the original reactant. An Arrhenius plot of the ET rate gave a straight line with an intercept (frequency factor) and a slope (activation energy) for the photoinduced ET and the back ET. Mixed-valence isomer states produced via photoinitiated ET rapidly decayed via back ET. A common and large frequency factor observed for Ru(II)-Rh(III) compounds is accounted for in terms of solvent-relaxation dynamics. For the back ET in the Ru(II)-Co(III) compounds, the frequency factors are reduced because of negative entropy change. ET within donor-acceptor ion-pair of Ru(bpy)₂³ and Co(CN)₃⁶ in crystal took place very rapidly compared with in water.     

Intervalence charge transfer transition in mixed valence complexes synthesised from RuIII(edta)- and FeII(CN)5-cores

Intervalence charge transfer properties were studied for a set of mixed valence complexes incorporating Ru(III) and Fe(II)-centres linked by various saturated and unsaturated bridging ligands (BL). Studies reveal that degree of ground state electronic interaction and coupling between Ru(III) and Fe(II)-centrescanbe attenuated by changing the nature of the bridging ligand. Further, inclusion of the bridging ligand with interrupted π-electron system in a β-CD cavity initiate an optical electron transfer from Fe(II) to Ru(III) which is otherwise not observed.     

Temperature dependence of the electronic factor in the nonadiabatic electron transfer at metal and semiconductor electrodes

The temperature dependence of the electronic contribution to the nonadiabatic electron transfer rate constant (k_ET) at metal electrodes is discussed. It is found in these calculations that this contribution is proportional to the absolute temperature T. A simple interpretation is given. We also consider the nonadiabatic rate constant for electron transfer at a semiconductor electrode. Under conditions for the maximum rate constant, the electronic contribution is also estimated to be proportional to T, but for different reasons than in the case of metals (Boltzmann statistics and transfer at the conduction band edge for the semiconductor versus Fermi–Dirac statistics and transfer at the Fermi level, which is far from the band edge, of the metal).     

Mass spectra of partially fluorinated β-diketonato complexes of Al(III), Cr(III), Fe(III) and Co(III)

The mass spectra of the Al(III), Cr(III), Fe(III) and Co(III) complexes of the anions of hexafluoroacetylacetone (hfac) trifluoroacetylacetone (ttac), benzoyltrifluoroacetone (btac) and thenoyltrifluoroacetone (ttac) have been determined and are discussed here. Emphasis is placed on discerning which of the observed reactions may properly be attributed to the influence of the metal in the complex and those which may be thought of as arising from the nature of the ligand. The most important influence of the coordinated metal is related to its ability to be reduced²; however, the presence of the metal serves to prohibit or facilitate certain rearrangement reactions relative to the free protonated ligand. Since essentially all fragmentation occurs within or by loss of a ligand, the nature of the ligand determines the nature of the observed fragments. Where intramolecular com-petition of fragment types is possible (tfac, btac and ttac complexes), the most probable fragment of a given class, odd electron or even electron, is easily determined. The most commonly eliminated fragments are CF₃and CF₂. Certain metastable peaks are associated with consecutive decomposition in the field free region. Such a phenomenon appears to be common for molecules of the type studied here.     

Electronic structure, spectrum, and intramolecular electron transfer model of [(NH3)5Ru-(4,4’-bipy)-Ru(NH3)5]5+

The electronic structure of the ground and excited states of the binuclear mixed-valence complex [Ru(NH₃)₅]₂(4,4’-bipy)⁵⁺ is calculated by the semiempirical INDO + CI method, and an electronic spectrum assignment is given. A theoretical model of electron transfer between the Ru(II) and Ru(III) metal centers is constructed on the basis of many-electron wave functions. The dependence of the electron transfer characteristics on the angles between the planes of the pyridine rings and also between the pyridine rings and the planes of cis(NH₃)-Ru-cis(NH₃) is analyzed. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 3, pp. 447–456, May–June, 1997.     

Surface Chemistry on Small Ruthenium Nanoparticles: Evidence for Site Selective Reactions and Influence of Ligands

