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+)

Abstract

The symmetrically dinuclear title compounds were isolated as diamagnetic [(bpy)₂Ru(μ-H₂L)Ru(bpy)₂](ClO₄)₂ (1-(ClO₄)₂) and as paramagnetic [(acac)₂Ru(μ-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 2.2 H₂O reveals an intricate hydrogen-bonding network: Two symmetry-related molecules 2 are closely connected through two NH(H₂L^2-)...O(acac^-) interactions, while the oxygen atoms of H₂L^2- 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 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(III) centers in 2 was established by using SQUID magnetometry and EPR spectroscopy. Geometry optimization by means of DFT calculations was carried out for 1²⁺ 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>10⁸. UV-visible-NIR spectroelectrochemistry and EPR spectroscopy of the electrogenerated paramagnetic intermediates were used to ascertain the oxidation-state distribution. In general, the complexes 1ⁿ⁺ prefer the ruthenium(II) configuration with electron transfer occurring largely at the bridging ligand (μ-H₂L^n-), as evident from radical-type EPR spectra for 1³⁺ and ¹⁺. Higher metal oxidation states (iii, 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^-.