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] (<b>1</b>^<b>red</b>) was synthesized by the reaction of [Ph₄P]₂[S₂MoS₂CuCl] with H₂bdt (bdt = benzene-1,2-dithiolate) in basic medium. <b>1</b>^<b>red</b> is highly susceptible toward dioxygen, affording the one electron oxidized diamagnetic compound [Ph₄P]­[Cu­(bdt)₂] (<b>1</b>^<b>ox</b>). 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 <b>1</b>^<b>red</b> during its synthesis, since in its absence, <b>1</b>^<b>ox</b> is isolated. Both <b>1</b>^<b>red</b> and <b>1</b>^<b>ox</b> were structurally characterized by X-ray crystallography. EPR experiments showed that <b>1</b>^<b>red</b> 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 <b>1</b>^<b>red</b> has a “thiyl radical character”. Time dependent DFT calculations identified such ligand to ligand charge transfer transitions. Accordingly, <b>1</b>^<b>red</b> is better described by the two isoelectronic structures [Cu^I(bdt₂, 4S^3–,*)]^2– ↔ [Cu^II(bdt₂, 4S^4–)]^2–. On thermodynamic grounds, oxidation of <b>1</b>^<b>red</b> (doublet state) leads to <b>1</b>^<b>ox</b> singlet state, [Cu^III(bdt₂, 4S^4–)]^1–