The reactivity of two classes of ruthenium nanoparticles (Ru NPs) of small size, either sterically stabilized by a polymer (polyvinylpyrrolidone, PVP) or electronically stabilized by a ligand (bisdiphenylphosphinobutane, dppb) was tested towards standard reactions, namely CO oxidation, CO₂ reduction and styrene hydrogenation. The aim of the work was to identify the sites of reactivity on the nanoparticles and to study how the presence of ancillary ligands can influence the course of these catalytic reactions by using NMR and IR spectroscopies. It was found that CO oxidation proceeds at room temperature (RT) on Ru NPs but that the system deactivates rapidly in the absence of ligands because of the formation of RuO₂. In the presence of ligands, the reaction involves exclusively the bridging CO groups and no bulk oxidation is observed at RT under catalytic conditions. The reverse reaction, CO₂ reduction, is achieved at 120 °C in the presence of H₂ and leads to CO, which coordinates exclusively in a bridging mode, hence evidencing the competition between hydrides and CO for coordination on Ru NPs. The effect of ligands localized on the surface is also evidenced in catalytic reactions. Thus, styrene is slowly hydrogenated at RT by the two systems Ru/PVP and Ru/dppb, first into ethylbenzene and then into ethylcyclohexane. Selectively poisoning the nanoparticles with bridging CO groups leads to catalysts that are only able to reduce the vinyl group of styrene whereas a full poisoning with both terminal and bridging CO groups leads to inactive catalysts. These results are interpreted in terms of location of the ligands on the particles surface, and evidence site selectivity for both CO oxidation and arene hydrogenation.     

Asymmetric salt effects on anion/cation reactions: A comparative study of the [Fe(CN)6]4− + [Co(NH3)5pz]3+ and [Ru(NH3)5pz]2+ + [Co(C2O4)3]3− reactions

The kinetics of electron transfer reactions between [Fe(CN)₆]^4? and [Co(NH₃)₅pz]³⁺ and between [Ru(NH₃)₅pz]²⁺ and [Co(C₂O₄)₃]^3? was studied in concentrated salt solutions (Na₂SO₄, LiNO₃, and Ca(NO₃)₂). An analysis of the experimental kinetic data, k_obs, permits us to obtain the true (unimolecular) electron transfer rate constants corresponding to the true electron transfer process (precursor complex → successor complex), k_et. The variations of both, k_obs and k_et, with salt concentrations are opposite for these reactions. These opposite tendencies can be rationalized by using the Marcus–Hush treatment for electron transfer reactions. The conclusion is that the negative salt effect found for the first reaction ([Fe(CN)₆]^4? + [Co(NH₃)₅pz]³⁺) is due to the increase of the reaction and reorganization free energies when the concentration of salt increases. In the case of the second reaction ([Ru(NH₃)₅pz]²⁺ + [Co(C₂O₄)₃]^3?), the positive salt effect observed is caused by the fact that the driving force becomes more favorable when the concentration of salt increases. Thus, it is shown that for anion/cation electron transfer reactions the kinetic salt effect depends on the charge sign of the oxidant (and the reductant). © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 37: 81–89, 2005     

Theory of Interrelated Electron and Proton Transfer Processes

A simple theory of elementary act of interrelated reactions of electron and proton transfer is developed. Mechanisms of synchronous and multistage transfer and coherent transitions via a dynamically populated intermediate state are discussed. Formulas for rate constants of adiabatic and nonadiabatic reactions are derived.     

Effect of the Conformation of Polymer Chain on the Reduction Rate of Polyacrylatopentaamminecobalt(III) by Polymer-Bound Ferrous Chelates and the Excited State of Tris(Bipyridine)Ruthenium(Ii)

Electron transfer reactions of Co(NH₃)₅PAA (PAA = polyacrylic acid) with either the polyanionic polymer-bound ferrous chelate, Fe¹¹P-SS (P-SS = vinylbenzylaminediacetate-co-styrenesulfonate) or the uncharged polymer-bound ferrous chelate, Fe¹¹P-VPRo (P-VPRo = vinylbenzylaminediacetate-co-vinylpyrrolidone), and the Ru(bpy)²⁺ ₃ photosensitized reduction of Co(NH₃)₅PAA have been investigated in aqueous solutions at pH 5.4, I = 0.06 (I is ionic strength), and 25°C. For the ferrous chelate reductions, the second-order rate constants for Fe¹¹-PSS and Fe¹¹P-VPRo were almost equal to that for the corresponding nonpolymer-bound ferrous chelate, Fe¹¹BDA (BDA = benzylaminediacetate). The results indicate that there is no appreciable steric hindrance due to the polymer chains of the polymer-bound ferrous chelates and that the effect of columbic repulsion force between the polyanion chains can be ignored for the reaction of Co(NH₃)₅PAA with Fe¹¹P-SS. The results also suggest that there are two kinds of the pendant Co(III) species, “reactive” and “inert.” The inert Co(III) species are shielded by the polymer chains from attack of the Fe(II) chelates that are present in the bulk solutions. A similar reaction behavior was observed in the Ru(bpy)²⁺ ₃ photosensitized reduction of Co(NH₃)₅PAA at pH 5.4. For the Co(III) complex having an extremely few Co(III) complex moieties on the polymer chain, almost all of the Co(III) groups were hardly reduced by the excited state of Ru(bpy)²⁺ ₃, and reverse quenching occurred due to binding of the Ru(bpy)²⁺ ₃ to the polyacrylic acid chain of the polymer complex. On the other hand, for Co(NH₃)₅PAA with a relatively large number of the Co(III) moieties on the polymer chain, lifetime measurements at a higher concentration of the Ru(bpy)²⁺ ₃ showed a double-exponential fit, which suggests that there are two parallel decay processes. The fast and slow components mainly correspond to the decays: Ru(bpy)²⁺ ₃ quenched by Co(III) and reverse quenching due to binding of Ru(bpy)²⁺ ₃ into the compact polymer chains.     

