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Ligands containing the azo group are often used in various metal complexes owing to their facile one-electron reduction, which in effect extends the means of degrading environmentally harmful azo dyes. In order to probe the idea of the generally accepted ease of reduction of azo-containing compounds, we present here three different diruthenium complexes [(acac)(2)Ru-III(mu-L2-)Ru-III(acac)(2)] (diastereomeric 1/2), [(bpy)(2)Ru-II(mu-L2-)Ru-II(bpy)(2)](ClO4)(2) ([3](ClO4)(2)), and [(pap)(2)Ru-II(mu-L2-)Ru-II(pap)(2)](ClO4)(2) ([4](ClO4)(2)) with a bridging ligand (L2- = 1,8-bis((E)-phenyldiazenyl)naphthalene-2,7-dioxido) that contains azo groups in addition to phenoxide-type donors. The Ru-III-Ru-III complexes (1/2) display interesting one-dimensional-chain effects, as revealed by temperature-dependent magnetic studies. The stability of the Ru-III oxidation state in 1/2 under ambient conditions correlates well with the sigma-donating acetylacetonato (acac) coligands. However, with pi-accepting 2,2'-bipyridine (bpy) or phenylazopyridine (pap) the Run state is preferably stabilized in 3(2+) or 4(2+), respectively, but there are interesting differences in their oxidative chemistry. The moderately pi accepting bpy allows for the Ru-II to Ru-III oxidation at reasonably low anodic potentials. However, for the strongly pi accepting pap, no Ru-II to Ru-III oxidation is observed within the solvent window. Instead, a phenoxide to phenoxyl radical type of oxidation based on the bridging ligand is observed. Surprisingly, the reductive chemistry of all three complexes is dominated by either the ruthenium centers or the coligands (bpy or pap), with no reductions observed on the azo function associated with the central bridging ligand (L2-). All of the above conclusions were drawn from combined structural, electrochemical, magnetic, spectroelectrochemical, and DFT investigations. Our results thus conclusively establish that the ease of reduction of an azo group in a particular compound is critically dependent on its substituents and that the noninnocence of the bridging ligands (L2-) in the dinuclear complexes can be decisively tuned by the appropriate choice of ancillary ligands.

After double deprotonation, 2,6-diaryl-p-benzoquinonodiimidazoles (aryl=4-tolyl (I) or 2-pyridyl (II)) were shown to bridge two [Ru(bpy)(2)](2+) (bpy=2,2'-bipyridine) complex fragments through the imidazolate N and p-quinone O (I -> 1(2+)) or through the imidazolate N and pyridyl N donor atoms (II -> 2(2+)). Characterization by crystal structure analysis, H-1/(CNMR)-C-13 spectroscopy, cyclic and differential pulse voltammetry, and spectroelectrochemistry (UV/Vis/NIR, IR, EPR) in combination with TD-DFT calculations revealed surprisingly different electronic structures for redox systems 1(n) and 2(n). Whereas 1(2+) is reduced to a radical complex with considerable semiquinone character, the reduction of 2(2+) with its exclusive N coordination exhibits little spin on the now redox-innocent quinone moiety, compared with the electron uptake by the pyridyl-imidazolate chelating site. The first of two close-lying oxidation processes occurs at the bridging heteroquinone ligand, whereas the second oxidation is partly (1(4+)) or predominantly (2(4+)) centered on the metal atoms.

The complex dication of the diruthenium(II) compound {(mu-tppz)[Ru(bik)Cl](2)}(ClO4)(2) can be oxidized and reduced in two one-electron steps each. In CH3CN/0.1 M Bu4NPF6, the odd-electron intermediates{(mu-tppz)[Ru(bik)Cl](2)}(n+), n = 1 and 3, have comproportionation constants of 7 x 10(8) and 1 x 10(5), respectively. Both exhibit near-infrared absorptions, in the case of n = 3 the 1640 nm band (epsilon = 1200 M-1 cm(-1), Delta nu(1/2) = 1560 cm(-1)) is attributed to an intervalence charge-transfer transition. While the mixed-valent intermediate (n = 3) is EPR silent even at 4 K, the n = 1 form shows g(II) 2.005 and g(perpendicular to) 1.994 at that temperature, signifying a diruthenium(II) complex of the tppz(center dot-) radical anion. The variation of energy and intensity of nu(CO) and of the ring vibration band around 1590 cm(-1) has been monitored not only for {(mu-tppz)[Ru(bik)Cl](2)}(n+), n = 0-4, but also for the mononuclear {(tppz)Ru(bik)Cl}(n+), n = 0-2. In the dinuclear complex the carbonyl stretching bands of the spectator ligand bik are shifted by about 15 cm(-1) on each one-electron-transfer step, increasing with the positive charge. The mixed-valent {(mu-tppz)[Ru(bik)Cl](2)}(3+) shows a perceptibly broader nu(CO) band, suggesting incomplete valence averaging (partial localization).

