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The diastereoisomeric complexes [[[(acac).sub.2]Ru([mu-QL)Ru[(acac).sub.2]].sup.n] (Q[L.sup.2-] = 1,4-dioxido-9,10-anthraquinone, n = +2, +1, 0, -1, -2) are obtained in neutral form as rac and meso isomers. The metal-ligand valence-state distribution in these complexes is assessed by using ultraviolet-visible-nearinfrared spectroelectrochemistry and EPR methods.

The combination of trpy and coligands in the complex framework of {Ru(trpy)(L)(NO)} (trpy = 2,2':6',2'-terpyridine, L = 2-phenylimidazo[4,5-f]1,10-phenanthroline) has facilitated the stabilization of the nitrosyl group in both the N[O.sup.+] and [NO.sup..] redox states that are reversibly interconvertible via one-electron transfer. The studies have shown that the photoreleased NO is trapped as an Mb-NO adduct.

High-oxidation-state decay of mononuclear complexes [RuTB(H2O)]2+ (X2+, where B = 2,2′-bpy or bpy for X = 1; B = 5,5′-F2-bpy for X = 2; B = 6,6′-F2-bpy for X = 3; T = 2,2′:6′,2″-terpyridine) oxidized with a large excess of CeIV generates a manifold of polynuclear oxo-bridged complexes. These include the following complexes: (a) dinuclear [TB-RuIV-O-RuIV-(T)(O)OH2]2+ (1-dn4+), [TB-RuIII-O-RuIII-T(MeCN)2]4+ (1-dn-N4+), and {[RuIII(trpy)(bpy)]2(µ-O)}4+ (1-dm4+); (b) trinuclear {[RuIII(trpy)(bpy)(µ-O)]2RuIV(trpy)(H2O)}(ClO4)56+ (1-tr6+) and {[RuIII(trpy)(bpy)(µ-O)]2RuIV(pic)2}(ClO4)4 (1-tr-P4+, where P is the 2-pyridinecarboxylate anion); and (c) tetranuclear [TB-RuIII-O-TRuIV(H2O)-O-TRuIV(H2O)-O-RuIII-TB]8+ (1-tn8+), [TB-RuIII-O-TRuIV(AcO)-O-TRuIV(AcO)-O-RuIII-TB]6+ (1-tn-Ac6+), and [TB-RuII-O-TRuIV(MeCN)-O-TRuIV(MeCN)-O-RuII-TB]6+ (1-tn-N6+). These complexes have been characterized structurally by single-crystal X-ray diffraction analysis, and their structural properties were correlated with their electronic structures. Dinuclear complex 1-dm4+ has been further characterized by spectroscopic and electrochemical techniques. Addition of excess CeIV to 1-dm4+ generates dioxygen in a catalytic manner. However, resonance Raman spectroscopy points to the in situ formation of 1-dn4+ as the active species. [ABSTRACT FROM AUTHOR]

The title complexes were obtained in neutral form (n = 0) as rac (1) and meso isomers (2). 2 was crystallized for X-ray diffraction and its temperature-dependent magnetism studied. It contains two antiferromagnetically coupled ruthenium(Ill) ions, bridged by the quinizarine dianion QL2- (quinizarine = 1,4-dihydroxy-9,10-anthraquinone). The potential of both the ligand (QL0 → QL4-) and the metal complex fragment combination [(acac)2RuII]2 → {[(acac)2RuIV]2}4+ to exist in five different redox states creates a large variety of combinations, which was assessed for the electrochemically reversibly accessible 2+, 1+, 0, 1-, 2- forms using cyclic voltammetry as well as EPR and UV-vis-NIR spectroelectrochemistry. The results for the two isomers are similar: Oxidation to 1+ or 2+ causes the emergence of a near-infrared band (1390 nm), without revealing an EPR response even at 4 K. Reduction to 1- or 2- produces an EPR signal, signifying metal-centered spin but no near-infrared absorption. Tentatively, we assume metal-based oxidation of [(acac)2RuIII(μ-QL2-)RuIII(acac)2] to a mixed-valent intermediate [(acac)2RuIII(μ-QL2-)RuIV(acac)2]+ and ligand-centered reduction to a radical complex [(acac)2RuIII(μ-QL · 3-)RuIII(acac)2]- with antiferromagnetic three-spin interaction. [ABSTRACT FROM AUTHOR]

