Synthesis and redox properties of [{CpRu(L(2))}(2)-(mu-fumaronitrile)][OTf](2) and [CpRu(L(2))(sigma N-fumaronitrile)][OTf] with L(2) equals N,N'-diisopropyl-1,4-diaza-1,3-butadiene (iPr-DAB) or L=PPh(3)

The reaction of [CpRu(iPr-DAB)(OTf)] (iPr-DAB=N,N'-diisopropyl-1,4-diaza-1,3-butadiene with excess fumaronitrile yields [CpRu(iPr-DAB)(sigma N-fumaronitrile)][OTf] (1), which is in equilibrium with the binuclear complex [{CpRu(iPr-DAB)}(2)(mu-fumaronitrile)][OTf](2) (2) and free fumaronitrile. Complex 2 could be prepared quantitatively by the reaction of [CpRu(iPr-DAB)][OTf] with 0.5 equiv. of fumaronitrile. The reaction of [CpRu(PPh(3))(2)][OTf] with excess fumaronitrile led to the monomeric complex [CpRu(PPh(3))(2)(sigma N-fumaronitrile)][OTf] (3), while reaction with 0.5 equiv. of fumaronitrile gave [{CpRu(PPh(3))(2)}(2)(mu-fumaronitrile)][OTf](2) (4) in 100% yield. Attempts to isolate the asymmetric compound [CpRu(iPr-DAB)(mu-fumaronitrile) CpRu(PPh(3))(2)][OTf](2) (5) failed since in all cases a mixture of 2, 4 and the desired complex 5 was formed. The redox behavior of complexes 1 and 2 was investigated by cyclic voltammetry and UV-Vis/IR spectroelectrochemistry. The chemically irreversible one-electron reduction of both complexes is localized on the fumaronitrile ligand, as was also found for the PPh(3)-substituted complexes 3 and 4. These results are in sharp contrast with the chemically reversible, iPr-DAB-localized one-electron reduction of the mononuclear derivatives [CpRu(iPr-DAB) (X)](+) (X=CO, PPh(3)). Oxidation of the Ru-II centers in 1 and 2 resulted in secondary chemical reactions. The final oxidation product in the case of the binuclear complex 2 has been unambiguously identified as the cation [CpRu(iPr-DAB)(OTf)](+). This result demonstrates that the Ru-III-(sigma N-fumaronitrile) bond in the studied complexes is rather weak and easily dissociates. The oxidation-induced reactivity of 2 hence rules out the spectroscopic characterization of the mixed-valence (RuRuIII)-Ru-II intermediate 2(+).