Rhenium dioxo complexes

High-valent rhenium dioxo complexes, ReC>2l(PR3)2, catalyse the hydrosilylation of carbonyl compounds.264 DFT calculations suggest rate-determining dissociative addition of the Si-H bond (of H-Si-alkyl3) across the Re=0 bond, with subsequent carbonyl coordination and reduction. 

A remarkable aspect of the rhenium oxo complex chemistry is the ability of terminal oxo functions to act as nucleophiles. This has been used to incorporate dioxo complexes such as [Re02(py)4] or [Re02(CN)4] between the negatively charged interlayers of silicates like hector-ite (pyridine complex) or between the positive layers of a hydrotalcite-like Mg/Al double hydroxide. " The same concept has been applied for the isolation of molecular units containing Re-O-M bridges where M is a Lewis acid. ... 

Rhenium forms a number of mono-oxo, CTs-dioxo, and trans-dioxo complexes in oxidation states IV through VII. Reversible, proton-independent, one-electron potentials for several systems are represented in Table 1 [13-22, 35, 36]. As is the case forTc, a large change in E° for a given redox step can be produced by changing the ancillary ligands. 

In contrast with the common activity of oxo-rhenium compounds in oxidation catalysis (see above), the Re -dioxo-complex [Re02l(PPh3)2] catalyzes the reductive hydrosilylation of aldehydes and ketones to give silyl ethers (reaction... 

A detailed kinetic study of the water exchange of the dioxo complex of rhenium(V) (206), however, gave very valuable information about the relative reactivities of the aqua and hydroxo complexes toward substitution The ratio of the water exchange rate of the aqua and hydroxo complexes of rhenium(V) was determined to be about 50. The 0 NMR studies of these complexes demonstrate its effectiveness for the characterization of these type of complexes in aqueous solutions (207). 

Dixon, J., Espenson, J. H., Kinetics and mechanism of oxygen atom abstraction from a dioxo-rhenium(VII) complex, 41 4727, 2002. 

Compared with the large number of rhenium(VII) trioxo complexes there are relatively few reports on dioxo and monoxo compounds. They are mainly derived from rhenium(VII) oxyfluor-ides or contain catecholato ligands. 

Fig. 17 Proposed reaction mechanism for the olefination of aldehydes catalyzed with (T -alkyne)methyl(dioxo)rhenium complexes. Reproduced with permission from [120]...

---