2,5-dioxido-1,4-benzoquinonediimine (H2L2-), a hydrogen-bonding noninnocent bridging ligand related to aminated topaquinone:: Different oxidation state distributions in complexes [{(bpy)2Ru}2(μ-H2L)]n (n=0,+,2+,3+,4+) and [{(acac)2Ru}2(μ-H2L)]m (m=2-,-,0,+,2+)

The symmetrically dinuclear title compounds were isolated as diamagnetic [(bpy)(2)Ru(mu-H2L)Ru(bpy)(2)]- (ClO4)(2) (1-(ClO4)(2)) and as paramagnetic [(acac)(2)Ru(mu-H2L)Ru(acac)(2)] (2) complexes (bpy = 2,2'-bipyridine; acac(-) -, acetylacetonate = 2,4-pentanedionato; H2L=2,5-dioxido-1,4-benzoquinonediimine). The crystal structure of 2(.)2H(2)O reveals an intricate hydrogen-bonding network: Two symmetry-related molecules 2 are closely connected through two NH(H2L2-)O-...- (acac-) interactions, while the oxygen atoms of H2L2- of two such pairs are bridged by an (H2O)(1) cluster at half-occupancy. The cluster consists of cyclic (H2O)(6) arrangements with the remaining two exo-H2O molecules connecting two opposite sides of the cyclo-(H2O)(6) cluster, and oxido oxygen atoms forming hydrogen bonds with the molecules of 2. Weak antiferromagnetic coupling of the two ruthenium(ill) centers in 2 was established by using SQUID magnetometry and EPR spectroscopy. Geometry optimization by means of DFT calculations was carried out for 1(2+) and 2 in their singlet and triplet ground states, respectively. 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 constants K-c > 10(8). UV-visible-NIR spectroelectrochemistry and EPR spectroscopy of the electrogenerated paramagnetic intermediates were used to ascertain the oxidation-state distribution. In general, the complexes 1(n+) prefer the ruthenium(ii) configuration with electron transfer occurring largely at the bridging ligand (mu-H2Ln-), as evident from radical-type EPR spectra for 1(3+) and 1(+). Higher metal oxidation states (111, iv) appear to be favored by the complexes 2(m); intense long-wavelength absorption bands and Ru-III-type EPR signals suggest mixed-valent dimetal configurations of the paramagnetic intermediates 2(+) and 2(-).