Nitric oxide production by visible light irradiation of aqueous solution of nitrosyl ruthenium complexes

[Ru(II)L(NH(3))(4)(pz)Ru(II)(bpy)(2)(NO)](PF(6))(5) (L is NH(3), py, or 4-acpy) was prepared with good yields in a straightforward way by mixing an equimolar ratio of cis-[Ru(NO(2))(bpy)(2)(NO)](PF(6))(2), sodium azide (NaN(3)), and trans-[RuL(NH(3))(4)(pz)] (PF(6))(2) in acetone. These binuclear compounds display nu(NO) at ca. 1945 cm(-)(1), indicating that the nitrosyl group exhibits a sufficiently high degree of nitrosonium ion (NO(+)). The electronic spectrum of the [Ru(II)L(NH(3))(4)(pz)Ru(II)(bpy)(2)(NO)](5+) complex in aqueous solution displays the bands in the ultraviolet and visible regions typical of intraligand and metal-to-ligand charge transfers, respectively. Cyclic voltammograms of the binuclear complexes in acetonitrile give evidence of three one-electron redox processes consisting of one oxidation due to the Ru(2+/3+) redox couple and two reductions concerning the nitrosyl ligand. Flash photolysis of the [Ru(II)L(NH(3))(4)(pz)Ru(II)(bpy)(2)(NO)](5+) complex is capable of releasing nitric oxide (NO) upon irradiation at 355 and 532 nm. NO production was detected and quantified by an amperometric technique with a selective electrode (NOmeter). The irradiation at 532 nm leads to NO release as a consequence of a photoinduced electron transfer. All species exhibit similar photochemical behavior, a feature that makes their study extremely important for their future application in the upgrade of photodynamic therapy in living organisms.     

Nitrosyl induces phosphorous-acid dissociation in ruthenium(II)

The trans-[Ru(NO)(NH(3))(4)(P(OH)(3))]Cl(3) complex was synthesized by reacting [Ru(H(2)O)(NH(3))(5)](2+) with H(3)PO(3) and characterized by spectroscopic ((31)P-NMR, δ = 68 ppm) and spectrophotometric techniques (λ = 525 nm, ε = 20 L mol(-1) cm(-1); λ = 319 nm, ε = 773 L mol(-1) cm(-1); λ = 241 nm, ε = 1385 L mol(-1) cm(-1); ν(NO(+)) = 1879 cm(-1)). A pK(a) of 0.74 was determined from infrared measurements as a function of pH for the reaction: trans-[Ru(NO)(NH(3))(4)(P(OH)(3))](3+) + H(2)O ? trans-[Ru(NO)(NH(3))(4)(P(O(-))(OH)(2))](2+) + H(3)O(+). According to (31)P-NMR, IR, UV-vis, cyclic voltammetry and ab initio calculation data, upon deprotonation, trans-[Ru(NO)(NH(3))(4)(P(OH)(3))](3+) yields the O-bonded linkage isomer trans- [Ru(NO)(NH(3))(4)(OP(OH)(2))](2+), then the trans-[Ru(NO)(NH(3))(4)(OP(H)(OH)(2))](3+) decays to give the final products H(3)PO(3) and trans-[Ru(NO)(NH(3))(4)(H(2)O)](3+). The dissociation of phosphorous acid from the [Ru(NO)(NH(3))(4)](3+) moiety is pH dependent (k(obs) = 2.1 × 10(-4) s(-1) at pH 3.0, 25 °C).     

cis-Bis(bipyridine)ruthenium Imidazole Derivatives: A Spectroscopic, Kinetic, and Structural Study

