Rhenium complexes applications

Although the number of applications of olefin metathesis to transition metal complexes is small compared to the number of applications in organic synthesis, this field is becoming increasingly important. Spectacular examples are the double RCM reactions of copper phenanthroline complexes as a synthetic route to catenanes [113] or a recently reported approach to steric shielding of rhenium complex terminated sp-carbon chains [114]. 

Structural studies on rhenium complexes containing complexone type ligands are of considerable interest in the light of the application of their technetium analogues in diagnostic nuclear... 

Rhenium complexes containing the metal in the oxidation state +III are comparatively numerous. This may be ascribed to the fact that the d configuration of the rhenium center can readily be stabilized by ligands with pronounced donor and rr-acceptor properties. Most of the rhenium(III) compounds are stable against hydrolysis rendering them suitable for nuclear medical applications. 

Recently, the interest in Re (I) complexes has been increased due to their potential utility for the activation and reduction of C02 into CO and C032 in a purpose of construction of artificial photosynthetic systems [11-13]. Rhenium Complexes such as ReX(CO)3(bpy) (X=C1, Br) and Re(CO)2(bpy)[P(OEt)3]2 have been used as photocatalysts for C02 reduction to CO in solvent mixture of triethanolamine/dimethylformamide [12,13]. Most of the research on photochemical activation and reduction of C02 using Re(I) complexes have focused on the homogeneous solution systems. There are few reports concerned about the encapsulation of rhenium complexes into molecular sieves and their photochemical application to the photochemical reduction of C02. 

Gillespie, A.M. and White, D.P. (2001) Understanding the steric control of stereoselective olefin binding in cyclopentadienyl complexes of rhenium an application of de novo ligand design. Organometallics, 20, 5149. 

The use of organometallic rhenium complexes has found a very broad scope as oxidation catalysts as described in the previous section, making MTO the catalyst of choice for many oxidation reactions of olefins. Interestingly, MTO and related rhenium compounds have also found application in the reverse reaction, the deoxygenation of alcohols and diols. Especially in recent years, this reaction has attracted much attention due to the increased interest in the use of biomass as feedstock for the chemical industry. This section provides an overview of the use of rhenium-based catalysts in the deoxygenation reaction of renewables. 

Where rhenium-catalyzed deoxydehydration has attracted a lot of interest, only two reports concerning dehydration catalyzed by rhenium complexes are noteworthy in view of their application on biomass-derived substrates. The first was published in 1996 by Zhu and Espenson and uses MTO as catalyst for the dehydration reaction of various alcohols, either aliphatic or aromatic, to obtain the corresponding olefins. Using MTO in benzene or in the alcohol itself at room temperature after 3 days gives reasonable turnovers and, in the case of benzylic alcohols, good yields. In the same paper, MTO is used for the amination, etherification, and disproportionation of alcohols, which are all reactions interesting in the viewpoint of biomass transformation [123]. 

Abstract This review presents a comprehensive overview of the most significant developments in the chemistry of technetium and rhenium complexes anchored by heterofunc-tionalized phosphines and by hard and soft scorpionates. The main goal is to provide the reader with chemical,radiochemical and biological knowledge which is expected to enhance the application of these types of compounds in the development of target-specific radiopharmaceuticals. 

K. Hashimoto and K. Yoshihara, Rhenium Complexes Labeled with 186/188Re for Nuclear Medicine, in Technetium and Rhenium, Their Chemistry and its Applications (Topics in Current Chemistry, Vol. 176), eds. K. Yoshihara and T. Omori, Springer-Verlag, Berlin, 1996, p.275. 

A general review on rhenium complexes containing poly(pyrazolyl)borates appeared in 1998.110 In 2004, an overview of the progress involving group VII elements and actinide was presented by Santos and coworkers,111 who very recently described also rhenium and technetium complexes anchored by scorpionates prepared for radiopharmaceutical application.112 Keane and Harman described exhaustively the coordination chemistry of a new generation of Tt-basic dearomatization agents M(Tp)(L)(7t-acid). 113 Jones et al.114 reported a review on the role of unstable alkane... 

Fig. 8.24. Rhenium complexes anchored by scorpionates investigated for their potential use in radiopharmaceutical applications.

Although they have not yet been developed to the point of synthetic application, pyrrole-rhenium complexes display the reactivity expected for a very strongly electron-attracting substituent. Complex (49) undergoes ring lithiation readily (Equation (128)). Use of the iodo rhenium complex (50) leads to addition at C2 (Equation (129)) <87JOM(333)7l, 89JOM(362)C3l>. 

Saladino R, Crestini C, Costanzo G, DiMauro E (2005) On the Prebiotic Synthesis of Nucle-obases, Nucleotides, Oligonucleotides, Pre-RNA and Pre-DNA Molecules. 259 29-68 Santos I, Paulo A, Correia JDG (2005) Rhenium and Technetium Complexes Anchored by Phosphines and Scorpionates for Radiopharmaceutical Applications. 252 45-84 Santos M, see Szathmdry E (2005) 259 167-211... 

These metals form chalcogenolate complexes in several oxidation states, and from the application-oriented point of view manganese compounds have been synthesized as models for hydrodesulfurization processes and rhenium and technetium derivatives as models for radiopharmaceuticals. 

Despite a common perception that organometallic chemistry essentially belongs in the province of catalysis rather than in vivo applications because of high reactivity of metal carbon bonds, certain organometallic species have very high kinetic stability. Indeed, this point is highlighted by the present wide application of the isonitrile complex Tc(ses-tamibi), [Tc(2-methoxyisobutylisonitrile)6]+, in myocardial imaging. Rhenium tricarbonyl and cyclopentadienyl complexes offer further ex-... 

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