Low-Temperature Nucleophilic Attack of Me3SiO- and MeO- on Rhenium(I) and Rhenium(0) Carbonyl Complexes

The reactivity of [Re(CO)5Cl] and [Re2(CO)10] with nucleophiles such as Me3SiO- and MeO- has been investigated by low-temperature NMR and by infrared spectroscopy particularly aimed at detecting the [Re(CO)5OR] species, which so far have eluded isolation, proposed as intermediates in the synthesis of the dimeric species [Re2(CO)6(mu-OR)3]- (R = SiMe3, Me). For both nucleophiles the first step involves an interaction with one specific carbonyl ligand and formation of new anionic complexes carrying a metal carboxy silyl or methyl ester, such as cis-[Re(CO)4(COOSiMe3)Cl]-, cis-[Re(CO)4(COOMe)Cl]-, [Re2(CO)9(COOSiMe3)]-, and [Re2(CO)9(COOMe)]-. The Re(I) complexes, stable only at low temperature, by further nucleophilic attack generate the well-known dimeric [Re2(CO)6(mu-OR)3]- complexes. The Re(0) metal carboxy-methyl esters derived from [Re2(CO)10] are thermally more stable and can be isolated in the solid state. In particular, the new complex [Re2(CO)9(COOMe)][NBu4] has been fully characterized by X-ray diffraction. In contrast, when the nucleophilic attack involves a weaker OR- nucleophile such as [(c-C6H11)7Si8O12O]- or a Re(I) pentacarbonyl substrate which bears a better leaving group such as the triflate, the low-temperature nucleophilic attack on a specific carbonyl ligand does not occur.