1,5-Diamido-9,10-Anthraquinone, A Centrosymmetric Redox-Active Bridge With Two Coupled Beta-Ketiminato Chelate Functions: Symmetric And Asymmetric Diruthenium Complexes

The dinuclear complexes {(mu-H2L)[Ru(bpy)(2)](2)}(ClO4)(2) ([3] (ClO4)(2)), {(mu-H2L)[Ru(pap)(2)](2)}(ClO4)(2) ([4](ClO4)(2)), and the asymmetric [(bpy)(2)Ru(mu-H2L)Ru(pap)(2)]-(ClO4)(2) ([5](ClO4)(2)) were synthesized via the mononuclear species [Ru(H3L)(bpy)(2)]ClO4 ([1]ClO4) and [Ru(H3L)(pap)(2)]ClO4 ([2]ClO4), where H4L is the centrosymmetric 1,5-diamino-9,10-anthraquinone, bpy is 2,2'-bipyridine, and pap is 2-phenylazopyridine. Electrochemistry of the structurally characterized [1]ClO4, [2]ClO4, [3](ClO4)(2), [4](ClO4)(2), and [5] (ClO4)(2) reveals multistep oxidation and reduction processes, which were analyzed by electron paramagnetic resonance (EPR) of paramagnetic intermediates and by UV vis NIR spectro-electrochemistry. With support by time-dependent density functional theory (DFT) calculations the redox processes could be assigned. Significant results include the dimetal/bridging ligand mixed spin distribution in 3(3+) versus largely bridge-centered spin in 4(3+)-a result of the presence of Ru-II-stabilizig pap coligands. In addition to the metal/ligand alternative for electron transfer and spin location, the dinuclear systems allow for the observation of ligand/ligand and metal/metal site differentiation within the multistep redox series. DFT-supported EPR and NIR absorption spectroscopy of the latter case revealed class II mixed-valence behavior of the oxidized asymmetric system 5(3+) with about equal contributions from a radical bridge formulation. In comparison to the analogues with the deprotonated 1,4-diaminoanthraquinone isomer the centrosymmetric H2L2- bridge shows anodically shifted redox potentials and weaker electronic coupling between the chelate sites.