The ruthenium bis(bipyridine) complexes cis-[Ru(bpy)(2)Im(OH(2))](2+), cis-[Ru(bpy)(2)(Im)(2)](2+), cis-[Ru(bpy)(2)(N-Im)(2)](2+), cis-[Ru(dmbpy)(2)Im(OH(2))](2+), cis-[Ru(dmbpy)(2)(N-Im)(OH(2))](2+)(bpy = 2,2'-bipyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine, Im = imidazole, N-Im = N-methylimidazole), have been synthesized under ambient conditions in aqueous solution (pH 7). Their electrochemical and spectroscopic properties, absorption, emission, and lifetimes were determined and compared. The substitution kinetics of the cis-[Ru(bpy)(2)Im(OH(2))](2+) complexes show slower rates and have lower affinities for imidazole ligands than the corresponding cis-[Ru(NH(3))(4)Im(OH(2))](2+) complexes. The crystal structures of the monoclinic cis-[Ru(bpy)(2)(Im)(2)](BF(4))(2), space group = P2(1)/a, Z = 4, a = 11.344(1) ?, b = 17.499(3) ?, c = 15.114(3) ?, and beta = 100.17(1) degrees, and triclinic cis-[Ru(bpy)(2)(N-Im)(H(2)O)](CF(3)COO)(2).H(2)O, space group = P&onemacr;, Z = 2, a = 10.432(4) ?, b = 11.995(3) ?, c = 13.912(5) ?, alpha = 87.03(3) degrees, beta = 70.28(3) degrees, and gamma = 71.57(2) degrees, complexes show that these molecules crystallize as complexes of octahedral Ru(II) to two bidentate bipyridine ligands with two imidazole ligands or a water and an N-methylimidazole ligand cis to each other. The importance of these molecules is associated with their frequent use in the modification of proteins at histidine residues and in comparisons of the modified protein derivatives with these small molecule analogs.     

Regulation of geometry around the ruthenium center of bis(2-pyridinecarboxylato) complexes by the nitrosyl moiety: syntheses, structures, and theoretical studies

cis-[Ru(NO)Cl(pyca)(2)] (pyca = 2-pyridinecarboxylato), in which the two pyridyl nitrogen atoms of the two pyca ligands coordinate at the trans position to each other and the two carboxylic oxygen atoms at the trans position to the nitrosyl ligand and the chloro ligand, respectively (type I shown as in Chart 1), reacted with NaOCH(3) to generate cis-[Ru(NO)(OCH(3))(pyca)(2)] (type I). The geometry of this complex was confirmed to be the same as the starting complex by X-ray crystallography: C(13.5)H(13)N(3)O(6.5)Ru; monoclinic, P2(1)/n; a = 8.120(1), b = 16.650(1), c = 11.510(1) A; beta = 99.07(1) degrees; V = 1536.7(2) A(3); Z = 4. The cis-trans geometrical change reaction occurred in the reactions of cis-[Ru(NO)(OCH(3))(pyca)(2)] (type I) in water and alcohol (ROH, R = CH(3), C(2)H(5)) to form [[trans-Ru(NO)(pyca)(2)](2)(H(3)O(2))](+) (type V) and trans-[Ru(NO)(OR)(pyca)(2)] (type V). The reactions of the trans-form complexes, trans-[Ru(NO)(H(2)O)(pyca)(2)](+) (type V) and trans-[Ru(NO)(OCH(3))(pyca)(2)] (type V), with Cl(-) in hydrochloric acid solution afforded the cis-form complex, cis-[Ru(NO)Cl(pyca)(2)] (type I). The favorable geometry of [Ru(NO)X(pyca)(2)](n)(+) depended on the nature of the coexisting ligand X. This conclusion was confirmed by theoretical, synthetic, and structural studies. The mono-pyca-containing nitrosylruthenium complex (C(2)H(5))(4)N[Ru(NO)Cl(3)(pyca)] was synthesized by the reaction of [Ru(NO)Cl(5)](2)(-) with Hpyca and characterized by X-ray structural analysis: C(14)H(24)N(3)O(3)Cl(3)Ru; triclinic, Ponemacr;, a = 7.631(1), b = 9.669(1), c = 13.627(1) A; alpha = 83.05(2), beta = 82.23(1), gamma = 81.94(1) degrees; V = 981.1(1) A(3); Z = 2. The type II complex of cis-[Ru(NO)Cl(pyca)(2)] was synthesized by the reaction of [Ru(NO)Cl(3)(pyca)](-) or [Ru(NO)Cl(5)](2)(-) with Hpyca and isolated by column chromatography. The structure was determined by X-ray structural analysis: C(12)H(8)N(3)O(5)ClRu; monoclinic, P2(1)/n; a = 10.010(1), b = 13.280(1), c = 11.335(1) A; beta = 113.45(1) degrees; V = 1382.4(2) A(3); Z = 4.     

