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‐BL2–)]n+ (aap = 2‐Arylazopyridine)
Authors: Ghumaan Sandeep , Mukherjee Sriparna , Kar Sanjib , Roy Dipankar , Mobin Shaikh M. , Sunoj Raghavan B. , Lahiri Goutam Kumar 
Published: August 2006

A group of ten quinonoid‐bridged diruthenium(II) complexes [(aap)2RuII(μ‐BL12–)RuII(aap)2](ClO4)2, [1a–1c](ClO4)2 and [(pap)2RuII(μ‐BLn2–)RuII (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}; BL2– = 5,8‐dioxido‐1,4‐napthoquinone (BL12–), 2,3‐dichloro‐5,8‐dioxido‐1,4‐napthoquinone (BL22–), 6,11‐dioxido‐5,12‐naphthacenedione (BL32–), 1,4‐dioxido‐9,10‐anthraquinone (BL42–), 2,3‐dimethyl‐1,4‐dioxido‐9,10‐anthraquinone (BL52–), 6,7‐dichloro‐1,4‐dioxido‐9,10‐anthraquinone (BL62–), 1,4‐diimino‐9,10‐anthraquinone (BL72–), 1,5‐dioxido‐9,10‐anthraquinone (BL82–)] 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 BL12– [C2–O1/C4–O2 1.278(5)/1.291(4) Å] and BL32– [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 BL2–. [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 BL2– 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 (Kc) 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 {RuII(μ‐BL·–)RuII} instead of the usually expected alternative mixed‐valence formulation of {RuII(μ‐BL2–)RuIII}. Consequently, [1–7]3+ display intense near‐infrared transitions in the range 1200–1500 nm with a band width at half height (Δν1/2) 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, {RuII(μ‐BL·–)RuIII}. [1–8]2+ exhibit multiple ligand‐based reductions involving coordinated BL2– 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+. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)