Proton-coupled electron transfer in solution, proteins, and electrochemistry

Recent advances in the theoretical treatment of proton-coupled electron transfer (PCET) reactions are reviewed. These reactions play an important role in a wide range of biological processes, as well as in fuel cells, solar cells, chemical sensors, and electrochemical devices. A unified theoretical framework has been developed to describe both sequential and concerted PCET, as well as hydrogen atom transfer (HAT). A quantitative diagnostic has been proposed to differentiate between HAT and PCET in terms of the degree of electronic nonadiabaticity, where HAT corresponds to electronically adiabatic proton transfer and PCET corresponds to electronically nonadiabatic proton transfer. In both cases, the overall reaction is typically vibronically nonadiabatic. A series of rate constant expressions have been derived in various limits by describing the PCET reactions in terms of nonadiabatic transitions between electron-proton vibronic states. These expressions account for the solvent response to both electron and proton transfer and the effects of the proton donor-acceptor vibrational motion. The solvent and protein environment can be represented by a dielectric continuum or described with explicit molecular dynamics. These theoretical treatments have been applied to numerous PCET reactions in solution and proteins. Expressions for heterogeneous rate constants and current densities for electrochemical PCET have also been derived and applied to model systems.     

Catalytic Electron‐Transfer Oxygenation of Substrates with Water as an Oxygen Source Using Manganese Porphyrins

Manganese(V)–oxo–porphyrins are produced by the electron‐transfer oxidation of manganese–porphyrins with tris(2,2′‐bipyridine)ruthenium(III) ([Ru(bpy)₃]³⁺; 2 equiv) in acetonitrile (CH₃CN) containing water. The rate constants of the electron‐transfer oxidation of manganese–porphyrins have been determined and evaluated in light of the Marcus theory of electron transfer. Addition of [Ru(bpy)₃]³⁺ to a solution of olefins (styrene and cyclohexene) in CH₃CN containing water in the presence of a catalytic amount of manganese–porphyrins afforded epoxides, diols, and aldehydes efficiently. Epoxides were converted to the corresponding diols by hydrolysis, and were further oxidized to the corresponding aldehydes. The turnover numbers vary significantly depending on the type of manganese–porphyrin used owing to the difference in their oxidation potentials and the steric bulkiness of the ligand. Ethylbenzene was also oxidized to 1‐phenylethanol using manganese–porphyrins as electron‐transfer catalysts. The oxygen source in the substrate oxygenation was confirmed to be water by using ¹⁸O‐labeled water. The rate constant of the reaction of the manganese(V)–oxo species with cyclohexene was determined directly under single‐turnover conditions by monitoring the increase in absorbance attributable to the manganese(III) species produced in the reaction with cyclohexene. It has been shown that the rate‐determining step in the catalytic electron‐transfer oxygenation of cyclohexene is electron transfer from [Ru(bpy)₃]³⁺ to the manganese–porphyrins.     

Electron Transfer in Metal‐Organic Molecules. A Time Resolved EXAFS and Optical Spectroscopy Study

We have measured, by means of ultrafast x‐ray absorption and optical spectroscopy, the M‐O (M=Fe, Co) and Co‐N metal to ligand bond length change as a function of time and the formation and decay of the excited states and intermediate species, after excitation with a 267 nm femtosecond pulse. These experimental data combined with DFT calculations allowed us to determine the mechanism of electron transfer operating in the redox reaction of two metal‐ligand complexes, [M(III)(C₂O₄)₃]^3‐ and [Co(III)(NH₃)₆ ]³⁺. Based on the data we find that, even though both molecules are excited into their charge transfer band, the redox reaction of [M(III)(C₂O₄)₃]^3‐ proceeds via intermolecular electron transfer while [Co(III)(NH₃)₆ ]³⁺ electron transfer mechanism is intramolecular.     