NMR spectroscopic and X-ray crystallographic study of methylcobalt(III) compounds with saturated amine ligands

The (13)C chemical shifts of methylcobalt(III) compounds with saturated amine ligands in cis positions to the methyl group and a monodentate ligand, L = CN(-), NH(3), NO(2)(-), N(3)(-), H(2)O, or OH(-), in the trans position are reported. The amine ligands used, 1,2-ethanediamine (en), 1,3-propanediamine (tn), N,N'-bis(2-aminoethyl)-1,3-propanediamine (2,3,2-tet), N,N'-bis(3-aminopropyl)-1,2-ethanediamine (3,2,3-tet), and 1,4,8,11-tetraazacyclotetradecane (cyclam), all exert an apparent cis influence on the (13)C resonance signal of the coordinated methyl group. In the trans-[Co(en)(2)(CH(3))(L)](n+) series the (15)N resonance frequency of the coordinated en has also been measured. The influence of L on the en (15)N chemical shifts is reverse the influence on the methyl (13)C chemical shifts except in the case of L = NO(2)(-), which affects a further deshielding of the amine nitrogen nucleus. The methyl (1)J(CH) coupling constants in the trans-[Co(en)(2)(CH(3))(L)](n+) series range from 128.09 Hz (L = CN(-)) to 134.11 Hz (L = H(2)O). The crystal structures of trans-[Co(en)(2)(CH(3))(ClZnCl(3))], trans-[Co(3,2,3-tet)(CH(3))(N(3))]ClO(4), trans,trans-[(CH(3))(en)(2)Co(CN)Co(en)(2)(CH(3))](PF(6))(3)(CH(3)CN), and cis-[Co(en)(2)(CH(3))(NH(3))]ZnCl(4) were determined from low-temperature X-ray diffraction data.     

Coordination and Redox Chemistry of Substituted-Polypyridyl Complexes of Ruthenium

