Self-exchange Electron-transfer Rate Research Articles

Mechanism of Cobalt Self-Exchange Electron Transfer

A heptanuclear cobalt cluster was synthesized in two different oxidation states, Co(II)7 and a mixed valence Co(III)Co(II)6, as a soluble model of a cobalt-phosphate/borate (Co-OEC) water splitting catalyst. Crystallographic characterization indicates similar cluster cores, distinguished primarily at the central Co atom. An anion associates to the cluster cores via hydrogen bonding. Using an isotope exchange method, an anomalously slow self-exchange electron transfer rate constant (k(obs) = 1.53 × 10(-3) M(-1) s(-1) at 40 °C and 38 mM [OTf] in MeCN), as compared to that predicted from semiclassical Marcus theory, supports a charge transfer process that is accelerated by dissociation of the anion from the oxidized cluster. This mechanism sheds light on the inverse dependence of anions in the self-repair mechanism of Co-OECs. Moreover, because H2O cannot directly bridge cobalt centers, owing to the encapsulation of the central Co within the cluster core, the observed results address a long-standing controversy surrounding the Co(2+/3+) self-exchange electron transfer reaction of the hexaaqua complex.

Nonequilibrium Thermodynamics Formalism for Marcus Theory of Heterogeneous and Self-Exchange Electron-Transfer Rate Constants

The cross-exchange electron-transfer rate constant expression of Marcus is derived from the Flux-force formalism of non-equilibrium thermodynamics. The relationship governing the Onsager's phenomenological coefficients for cross-exchange and self-exchange electron-transfer processes is deduced. Onsager's phenomenological coefficient pertaining to the Butler-Volmer equation is derived and estimated from the experimental exchange current densities. The correlation between the heterogeneous and the homogeneous electron-transfer rate constants derived by Marcus is analyzed in terms of the corresponding phenomenological coefficients.

Indirect Determination of Self-Exchange Electron Transfer Rate Constants

Second-order rate constants kij(obsd) measured at 25 °C in acetonitrile by stopped-flow spectrophotometry for forty-four electron transfer (ET) reactions among fourteen 0/+1 couples [three aromatic compounds (tetrathiafulvalene, tetramethyltetraselenafulvalene, and 9,10-dimethyl-9,10-dihydrophenazine), four 2,3-disubstituted 2,3-diazabicyclo[2.2.2]octane derivatives, six acyclic hydrazines, and the bridgehead diamine 1,5-diazabicyclo[3.3.3]undecane] and seventeen compounds and forty-seven reactions from a previous study (J. Am. Chem. Soc. 1997, 119, 5900) [three p-phenylenediamine derivatives, four ferrocene derivatives, and ten tetraalkylhydrazines] are discussed. When all 91 kij(obsd) values are simultaneously fitted to Marcus's adiabatic cross rate formula kij(calcd) = (kiikjjKijfij)1/2, ln fij = (ln Kij)2/4 ln(kiikjj/Z2), best-fit self-exchange rate constants, kii(fit), are obtained that allow remarkably accurate calculation of kij(obsd); kij(obsd)/kij(calcd) is in the range 0.5−2.0 for all 91 reactions. The average difference without regard to sign, |ΔΔGij|, between observed cross reaction activation free energy and that calculated using the kii(fit) values and equilibrium constants is 0.13 kcal/mol. The ΔGii(fit) values obtained range from 2.3 kcal/mol for tetramethyltetraselenafulvalene0/+ to 21.8 kcal/mol for tetra-n-propylhydrazine0/+, corresponding to a factor of 2 × 1014 in kii(fit). The principal factor affecting kii(fit) for our data appears to be the internal vertical reorganization energy (λv), but kii(fit) values also incorportate the effects of changes in the electronic matrix coupling element (V). Significantly smaller V values for ferrocenes and for hydrazines with alkyl groups larger than methyl than for aromatics and tetramethylhydrazine are implied by the observed ΔGii(fit) values.

