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The newly designed electrically neutral complexes [Ru(acac)(2)(Q)] (1-3) involving redox-active polycyclic aromatic hydrocarbon (PAH) derived quinonoids (Q): Q(1)((O,O)) (1) Q(2)((O,NH)) (2) Q(2)((NH,NH)) (3) (acac = acet- ylacetonate) were prepared from the metal precursor [Ru-II(acac)(2)(CH3CN)(2)] and preformed pyrene-4,5-dione (Q(1)) and partially deprotonated pyrene-4,5-diamine (H(4)Q(2)), respectively. The structural characterization of 1-3 established their molecular identities including intermolecular pi-pi stacking interactions between the extended pi-system of pyrene in the adjacent molecules and the hydrogen bonded 1D-polymeric form of 3. The redox sensitive C-O and C-N bond distances of Q in 1, 2 and 3 revealed the dominating ground state electronic forms of [(acac)(2)Ru-III-Q(1)((O,O))(center dot-)] (S = 0), [(acac)(2)Ru-III-Q(2)((O,NH)center dot-)] (S = 0) and [(acac)(2)Ru(II)Q(2)((NH,NH)o)] (S = 0), respectively, where strong antiferromagnetic coupling between Ru-III(t(2g)(5)) and Q(center dot-) resulted in S = 0 state in 1 or 2. Complexes 1-3 exhibited reversible single oxidation and reduction within the potential window of +/- 1.5 V versus SCE in CH3CN, which progressively shifted to the negative potential on moving from 1 to 2 to 3, primarily due to the difference in electronegativity between O and N donors of Q. The collective consideration of experimental (EPR, electronic spectra) and theoretical (DFT, TD-DFT) results of 1(n)-3(n) (n = +1, 0, -1) revealed (i) extensive mixing of metal-ligand orbitals due to the inherent covalency factor and (ii) Q(center dot-) and Ru-II based oxidations of 1/2 and 3, respectively, led to the {Ru-III-Q(o)} electronic form at the metal-ligand interface of the oxidized state (1(-).3(-)), while the reduced state (1(-)-3(-)) could best be described by the resonating form of {Ru-II-Q(center dot-)} {Ru-III-Q(2-)}.

The doubly deprotonated form L2- of indigo=H2L can bind two [Ru(acac)(2)] complex fragments in the cis (1) and trans configuration (2), as evidenced from crystal structure analysis. While the latter type of N,O; N',O' coordination has been observed earlier, for example, with [Ru(bpy)(2)](2+), leading to two equivalent six-membered ring chelates, the cis arrangement in 1 is observed here for the first time in a dinuclear complex, producing one five-membered ring chelate with N,N-' coordination and one seven-membered chelate with O,O-' coordination. The different structures of the isomers result in differing electrochemical and spectroelectrochemical (EPR, UV-Vis-NIR) responses for various accessible charge states 1(n) and 2(n), n=-, 0, +, 2+. The associated electronic structures were analyzed by DFT (structures, spin density) and TD-DFT calculations (electronic transitions), revealing mainly metal-based reduction but largely indigo ligand-based oxidation of both neutral precursors.

Thermolysis of (Z)-1-ferrocenyltelluro-1-ferrocenyl-4-ferrocenyl-1-buten-3-yne with [Os-3(CO)(12)] in toluene gave two new clusters: non-planar six-membered metallacycle complex [Os3H(CO)(7)(mu(3)-Te){(1):(1):(2):(2)-FcCCC(H)(Fc)(2)}] (1, Fc = ferrocenyl) and five-membered cyclometallated ring complex [Os-3(CO)(7)(mu(3)-Te){(1):(1):(3)-FcC(H)C(H)CC(Fc)(2)}] (2). Compounds 1 and 2 may be thought to form by orthometallation of ferrocene with a closed Os-3 triangle by intramolecular rearrangement of atoms within the cluster complexes. Typically the formation of a non-planar metallabenzene such as 1 may involve the orthometallation of another aromatic ring. Also, conversion of 1 to 2 shows the initial step required for alkene hydrogenation by a tellurium-bridged triosmium cluster. Both compounds were characterized by IR and H-1 NMR spectroscopy, and their structures established by single-crystal XRD.