Mixed-valence properties of Ruthenium–Polypyridine dimers bridged by Imidazolate and Triazolate Ligands

The dinuclear complex cis,cis-[(bpy)₂ClRu(μ-bim)RuCl(bpy)₂] ^{n +} (bpy = 2,2′-bipyridine; bim = benzimidazolate; n = 1, 2, or 3) was synthesized, isolated as a hexafluorophosphate salt, and investigated in organic solutions by cyclic voltammetry and UV/visible/NIR spectroelectrochemistry. The mixed-valent species (n = 2) displays significant metal–metal electronic coupling in the ground state but exhibits localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge transfer (IVCT) band and redox parameters. On the basis of the resonance energy (H _AB) estimated in the context of Hush's semiclassical theory, the extent of intermetallic communication was found to be larger than that recently reported for the bta-bridged analog (bta = benzotriazolate). Some differences between the IVCT features of these systems have been rationalized in terms of the degree of σ,π-basic character of the bridging ligands, according to an electron superexchange mechanism of the “hole-transfer” type. The stabilization of the mixed-valent complexes is attributed mainly to cooperative metal-to-ligand/ligand-to-metal charge-transfer effects. The combined π-acceptor and σ,π-donor abilities of the ancillary (bpy) and bridging (bim or bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (Ru^III–L–Ru^III) and fully reduced (Ru^II–L–Ru^II) isovalent species.     

A biomimetic approach to artificial photosynthesis: Ru(II)-polypyridine photo-sensitisers linked to tyrosine and manganese electron donors

The paper describes recent advances towards the construction of functional mimics of the oxygen evolving complex in photosystem II (PSII) that are coupled to photoinduced charge separation. Some key principles of PSII and artificial systems for light-induced charge accumulation are discussed. Systems are described where biomimetic electron donors--manganese complexes and tyrosine--have been linked to a Ru(II)-polypyridine photosensitiser. Oxidation of the donors by intramolecular electron transfer from the photo-oxidised Ru(III) complex has been studied using optical flash photolysis and EPR experiments. A step-wise electron transfer Mn(III,III)-->tyrosine Ru(III) has been demonstrated, in analogy to the reaction on the donor side of PSII. Electron transfer from the tyrosine to Ru(III) was coupled to tyrosine deprotonation. This resulted in a large reorganisation energy and thus a slow reaction rate, unless the tyrosine was hydrogen bonded or already deprotonated. A comparison with analogous reactions in PSII is made. Finally, light-induced oxidation of a manganese dimer linked to a Ru(II)-photosensitiser has been observed. Preliminary results suggest the possibility of photo-oxidising manganese dimers in several steps, which is an important advancement towards water oxidation.     

金属羰基化合物的结构对氧原子转移反应速率的影响

本文研究了氧化三甲胺Ｍｅ３ＮＯ与羰基簇合物Ｍ４（ＣＯ）１２－ｎＬｎ（Ｍ＝Ｃｏ，Ｉｒ；ｎ＝１，２；Ｌ＝磷配体）的氧转移反应动力学，讨论了反应机理。反应符合二级速率方程：ｒ＝Ｋ２［Ｍｅ３ＮＯ］［Ｍ４（ＣＯ）１２－ｎＬｎ］Ｍ４（ＣＯ）１２－ｎＬｎ的氧转移反应活性呈现如下顺序：中心元素不同时Ｃｏ４（ＣＯ）１２－ｎＬｎ＜Ｉｒ４（ＣＯ）１２－ｎＬｎ；取代配体不同时Ｍ４（ＣＯ）１２－ｎ（Ｐ（ＯＭｅ）３）ｎ＞Ｍ４     

Redox kinetics of metal complexes in nonaqueous solutions. V. Kinetics and mechanism of the iron (II) reduction of the acetylacetonates of manganese (III) and cobalt (III) in acetonitrile

The electron transfer step of the reduction of Mn(acac)₃ and Co(acac)₃ by Fe(II) in acetonitrile is preceded by the one-ended dissociation of an acac ligand and the formation of a binuclear bridged complex. After the electron transfer has taken place through the bridging ligand, the complex dissociates into the products M(acac)₂ (M = Mn, Co) and Fe(acac)²⁺. These primary reaction products could not be identified, since the transfer of acac from M(acac)₂ to Fe(acac)²⁺ is too rapid, producing ultimately Fe(acac)₃ and M²⁺. The M(III)-oxygen cleavage is accelerated by M(acac)₂. Furthermore, the dissociation of the binuclear intermediate is catalyzed by the M(acac)₃ reactant. Mn(acac)₃ is reduced more than a thousand times faster than Co(acac)₃.