Synthesis and some properties of binuclear ruthenocenes bridged by oligoynes: formation of bis(cyclopentadienylidene)cumulene diruthenium complexes in the two-electron oxidation

The monoynes Rc*Ctriple bond]CRc*] and Rc'Ctriple bond]CRc'] were obtained in improved yields using Mo(CO)6]/2-FC6H5OH as a catalyst in the alkyne metathesis of Rc*Ctriple bond]CMe] and Rc'Ctriple bond]CMe], respectively (Rc = ruthenocenyl, Rc* = 1',2',3',4',5'-pentamethylruthenocenyl, and Rc' = 2',3',4',5'-tetramethylruthenocenyl groups). The diynes Rc*(Ctriple bond]C)2Rc*] and Rc'(Ctriple bond]C)2Rc'] were synthesized by the oxidative coupling of the corresponding terminal ethynes in good yields. The triyne Rc*(Ctriple bond]C)3Rc*] and the tetrayne Rc*(Ctriple bond]C)4Rc*] were prepared by the hetero- and homocoupling of Rc*Ctriple bond]CCtriple bond]CH], which was obtained from the reaction of Rc*Ctriple bond]CCHO] with LiN2CSiMe3], respectively. Although the oxidation waves did not always exhibit a clear two-electron oxidation process, the oxidation potentials shifted to a lower potential with an increase in the number of methyl substituents on the ruthenocenyl ring, and shifted to a higher potential with the increase in the number of Ctriple bond]C units; this result is in contrast to that found in the Rc(CH=CH)(n)Rc] series. The chemical oxidation of Rc'Ctriple bond]CRc'] yielded a stable two-electron-oxidized species, the structure of which was confirmed by X-ray crystallography to be Ru2(mu2-eta(6):eta(6)-C5Me4C=CC5Me4)(eta-C5H5)2](BF4)2. Changing the substituents (Rc, Rc*, and Rc') had no effect on the chemical oxidation, but in the case of the Rc' series the Me substituent increased the stability of the two-electron-oxidized species in solution. The diyne Rc*(Ctriple bond]C)2Rc*] and the triyne Rc*(Ctriple bond]C)3Rc*] also gave a similar but unstable two-electron-oxidized species. In acetone or acetonitrile, the two-electron-oxidized species of Rc*Ctriple bond]CRc*] and Rc*(Ctriple bond]C)2Rc*] gradually formed the corresponding bis(fulvene)-type complexes. This implies that the two-electron-oxidized species of Rc*(Ctriple bond]C)(n)Rc*] are destabilized with the increasing n.