- Cobalt(II) complexes with nitrogen donors and their dioxygen affinity in dimethyl sulfoxide
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The formation of CoII complexes with differently methylated N-donor ligands ethylenediamine (en), N,N′-dimethylethyl-enediamine (dmen), N,N,N′-trimethylethylenediamine (trmen), and N,N,N′,N′-tetramethylethylenediamine (tmen), has been studied at 298 K in both the aprotic solvent dimethyl sulfoxide (dmso) and in an ionic medium set to 0.1 mol dm-3 with Et4NClO4 under anaerobic conditions. UV/Vis spectrophotometric and calorimetric measurements were carried out to obtain the thermodynamic parameters for the systems investigated. Only mononuclear CoLj2+ complexes were formed (j = 1-3 for en, j = 1, 2 for dmen; and j = 1 for trmen and tmen) where the diamines act as bidentate agents. All the complexes are enthalpy-stabilized whereas the entropy changes counteract the complex formation. The results are discussed in terms of different basicities and steric requirements both of the ligands and the complexes formed. Voltammetric measurements were carried out to correlate the electrochemical properties of the anaerobic complexes with their different dioxygen affinities. The X-ray structure of the starting adduct [Co(dmso)6][ClO4]2, determined on a monocrystal, is also reported. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.
- Comuzzi, Clara,Melchior, Andrea,Polese, Pierluigi,Portanova, Roberto,Tolazzi, Marilena
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p. 2194 - 2201
(2007/10/03)
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- A pulse radiolysis study of the rate of ligand dissociation from mixed ligand cobalt(II) complexes
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The reduction and subsequent reactions of the series of mixed ligand cobalt(III) complex cations, [Co(bipy)3]3+, [Co(phen)3]3+, [Co(en)3]3+, [Co(en)2(bipy)]3+, [Co(en)
- Funston, Alison M.,McFadyen, W. David,Tregloan, Peter A.
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p. 2053 - 2060
(2007/10/03)
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- Magnetodynamic effects on outer-sphere electron-transfer reactions: A paramagnetic transition state
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The effect of the magnetic field on the rate of outer-sphere electron-transfer reactions has been investigated as a function of the field intensity, between 0 and 9 T, and at a given temperature. In complexes of d6 metal ions, i.e., Ru(II) and Co(III), the rate constant exhibits a complex dependence on the field: a complexity associated with field-induced changes of the electronic matrix element and the activation energy. Changes in the activation energy have been investigated as a function of the temperature at a given field intensity. These measurements have shown that the magnetic susceptibility of activation has the large positive values that are expected for a strongly paramagnetic transition state. The magnetic field effects are discussed in terms of symmetry-determined selection rules for the coupling of the initial and final electronic states of the reactions.
- Ronco,Ferraudi
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p. 3961 - 3967
(2008/10/08)
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- Effect of steric crowding on the rates of reactions of a nickel(I) tetraaza macrocycle with organic halides and hydroperoxides
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The reactions of the sterically crowded decamethylcyclam complex of nickel(I), Ni(dmc)+, with organic halides, hydrogen peroxide, and tert-butyl hydroperoxide occur some 104 times more slowly than the corresponding reactions of Ni(tmc)+, a tetramethylcyclam complex. This supports the assignment of an inner-sphere mechanism in both cases, because reactions that necessarily adopt an outer-sphere mechanism (e.g., those of cobalt(III) amine complexes) differ no more in their relative rates than can easily be explained by the small difference in driving force.
