Ruthenium temperature dependence

Studies like those mentioned here on the osmium complexes are more difficult for related complexes of ruthenium because of the intervention of a lowlying, thermally populable d-d excited state. However, it is possible to separate the two contributions to excited state decay by temperature dependent measurements. In the case of Ru(bpy>32+, temperature dependent lifetime studies have been carried out in a series of solvent, and the results obtained for the variation of knr with Eem are in agreement with those obtained for the Os complexes (19). 

Kinetic experiments were performed in other ruthenium-derivative proteins [115, 116, 117, 118], but it is difficult to compare their results with those previously reported as long as the temperature dependence of the rate has not been measured. 

A similar study was performed on ruthenium-modified myoglobins, in which AG variations were obtained by changing the nature of the ruthenium complex covalently bound to the protein, and by substituting a porphyrin to the heme [137]. It is gratifying to observe that, in spite of the rather heterogeneous character of this series, the study leads to an estimation of 1.9 to 2.4 eV for A which is consistent with the value 2.3 eV derived in section 3.2.1 from temperature dependent experiments. Satisfactory agreement between the results given by the two methods is also observed in the case of ruthenium-modified cytochrome c [138]. 

A variety of physical methods has been used to ascertain whether or not surface ruthenation alters the structure of a protein. UV-vis, CD, EPR, and resonance Raman spectroscopies have demonstrated that myoglobin [14, 18], cytochrome c [5, 16, 19, 21], and azurin [13] are not perturbed structurally by the attachment of a ruthenium complex to a surface histidine. The reduction potential of the metal redox center of a protein and its temperature dependence are indicators of protein structure as well. Cyclic voltammetry [5, 13], differential pulse polarography [14,21], and spectroelectrochemistry [12,14,22] are commonly used for the determination of the ruthenium and protein redox center potentials in modified proteins. 

Intramolecular electron transfer from Ru(II) to Fe(III) in (NH3)3Ru(II) (His-33)cyt(Fe(III)) induced by pulse-radiolysis reduction of Ru(III) in the (NH3)5Ru(III) (His-33)cyt(Fe(III)) complex were investigated [84]. The results obtained differ from those of refs. 77-80 where flash photolysis was used to study the similar electron transfer reaction. It was found [84] that, over the temperature range 276-317 K the rate of electron transfer from Ru(II) to Fe(III) is weakly temperature dependent with EA 3.3 kcal mol 1. At 298 K the value of kt = 53 2 s"1. The small differences in the temperature dependence of the electron tunneling rate in ruthenium-modified cytochrome c reported in refs. 77-80 and 84 was explained [84] by the different experimental conditions used in these two studies. 

In the following year Caspar and Meyer184 applied these considerations to MLCT excited states of Re(I), d-d excited states of Rh(III) and then to MLCT states of a series of Ru-bpy complexes185. In the case of ruthenium complexes there was an important development three rate constants for return to the ground state could be delineated. In addition to kr and knr, which are essentially temperature independent, a new temperature-dependent term, k °, was found to be important. Thus, the observed rate constant in these cases was described as... 

The temperature dependence of the actual oxidation state of the Ru(0001) surface raises the question of whether only the Ru02 phase is catalytically active. To confirm or correct the suggested mechanism of CO oxidation on a ruthenium catalyst, the evolution of the oxidation state of the catalyst with reaction temperature and the C02 production at CO + 02 pressures comparable to those of realistic catalytic conditions were followed simultaneously. The dynamic response of the Ols and Ru3d5/2 core level spectra was used for a precise assignment of the state of oxidation of the catalyst in the course of the reaction—correlated to the corresponding C02 yields measured with a mass spectrometer. The already determined Ru3d5/2 and Ols core-level spectra summarized in Figure 26 provided the necessary basis for identification of the adsorbed, "incorporated," and oxide states and verification of their roles in the CO oxidation reaction. 

Although the previously observed phosphorescence of ferrocene now appears to have been artlfactual, a fairly strong luminescence has been observed from the analogous ruthenium(II) compound (53,210). The emission spectrum of ruthenocene, measured at low temperatures either from the pure solid or from glassy media, appears as a rather broad but highly structured band centered around a maximum at about 17 kK. The lifetime of the emission from the solid is strongly temperature dependent. 

More recently, Dalla Betta and Shelef (51) performed in situ IR measurements with AljOj-supported ruthenium, exposed to 1 bar total pressure mixtures of H2 CO He = 0.075 0.025 0.9 at temperatures from 250°C upward. Up to 250°C adsorbed CO was present at almost complete monolayer coverage. At higher temperatures the coverage decreased. As the phenomenon is irreversible with respect to a lowering of the reaction temperature, the authors conclude that it reflects surface blocking by a reaction residue rather than a temperature dependence of the carbon monoxide adsorption-desorption equilibrium. 

Keywords Rhodium, Ruthenium, Diphosphines, Enamides, Substrate chelation, Pressm e and temperature dependence. Dihydrides, Alkyl hydrides. Transient NMR... 

Cyclooctatetraene (COT) iron carbonyl complexes and ruthenium carbonyl complexes (Table 5) have raised a good deal of interest as flux-ional molecules which display temperature dependent NMR spectra e.g. COT-Fe(CO)3 shows only one sharp peak at room temperature, but a pattern conceivable with... 

Nonradiative Deactivation Involving a Second Excited State. A somewhat different situation is presented by the pressure effects reported for the MLCT emissions from the ruthenium(Il) complex Ru(bpy)f+. At ambient temperature, in a fluid solution this species shows little unimolecular photochemistry and relatively small emission quantum yields (ff>r < 0.1) [32]. Initial pressure studies on the luminescence of this ion in 18°C aqueous solution detected little sensitivity to pressure [60], as might be expected for a weakly coupled nonradiative mechanism owing to the low compressibility of water. However, detailed studies by Fetterolf and Offen [32,61] painted a more complex picture. These workers probed the temperature dependence of AF and confirmed the small negative value at low temperature but also demonstrated a remarkable temperature dependence for this parameter. 

The supported ruthenium catalysts can be synthesized by OMCVD using Ru3(CO)i2 as precumor in static and fluidized-bed conditions. It was found that the ruthenium particles are distributed uniformly on the supports in the fluidized-bed conditions. The ruthenium loadings depend on deposition temperature and pressure, and adsorption time. Under the static conditions, various differently loaded ruthenium catalysts were prepared by controlling the initial amount of the ruthenium precursor under high-vacuum conditions. The size of the ruthenium particles can be controlled by changing the support. The as-prepared catalysts were highly catalytically active and stable for the CO oxidation reaction. 

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