Estimation of Self-Exchange Electron Transfer Rate Constants for Organic Compounds from Stopped-Flow Studies

Second-order rate constants k12(obsd) measured at 25 °C in acetonitrile by stopped-flow for 47 electron transfer (ET) reactions among ten tetraalkylhydrazines, four ferrocene derivatives, and three p-phenylenediamine derivatives are discussed. Marcus's adiabatic cross rate formula k12(calcd) = (k11 k22 k12 f12)1/2, ln f12 = (ln K12)2/4 ln(k11k22/Z2) works well to correlate these data. When all k12(obsd) values are simultaneously fitted to this relationship, best-fit self-exchange rate constants, kii(fit), are obtained that allow remarkably accurate calculation of k12(obsd); k12(obsd)/k12‘(calcd) is in the range of 0.55−1.94 for all 47 reactions. The average ΔΔGij⧧ between observed activation free energy and that calculated using kii(fit) is 0.13 kcal/mol. Simulations using Jortner vibronic coupling theory to calculate k12 using parameters which produce the wide range of kii values observed predict that Marcus's formula should be followed even when V is as low as 0.1 kcal/mol, in the weakly nonadiabatic region. Tetracyclohexylhydrazine has a higher kii than tetraisopropylhydrazine by a factor of ca. 10. Replacing the dimethylamino groups of tetramethyl-p-phenylenediamine by 9-azabicyclo[3.3.1]nonyl groups has little effect on kii, demonstrating that conformations which have high intermolecular aromatic ring overlap are not necessary for large ET rate constants. Replacing a γ CH2 group of a 9-azabicyclo[3.3.1]nonyl group by a carbonyl group lowers kii by a factor of 17 for the doubly substituted hydrazine and by considerably less for the doubly substituted p-phenylenediamine.

Structural and Electron Self-Exchange Rate Variations in Isomeric (Hexaamine)cobalt(III/II) Complexes.

The syntheses and characterisation of the new macrocyclic hexaamine trans-(5(S),7(S),12(R),14(R)-tetramethyl)-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (L(6)) and its Co(III) complex are reported. The X-ray crystal structural analyses of [CoL(6)]Cl(2)(ClO(4)) [monoclinic, space group C2/c, a = 16.468(3) Å, b = 9.7156(7) Å, c = 15.070(3) Å, beta = 119.431(8) degrees, Z = 4] and the closely related cis-diamino-substituted macrocyclic complex [CoL(2)](ClO(4))(3). 2H(2)O (L(2) = cis-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine) [orthorhombic, space group Pna2(1), a = 16.8220(8) Å, b = 10.416(2) Å, c = 14.219(3) Å, Z = 4] reveal significant variations in the observed Co-N bond lengths and coordination geometries, which may be attributed to the trans or cis disposition of the pendent primary amines. The Co(III/II) self-exchange electron transfer rate constants for these and other closely related hexaamines have been determined, and variations of some 2 orders of magnitude are found between pairs of trans and cis isomeric Co(III) complexes.

EPR-Spectroscopic Investigation of the Self-Exchange Electron Transfer Rate Constants and Reorganization Energies for some Electrochemically Generated Radicals.

EPR-Spectroscopic Investigation of the Self-Exchange Electron Transfer Rate Constants and Reorganization Energies for some Electrochemically Generated Radicals.

Intermolecular Electron-Transfer Reactions Involving Hydrazines

The self-exchange electron-transfer (ET) rate constant k22 for 1,2,3,4,5-pentamethylferrocene was determined by NMR line broadening to be 8.5(±0.8) × 106 M-1 s-1 (25 °C, CD3CN/0.09 M Et4NBF4) and k11 for the bis-N,N-bicyclic hydrazine, 9,9‘-bi-9-azabicyclo[3.3.1]nonane, to be 8.7(0.5) × 103 M-1 s-1 (25 °C, CH2Cl2). These self-exchange rate constants are used to analyze cross reactions of these molecules with hydrazines, ferrocenes, and tetramethyl-p-phenylenediamine (TMPD) using Marcus theory. Cross-reaction rate constants for eight reactions are reported. Included are six cross-reactions between methylferrocenes and four cyclic hydrazines, one hydrazine, hydrazine−reaction, and the reduction of TMPD+ by a cyclic hydrazine. These are the first cross-reaction rate constants reported for hydrazine−hydrazine and hydrazine−TMPD+ ET reactions. The cross-reaction rate constants are used to test the application of Marcus theory to hydrazine ET reactions and to extract estimates of hydrazine self-exchange ET rate constants in systems for which direct measurement is presently impossible.

