An experimental and density functional theory approach towards the establishment of preferential metal- or ligand-based electron-transfer processes in large quinonoid-bridged diruthenium complexes [{(aap)₂ru}₂(μ-bl^2–)]ⁿ⁺ (aap = 2-arylazopyridine)

Abstract

A group of ten quinonoid-bridged diruthenium(II) complexes [(aap)₂Ru^II(μ-BL₁^2–)Ru^II(aap)₂](ClO₄)₂, [1a–1c](ClO₄)₂ and [(pap)₂Ru^II(μ-BL_n^2–)Ru^II (pap)₂](ClO₄)₂ [2–8](ClO₄)₂ [aap = 2-arylazopyridine, NC₅H₄–N = N–C₆H₄(R) {R = H (pap) [1a](ClO₄)₂, m-Me [1b](ClO₄)₂, m-Cl [1c](ClO₄)₂}; BL^2– = 5,8-dioxido-1,4-napthoquinone (BL₁^2–), 2,3-dichloro-5,8-dioxido-1,4-napthoquinone (BL₂^2–), 6,11-dioxido-5,12-naphthacenedione (BL₃^2–), 1,4-dioxido-9,10-anthraquinone (BL₄^2–), 2,3-dimethyl-1,4-dioxido-9,10-anthraquinone (BL₅^2–), 6,7-dichloro-1,4-dioxido-9,10-anthraquinone (BL₆^2–), 1,4-diimino-9,10-anthraquinone (BL₇^2–), 1,5-dioxido-9,10-anthraquinone (BL₈^2–)] have been synthesized. The crystal structures of [1a](ClO₄)₂•H₂O and [3](ClO₄)₂ suggest the preferential crystallization of the meso isomer in both cases. The two similar C–O distances in coordinated BL₁^2– [C2–O1/C4–O2 1.278(5)/1.291(4) Å] and BL₃^2– [C2–O1/C4–O2 1.282(7)/1.280(7) Å] in [1a](ClO₄)₂ and [3](ClO₄)₂, respectively and the corresponding intraring distances suggest a delocalized keto-enol state of the coordinated BL^2–. [1–8]²⁺ exhibit two successive one-electron oxidation processes and multiple reductions in both CH₃CN and CH₂Cl₂. The first oxidation potential varies substantially depending on a variety of factors associated with BL^2– and follows the order: [8]²⁺ ≫ [2]²⁺ > [6]²⁺ > [1a]²⁺ ≥ [4]²⁺ > [3]²⁺ > [5]²⁺ ≫ [7]²⁺. The separation in potentials between the successive oxidation processes translates to comproportionation constant (K_c) values in the ranges 2.5 × 10⁴ – 2.6 × 10⁵ and 1.7 × 10³ – 1.3 × 10⁶ in CH₃CN and CH₂Cl₂, respectively. The intermediate paramagnetic species [1–8]³⁺ systematically exhibit closely spaced rhombic or axial-type EPR spectra at 77 K corresponding to g values close to the free-electron value of 2.0023, thereby suggesting a radical complex formulation of {Ru^II(π-BL^–)Ru^II} instead of the usually expected alternative mixed-valence formulation of {Ru^II(π-BL^2–)Ru^III}. Consequently, [1–7]³⁺ display intense near-infrared transitions in the range 1200–1500 nm with a band width at half height (Δν_1/2) of 1900–3800 cm^–1 which is lower than the calculated value of 3800–4600 cm^–1 obtained using the Hush formula for a localized class II mixed-valence system. Electrogenerated EPR-inactive second-step oxidized species [1–8]⁴⁺ have been described as spin-coupled radical-bridged mixed-valence ruthenium (II)(III) species, {Ru^II(π-BL^–)Ru^III}. [1–8]²⁺ exhibit multiple ligand-based reductions involving coordinated BL^2– as well as aap. The above preferential metal- or ligand-based accessible electron-transfer processes in the complexes have been further substantiated by DFT calculations on the geometry-optimized structure of [1a]²⁺.