Redox reactivity and comprehensive synthetic chemistry of the perchloroditungstate [W2(μ-Cl)3Cl6]n− (n = 3, 2, 1) anions in organic media

The catalytic reactivity of the perchloroditungstate anions [W2(μ-Cl)3Cl6]n− (n = 3, 2, 1) [n = 3, 2 (W26+,a′2e′4); n = 2, 3 (W27+,a′2e′3); n = 1, 4 (W28+,a′2e′2)] in organic media remains unexplored, despite the facile redox activity of the W2n+ core (n = 4, 6–8), due to, among other reasons, the lack of simple and efficient methods for their synthesis. The preparation of salts of 2 with the bulky organic cations [nBu4N+ (TBA+, a); Ph4P+ (TPP+, b)] can be achieved in high yield from the room temperature reduction of WCl4 with Na/Hg in toluene (∼90%), or from the reaction of W(CO)6 with Me3SiCl in boiling chlorobenzene (74%). Their oxidized counterparts 3a,b can be isolated in high yield directly from the oxidative decarbonylation of W(CO)6 with 1,2-dichloroethane in the presence of TBACl or TPPCl in boiling chlorobenzene, or indirectly from the facile one-electron oxidation of 2a,b induced by (a) HCl(g) in organic aprotic solvents (CH2Cl2, THF, MeCN) or in biphasic media (aqueous HCl 6–12 M/CH2Cl2); (b) Lewis acids (SnCl2, AlCl3) in CH2Cl2; (c) various metal halide oxidants (FeCl3, CuCl2, HgCl2) in organic media (CH2Cl2, THF, MeCN); and (d) carbon tetrachloride in solution (CH2Cl2, CCl4) or in the solid state (2a,b/CCl4). The redox chemistry (chemical or electrochemical) of the complexes in organic media, as well as their UV–Vis, ESI–MS, CV and EPR spectra are also reported. Examination of 2b by cyclic voltammetry in CH2Cl2 gives two reversible waves at −0.741 and −0.021 V versus Fc+/Fc  , which are related spectroelectrochemically to the following successive one-electron processes: W26+ → W27+ → W28+. Thus, the synthesis of 3b can be accomplished by bulk electrolysis of 2b at 0.750 V. Whereas 2a,b do not react anaerobically with PhCH2OH at ambient temperature, complexes 3a,b are reduced to 2a,b producing a mixture of PhCHO and PhCH2Cl. The facile 2/3 redox chemistry and the capability of 3 in oxidizing PhCH2OH makes this redox couple functional and of potential utility in catalytic oxidations of organic substrates.

New simple high-yield synthetic routes for complexes 2a,b and 3a,b, otherwise not easily accessible, have been developed and their redox chemistry in organic media is presented. Their facile chemical or electrochemical redox interconversion in conjunction to their stability and the ability of 3 to oxidize alcohols (PhCH2OH → PhCHO), while it is reduced to 2, makes the W2n+ (n = 6, 7) couple functional and of potential utility in catalytic oxidations of organic substrates.Figure optionsDownload as PowerPoint slide