- Sadler, Novelette,Scott, Susannah L.,Bakac, Andreja,Espenson, James H.,Ram
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p. 3951 - 3954
(2008/10/08)
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- Potentiometric determination of the stabilities of cobalt(II) complexes of polyamine Schiff bases and their dioxygen adducts
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Potentiometric measurement of hydrogen ion concentration, previously employed for the determination of oxygenation constants of cobalt(II) complexes in aqueous and mixed aqueous solvents, is now extended to the determination of equilibrium constants for Schiff base ligands and for the determination of oxygenation constants for systems in which the ligands and/or their metal complexes are not completely formed in the absence of dioxygen or in the absence of the cobalt(II) ion. The method is illustrated by the potentiometric measurement of oxygenation constants for solutions in 70% dioxane-30% water (v/v) containing cobalt(II) chloride and the following ligarid components: 1, salicylaldehyde and N,N′-bis(2-aminoethyl)dipicolinic acid diamide; 2, salicylaldehyde and N,N-bis(2-aminophenyl)methylamine; 3, salicylaldehyde, ethylenediamine, and 4-methylpyridine; and 4, 3-fluorosalicylaldehyde, o-phenylenediamine, and 4-methylpyridine. The stability constant and dioxygen affinity are also reported for the cobalt(II) complex of a pentadentate ligand, N,N-bis(2-((2-hydroxybenzyl)amino)phenyl)methylamine, produced by hydrogenation of the Shiff base formed from salicylaldehyde and N,N-bis(2-aminophenyl)methylamine. The equilibrium parameters obtained provide all the information necessary to calculate the concentrations of all molecular species present in solution at 25.00°C and 0.100 M ionic strength as a function of p[H] and solution composition. The results are displayed for each system as distribution curves consisting of species concentration profiles vs p[H] at specific concentrations of solution components.
- Motekaitis, Ramunas J.,Martell, Arthur E.
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p. 2718 - 2724
(2008/10/08)
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- Oxidation of a series of tris(polypyridyl)chromium(II) ions by several cobalt(III) complexes
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The kinetics of the outer-sphere oxidation of Cr(NN)32+ ions (NN = 2,2′-bipyridine, 1,10-phenanthroline, and their substituted derivatives) by tris(chelating ligand) complexes of Co(III) were studied. The second-order rate constants increase with the driving force of the reaction. The rate constants for Co(en)33+ at 23 ± 2°C increase from 2.2 (±0.3) × 103 M-1 s-1 for Cr(bpy)32+ to 1.1 (±0.2) × 105 M-1 s-1 for Cr(4,7-Me2-phen)32+ and agree very well with the values calculated by the Marcus cross relation. For Co(bpy)33+ and Co(phen)33+, on the other hand, the experimental rate constants are lower than the calculated values. It is suggested that the reactions of these two species are mildly nonadiabatic, the extent of nonadiabaticity increasing with an increase in the standard free energy of the reaction.
- Zahir, Khurram,Espenson, James H.,Bakac
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p. 3144 - 3146
(2008/10/08)
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- Kinetics of Reduction of Tris(ethylenediamine)cobalt(III) by Ethylenediamine-NNN'N'-tetra-acetochromate(II)
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The kinetics of the reduction of 3+ (en=ethylenediamine) by 2- have been studied by stopped-flow methods under a variety of conditions.The reaction is first order in both reagents, with a second-order rate constant of 3.37*103 dm3 mol-1 s-1 at 25 deg C, pH 4.81, unit ionic strength and =5*10-2 mol dm-3 (edta=ethylenediamine-NNN'N'-tetra-acetate).Activation parameters (ΔH=56 kJ mol-1, ΔS=12 J K1- mol-1) are consistent with weak electrostatic attraction between the reactants.The rate shows an inverse dependence on the concentration of free edta, attributed to formation of an ion pair or hydrogen-bonded adduct with the oxidant.The pH dependence is a double sigmoid, consistent with two protonation equilibria in the reductant with pKa values 2.7 and 5.5, and the reaction rate increases with increasing numbers of negative donor groups in the complex.
- Segal, Michael G.
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p. 2485 - 2488
(2007/10/02)
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- Kinetics and mechanism of electron transfer to transition-metal complexes by photochemically produced tris(bipyridyl)ruthenium(l+) Ion
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Rate constants were determined for the one-electron reduction of Cr(H2O)63+, several organochromium cations of the family (H2O)5CrR2+, several substituted pyridine complexes in the series (H2O)5CrNC5H4X3+, cobalt(III) amine complexes, and miscellaneous species including Ybaq3+ and (1R,4R,8S,11S)-Ni(tmc)2+ (where tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The results are considered in light of the Marcus equation. The data for the pyridine complexes are correlated by the Hammett equation; the reaction constant in comparison with those of other complexes indicates that electron transfer occurs directly to the metal and not, as in certain other instances, by initial reduction of the pyridine ligand bound to chromium. The qualitative differences in rates can be rationalized by a simple MO scheme.
