The one-electron reduction of dithiolate and diselenolate ligands

Herein we present an assessment to determine which of nine well-established DFT functionals best describes the reduction of C2H2Se2(-)˙. In addition, we have also studied the effects of changing the substituents bound to the alkene functional group of dithiolene and diselenolene ligands. Such ligands are important due to their unique electrochemical and physical properties when ligated to metals. The M06-L functional shows best agreement with the QCISD/cc-pVTZ value of -2.45 V for the reduction potential of the (C2H2Se2˙(-)/C2H2Se2(-2)) redox couple. At the M06-L/6-311+G(d,p) level of theory the calculated reduction potential for the (C2H2Se2˙(-)/C2H2Se2(-2)) redox couple is only 0.09 V in error. However, as a result of the nature of the oxidized species for the respective ligands the absolute reduction potential of the (C2H2Se2˙(-)/C2H2Se2(-2)) redox couple is 0.57 V more oxidizing than the (C2H2S2˙(-)/C2H2S2(-2)) redox couple. This is due to the radical electron in C2H2S2˙(-) being delocalized within the alkene backbone, whereas in C2H2Se2˙(-) the electron is largely localized on the Se atoms. The relative reducing power of the S- and Se-containing redox couples is shown to vary depending on the choice of substituents. In particular the reduction potential of the various S-containing redox couples range from being 0.34 V more reducing to 0.28 V more oxidizing than the analogous Se-containing redox couples. This difference in the relative reducing power appears to be a result of the nature of the oxidized ligand. Thus, depending on the choice of moiety very different chemistry is seen between the analogous dithiolate and diselenolate ligands.