Conformationally-controlled electron-transfer reactions as manifested with copper(II)/(I) macrocyclic ligand complexes

Abstract

Self-exchange electron transfer rate constants and reorganization energies for some aromatic compounds in N,N-dimethylformamide determined by elect

The rate constants for homogeneous self-exchange electron transfer reactions of nine aromatic and heteroaromatic compounds in N,N-dimethylformamide have been measured by electron spin resonance line-broadening experiments. The aromatic anion radicals were generated electrochemically in a flow cell. The measured rate constants are compared with literature values when possible and self-exchange reorganization energies λ have been calculated. It is found that for all the aromatic and heteroaromatic compounds, except benzophenone, λ 11 ± 1.5 kcal mol −1. For benzophenone a significantly higher value for λ is found (14.7 kcal mol −1. An electrochemical flow cell connected with the cavity of an ESR spectrometer has been used to measure self-exchange ET rate constants and self-exchange reorganization energies for a number of aromatic and heteroaromatic compounds. The obtained values agree with some of the literature values, but with others the differences observed are unsatisfactory and inexplicable. It seems a general problem with such measurements that although accurate measurements (± 10%) can be made in several laboratories, the results obtained in different laboratories may deviate by more than 100%. When comparing reorganization energies for different compounds it seems safest to compare values obtained in the same investigation.

Direct determination of the self-exchange electron-transfer rate constant for a copper(II/I) macrocyclic pentathiaether complex

Direct determination of the self-exchange electron-transfer rate constant for a copper(II/I) macrocyclic pentathiaether complex

Electrolyte effects in electron transfer reactions of 1,8-bis(2-pyridyl)-3,6-dithiaoctane copper(II) with ferrocene in acetonitrile. Specific rate enhancement by tetrafluoroborate ion

The kinetics of electron transfer between 1,8-bis(2-pyridyl)-3,6-dithiaoctane copper(II), Cu(pdto) 2+, and ferrocene, FeCp 2, in acetonitrile have been studied by stopped-flow spectrophotometry, as a function of ferrocene concentration and the concentration of added electrolytes, n-Bu 4NPF 6, n-Bu 4NBF 4 and NaCF 3CO 2. The reactions exhibit different rate dependencies with the three salts. The second-order rate constant, k 2=(2.1±0.2)x10 4 M −1 S −1, measured in CH 3CN at 25 °C with excess ferrocene between 1.2x10 −3 and 7.5x10 −3 M is unaffected by n-Bu 4NPF 6 added up to 0.10 M. A lowered second-order rate constant, k 2=(1.56±0.15)x10 3 M −1 S −1 is obtained with [NaCF 3CO 2] between 0.01 and 0.10 M. Inner-sphere association of CF 3CO 2− with the Cu(II) center is proposed to explain the rate inhibition in trifluoroacetate solutions. Values of 15 and 0.25 M −1 S −1 have been estimated for the Cu(II)/Cu(I) self-exchange electron transfer rate constant in n-Bu 4NPF 6 and NaCF 3CO 2 media, using the Marcus cross relations. With added n-Bu 4NBF 4, the measured second order rate constant, k 2, increases with [BF 4 −] reaching 1.75x10 5 M −1 S −1 at 0.050 M salt concentration. The rate data in BF 4 − solutions is fitted to a rate law involving ion-pair formation between BF 4 − and Cu(pdto) 2+, with the ion-paired species showing enhanced electron transfer reactivity. Spectral changes that accompany the addition of BF 4 − to Cu(pdto) 2+ solutions suggest that the association of BF 4 − facilitates electron transfer by modifying the energy of the electron acceptor orbital in the redox change.

Direct determination of self-exchange electron-transfer rate constants of nickel(III,II) complexes by nickel-61 ESR line broadening

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDirect determination of self-exchange electron-transfer rate constants of nickel(III,II) complexes by nickel-61 ESR line broadeningJean Fu Wang, Krishan Kumar, and Dale W. MargerumCite this: Inorg. Chem. 1989, 28, 18, 3481–3484Publication Date (Print):September 1, 1989Publication History Published online1 May 2002Published inissue 1 September 1989https://doi.org/10.1021/ic00317a017RIGHTS & PERMISSIONSArticle Views194Altmetric-Citations13LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (509 KB) Get e-Alerts Get e-Alerts