- Connolly, Philip,Espenson, James H.,Bakac, Andreja
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p. 2169 - 2175
(2008/10/08)
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- Kinetics of reduction of cobalt(III) complexes by viologen radicals
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The second-order rate constants are reported for reduction of 11 cobalt(III) complexes by viologen radicals, derived from diquaternary salts of 4,4′-bipyridine and 2,2′-bipyridine. Rate constants vary from 1.5 × 102 to 6.0 × 108 M-1 s-1, and an excellent correlation with the Marcus expression for 14 redox reactions is observed. These data conform, on the basis of a unity transmission coefficient, to a self-exchange rate constant for the viologens (X2+/+ couple) of 1.5 × 106 M-1 s-1. It is suggested however that this value is about 102 too low.
- Tsukahara,Wilkins
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p. 3399 - 3402
(2008/10/08)
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- Kinetics of aqueous outer-sphere electron-transfer reactions of superoxide ion. Implications concerning the O2/O2- self-exchange rate constant
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The kinetics and mechanism of the reactions of aqueous superoxide ion and several Co(III) amine complexes and ferrocenium ion are reported. The initial and rate-limiting step (and the only step for the ferrocenium ion reaction) is outer-sphere electron transfer from superoxide ion to the metal complex. Second-order rate constants at 25.0°C are 31.3, 23.8, and 16.4 M-1 s-1 for Co(NH3)63+, Co(en)33+, and Co(trans-1,2-cyclohexanediamine)33+, respectively, and 8.6 × 106 M-1 s-1 for Fe(C5H5)2+. For the Co(III) reactions, several physical (temperature, pH) and chemical (isotopic labeling of complex and solvent, choice of counteranion, presence and absence of dioxygen) effects were investigated. Dioxygen has a marked effect upon the Co(III) reactions, resulting in second-order rate constants that are greater by a factor of 2 under anaerobic conditions than under aerobic. This dioxygen effect is explained by the competing reactions of the Co(II) amine complexes produced in the electron-transfer step with O2 or O2-. The Marcus cross-relation permits calculation of a parameter that might be interpreted as the O2(aq)/O2- self-exchange rate constant from data for these and other reactions, but widely variant values are obtained, 10-8-105.7 M-1 s-1. This phenomenon is attributed to the very different solvation of dioxygen and superoxide ion.
- Mcdowell, M. Steven,Espenson, James H.,Baka?, Andreja
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p. 2232 - 2236
(2008/10/08)
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- Electron-transfer reactions of tris(picolinato)vanadate(II), a LOMI reagent
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Reductions of Co(edta)-, Co(NH3)63+, Co(en)33+, and Co(sep)3+ by tris(picolinato)vanadate(II), V(pic)3-, take place with second-order rate constants 7.7 × 104 M-1 s-1, 8.0 × 103 M-1 s-1, 8.2 × 102 M-1 s-1, and 8.4 × 104 M-1 s-1, respectively, at 25.0°C and at 0.5 M ionic strength. Application of the Marcus theory to the latter three results leads to a self-exchange rate for V(pic)30/- of 3.1 × 106 M-1 s-1. For Co(edta)- the observed rate is an order of magnitude slower than predicted by the Marcus theory and reasons for this discrepancy are proposed.
- Lannon, A. Martin,Lappin, A. Graham,Segal, Michael G.
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p. 4167 - 4170
(2008/10/08)
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- Electron-transfer reactivity in some simple cobalt(III)-cobalt(II) couples. Franck-Condon vs. electronic contributions
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Electron-transfer rates involving the Co(sep)3+,2+ (sep = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) and Co(en)33+,2+ couples have been examined for any differences in contributions of Franck-Condon and electronic terms to the rate constants. Rate constants for cross-reaction oxidations of Co(sep)2+ follow the classically predicted dependence of the Franck-Condon factor on ΔGab°. Far-infrared and Raman vibrational spectra of M(sep)3+/M(en)33+ (M = Co, Rh) are in very close correspondence. As a consequence, the force constants of the M-N stretching modes must also be comparable (differing by less than 10%) in the Co(sep)3+ and Co(en)33+ complexes. Strain energy calculations based on these and other published force constants are consistent with the differences in self-exchange rates and indicate upper limits on the Franck-Condon term in the rate constants consistent with significant contributions from electronic factors in both exchange reactions.
- Endicott, John F.,Brubaker, George R.,Ramasami,Kumar, Krishan,Dwarakanath, Kaddi,Cassel, Jonathan,Johnson, David
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p. 3754 - 3762
(2008/10/08)
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