Fully Delocalized (Ethynyl)(vinyl)phenylene Bridged Triruthenium Complexes in up to Five Different Oxidation States

Triruthenium [(dppe)2Ru­{−CC–1,4-C6H2–2,5-R2–CHCH–RuCl­(CO)­(PiPr3)2}2]n+ (4a, R = H; 4b, R = OMe) containing unsymmetrical (ethynyl)­(vinyl)­phenylene bridging
ligands and displaying five well-separated redox states (n = 0–4) are compared to their bis­(alkynyl)ruthenium precursors
(dppe)2Ru­{−CC–1,4-C6H2–2,5-R2–CCR′} (2a,b: R′ = TMS; 3a,b: R′ = H) and their symmetrically substituted bimetallic
congeners, complexes {Cl­(dppe)2Ru}2{μ-CC–1,4-C6H2–2,5-R2–CC}
(Aa, R = H; Ab, R = OMe) and {RuCl­(CO)­(PiPr3)2}2{μ-CHCH–1,4-C6H2–2,5-R2–CHCH}
(Va, R = H; Vb, R = OMe) as well as the mixed (ethynyl)­(vinyl)­phenylene
bridged [Cl­(dppe)2Ru–CC–1,4-C6H4–CHCH–RuCl­(CO)­(PiPr3)2] (Ma). Successive one-electron transfer steps were
studied by means of cyclic voltammetry, EPR and UV–vis–NIR–IR
spectroelectrochemistry. These studies show that the first oxidation
mainly involves the central bis­(alkynyl) ruthenium moiety with only
limited effects on the appended vinyl ruthenium moieties. The second
to fourth oxidations (n = 2, 3, 4) involve the entire
carbon-rich conjugated path of the molecule with an increased charge
uniformly distributed between the two arms of the molecules, including
the terminal vinyl ruthenium sites. In order to assess the charge
distribution, we judiciously use 13CO labeled analogues
to distinguish stretching vibrations due to the acetylide triple bonds
and the intense and charge-sensitive Ru­(CO) IR probe in different
oxidation states. The comparison between complex pairs 4a,bn+ (n =
0–3), Aa,bn+ and Va,bn+ (n =
0–2) serves to elucidate the effect of the methoxy donor substituents
on the redox and spectroscopic properties of these systems in their
various oxidation states and on the metal/ligand contributions to
their frontier orbitals.