One Electron Reduced Square Planar Bis(benzene-1,2-dithiolato) Copper Dianionic Complex and Redox Switch by O2/HO

The complex [Ph4P]2[Cu(bdt)2] (1 red ) was synthesized by the reaction of [Ph4P]2[S2MoS2CuCl] with H2bdt (bdt = benzene-1,2-dithiolate) in basic medium. 1 red is highly susceptible toward dioxygen, affording the one electron oxidized diamagnetic compound [Ph4P][Cu(bdt)2] (1 ox ). The interconversion between these two oxidation states can be switched by addition of O2 or base (Et4NOH = tetraethylammonium hydroxide), as demonstrated by cyclic voltammetry and UV–visible and EPR spectroscopies. Thiomolybdates, in free or complex forms with copper ions, play an important role in the stability of 1 red during its synthesis, since in its absence, 1 ox is isolated. Both 1 red and 1 ox were structurally characterized by X-ray crystallography. EPR experiments showed that 1 red is a Cu(II)–sulfur complex and revealed strong covalency on the copper–sulfur bonds. DFT calculations confirmed the spin density delocalization over the four sulfur atoms (76%) and copper (24%) atom, suggesting that 1 red has a “thiyl radical character”. Time dependent DFT calculations identified such ligand to ligand charge transfer transitions. Accordingly, 1 red is better described by the two isoelectronic structures [CuI(bdt2, 4S3–,*)]2– ↔ [CuII(bdt2, 4S4–)]2–. On thermodynamic grounds, oxidation of 1 red (doublet state) leads to 1 ox singlet state, [CuIII(bdt2, 4S4–)]1–.