The complexes [Ru(tpy)(acac)(Cl)], [Ru(tpy)(acac)(H(2)O)](PF(6)) (tpy = 2,2',2"-terpyridine, acacH = 2,4 pentanedione) [Ru(tpy)(C(2)O(4))(H(2)O)] (C(2)O(4)(2)(-) = oxalato dianion), [Ru(tpy)(dppene)(Cl)](PF(6)) (dppene = cis-1,2-bis(diphenylphosphino)ethylene), [Ru(tpy)(dppene)(H(2)O)](PF(6))(2), [Ru(tpy)(C(2)O(4))(py)], [Ru(tpy)(acac)(py)](ClO(4)), [Ru(tpy)(acac)(NO(2))], [Ru(tpy)(acac)(NO)](PF(6))(2), and [Ru(tpy)(PSCS)Cl] (PSCS = 1-pyrrolidinedithiocarbamate anion) have been prepared and characterized by cyclic voltammetry and UV-visible and FTIR spectroscopy. [Ru(tpy)(acac)(NO(2))](+) is stable with respect to oxidation of coordinated NO(2)(-) on the cyclic voltammetric time scale. The nitrosyl [Ru(tpy)(acac)(NO)](2+) falls on an earlier correlation between nu(NO) (1914 cm(-)(1) in KBr) and E(1/2) for the first nitrosyl-based reduction 0.02 V vs SSCE. Oxalate ligand is lost from [Ru(II)(tpy)(C(2)O(4))(H(2)O)] to give [Ru(tpy)(H(2)O)(3)](2+). The Ru(III/II) and Ru(IV/III) couples of the aqua complexes are pH dependent. At pH 7.0, E(1/2) values are 0.43 V vs NHE for [Ru(III)(tpy)(acac)(OH)](+)/[Ru(II)(tpy)(acac)(H(2)O)](+), 0.80 V for [Ru(IV)(tpy)(acac)(O)](+)/[Ru(III)(tpy)(acac)(OH)](+), 0.16 V for [Ru(III)(tpy)(C(2)O(4))(OH)]/[Ru(II)(tpy)(C(2)O(4))(H(2)O)], and 0.45 V for [Ru(IV)(tpy)(C(2)O(4))(O)]/[Ru(III)(tpy)(C(2)O(4))(OH)]. Plots of E(1/2) vs pH define regions of stability for the various oxidation states and the pK(a) values of aqua and hydroxo forms. These measurements reveal that C(2)O(4)(2)(-) and acac(-) are electron donating to Ru(III) relative to bpy. Comparisons with redox potentials for 21 related polypyridyl couples reveal the influence of ligand changes on the potentials of the Ru(IV/III) and Ru(III/II) couples and the difference between them, DeltaE(1/2). The majority of the effect appears in the Ru(III/II) couple. ()A linear correlation exists between DeltaE(1/2) and the sum of a set of ligand parameters defined by Lever et al., SigmaE(i)(L(i)), for the series of complexes, but there is a dramatic change in slope at DeltaE(1/2) approximately -0.11 V and SigmaE(i)(L(i)) = 1.06 V. Extrapolation of the plot of DeltaE(1/2) vs SigmaE(i)(L(i)) suggests that there may be ligand environments in which Ru(III) is unstable with respect to disproportionation into Ru(IV) and Ru(II). This would make the two-electron Ru(IV)O/Ru(II)OH(2) couple more strongly oxidizing than the one-electron Ru(IV)O/Ru(III)OH couple.     

Dinitrogen formation by oxidative intramolecular N---N coupling in cis,cis-[(bpy)2(NH3)RuORu(NH3)(bpy)2]4+

The (15)N-labeled diammine(mu-oxo)ruthenium complex cis,cis-[(bpy)(2)(H(3)(15)N)Ru(III)ORu(III)((15)NH(3))(bpy)(2)](4+) ((2-(15)N)(4+)) was synthesized from cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(H(2)O)(bpy)(2)](4+) by using ((15)NH(4))(2)SO(4) and isolated as its perchlorate salt in 17% yield. A 1:1 mixture of (2-(15)N)(4+) and nonlabeled cis,cis-[(bpy)(2)(H(3)(14)N)Ru(III)ORu(III)((14)NH(3))(bpy)(2)](4+) were electrochemically oxidized in aqueous solution. The gaseous products (14)N(2) and (15)N(2) were formed in equimolar amounts with only a small amount of (14)N(15)N detected. This demonstrates that dinitrogen formation by oxidation of the diammine complex proceeds by intramolecular N---N coupling.     

Enhancement of metal-metal coupling at a considerable distance by using 4-pyridinealdazine as a bridging ligand in polynuclear complexes of rhenium and ruthenium