Ruthenium nitrosyl complexes [Ru(trpy)(L1-4)(NO)](3+) (13-16) [trpy = 2,2':6',2"-terpyridine, L-1 = 2-(2-pyridyl)-benzoxazole, L-2 = 2-(2-pyridyl)benzthiazole, L-3 = 2-(2-pyridyl)benzimidazole, L-4 = 1-methyl-2-(2-pyridyl)-1H-benzimidazole] were obtained in a stepwise manner starting from [Ru-II(trpy)(L1-4) (Cl)]ClO4 (1-4) → [Ru-II(trpy)(L1-4) (H2O)](ClO4)(2) (5-8) → [Ru-II(trpy)(L1-4) (NO2)ClO4 (9-12) → [Ru-II(trpy)(L-1,L-2,L-4) (NO)](ClO4)(3) (13, 14, 16)/[Ru-II(trpy)(L-3) (NO)](Cl)(4))(2)(NO3) (15). Crystal structures of 1, 2, 4, 9, 12, 13, 15, and 16 established the stereoretentive nature of the transformation processes. Though the complexes of L1, L3, and L 4 were isolated in the isomeric form A (π-acceptor trpy and azole ring in the equatorial plane and the pyridine and chloride donors in the axial positions), complexes of L 2 preferentially stabilized in form B (trpy and pyridine in the equatorial plane and the azole ring and chloride donors in the axial positions). The v(NO) stretching frequency varied in the range of 1957-1932 cm(-1), 13 &MGT; 14 &MGT; 15 > 16, primarily depending on the electronic aspects of L as well as the isomeric structural forms. The coordinated nitrosyl function underwent successive reductions of [Ru-II-NO+](3+) → [Ru-II-NO•](2+) and [Ru-II-NO•](2+) - [Ru-II-NO-](+), and the first reduction potential follows the order 14 > 13 &MGT; 15 ≈ 16. The nearly axial EPR spectra having nitrogen hyperfine splittings (A ≈ 26 G) at 77 K of 13(-)-16(-) with (g) ≈ 2.0 established that the reduction process is largely centered around the nitrosyl function. Despite an appreciably high v(NO), the complexes were found to be unusually stable even in the aqueous medium. They transformed slowly and only partially into the corresponding nitro derivatives in H2O (k ≈ 10(-4) s(-1) and K = 0.4-3.8). The chloro (1-4), aqua (5-8), and nitro (9-12) derivatives displayed reasonably strong emissions near 700 nm at 77 K (φ = 10(-1)-10(-2)). The aqua derivative 7 was found to interact with the calf thymus and the circular form of p-Bluescript SK DNA.

The reaction of cis-Ru(acac)(2)(CH3CN)(2) (acac = acetylacetonate) with 2,2'-dipyridylamine (L) in ethanolic medium resulted in facile one-pot synthesis of stable [(acac)(2)Ru-III(L)]ClO4 ([1]ClO4), trans-[(acac)(2)Ru-II(L)2] (2), trans[(acac)(2)Ru-III(L)(2)[ClO4 ([2]ClO4), and cis-[(acac)(2)Ru-II(L)(2)] (3). The bivalent congener 1 was generated via electrochemical reduction Of [1]ClO4. Although in [1](+) the dipyridylamine ligand (L) is bonded to the metal ion in usual bidentate fashion, in 2/[2](+) and 3, the unusual monodentate binding mode of L has been preferentially stabilized. Moreover, in 2/[2](+) and 3, two such monodentate L's have been oriented in the trans- and cis-configurations, respectively. The binding mode of L and the isomeric geometries of the complexes were established by their single-crystal X-ray structures. The redox stability of the Ru(II) state follows the order 1 < 2 much less than 3. In contrast to the magnetic moment obtained for [1]ClO4, mu = 1.84 mu(B) at 298 K, typical for low-spin Ru(III) species, the compound [2]ClO4 exhibited an anomalous magnetic moment of 2.71 mu(B) at 300 K in the solid state. The variable-temperature magnetic measurements showed a pronounced decrease of the magnetic moment with the temperature, and that dropped to 1.59 mu(B) at 3 K. The experimental data can be fitted satisfactorily using eq 2 that considered nonquenched spin-orbit coupling and Weiss constant in addition to the temperature-independent paramagnetism. [1]ClO4 and [2]ClO4 displayed rhombic and axial EPR spectra, respectively, in both the solid and the solution states at 77 K.

The title compounds involving the structurally characterized bridging ligand bpytz were characterized, showing very strong electrochemical stabilization of the mixed-valent (RuRuIII)-Ru-II state (K-c = 10(13.9)) but no detectable (epsilon < 20 M-1 cm(-1)) intervalence charge-transfer band in the infrared region. In situ reduction of the neutral precursor produces a diruthenium(II) complex of the bpytz radical anion according to EPR spectroscopy, whereas oxidation of the mixed-valent form leads to a diruthenium(III) species.