Dehydrogenative Route to Metallo Dithiolenes: The Reaction of Zinc Polysulfides and Alkenes

Complexes of the type ZnS6(TMEDA), ZnS4(PMDETA), and ZnS4(Me3TACN) react with electrophilic alkenes to give dithiolene complexes ZnS2C2R2(amine)n. The Me3TACN complex is the most reactive, while the more conveniently prepared PMDETA complex also undergoes this reaction at useful rates. The following alkenes were successfully tested: C2H2(CO2Me)2 (cis and trans isomers), C2H3(CO2Me), C2H3(CN), 1,2-C2H2Me(CN), C2H3(CHO), and 1,2-C2H2(CN)(Ph). Crystallographic analysis shows that the highly reactive complex ZnS4(Me3TACN) is structurally similar to ZnS4(PMDETA), including the presence of an elongated Zn-Nax bond. Model studies indicate that the reaction of alkenes with LnZnSx proceeds via the reversible formation of a dipolar intermediate, as indicated by the ability of the polysulfido complexes to catalyze the isomerization of cis-C2H2(CO2Me)2. It is proposed that such dipolar species undergoes ring closure to give alkanedithiolato intermediates, e.g., Zn[S2C2H2(CO2Me)2](PMDETA). The dithiolato complexes Zn[S2C2H2(CO2Me)2](PMDETA) and Zn[S2C2H2(CO2Me)2](TMEDA) were prepared from ZnMe2, the diand triamines, and the dithiol meso-(HS)2C2H2(CO2Me)2. These dithiolates undergoes dehydrogenation upon treatment with S8 to give the dithiolene Zn[S2C2(CO2Me)2]Ln at a rate that is independent of the ancillary ligand L. The dithiolene ligands can be removed from the Zn center by treatment with [COCl2]3 and Cp2TiCl2; in this way Zn[S2C2H(CN)](PMDETA) was converted to OCS2C2H(CN) and Cp2TiS2C2H(CN).