Novel polynuclear complexes of rhenium and ruthenium containing PCA (PCA = 4-pyridinecarboxaldehyde azine or 4-pyridinealdazine or 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a bridging ligand have been synthesized as PF(6-) salts and characterized by spectroscopic, electrochemical, and photophysical techniques. The precursor mononuclear complex, of formula [Re(Me(2)bpy)(CO)(3)(PCA)](+) (Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine), does not emit at room temperature in CH(3)CN, and the transient spectrum found by flash photolysis at lambda(exc) = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me(2)bpy)(CO)(3)Re(II)(PCA(-))](+), with lambda(max) = 460 nm and tau < 10 ns. The spectral properties of the related complexes [[Re(Me(2)bpy)(CO)(3)}(2)(PCA)](2+), [Re(CO)(3)(PCA)(2)Cl], and [Re(CO)(3)Cl](3)(PCA)(4) confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(II)(NH(3))(5)](3+), presents an intense absorption in the visible spectrum that can be assigned to a MLCT d(pi)(Ru) --> pi(PCA); in CH(3)CN, the value of lambda (max) = 560 nm is intermediate between those determined for [Ru(NH(3))(5)(PCA)](2+) (lambda(max) = 536 nm) and [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](4+) (lambda(max) = 574 nm), indicating a significant decrease in the energy of the pi-orbital of PCA. The mixed-valent species, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(III)(NH(3))(5)](4+), was obtained in CH(3)CN solution, by bromine oxidation or by controlled-potential electrolysis at 0.8 V in a OTTLE cell of the [Re(I),Ru(II)] precursor; the band at lambda(max) = 560 nm disappears completely, and a new band appears at lambda(max) = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) Re(I) --> Ru(III). By using the Marcus-Hush formalism, both the electronic coupling (H(AB)) and the reorganization energy (lambda) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (approximately 15.0 Angstroms), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](5+) (K(c) approximately 10(2), in CH(3)CN), puts into evidence an unusual enhancement of the metal-metal coupling in the bridged PCA complexes. This effect can be accounted for by the large extent of "metal-ligand interface", as shown by DFT calculations on free PCA. Moreover, lambda is lower than the driving force -DeltaG degrees for the recombination charge reaction [Re(II),Ru(II)] --> [Re(I),Ru(III)] that follows light excitation of the mixed-valent species. It is then predicted that this reverse reaction falls in the Marcus inverted region, making the heterodinuclear [Re(I),Ru(III)] complex a promising model for controlling the efficiency of charge-separation processes.     

Study of the ligand substitution reactions of cis,cis-[(bpy)(2)(L)RuORu(L')(bpy)(2)](n+) (L,L' = H(2)O,OH(-), NH(3)) using electrospray ionization mass spectrometry and (1)H NMR

We have successfully applied electrospray ionization mass spectrometry (ESI-MS) and (1)H NMR analyses to study ligand substitution reactions of mu-oxo ruthenium bipyridine dimers cis,cis-[(bpy)(2)(L)RuORu(L')(bpy)(2)](n+) (bpy = 2,2'-bipyridine; L and L' = NH(3), H(2)O, and HO(-)) with solvent molecules, that is, acetonitrile, methanol, and acetone. The results clearly show that the ammine ligand is very stable and was not substituted by any solvents, while the aqua ligand was rapidly substituted by all the solvents. In acetonitrile and acetone solutions, the substitution reaction of the aqua ligand(s) competed with a deprotonation reaction from the ligand. The hydroxyl ligand was not substituted by acetonitrile or acetone, but it exchanged slowly with CH(3)O(-) in methanol. The substitution reaction of the aqua ligands in [(bpy)(2)(H(2)O)Ru(III)ORu(III)(H(2)O)(bpy)(2)](4+) was more rapid than that of the hydroxyl ligand in [(bpy)(2)(H(2)O)Ru(III)ORu(IV)(OH)(bpy)(2)](4+). In methanol, slow reduction of Ru(III) to Ru(II) was observed in all the mu-oxo dimers, and the Ru-O-Ru bridge was then cleaved to give mononuclear Ru(II) complexes.     

Insight into the photoinduced ligand exchange reaction pathway of cis-[Rh2(μ-O2CCH3)2(CH3CN)6]2+ with a DNA model chelate

