The effect of chlorine and fluorine substitutions on tuning the ionization potential of benzoate-bridged paddlewheel diruthenium(II, II) complexes

A series of paddlewheel diruthenium(II, II) complexes with various chlorine-substituted benzoate ligands (Cl-series) was synthesized as tetrahydrofuran (THF) adducts [Ru₂(Cl_xPhCO₂)₄(THF)₂]; where Cl_xPhCO₂^- = o-chlorobenzoate, o-Cl; m-chlorobenzoate, m-Cl; p-chlorobenzoate, p-Cl; 2,3-dichlorobenzoate, 2,3-Cl₂; 2,4-dichlorobenzoate, 2,4-Cl₂; 2,5-dichlorobenzoate, 2,5-Cl₂; 2,6-dichlorobenzoate, 2,6-Cl₂; 3,4-dichlorobenzoate, 3,4-Cl₂; 3,5-dichlorobenzoate, 3,5-Cl₂; 2,3,4-trichlorobenzoate, 2,3,4-Cl₃; 2,3,5-trichlorobenzoate, 2,3,5-Cl₃; 2,4,5-trichlorobenzoate, 2,4,5-Cl₃; 3,4,5-trichlorobenzoate, 3,4,5-Cl₃; 2,3,4,5-tetrachlorobenzoate, 2,3,4,5-Cl₄. This Cl-series and the previously synthesized F-series together with four new fluorine-substituted derivatives, [Ru₂(F_xPhCO₂)₄(THF)₂] (where F_xPhCO₂^- = 2,3-difluorobenzoate, 2,3-F₂; 2,4-difluorobenzoate, 2,4-F₂; 2,5-difluorobenzoate, 2,5-F₂; 2,3,5-trifluorobenzoate, 2,3,5-F₃), were experimentally characterized with respect to solid-state structure, magnetic properties and electrochemistry. By tuning the substituents of the benzoate ligands using chlorine or fluorine atoms, the redox potential (E_1/2) for [Ru₂^II,II]/[Ru₂^II,III]⁺ varied over a wide range of potentials from -40 mV to 360 mV (vs. Ag/Ag⁺ in THF). This was dependent on (i) the number of ortho-substituents, i.e. non-, mono- and di-o-substituted groups, with quasi-Hammett parameters for ortho-Cl and -F substitutions (σ_o = -0.272 and -0.217, respectively) and (ii) the general Hammett constants, σ_m and σ_p, for each group. The HOMO energy level calculated on the basis of the atomic coordinates of the solid-state structure was strongly affected by Cl- and F-substitutions as well as the redox potential in solution, which emphasizes the steric contribution of ortho-substituents in the energy level giving a deviation of E_HOMO < 0.3 eV and <0.55 eV for the Cl- and F-series, respectively.