2,5-Dioxido-1,4-benzoquinonediimine (H2L2-), a hydrogen-bonding noninnocent bridging ligand related to aminated topaquinone: Different oxidation state distributions in complexes (bpy)2Ru2(m-H2L)n (n = 0 , +, 2+, 3+, 4+) and (acac)2Ru2(m-H2L)m (m = 2-, -, 0, +, 2+)
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Chemistry - A European Journal 11 (17): 4901--4911 (2005)

The sym. dinuclear title compds. were isolated as diamagnetic (bpy)2Ru(m-H2L)Ru(bpy)2(ClO4)2 (1-(ClO4)2) and as paramagnetic (acac)2Ru(m-H2L)Ru(acac)2 (2) complexes (bpy = 2,2'-bipyridine; acac- = acetylacetonate; H2L = 2,5-dioxido-1,4-benzoquinonediimine). The crystal structure of 2·2H2O reveals an intricate H-bonding network: two symmetry-related mols. 2 are closely connected through two NH(H2L2-)···O-(acac-) interactions, while the O atoms of H2L2- of two such pairs are bridged by an (H2O)8 cluster at half-occupancy. The cluster consists of cyclic (H2O)6 arrangements with the remaining two exo-H2O mols. connecting two opposite sides of the cyclo-(H2O)6 cluster, and oxido O atoms forming H bonds with the mols. of 2. Weak antiferromagnetic coupling of the two Ru(III) centers in 2 was established by using SQUID magnetometry and EPR spectroscopy. Geometry optimization by DFT calcns. was carried out for 12+ and 2 in their singlet and triplet ground states, resp. The nature of low-energy electronic transitions was explored by using time-dependent DFT methods. Five redox states were reversibly accessible for each of the complexes; all odd-electron intermediates exhibit comproportionation consts. Kc \textgreater 108. UV-visible-NIR spectroelectrochem. and EPR spectroscopy of the electrogenerated paramagnetic intermediates were used to ascertain the oxidn.-state distribution. In general, the complexes 1n+ prefer the Ru(II) configuration with electron transfer occurring largely at the bridging ligand (m-H2Ln-), as evident from radical-type EPR spectra for 13+ and 1+. Higher metal oxidn. states (III, IV) appear to be favored by the complexes 2m; intense long-wavelength absorption bands and RuIII-type EPR signals suggest mixed-valent dimetal configurations of the paramagnetic intermediates 2+ and 2-. on SciFinder(R)
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