ChemInform Abstract: Redox Behavior of Copper(II) and Copper(I) Complexes with Tetradentate Bis(pyridyl)‐dithiaether and Bis(pyridyl)‐diaza Ligands Towards Ruthenium Ammine and Bipyridyl Complexes

Abstract Electron transfer reactions involving the 1,8-bis- (2-pyridyl)-3,6-dithiaoctane copper(II) complex, Cu- (pdto) 2+ , and a series of Ru(II) ammine and bipyridyl complexes have been studied kinetically in MES buffered 0.10 M LiCF 3 CO 2 . Values of the second- order rate constant, k 2 (M -1 s -1 ) are Ru(NH 3 )5 py 2+ , 3.8 × 10 4 ; Ru(NH 3 ) 5 isn 2+ , 1.4 × 10 4 ; Ru(NH 3 ) 4 - bpy 2+ , 2.0 × 10 3 ; cis -Ru(NH 3 ) 4 (isn) 2 2+ , 2.3 × 10 2 ; cis - Ru(bpy) 2 Cl 2 , 4.4 × 10 4 ; Ru(bpy) 2 C 2 O 4 , 4.5 × 10 5 ; at 25.0 °C in 50% aqueous methanol. Data for the ammine complexes, which follow a linear free energy relationship between log k 2 and E°, the Ru(III)/ Ru(II) reduction potential, suggest an outer-sphere mechanism. Inner-sphere electron transfer is indicated with Ru(bpy) 2 Cl 2 and Ru(bpy) 2 C 2 O 4 as reductants. Oxidation of coordinated oxalate occurs with Ru(bpy) 2 C 2 O 4 . The rate constant has also been determined for the oxidation of the 1,8-bis(2-pyridyl)- 3,6-dimethyl-3,6-diazaoctane copper(I) complex, Cu(pdao) + by Ru(NH 3 ) 5 py 3+ in 100% aqueous 0.10 M LiCF 3 CO 2 . A value of 1.8 × 10 4 M -1 s -1 was obtained at 25.0 °C and pH 6.1. Application of the Marcus relations has yielded estimated self-exchange electron-transfer rate constants of 0.63 and 8.1 M -1 s -1 for the Cu(pdto) 2+/+ and Cu(pdao) 2+/+ couples.

Oxidation of (hydroxyethyl)ferrocene by [2-pyridylmethylbis(2-ethylthioethyl)/amine]copper(II)

A kinetic study of the oxidation of (hydroxyethyl)ferrocene (HEF) by [2-pyridylmethylbis(2-ethyl-thioethyl)ainine]copper(II) (Cu(pmas) 2+) is reported, with the objective of documenting the influence of the two thioether sulfur ligands on the electron transfer rate. Both reactants exhibit a first-order dependence at pH 6, I = 0.1 M(NaNO 3); k(25°C) = 1.3 × 10 4 M −1sec −1, Δ H‡ = 10.1 kcal/mole, Δ S‡ = −6 eu. The apparent Cu(pmas) 2+/+ self-exchange electron transfer rate constant calculated from this reaction on the basis of relative Marcus theory (4.7 × 10 1 M −1 sec −1) agrees well with previous findings on ferrocytochrome c, Fe(CN) 6 4−, and Ru(NH 3) 5py 2+ oxidations. Spectrophotometric titrations of Cu(pmas) 2+ and Cu(tmpa) 2+ (tmpa = tris(2-pyridylmethyl)amine) with azide ion showed that both Cu(pmas)N 3) + ( K f1 = 3.1 × 10 3 M −1) and Cu(pmas)(N 3) 2 ( K f2 = 3.5 × 10 1 M −1) but Cu(tmpa)(N 3) + ( K f = 6.6 × 10 2 M −1) are formed up to 0.15 M N 3 − (25°C, pH 6, I = 0.2 M), suggesting that a thioether sulfur atom is displaced in the uptake of a second N 3 − ion by Cu(pmas)(N 3) +. The effect of thioether sulfur displacement by azide ion on the HEF-Cu(pmas) 2+ reaction rate may be understood entirely through the tendency of N 3 − to shift the position of the redox equilibrium towards the reactant side, without invoking any special role for the sulfur ligand in promoting electron transfer reactivity.

