Substituent-Induced Reactivity in Quinonoid-Bridged Dinuclear Complexes: Comparison between the Ruthenium and Osmium Systems

The ligand 2,5-bis­[2-(methylthio)­anilino]-1,4-benzoquinone
(L) was used in its doubly deprotonated form to synthesize
the complex [{Cl­(η6-Cym)­Os}2(μ-η2:η2-L‑2H)] (1; Cym = p-cymene = 1-isopropyl-4-methylbenzene). Spectroscopic characterization
and elemental analysis confirms the presence of the chloride ligands
in 1, which indirectly shows that the bridging ligand L‑2H acts in a bis-bidentate
fashion in 1, with the thioether substituents on the
bridge remaining uncoordinated. Abstraction of the chloride ligands
in 1 by AgBF4 in CH3CN leads not
only to the release of those chloride ligands but also to a simultaneous
substituent-induced release of Cym with the bridging ligand changing
its coordination mode to bis-tridentate. In the resulting complex
[{(CH3CN)3Os}2(μ-η3:η3-L‑2H)]2+ (22+),
the thioether groups of L‑2H are now coordinated to the osmium centers with the bridging
ligand coordinating to the metal center in a bis-meridional form.
The coordination mode of L‑2H in 22+ was confirmed
by single-crystal X-ray diffraction data. A structural analysis of 22+ reveals localization of double
bonds within the “upper” and “lower” parts
of the bridging ligand in comparison to bond distances in the free
ligand. Additionally, the binding of the bridge to the osmium centers
is seen to occur through O– and neutral imine-type
N donors. The complexes 1 and 22+ were investigated by cyclic voltammetry and UV–vis–near-IR
and EPR spectroelectrochemistry. This combined approach was used to
unravel the redox-active nature of the ligand L‑2H, to determine the sites of electron transfer
(ligand radical versus mixed valency), and to compare the present
systems with their ruthenium analogues 3 and 42+ (Schweinfurth, D. Inorg. Chem. 2011, 50, 1150). The effect of replacing ruthenium by its
higher homologue osmium on the reactivity and the electrochemical
and spectroscopic properties were explored, and the differences were
deciphered by taking into account the intrinsic dissimilarities between
the two homologues. The usefulness of incorporating additional donor
substituents on potentially bridging quinonoid ligands was probed
in this work.