We previously showed that [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) binds to dsDNA only upon irradiation with visible light and that photolysis results in a 34-fold enhancement of its cytotoxicity toward Hs-27 human skin fibroblasts, making it potentially useful for photodynamic therapy (PDT). With the goal of gaining further insight on the photoinduced binding of DNA to the complex, we investigated by NMR spectroscopy the mechanism by which 2,2'-bipyridine (bpy), a model for biologically relevant bidentate nitrogen donor ligands, binds to [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) upon irradiation in D(2)O. The photochemical results are compared to the reactivity in the dark in D(2)O and CD(3)CN. The photolysis of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) with equimolar bpy solutions in D(2)O with visible light affords [Rh(2)(O(2)CCH(3))(2)(eq/eq-bpy)(CH(3)CN)(2)(D(2)O(ax))(2)](2+) (eq/eq) with the reaction reaching completion in ~8 h. Only vestiges of eq/eq are observed at the same time in the dark, however, and the reaction is ~20 times slower. Conversely, the dark reaction of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) with an equimolar amount of bpy in CD(3)CN affords [Rh(2)(O(2)CCH(3))(2)(η(1)-bpy(ax))(CH(3)CN)(5)](2+) (η(1)-bpy(ax)), which remains present even after 5 days of reaction. The photolysis results in D(2)O are consistent with the exchange of one equiv CH(3)CNeq for solvent, and the resulting species quickly reacting with bpy to generate eq/eq; the initial eq ligand dissociation is assisted by absorption of a photon, thus greatly enhancing the reaction rate. The photolytic reaction of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+):bpy in a 1:2 ratio in D(2)O affords the eq/eq and (eq/eq)(2) adducts. The observed differences in the reactivity in D(2)O vs CD(3)CN are explained by the relative ease of substitution of eq D(2)O vs CD(3)CN by the incoming bpy molecule. These results clearly highlight the importance of dissociation of an eq CH(3)CN molecule from the dirhodium core to attain high reactivity and underscore the importance of light for the reactivity of these compounds, which is essential for PDT agents.     

The inducing NO-vasodilation by chemical reduction of coordinated nitrite ion in cis-[Ru(NO(2))L(bpy)(2)](+) complex

The synthesis of [Ru(NO(2))L(bpy)(2)](+) (bpy = 2,2'-bipyridine and L = pyridine (py) and pyrazine (pz)) can be accomplished by addition of [Ru(NO)L(bpy)(2)](PF(6))(3) to aqueous solutions of physiological pH. The electrochemical processes of [Ru(NO(2))L(bpy)(2)](+) in aqueous solution were studied by cyclic voltammetry and differential pulse voltammetry. The anodic scan shows a peak around 1.00 V vs. Ag/AgCl attributed to the oxidation process centered on the metal ion. However, in the cathodic scan a second peak around -0.60 V vs. Ag/AgCl was observed and attributed to the reduction process centered on the nitrite ligand. The controlled reduction potential electrolysis at -0.80 V vs. Ag/AgCl shows NO release characteristics as judged by NO measurement with a NO-sensor. This assumption was confirmed by ESI/MS(+) and spectroelectrochemical experiment where cis-[Ru(bpy)(2)L(H(2)O)](2+) was obtained as a product of the reduction of cis-[Ru(II)(NO(2))L(bpy)(2)](+). The vasorelaxation observed in denuded aortic rings pre-contracted with 0.1 mumol L(-1) phenylephrine responded with relaxation in the presence of cis-[Ru(II)(NO(2))L(bpy)(2)](+). The potential of rat aorta cells to metabolize cis-[Ru(II)(NO(2))L(bpy)(2)](+) was also followed by confocal analysis. The obtained results suggest that NO release happens by reduction of cis-[Ru(II)(NO(2))L(bpy)(2)](+) inside the cell. The maximum vasorelaxation was achieved with 1 x 10(-5) mol L(-1) of cis-[Ru(II)(NO(2))L(bpy)(2)](+) complex.     

