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

The known complex {Cp(PPh3)(2)Ru}(2)(mu-C=CC=C) (3-Ph) and its PMe3-substitution product {Cp(PPh3)(PMe3)Ru}(2)(mu-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 C-4 bridge are affected, with removal of electrons housed in MOs delocalized over all atoms of the Ru-C-4-Ru chain. Comparison of models with different ligand surroundings suggests that molecules containing strong electron-donating ligands should be more easily oxidized.