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

Abstract

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)(σN-fumaronitrile)][OTf] ( 1), which is in equilibrium with the binuclear complex [{CpRu(iPr-DAB)} 2(μ-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(σN-fumaronitrile)][OTf] ( 3), while reaction with 0.5 equiv. of fumaronitrile gave [{CpRu(PPh 3) 2) 2(μ-fumaronitrile)][OTf] 2 ( 4) in 100% yield. Attempts to isolate the asymmetric compound [CpRu(iPr-DAB)(μ-fumaronitrile)CpRu(PPh 3) 2][OTf] 2 ( 5) failed since in all cases a mixture of 2, 4 and the desired complex of 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-(σN-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 Ru IIRu III intermediate 2 +.