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(H₂L²⁻)⋅⋅⋅O(acac⁻) interactions, while the oxygen 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 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-H₂Ln⁻), as evident from radical-type EPR spectra for 1³⁺ and 1⁺. Higher metal oxidation states (III, IV) appear to be favored by the complexes 2m; intense long-wavelength absorption bands and Ru^III-type EPR signals suggest mixed-valent dimetal configurations of the paramagnetic intermediates 2⁺ and 2⁻.