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.

Synthetic organic chemistry has been revolutionized by transition metal mediated C-H activation in the last few decades. Ortho C-H activation has shown widespread growth in this regard primarily due in fact to the favourable 5-6 membered metallacycle required to direct the ortho C-H functionalization. Reaching out to the distal positions thus require special improvised techniques. In this regard, the use of directing group in reaching out to the para position has acquired a special place in the field of C-H activation in particular and synthetic organic chemistry in general. As such, the efficient use of such directing groups to reach out to the distal para position are listed in this review, thereby providing an alternative to withstanding general para electrophilic/nucleophilic reactions in synthetic organic chemistry.

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 article examines the newly designed and structurally characterized redox-active BIAN-derived [Ru(trpy)(R-BIAN)Cl]ClO4 ([1a]ClO4-[1c]ClO4), [Ru(trpy)(R-BIAN)(H2O)](ClO4)(2) ([3a] (ClO4)(2)-[3c](ClO4)(2)), and BIAO-derived [Ru(trpy)(BIAO)Cl]ClO4 ([2a]ClO4) (trpy = 2,2':6',2 ''-terpyridine, R-BIAN = bis(arylimino)acenaphthene (R = H (1a(+), 3a(2+)), 4-OMe (1b(+), 3b(2+)), 4-NO2 (1c(+), 3c(2+)), BIAO = [N-(phenyl)imino]acenapthenone). The experimental (X-ray, H-1 NMR, spectroelectrochemistry, EPR) and DFT/TD-DFT calculations of 1a(n)-1c(n) or 2a(n) collectively establish {Ru-II-BIAN(0)} or {Ru-II- BIAO(0)} configuration in the native state, metal-based oxidation to {Ru-III-BIAN(0)} or {Ru-III-BIAO(0)}, and successive electron uptake processes by the alpha-diimine fragment, followed by trpy and naphthalene pi-system of BIAN or BIAO, respectively. The impact of the electron-withdrawing NO2 function in the BIAN moiety in 1c(+) has been reflected in the five nearby reduction steps within the accessible potential limit of-2 V versus SCE, leading to a fully reduced BIAN(4-) state in [1c](4-). The aqua derivatives ({Ru-II-OH2}, 3a(2+)-3c(2+)) undergo simultaneous 2e(-)/2H(+) transfer to the corresponding {Ru-IV=0} state and the catalytic current associated with the Ru-IV/Ru-V response probably implies its involvement in the electrocatalytic water oxidation. The aqua derivatives (3a(2+)-3c(2+)) are efficient and selective precatalysts in transforming a wide variety of alkenes to corresponding epoxides in the presence of PhI(OAc)(2) as an oxidant in CH2Cl2 at 298 K as well as oxidation of primary, secondary, and heterocyclic alcohols with a large substrate scope with H2O2 as the stoichiometric oxidant in CH3CN at 343 K. The involvement of the {Ru-IV=0} intermediate as the active catalyst in both the oxidation processes has been ascertained via a sequence of experimental evidence.

