Tuning of the ionization potential of paddlewheel diruthenium(ii, ii) complexes with fluorine atoms on the benzoate ligands

A series of paddlewheel diruthenium(ii, ii) complexes with various fluorine-substituted benzoate ligands were isolated as THF adducts and structurally characterized: [Ru₂(F_xPhCO₂) ₄(THF)₂] (F_xPhCO₂^- = o-fluorobenzoate, o-F; m-fluorobenzoate, m-F; p-fluorobenzoate, p-F; 2,6-difluorobenzoate, 2,6-F₂; 3,4-difluorobenzoate, 3,4-F ₂; 3,5-difluorobenzoate, 3,5-F₂; 2,3,4-trifluorobenzoate, 2,3,4-F₃; 2,3,6-trifluorobenzoate, 2,3,6-F₃; 2,4,5-trifluorobenzoate, 2,4,5-F₃; 2,4,6-trifluorobenzoate, 2,4,6-F₃; 3,4,5-trifluorobenzoate, 3,4,5-F₃; 2,3,4,5-tetrafluorobenzoate, 2,3,4,5-F₄; 2,3,5,6-tetrafluorobenzoate, 2,3,5,6-F₄; pentafluorobenzoate, F₅). By adding fluorine atoms on the benzoate ligands, it was possible to tune the redox potential (E_1/2) for [Ru₂^II,II]/[Ru₂ ^II,III]⁺ over a wide range of potentials from -40 mV to 350 mV (vs. Ag/Ag⁺ in THF). 2,3,6-F₃, 2,3,4,5-F ₄, 2,3,5,6-F₄ and F₅ were relatively air-stable compounds even though they are [Ru₂^II,II] species. The redox potential in THF was dependent on an electronic effect rather than on a structural (steric) effect of the o-F atoms, although more than one substituent in the m- and p-positions shifted E_1/2 to higher potentials in relation to the general Hammett equation. A quasi-Hammett parameter for an o-F atom (σ_o) was estimated to be ∼0.2, and a plot of E _1/2vs. a sum of Hammett parameters including σ_o was linear. In addition, the HOMO energy levels, which was calculated based on atomic coordinates of solid-state structures, as well as the redox potential were affected by adding F atoms. Nevertheless, a steric contribution stabilizing their static structures in the solid state was present in addition to the electronic effect. On the basis of the electronic effect, the redox potential of these complexes is correlated to the HOMO energy level, and the electronic effect of F atoms is the main factor controlling the ionization potential of the complexes with ligands free from the rotational constraint, i.e. complexes in solution.