Rhenium complexes addition

The main routes for the synthesis of rhenium(II) compounds involve oxidation of rhenium(I) or the reduction of rhenium(III) complexes. Additionally a few examples are known where Re" species are formed during the cleavage of Re—Re bonds of bimetallic units. 

Structurally these complexes (Fig. 16.59) are clearly related to IRe ClJ. the only difference being (for the rhenium complexes) the addition of ligands to overlap wiih the metal dp hybrid orbitals which were nonbonding in [RevCl )2. ... 

In general, emission intensity in the molecular polygons examined here was significantly diminished from that of the monomolecular rhenium complexes. This can be attributed to a decrease in the emission lifetime due to quenching by the additional vibronic components. 

The luminescence spectra of all receptors in CH3CN were found to be dramatically affected by the addition of acetate or chloride. While compound 19 exhibits an emission decrease, the other receptors 17,18 and 20 show a remarkable intensity increase (up to 500%) with a slight concomitant blue shift of the emission maximum (660 nm for 17). The anion-induced enhancement of luminescence intensity in the case of 17 is clearly due to the decrease of the electron transfer between the ruthenium(II) bipyridyl centre and the quinone moieties. Alternatively, receptors bearing ruthenium or rhenium complexes on the upper rim were also described [20]. 

A rhenium complex, [ReBr(CO)3(thf)]2, has been found able to catalyse the inter-molecular reactions of 1,3-dicarbonyl compounds with terminal alkynes to give the corresponding alkenyl derivatives in excellent yields (Scheme 6).35 These reactions could apply to an intramolecular version and gave the corresponding cyclic compounds quantitatively. Tributylphosphine has been found to be a superior catalyst for the a-C-addition of 1,3-dicarbonyl compounds to electron-deficient alkynes.36... 

In addition to described heterogeneous systems a homogeneous photocatalytic C02 reduction was tested using heteroleptic rhenium complexes [99, 100] and supramolecular ruthenium and rhenium bi- and tetranuclear complexes their excited states were quenched by 1-benzyl-1,4-dihydronicotinamide (BNAH) and C02 was reduced by the electron donor intermediate species [101]. 

Analogous alkoxy and aryloxyrhenium complexes do not show the same reactivity. However, Brown and Mayer discovered soon thereafter that a different rhenium complex could mediate C—O bond formation.68 Photolysis of the Re(V) complex 13 led to C—H activation and formation of a phenyl rhenium oxo, 14. Yields were improved from 30-40% to 90% upon addition of pyridine to the photolysis mixture. The role of pyridine was unclear, because other tertiary amines provided no such improvement in yield. Substituted benzenes showed a preference for para activation over meta fluorobenzene also gave a significant amount of ortho activation. 

When the cyclopropenyl rhenium complex (CO)5Re( 7 -C3Ph3) was refluxed in hexane for 3-4 h, the isomeric / -oxocyclobutenyl and tricarbonyl( 7 -hexaphenylcyclohexadi-enyl)rhenium complexes are obtained, in addition to the tetracarbonyl rhenacyclobutadi-ene complex. In the presence of excess Me3NO, a mixture of two interconvertible triphenyl rhenafuran complexes is obtained. Furthermore, if the thermal reaction is conducted in the presence of phosphorus ligands, addition and ring expansion occur, affording octahedral -cyclobutenone complexes (equation 206) . ... 

These -propargyl rhenium complexes undergo kinetic addition of nucleophiles at the central C atom to produce rhenacyclobutenes. The nucleophiles range from PR3 to malonate, acetylides, pyridines, and water. The derivatives of the addition of pyridines, however, are unstable and undergo further rearrangements to allene or acetylene complexes. Protonation of the metaUacyclobutenes produces u -allyl complexes. 

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