The neutral title complexes [(L(1-3))ClRu(II)(mu-tppz)Ru(II)Cl(L(1-3))] [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine with L(1) = 2-picolinate, L(2) = 2-quinolinecarboxylate (quinaldate) and with the hitherto little used L(3) = 8-quinolinecarboxylate] have been structurally characterized as approximately anti- (1 and 3) and syn-configured isomers (2) with L ligand N (1 and 3) or 0 atoms (2) trans to the pyrazine N atoms of tppz. In contrast to 1 and 2 with five-membered chelate rings, complex 3 (which is isomeric with 2) contains six-membered chelate rings. Each system 1-3 thus features a significantly different coordination situation, and all complexes exhibit a considerably distorted tppz bridge, including a twisted central pyrazine ring. In spite of this, double one-electron reduction of the bridge is always possible, as is evident from electron paramagnetic resonance (EPA) and UV/vis spectroelectrochemistry. Two separate (Delta E similar to 0.4 V and K(c) similar to 10(7)) one-electron oxidations occur on the metals, producing invariably EPA-silent (4 K) Ru(III)Ru(II) intermediates with moderately intense near-IR absorptions, ranging from 1500 to 1900 nm. IA spectroelectrochemistry of the carboxylato carbonyl stretching bands did not result in any noticeable shift, in contrast to what was observed with dipyridyl ketones and related coligands. Density functional theory (DFT)/time-dependent OFT calculations confirm the experimental structures and explain the noted spectroscopic trends: destabilized and closer-spaced frontier orbitals for 3 over 2, with the comparison to 1 suggesting that the configuration is a major factor. Nevertheless, the rather unperturbed electronic structure of the [Ru(mu-tppz)Ru](n) entity, despite different coordination situations at the metal sites, is remarkable and suggests further use of this entity as a robust, carboxylate-tolerant redox-active platform in extended frameworks.

Nitrosylruthenium derivatives having NO+ and NO. states have been synthesized in a stepwise manner starting from [Ru-II(trpy)(pdt)(Cl)](ClO4) {[1](ClO4)} -> [Ru-II(trpy)(pdt)(CH3CN)](ClO4)(2) {[2](ClO4)(2)} -> [Ru-II(trpy)(pdt)(NO2)](ClO4) {[3](ClO4)} -> [Ru-II(trpy)(pdt)(NO+)](ClO4)(3) {[4](ClO4)(3)} -> [Ru-II(trpy)(pdt)(NO.)](ClO4)(2) {[4](ClO4)(2)} [trpy = 2,2':6',2 ''-terpyridine, pdt = 5,6-diphenyl-3-pyridyl-as-triazine]. The molecular identity of [1](ClO4) and [2](ClO4)(2), and the subsequent stereoretentive transformation of [1](ClO4) -> [2](ClO4)(2) have been authenticated by single-crystal X-ray structures. Electrochemical and spectral features are investigated as a function of the monodentate ligands (Cl-, CH3CN, NO2-, NO+, NO.). The kinetic and thermodynamic aspects of the reaction of the moderately strong electrophilic {Ru-II-NO+} center [v(NO): 1944 cm(-1)] in [4](3+) with a nucleophile such as OH- have been studied. The nitrosyl species [Ru-II(trpy)(pdt)(NO+)](3+) ([4](3+)) can be selectively reduced to [Ru-II(trpy)(pdt)(NO.)](2+) ([4](2+)) electrochemically as well as chemically by hydrazine hydrate. On exposure to light an acetonitrile solution of [Ru-II(trpy)(pdt)(NO.)](2+) ([4](2+)) undergoes slow photocleavage of the Ru-II-NO bond over a period of 4 h, resulting in the corresponding solvated species [Ru-II(trpy)(pdt)(CH3CN)](2+) ([2](2+)). The rate of photolability of the Ru-II-NO bond in [4](2+) has been monitored spectrophotometrically. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).