Redox behavior of copper(II) and copper(I) complexes with tetradentate bis(pyridyl)-dithiaether and bis(pyridyl)-diaza ligands towards ruthenium ammine and bipyridyl complexes

Electron transfer reactions involving the 1,8-bis- (2-pyridyl)-3,6-dithiaoctane copper(II) complex, Cu- (pdto) 2+, and a series of Ru(II) ammine and bipyridyl complexes have been studied kinetically in MES buffered 0.10 M LiCF 3CO 2. Values of the second- order rate constant, k 2 (M -1 s -1) are Ru(NH 3)5 py 2+, 3.8 × 10 4; Ru(NH 3) 5isn 2+, 1.4 × 10 4; Ru(NH 3 ) 4 - bpy 2+, 2.0 × 10 3; cis-Ru(NH 3) 4(isn) 2 2+, 2.3 × 10 2; cis- Ru(bpy) 2Cl 2, 4.4 × 10 4; Ru(bpy) 2C 2O 4, 4.5 × 10 5; at 25.0 °C in 50% aqueous methanol. Data for the ammine complexes, which follow a linear free energy relationship between log k 2 and E°, the Ru(III)/ Ru(II) reduction potential, suggest an outer-sphere mechanism. Inner-sphere electron transfer is indicated with Ru(bpy) 2Cl 2 and Ru(bpy) 2C 2O 4 as reductants. Oxidation of coordinated oxalate occurs with Ru(bpy) 2C 2O 4. The rate constant has also been determined for the oxidation of the 1,8-bis(2-pyridyl)- 3,6-dimethyl-3,6-diazaoctane copper(I) complex, Cu(pdao) + by Ru(NH 3) 5py 3+ in 100% aqueous 0.10 M LiCF 3CO 2. A value of 1.8 × 10 4 M -1 s -1 was obtained at 25.0 °C and pH 6.1. Application of the Marcus relations has yielded estimated self-exchange electron-transfer rate constants of 0.63 and 8.1 M -1 s -1 for the Cu(pdto) 2+/+ and Cu(pdao) 2+/+ couples.

Determination of the self-exchange electron-transfer rate constant for a copper(III/II) tripeptide complex by proton NMR line broadening

Determination of the self-exchange electron-transfer rate constant for a copper(III/II) tripeptide complex by proton NMR line broadening

ChemInform Abstract: FLASH‐PHOTOLYSIS STUDIES OF THE ELECTRON‐TRANSFER REACTIONS OF DIOXYGEN COMPLEXES OF COBALT(III) WITH TRIS(2,2′‐BIPYRIDYL)RUTHENIUM(III)

AbstractDie Elektronentransfer‐ Reaktionen zwischen den u‐Peroxo‐Co(III)‐Komplexen (I) und Tris‐[bipyridyl]‐ Ru(III) (II) unter Bildung der u‐Superoxo‐Komplexe (III) und des Ru(II)‐Analogen (IV) werden mittels Spektrophotometrie untersucht.

Flash-photolysis studies of the electron-transfer reactions of dioxygen complexes of cobalt(III) with tris(2,2′-bipyridyl)ruthenium(III)

Electron-transfer reactions between the µ-peroxo-cobalt(III) complexes [(H3N)5Co(µ-O2)Co(NH3)5]4+(1), [(H3N)4Co(µ-O2)(µ-NH2)Co(NH3)4]3+(2), and [(L–L)2Co(µ-O2)(µ-NH2)Co(L–L)2]3+[L–L = ethylenediamine (3), 2,2′-bipyridyl (4), or 1,10-phenanthroline (5)] and [Ru(bipy)3]3+(bipy = 2,2′-bipyridyl) have been investigated. The reactants have been generated by the electron-transfer quenching of the excited state of [Ru(bipy)3]2+ by the corresponding µ-superoxo-complexes using flash photolysis. The half-wave potentials measured by polarography for complexes (3), (4), and (5) are 1.00 ± 0.05, 1.04 ± 0.05, and 1.04 ± 0.05 V respectively. The observed rate constants are used to calculate the self-exchange electron-transfer rate constants for the bridged cobalt(III) complexes. The Marcus cross relationship has been used to predict the rate constants determined earlier for the electron-transfer reaction between (3) and the superoxo-forms of complexes (1), (4), and (5).