Oxidation Chemistry of Metal-Bonded C₄Chains: A Combined Chemical, Spectroelectrochemical, and Computational Study

Abstract

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 3n+ (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.