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 bis-bidentate bridging function of gbha(2-) with N,O-/N,O- coordination was observed for the first time in the complex (mu-gbha)[Ru-III(acac)(2)](2) (1). Density functional theory calculations of 1 yield a triplet ground state with a large (Delta E > 6000 cm(-1)) singlet-triplet gap. Intermolecular antiferromagnetic coupling was observed (J approximate to -5.3 cm-1) for the solid. Complex 1 undergoes two one-electron reduction and two one-electron oxidation steps; the five redox forms {(mu-gbha)[Ru(acac)2]2}(n) (n = -2, -1, 0, +1, +2) were characterized by UV-vis-NIR spectroelectrochernistry (NIR = near infrared). The paramagnetic intermediates were also investigated by electron paramagnetic resonance (EPR) spectroscopy. The monoanion with a comproportionation constant K-c of 2.7 x 10(8) does not exhibit an NIR band for a Ru-III/Ru-II mixed-valent situation; it is best described as a 1,4-diazabutadiene radical anion containing ligand gbha(center dot 3-), which binds two ruthenium(III) centers. A Ru-III-type EPR spectrum with g(1) = 2.27, g(2) = 2.21, and g(3) = 1.73 is observed as a result of antiferromagnetic coupling between one Ru-III and the ligand radical. The EPR-active monocation (K-c = 1.7 x 10(6)) exhibits a broad (Delta v(1/2) = 2600 cm(-1)) intervalence charge-transfer band at 1800 nm, indicating a valence-averaged (Ru-3.5)(2) formulation (class III) with a tendency toward class II (borderline situation).

The complex framework [Ru(tpy)(dpk)](2+) has been used to study the generation and reactivity of the nitrosyl complex [Ru(tpy)(dpk)(NO)(3+) ([4](3+)). Stepwise conversion of the chloro complex [Ru(tpy)(dpk)(Cl)](+) ([1](+)) via [Ru(tpy)(dpk)(CH3CN)](2+) ([2](2+)) and the nitro compound [Ru(tpy)(dpk)(NO2)](+) ([3](+)) yielded [4](3+); all four complexes were structurally characterized as perchlorates. Electrochemical oxidation and reduction was investigated as a function of the monodentate ligand as was the IR and UV-vis spectroscopic response (absorption/emission). The kinetics of the conversion [4](3+)/[3](+) in aqueous environment were also studied. Two-step reduction of [4](3+) was monitored via EPR, UV-vis, and IR (v(NO), v(CO)) spectroelectrochemistry to confirm the {RuNO}(7) configuration of [4](2+) and to exhibit a relatively intense band at 505 nm for [4](+), attributed to a ligand-to-ligand transition originating from bound NO-.

Crystallographically characterised 3,6-bis(2'-pyridyl) pyridazine (L) forms complexes with {(acac)(2)Ru} or {(bpy)(2)Ru2+} via one pyridyl-N/pyridazyl-N chelate site in mononuclear Ru-II complexes (acac)(2)Ru(L), 1, and [(bpy)(2)Ru(L)] (ClO4)(2), [3] (ClO4)(2). Coordination of a second metal complex fragment is accompanied by deprotonation at the pyridazyl-C-5 carbon {L --> (L-H+)} to yield cyclometallated, asymmetrically bridged dinuclear complexes [(acac)(2)Ru-III (mu-L-H+) Ru-III (acac)(2)] (ClO4), [2] (ClO4), and [(bpy)(2)Ru-II (mu- L-H+) Ru-II (bpy)(2)] (ClO4)(3), [4] (ClO4)(3). The different electronic characteristics of the co-ligands, sigma donating acac(-) and pi accepting bpy, cause a wide variation in metal redox potentials which facilitates the isolation of the diruthenium(III) formin [2] (ClO4) with antiferromagnetically coupled Ru-III centres (J = 11.5 cm(-1)) and of a luminescent diruthenium(II) species in [4] (ClO4)(3). The electrogenerated mixed-valent (RuRuIII)-Ru-II states 2 and [4](4+) with comproportionation constants K-c > 10(8) are assumed to be localised with the Ru-III ion bonded via the negatively charged pyridyl-N/pyridazyl-C-5 chelate site of the bridging (L-H+)(-) ligand. In spectroelectrochemical experiments they show similar intervalence charge transfer bands of moderate intensity around 1300 nm and comparable g anisotropies (g(1)-g(3) approximate to 0.5) in the EPR spectra. However, the individual g tensor components are distinctly higher for the pi acceptor ligated system [4](4+), signifying stabilised metal d orbitals.

