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

The known complex {Cp(PPh 3 ) 2 Ru} 2 (μ-C⋮CC⋮C) ( 3-Ph ) and its PMe 3 -substitution product {Cp(PPh 3 )(PMe 3 )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 AgPF 6 in CH 2 Cl 2 /1,2-dimethoxyethane gave the one- and two-electron oxidized species [ 3-Ph ][PF 6 ] and [ 3-Ph ][PF 6 ] 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.