Redox rhenium complex

Although these complicated features of the redox reactions of technetium are important topics, the present article is limited to the susbtitution kinetics of technetium complexes. Some redox kinetics of technetium complexes have been discussed by Koltunov and Gomonova [6], The redox potentials for analogous technetium and rhenium complexes, based on their differences, have been compiled by Deutsch et al. [7],... 

The main goals of this research are (1) to characterize polynuclear rhenium complexes which are capable of multielectron transfer reactions (2) to come up with appropriate conclusions on the redox-initiated transformations of this synthetic analog through the use of a new design of spectroelectrochemical cell and (3) propose possible systems or investigations where infrared spectroelectrochemi-stiy can be very useful. 

ReCl3(PPh3)(benzil)] reacts with bipy and related ligands or terpy to form a number of rhe-nium(III) and rhenium(II) compounds which are useful precursors for the synthesis of lower-valent rhenium complexes. " Thus, reduction of [Re(bipy)3][PF6]2 with zinc amalgam results in the rhenium(I) compound [Re(bipy)3][PF6] in excellent yields. The corresponding terpyridyl bis-chelate [Re(terpy)2][PF6] has been prepared in a similar manner. " The electrochemistry of the products provides a convenient measure of the chemical reactivity associated with the redox processes. Thus, the one-electron oxidation of [Re(bipy)3]" is reversible at -0.33 V, whereas the Re"/Re" redox couple is irreversible and occurs at relatively low potentials (-1-0.61 V) which is consistent with the instability of [Re(bipy)3] + in solution. However, in the presence of a small coordinating molecule such as CNBu, oxidation to the rhenium(III) state is readily available by the formation of seven-coordinate complexes of the composition [Re(bipy)3(L)]. " ... 

Raman spectroscopy metal in water complexes, 309 Rare earth complexes acetylacetone synthesis, 377 guanidinium, 282 hydroxamic acids, 506 Redox properties bipyridyl metal complexes, 90 Reductive coupling nitrile metal complexes, 265 Resorcinol, 2,4-dinitro-metal complexes, 273 Rhenium complexes acetylacetone, 376 synthesis, 375, 378... 

Reactions using rhenium complexes as redox photosensitizers for hydrogen generation have also been reported. The coexistence of Co complexes (76,77) or Fe complexes (78) as catalyst induced efficient hydrogen generation. 

Photocatalytic CO2 reduction of a supramolecule with a Zn porphyrin unit, which is a redox photosensitizer that can absorb even wider ranges of visible light, connected to a rhenium complex (ZnTMP-ReCl) was considered (101,102). In this system, ultrafast (l.SxlO s ) electron transfer from the S2 excited state of the ZnTMP unit to the rhenimn imit was observed. Reduction of CO2 proceeded with generation of the OER species of the rhenium unit by the reduction of this intramolecular charge-transfer state by TEA. 

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

Table 7-14 reports the redox potentials of one-electron oxidation of the ferrocenyl ligands in all the discussed rhenium complexes. 

Herrmann and co-workers showed that the 17-electron complex 62 may be subjected to electrochemical oxidation as well as reduction (54.55). The redox potentials of the respective halo complexes follow the expected trend for change in the halide ligands. In accordance with these electrochemical studies, both chemical oxidation and reduction reactions of 62 were found [Eqs. (49) and (50)]. Oxidation of the rhenium complex Re(CCMe3)(// -C5Me5)Cl2 by dioxygen results in cleavage of the neopentylidyne ligand from the metal center as pivalic anhydride and pivaloyl chloride. 

Interestingly, the electron reservoir properties of redox active ligands are also found to be useful to impose one-electron transformation on late transition metals. Rhenium complexes are known to be powerful oxo-transfer reagents [27]. However, closed-shell... 

Scheme 15 Ethylene capture at the redox non-innocent thiyi radical of a rhenium complex...

Reported rhenium complexes possess one [70-74], two [75], and three [75] dithiocarboxylato ligands coordinated to the one rhenium atom to form a high coordination state, as in the case of the technetium complex [76]. One of these complexes 29 is synthesized from Re(VII)S4 and (PhCSS)2 (Fig. 10) [72]. An internal redox reaction occurs and Re( VII) is reduced to Re(III) in the synthetic process. Addition of sulfur-abstracting reagents, Ph3P or Et4NCN, produces the heptacoordinate complex 30 with the neutral capped octahedron structure or... 

Complex 4 and TpRe( = 0)Br2 (27) have been prepared by reaction of 3 with PPhs in the presence of Me3SiCl or Me3SiBr (Scheme 19). The reduction of 4 requires more forcing conditions relative to the technetium analog, consistent with the fact that reduction of rhenium complexes is typically less facile than corresponding technetium complexes. Replacement of chloride ligands with bromide does not significantly alter the redox properties of TpRe( = 0)X2 (X = Cl or Br). 

Technetium complexes with dtpa, dmsa, or mdp (methylene diphos-phonate) can be prepared by exchange reactions of the respective rhenium complexes with pertechnetate. Despite the complication that redox as well as substitution is involved here, rates correlate with metal-ligand bond strengths. A detailed kinetic study of these reactions would be welcome. Another type of ligand exchange reaction where kinetic studies are needed is the similar situation encountered in the preparation of technetium(III), (IV), or (V) complexes by reduction of pertechnetate with tin(II) complexes of the respective ligands. 

Cysteine, however, is able to strip ca. 50% of the technetium from the labelled protein after 4 hours of incubation [102]. Methods for complexation of antibodies with technetium must be modified for rhenium, because of its lower redox potential and-consequently-its greater tendency to reoxidize [103]. 

The bimetallic complex [Re(CO)3Cl]jtbpq synthesized in this work showed the typical spectroscopic and electrochemical behavior based on analogous polypyridyl complexes of rhenium(l). Re(l) dn tpbq n charge transfer transition and ligand-field n- n transitions are observed. Typical redox behavior of this system consists of Re /Re oxidation and tpbq/tpbq reduction. Such electrochemical activity, particularly in the reductive region, is found ideal for catalytic processes such as CO reduction. IR-SEC studies have shown that the reduction process occurring at -0.50... 

Scheme 145 pH-dependent redox couples of rhenium (V)-complexes. 

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