The trinuclear complexes [{(L-'/"')(2)Ru-II}(3)(mu(3)-L)](ClO4)(3), [1](ClO4)(3)-[3](ClO4)(3){L=trianionic form of 1,3,5-triazine-2,4,6-trithiol; NpC5H4N=N-a-C6H4(R), R=H (L'), m-Me (L"), p-Me (L"')} and the analogous mononuclear complex [(L')(2)Ru-II(NC5H4S-)]ClO4 [4] ClO4 were synthesized. Crystal structures of [1](ClO4)(3) and [4] ClO4 were determined. [1](3+)-[3](3+) exhibit three successive oxidative couples corresponding to Ru(II)Ru(II)Ru(III)reversible arrowRu(II)Ru(II)Ru(II); Ru(II)Ru(III)Ru(III)reversible arrowRu(II)Ru(II)Ru(III); Ru(III)Ru(III)Ru(III)reversible arrowRu(II)Ru(III)Ru(III) where the mixed valent states are moderately coupled. The complexes display multiple reductions associated with the azo functions of the ancillary ligands (L'-"'). The energy of the Ru-II-based lowest energy MLCT transitions (533-558 nm) involving the pi* level of azoimine chromophore of L'-"' varies depending on the nuclearity as well as substituents in the ligand framework and follows the order: [1](3+)>[2](3+)>[3](3+)>[4](+). The complexes exhibit reasonably high third-order non-linear optical properties with gamma=(0.90-2.45)x10(-29) esu. The interactions of the trinuclear complexes [{(L')(2)Ru-II}(3)(mu(3)-L)](3+) [1](3+), [{(bpy)(2)Ru-II} 3(mu(3)-L)](3+) [5](3+) and [{(phen)(2)Ru-II}(3)(mu(3)-L)](3+) [6](3+) (bpy=2,2'-bipyridine and phen=1,10-phenanthroline) with the circular and linear forms of p-Bluescript DNA show reduced ethidium bromide fluorescence on gel electrophoresis.

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.

The triruthenium complexes [{(bpy)(2)Ru-II}(3)L](3+) [1](3+) and [{(phen)(2)Ru-II}(3)L](3+) [2](3+) have been synthesized via the reactions of [Ru-II(bpy)(2)(EtOH)(2)](2+) and [Ru-II(phen)(2)(EtOH)(2)](2+) with the trisodium salt of 1,3,5-triazine-2,4,6 trithiol (Na3L) respectively. In CH3CN, the complexes [1](3+) and [2](3+) exhibit three reversible one-electron redox processes corresponding to successive Ru(II)/ Ru(III) couples. The 190 - 250 mV separation in potential between the successive Ru(II)/ Ru(III) couples is indicative of moderate intermetallic electronic coupling in the mixed valence states. The bipyridine and phenanthroline based reductions are observed at - 1.58, - 1.86 V and - 1.77, - 2.01, - 2.43 V versus SCE respectively. The spectroelectrochemical study on the bipyridine derivative [1](n+) ( n = 3 - 6) in acetonitrile medium at 243 K shows a broad and relatively weak intervalence charge-transfer transition (IVCT) near 1900 nm for both the mixed valence states (RuRuRuIII)-Ru-II-Ru-II [1](4+) and (RuRuRuIII)-Ru-II-Ru-III [1](5+), characteristic of class II behaviour. The calculated coupling constant (V-ab), 560 cm(-1) is also supportive of class II mixed-valence states. The electrochemically generated one-electron oxidised species [1](4+) or [2](4+) exhibits an EPR spectrum characteristic of low-spin Ru-III ion in a distorted octahedral environment (g(1) = 2.246, g(2) = 1.993 for [1](4+) and g(1) = 2.469, g(2) = 2.191 for [2](4+)). The complexes are moderately strongly luminescent at 77 K. Both the complexes have also shown third order non-linear optical properties with gamma = -4.5 x 10(-29) esu for [1](3+) and -5.09 x 10(-29) esu for [2](3+).