Oxygen atom transfer from a trans-dioxoruthenium(VI) complex to nitric oxide

In aqueous acidic solutions trans-[Ru(VI)(L)(O)(2)](2+) (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)](2+). When ≤1 mol equiv NO is used, the intermediate Ru(IV) species, trans-[Ru(IV)(L)(O)(OH(2))](2+), can be detected. The reaction of [Ru(VI)(L)(O)(2)](2+) with NO is first order with respect to [Ru(VI)] and [NO], k(2)=(4.13±0.21)×10(1) M(-1) s(-1) at 298.0 K. ΔH(≠) and ΔS(≠) are (12.0±0.3) kcal mol(-1) and -(11±1) cal mol(-1) K(-1), respectively. In CH(3)CN, ΔH(≠) and ΔS(≠) have the same values as in H(2)O; this suggests that the mechanism is the same in both solvents. In CH(3)CN, the reaction of [Ru(VI)(L)(O)(2)](2+) with NO produces a blue-green species with λ(max) at approximately 650 nm, which is characteristic of N(2)O(3). N(2)O(3) is formed by coupling of NO(2) with excess NO; it is relatively stable in CH(3)CN, but undergoes rapid hydrolysis in H(2)O. A mechanism that involves oxygen atom transfer from [Ru(VI)(L)(O)(2)](2+) to NO to produce NO(2) is proposed. The kinetics of the reaction of [Ru(IV)(L)(O)(OH(2))](2+) with NO has also been investigated. In this case, the data are consistent with initial one-electron O(-) transfer from Ru(IV) to NO to produce the nitrito species [Ru(III)(L)(ONO)(OH(2))](2+) (k(2)>10(6) M(-1) s(-1)), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)](2+) and NO(2)(-) (k(2)=54.7 M(-1) s(-1)).     

Electronic structure of oxidized complexes derived from cis-[Ru(II)(bpy)2(H2O)2]2+ and its photoisomerization mechanism

The geometry and electronic structure of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) and its higher oxidation state species up formally to Ru(VI) have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, cis-[Ru(IV)(bpy)(2)(OH)(2)](2+) (d(4)) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru(V)(bpy)(2)(O)(OH)](2+) (d(3)) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) to its trans-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.     

Stepwise oxidation of anilines by cis-[RuIV(bpy)2(py)(O)]2+

The net six-electron oxidation of aniline to nitrobenzene or azoxybenzene by cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (bpy is 2,2'-bipyridine; py is pyridine) occurs in a series of discrete stages. In the first, initial two-electron oxidation is followed by competition between oxidative coupling with aniline to give 1,2-diphenylhydrazine and capture by H(2)O to give N-phenylhydroxylamine. The kinetics are first order in aniline and first order in Ru(IV) with k(25.1 degrees C, CH(3)CN) = (2.05 +/- 0.18) x 10(2) M(-1) s(-1) (DeltaH(++) = 5.0 +/- 0.7 kcal/mol; DeltaS(++) = -31 +/- 2 eu). On the basis of competition experiments, k(H)2(O)/k(D)2(O) kinetic isotope effects, and the results of an (18)O labeling study, it is concluded that the initial redox step probably involves proton-coupled two-electron transfer from aniline to cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (Ru(IV)=O(2+)). The product is an intermediate nitrene (PhN) or a protonated nitrene (PhNH(+)) which is captured by water to give PhNHOH or aniline to give PhNHNHPh. In the following stages, PhNHOH, once formed, is rapidly oxidized by Ru(IV)=O(2+) to PhNO and PhNHNHPh to PhN=NPh. The rate laws for these reactions are first order in Ru(IV)=O(2+) and first order in reductant with k(14.4 degrees C, H(2)O/(CH(3))(2)CO) = (4.35 +/- 0.24) x 10(6) M(-1) s(-1) for PhNHOH and k(25.1 degrees C, CH(3)CN) = (1.79 +/- 0.14) x 10(4) M(-1) s(-1) for PhNHNHPh. In the final stages of the six-electron reactions, PhNO is oxidized to PhNO(2) and PhN=NPh to PhN(O)=NPh. The oxidation of PhNO is first order in PhNO and in Ru(IV)=O(2+) with k(25.1 degrees C, CH(3)CN) = 6.32 +/- 0.33 M(-1) s(-1) (DeltaH(++) = 4.6 +/- 0.8 kcal/mol; DeltaS(++) = -39 +/- 3 eu). The reaction occurs by O-atom transfer, as shown by an (18)O labeling study and by the appearance of a nitrobenzene-bound intermediate at low temperature.     

