Oxidation chemistry of metal-bonded C₄ chains: A combined chemical, spectroelectrochemical, and computational study

The known complex {Cp(PPh₃)₂Ru}₂(μ-C≡CC≡C) (3-Ph) and its PMe₃- substitution product {Cp(PPh₃)(PMe₃)Ru}₂(μ-C≡CC≡C) (3-Me) have been shown by cyclic voltammetry to undergo a series of four stepwise one-electron oxidation processes. Successive oxidation potentials (V) for the first three reversible processes of 3-Ph (3-Me) are -0.23 (-0.26), +0.41 (+0.33), and +1.03 (+0.97); the fourth, irreversible oxidation at +1.68 (+1.46) V is accompanied by chemical transformation followed by further oxidation. Chemical oxidation of 3-Ph with 1 or 2.5 equiv of AgPF₆ in CH₂Cl₂/1,2- dimethoxyethane gave the one- and two-electron oxidized species [3-Ph][PF₆] and [3-Ph][PF₆]₂, respectively. The chemical and electrochemical studies have been complemented by a series of detailed spectroelectrochemical experiments to obtain spectral data associated with the 3(n+) (n = 0-4) species from 1500 to 40 000 cm^-1, without necessitating the isolation of each individual species. Theoretical techniques have been employed in order to probe the structure of the conjugated all-carbon ligand at each stage of oxidation. Both the metal centers and the carbon atoms of the C₄ bridge are affected, with removal of electrons housed in MOs delocalized over all atoms of the Ru-C₄-Ru chain. Comparison of models with different ligand surroundings suggests that molecules containing strong electron-donating ligands should be more easily oxidized.