The reaction of mononuclear ruthenium precursor [Ru-II(acac)(2)(CH3CN)(2)] (acac = acetylacetonate) with the thiouracil ligand (2-thiouracil, H2L1 or 6-methyl -2-thiouracil, H2L2) in the presence of NEt3 as base in ethanol solvent afforded a trinuclear triangular complex Ru-3(O,O-acetylacetonate)(3)(mu- O,O,gamma- C-acetylaceto n ate) 3(u FS U If ido) (1). In 1, each ruthenium center is linked to one usual 0,0-bonded terminal acetylacetonate molecule whereas the other three acetylacetonate units act as bridging functions: each bridges two adjacent ruthenium ions through the terminal O,O-donor centers at one end and via the gamma-carbon center at the other end. Moreover, there is a mu(3)-Sulfido bridging in the center of the complex unit, which essentially resulted via the selective cleavage of the carbon-sulfur bond of the thiouracil ligand. In diamagnetic complex 1, the ruthenium ions are in mixed valent (RuRuRuII)-Ru-III-Ru-III state, where the paramagnetic ruthenium(III) ions are antiferromagnetically coupled. The single crystal X-ray structure of 1 showed two crystallographically independent C-3-symmetric molecules, Ru-3(O,O-acetylacetonate)(3)(mu-O,O,gamma-Cacetylacetonate)(3)(mu(3)-S) (1), in the asymmetric unit. Bond distances of both crystallographically independent molecules are almost identical, but there are some significant differences in bond angles (up to 60) and interplanar angles (up to 80). Each ruthenium atom exhibits a distorted octahedral environment formed by four oxygen atoms, two from each of the terminal and bridging acetylacetonate units, one gamma-carbon of an adjacent acetylacetonate ligand, and the sulfur atom in the center of the complex. In agreement with the expected Vold symmetry of the complex molecule, the H-1 and C-13 NIVIR spectra of 1 in CDCl3 displayed signals corresponding to two types of ligand units. In dichloromethane solvent, 1 exhibited three metal center based successive quasireversible redox processes, (RuRuRuIII)-Ru-III-Ru-III-(RuRuRuII)-Ru-III-Ru-II (couple I, 0.43 V vs SCE); (RuRuRuIV)-Ru-III-Ru-III-(RuRuRuIII)-Ru-III-Ru-III (couple II, 1.12 V); and (RuRuRuIII)-Ru-III-Ru-III-(RuRuRuII)-Ru-III-Ru-III (couple III, -1.21 V). However, in acetonitrile solvent, in addition to the three described couples [(couple 1), 0.34 V; (couple II), 1.0 V; (couple III), -1.0], one irreversible oxidative response ((RuRuRuIV)-Ru-III-Ru-III --> Ru-III- (RuRuIV)-Ru-IV or oxidation of the coordinated sulfide center) appeared at E-pa 1.50 V. The large differences in potentials between the successive couples are indicative of strong coupling between the ruthenium ions in the mixed-valent states. Compound 1 exhibited a moderately strong charge-transfer (CT) transition at 654 nm and multiple ligand based intense transitions in the UV region. In the (RuRuRuIII)-Ru-III-Ru-III (1(+)) state, the CT band was slightly blue shifted to 644 nm; however, the CT band was further blue shifted to 520 nm on two-electron oxidation to the (RuRuRuIV)-Ru-III-Ru-III (1(2+)) state with a reduction in intensity.

