Abstract
The quinonoid ligand-bridged diruthenium compds. [(acac)2Ru(m-L2-)Ru(acac)2] (acac- = acetylacetonato = 2,4-pentanedionato; L2- = 2,5-dioxido-1,4-benzoquinone, 1; 3,6-dichloro-2,5-dioxido-1,4-benzoquinone, 2; 5,8-dioxido-1,4-naphthoquinone, 3; 2,3-dichloro-5,8-dioxido-1,4-naphthoquinone, 4; 1,5-dioxido-9,10-anthraquinone, 5; and 1,5-diimido-9,10-anthraquinone, 6) were prepd. and characterized anal. The crystal structure anal. of 5 in the rac configuration reveals two tris(2,4-pentanedionato)ruthenium moieties with an extended anthracenedione-derived bis(ketoenolate) p-conjugated bridging ligand. The weakly antiferromagnetically coupled RuIII(μ-L2-) RuIII configuration in 1-6 exhibits complicated overall magnetic and EPR responses. The simultaneous presence of highly redox-active quinonoid-bridging ligands and of two ruthenium centers capable of adopting the oxidn. states +2, +3, and +4 creates a large variety of possible oxidn. state combinations. Accordingly, the complexes 1-6 exhibit two reversible one-electron oxidn. steps and at least two reversible redn. processes. Shifts to pos. potentials were obsd. on introduction of Cl substituents (1$\rightarrow$2, 3$\rightarrow$4) or through replacement of NH by O(6$\rightarrow$5). The ligand-to-metal charge transfer (LMCT) absorptions in the visible region of the neutral mols. become more intense and shifted to lower energies on stepwise redn. with two electrons. On oxidn., the para-substituted systems 1-4 exhibit monocation intermediates with intervalence charge transfer (IVCT) transitions of RuIIIRuIV mixed-valent species. In contrast, the differently substituted systems 5 and 6 show no such near IR (NIR) absorption. While the first redn. steps are thus assigned to largely ligand-centered processes, the oxidn. appears to involve metal-ligand delocalized MOs with variable degrees of mixing. [on SciFinder(R)]
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