Nitrosyl complexes with {Ru-NO}6 and {Ru-NO}7 configurations have been isolated in the framework of [Ru(trpy)(L)(NO)]n+ [trpy = 2,2′:6′,2″-terpyridine, L = 2-phenylimidazo[4,5-f]1,10-phenanthroline] as the perchlorate salts [4](ClO4)3 and [4](ClO4)2, respectively. Single crystals of protonated material [4-H+](ClO4)4 · 2H2O reveal a RuNO bond angle of 176.1(7)° and triply bonded NO with a 1.127(9) Å bond length. Structures were also determined for precursor compounds of [4]3+ in the form of [Ru(trpy)(L)(Cl)](ClO4) · 4.5H2O and [Ru(trpy)-(L-H)(CH3CN)](ClO4)3 · H2O. In agreement with largely NO centered reduction, a sizable shift in ν(NO) frequency was observed on moving from [4]3+ (1953 cm-1) to [4]2+ (1654 cm-1). The RuII-NO• in isolated or electrogenerated [4]2+ exhibits an EPR spectrum with g1 = 2.020, g2 = 1.995, and g3 = 1.884 in CH3CN at 110 K, reflecting partial metal contribution to the singly occupied molecular orbital (SOMO); 14N (NO) hyperfine splitting (A2 = 30 G) was also observed. The plot of ν(NO) versus E°({RuNO}6 → {RuNO}7) for 12 analogous complexes [Ru(trpy)(L′)(NO)]n+ exhibits a linear trend. The electrophilic Ru-NOt species [4]3+ is transformed to the corresponding Ru-NO2-system in the presence of OH- with k = 2.02 × 10-4 s-1 at 303 K. In the presence of a steady flow of dioxygen gas, the RuII-NO• state in [4]2+ oxidizes to [4]3+ through an associatively activated pathway (ΔS‡ = -190.4 J K-1 M-1) with a rate constant (k [s-1]) of 5.33 × 10-3. On irradiation with light (Xe lamp), the acetonitrile solution of paramagnetic [Ru(trpy)(L)(NO)]2+ ([4]2+) undergoes facile photorelease of NO (kNO = 2.0 × 10-1 min-1 and t1/2 ≈ 3.5 min) with the concomitant formation of the solvate [RuII(trpy)(L)(CH3CN)]2+ [2′]2+. The photoreleased NO can be trapped as an Mb-NO adduct. [ABSTRACT FROM AUTHOR]

Reaction of 3,6-diaryl-1,2,4,5-tetrazines (aryl = R = phenyl, 2-furyl or 2-thienyl) with 2 equiv of Ru(acac)2(CH3CN)2 results in reductive tetrazine ring opening to yield diruthenium complexes [(acac)2RuIII(dih-R²-)RuIII(acac)2] bridged by the new 1,2-diiminohydrazido(2-) (dih-R2- = HNC(R)NNC(R)NH2-) ligands, rac/meso diastereoisomers could be detected and separated for the compounds with R = phenyl and 2-thienyl, all species are diamagnetic and were characterized by ¹H NMR spectroscopy. Crystal structure determination of the meso isomers with R = phenyl and 2-thienyl confirmed the 1,2-diiminohydrazido formulation through long N-N (≈1.40 A) and short C=N(H) bonds (≈1.31 Å), implying two bridged ruthenium(III) centers at about 4.765 Å distance with strong antiferromagnetic coupling. The complexes undergo two reversible and well-separated one-electron reduction and oxidation processes, respectively. EPR Spectroscopy of the paramagnetic intermediates with comproportionation constants Kc > 1012 and UV-vis-NIR spectroelectrochemistry were used to identify the accessible redox states as [(acac)2RuII(dih-R2-)RuII(acac)2]2-, [(acac)2RuII(dih-R•-)RuII(acac)2]-, [(acac)2RuIII(dih-R2-)RuIII(acac)2], [(acac)2RuIII(dih-R•-)RuIII(acac)2]+, and [(acac)2RuIII(dih-R)RuIII(acac)2]2+. While the UV-vis-NIR spectroscopic response of [(acac)2Ru(dih-R)Ru(acac)2]0/-/2- is very similar to that of [(bpy)2Ru(adc-R)Ru(bpy)2]4+/3+/2+, adc-R2- = 1,2-diacylhydrazido(2-), the EPR result indicating ligand-centered spin for [(acac)2RuII(dih-R•-)RuII(acac)2]- despite deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures. [ABSTRACT FROM AUTHOR]

The title complexes were obtained in neutral form (n = 0) as rac (1) and meso isomers (2). 2 was crystallized for X-ray diffraction and its temperature-dependent magnetism studied. It contains two antiferromagnetically coupled ruthenium(III) ions, bridged by the quinizarine dianion QL(2-) (quinizarine = 1,4-dihydroxy-9,10-anthraquinone). The potential of both the ligand (QLo --> QL4-) and the metal complex fragment combination [(acac)2RuII]2 --> ([(acac)2RuIV]2)4+ to exist in five different redox states creates a large variety of combinations, which was assessed for the electrochemically reversibly accessible 2+, 1+, 0, 1-, 2- forms using cyclic voltammetry as well as EPR and UV-vis-NIR spectroelectrochemistry. The results for the two isomers are similar: Oxidation to 1+ or 2+ causes the emergence of a near-infrared band (1390 nm), without revealing an EPR response even at 4 K. Reduction to 1- or 2- produces an EPR signal, signifying metal-centered spin but no near-infrared absorption. Tentatively, we assume metal-based oxidation of [(acac)2RuIII(mu-QL2-)RuIII(acac)2] to a mixed-valent intermediate [(acac)2RuIII(mu-QL2-)RuIV(acac)2]+ and ligand-centered reduction to a radical complex [(acac)2RuIII(mu-QL.3-)RuIII(acac)2 (-) with antiferromagnetic three-spin interaction.