The reaction of Ru(trpy)Cl-3 (trpy = 2,2':6',2"-terpyridine) with the pyridine-based imine function NpC5H4-CH=NiNH-C6H5 (L), incorporating an NH spacer between the imine nitrogen (N) and the pendant phenyl ring, in ethanol medium followed by chromatographic work up on a neutral alumina column using CH3CN/CH2Cl2 (1:4) as eluent, results in complexes of the types [Ru(trpy)(L')](ClO4)(2) (1) and [Ru(trpy)(L)Cl]ClO4 (2). Although the identity of the free ligand (L) has been retained in complex 2, the preformed imine-based potentially bidentate ligand (L) has been selectively transformed into a new class of unusual imine-amidine-based tridentate ligand, NpC5H4-CH= N-i-N(C6H5)C(CH3)=NaH (L'), in 1. The single-crystal X-ray structures of the free ligand (L) and both complexes 1 and 2 have been determined. In 2, the sixth coordination site, that is, the Cl- function, is cis to the pyridine nitrogen (N-p) of L which in turn places the NH spacer away from the Ru-Cl bond, whereas, in 1, the corresponding sixth position, that is, the Ru-N-a (amidine) bond, is trans to the pyridine nitrogen (N-p) of L'. The trans configuration of N-a with respect to the N-p of L' in 1 provides the basis for the selective L --> L' transformation in 1. The complexes exhibit strong Ru(II) --> pi* (trpy) MLCT transitions in the visible region and intraligand transitions in the UV region. The lowest energy MLCT band at 510 nm for 2 has been substantially blue-shifted to 478 nm in the case of 1. The reversible Ru(III)-Ru(II) couples for 1 and 2 have been observed at 0.80 and 0.59 V versus SCE, respectively. The complexes are weakly luminescent at 77 K, exhibiting emissions at lambda(max), 598 nm [quantum yield (Phi) = 0.43 x 10(-2)] and 574 nm (Phi = 0.28 x 10(-2)) for 1 and 2, respectively.

The reactions of potassium salt of xanthates, R'OCS2-K+ 2 (R'=Me, Et, Pr-n, Pr-i, Bu-n, Bu-i, benzyl) with the low-spin ctc-Os-II(L)(2)Br-2, 1 [L=NC5H4-N=N-C6H4(R), R=H, m-Me; cfc: cis-trans-cis with respect to the bromides,pyridine and azo nitrogens, respectively] in boiling N,N-dimethylformamide solvent resulted in low-spin diamagnetic Os-II[o-SC6H3(R)-N=N-C5H4N](2), 3 complexes. In complex 3 the o-carbon hydrogen bond of the pendant phenyl ring of both the parent ligands L, has been selectively and directly thiolated via carbon-sulfur bond cleavage of the xanthate. The reaction is selective to the nature of the solvent used, taking place only in those having high boiling points and relative permittivities. The rate of the reaction is dependent on the nature of the R' group present in the thiolating agent 2, following the order: Et > Me > Pr-i > Bu-i > Pr-n > Bu-n > benzyl. The formation of the thiolated product (3) is authenticated by the single-crystal X-ray structure for the complex where R=H. Structure of the complex has revealed that the OsN4S2 coordination sphere has a meridional configuration, cis-trans-cis with respect to sulfur, azo and pyridine nitrogens, respectively. The molecular geometry of the complexes in solution has been established by H-1 and C-13 NMR spectroscopy. When one methyl group is present at the meta position of the pendant phenyl ring of L the reaction resulted in two isomeric complexes due to free rotation of the singly bonded meta-substituted phenyl ring with respect to the azo group. LH NMR study indicated the presence of an intimate mixture of the two isomers in solution in a 3:4 ratio. In dichloromethane solution, complex 3 exhibits two metal-to-ligand charge-transfer transitions in the visible region and intraligand (pi-pi*/n-pi*) transitions appear in the UV region. In dichloromethane solution the complexes display one reversible osmium(II)right arrow over left arrow osmium(III) oxidation couple near 0.35 V and an irreversible oxidative response near 1.0 V vs SCE due to the oxidation of the coordinated thiol group. Two successive one-electron reductions of the coordinated azo groups take place at the negative side of SCE. (C) 1999 . .

Two new ruthenium(II) mixed-ligand tris-chelated complexes of the type [Ru(bipy)(2)L]ClO4, (bipy = 2,2'-bipyridine; L = pyridine-2-thiolate 1 or pyridin-2-olate 2) have been synthesized. The complexes are essentially diamagnetic and behave as 1:1 electrolytes in acetonitrile solution. They display two metal-to-ligand charge-transfer (m.l.c.t.) transitions near 500 and 340 nm respectively along with intraligand transitions in the UV region. Both exhibit room-temperature emission from the highest-energy (m.l.c.t.) band. At room temperature the lifetime of the excited states for the thiolato (1) and phenolate (2) complexes are 100 and 90 ns respectively. The geometry of the complexes in solution has been assessed by high-resolution H-1 NMR spectroscopy. The molecular structure of complex 1 in the solid state has been determined by single-crystal X-ray diffraction. Tt shows the expected pseudo-octahedral geometry with considerable strain due to the presence of the sterically hindered ligand L-1. In acetonitrile solution the complexes show quasi-reversible ruthenium(II)-ruthenium(III) oxidation couples at 0.54 and 0.64 V versus saturated calomel electrode and quasi-reversible ruthenium(In)ruthenium(Iv) oxidations at 1.41 and 1.03 V respectively. Two reversible reductions are observed near -1.6 and -1.9 V for each complex due to electron transfer to the co-ordinated bipy units. The trivalent analogues of 1 and 2 are unstable at room temperature but can be generated in solution by coulometric oxidation at 263 K as evidenced by EPR spectroscopy.

A group of stable new ruthenium (II) mixed tris-chelated complexes of the type [Ru(bpy)(2)(L)]ClO4 . 2H(2)O, [bpy:2,2'-bipyridine; L:anionic form of KSSCOR, R = Me, Et, (n)pr, (n)Bu] have been prepared by the reaction of Ru(bpy)(2)CO3 with KSSCOR in methanol. The complexes are essentially diamagnetic and behave as 1:1 electrolytes in CH3CN. All the complexes show metal to ligand charge transfer transitions in the visible region (495-450 nm). The complexes do not exhibit any emission spectra at room temperature. The geometry of the complexes has been assessed by high-resolution H-1 NMR spectroscopy. Reductions of coordinated bpy units are observed in the range -1.5 and -1.8 V, respectively. All the complexes exhibit one reversible ruthenium (II)-ruthenium(III) oxidation couple near 0.7 V (versus SCE) in acetonitrile solution at platinum electrode. Coulometric oxidation of [Ru-II(bpy)(2)(L)](+) at 0.9 V.(vs. SCE) as well as chemical oxidation by Ce(IV) in 0.1M aqueous HClO4 produces unstable green coloured ruthenium(III) congener of [Ru-II(bpy)(2)(L)(+) i.e. [Ru-III(bpy)(2)(L)](+2). The presence of trivalent ruthenium in the oxidized solution is evidenced by the rhombic EPR spectrum with g values (g(1) = 2.720, g(2) = 2.370 and g(3) = 1.700).

Mixed ligand phenolate Schiff base complexes of ruthenium (II) of the types [Ru(pap)(2)L]ClO4, [pap = 2-(phenylazo) pyridine; L = deprotonated form of HL ligand, HO-C6H4-CH=N-C6H4-(R), where R = H, Me, OMe, Cl, NO2] and [Ru(bpy)(2)L(2)]ClO4H2O (where bpy = 2,2'-bipyridine) have been synthesised and characterised. The complexes are diamagnetic and in acetonitrile solution they behave as 1:1 electrolyte. All the complexes show strong MLCT transitions in the visible region 400-800 nm. The redox properties of all these complexes have also been studied.