Dehydration reaction of hydroxyl substituted alkenes and alkynes on the Ru(2)S(2) complex

A variety of inter- and intramolecular dehydration was found in the reactions of [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)(mu-S(2))](CF(3)SO(3))(4) (1) with hydroxyl substituted alkenes and alkynes. Treatment of 1 with allyl alcohol gave a C(3)S(2) five-membered ring complex, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH(2)CH(2)CH(OCH(2)CH=CH(2))S]](CF(3)SO(3))(4) (2), via C-S bond formation after C-H bond activation and intermolecular dehydration. On the other hand, intramolecular dehydration was observed in the reaction of 1 with 3-buten-1-ol giving a C(4)S(2) six-membered ring complex, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2) [mu-SCH(2)CH=CHCH(2)S]](CF(3)SO(3))(4) (3). Complex 1 reacts with 2-propyn-1-ol or 2-butyn-1-ol to give homocoupling products, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCR=CHCH(OCH(2)C triple bond CR)S]](CF(3)SO(3))(4) (4: R = H, 5: R = CH(3)), via intermolecular dehydration. In the reaction with 2-propyn-1-ol, the intermediate complex having a hydroxyl group, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH=CHCH(OH)S]](CF(3)SO(3))(4) (6), was isolated, which further reacted with 2-propyn-1-ol and 2-butyn-1-ol to give 4 and a cross-coupling product, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH=CHCH(OCH(2)C triple bond CCH(3))S]](CF(3)SO(3))(4) (7), respectively. The reaction of 1 with diols, (HO)CHRC triple bond CCHR(OH), gave furyl complexes, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SSC=CROCR=CH]](CF(3)SO(3))(3) (8: R = H, 9: R = CH(3)) via intramolecular elimination of a H(2)O molecule and a H(+). Even though (HO)(H(3)C)(2)CC triple bond CC(CH(3))(2)(OH) does not have any propargylic C-H bond, it also reacts with 1 to give [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH(2)C(=CH(2))C(=C=C(CH(3))(2))]S](CF(3)SO(3))(4) (10). In addition, the reaction of 1 with (CH(3)O)(H(3)C)(2)CC triple bond CC(CH(3))(2)(OCH(3)) gives [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(2)][mu-S=C(C(CH(3))(2)OCH(3))C=CC(CH(3))CH(2)S][Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)]](CF(3)SO(3))(4) (11), in which one molecule of CH(3)OH is eliminated, and the S-S bond is cleaved.     

New dicarboxylic acid bipyridine ligand for ruthenium polypyridyl sensitization of TiO2

An ambidentate dicarboxylic acid bipyridine ligand, (4,5-diazafluoren-9-ylidene) malonic acid (dfm), was synthesized for coordination to Ru(II) and mesoporous nanocrystalline (anatase) TiO(2) thin films. The dfm ligand provides a conjugated pathway from the pyridyl rings to the carbonyl carbons of the carboxylic acid groups. X-ray crystal structures of [Ru(bpy)(2)(dfm)]Cl(2) and the corresponding diethyl ester compound, [Ru(bpy)(2)(defm)](PF(6))(2), were obtained. The compounds displayed intense metal-to-ligand charge transfer (MLCT) absorption bands in the visible region (ε > 11,000 M(-1) cm(-1) for [Ru(bpy)(2)(dfm)](PF(6))(2) in acetonitrile). Significant room temperature photoluminescence, PL, was absent in CH(3)CN but was observed at 77 K in a 4:1 EtOH:MeOH (v:v) glass. Cyclic voltammetry measurements revealed quasi-reversible Ru(III/II) electrochemistry. Ligand reductions were quasi-reversible for the diethyl ester compound [Ru(bpy)(2)(defm)](2+), but were irreversible for [Ru(bpy)(2)(dfm)](2+). Both compounds were anchored to TiO(2) thin films by overnight reactions in CH(3)CN to yield saturation surface coverages of 3 × 10(-8) mol/cm(2). Attenuated total reflection infrared measurements revealed that the [Ru(bpy)(2)(dfm)](2+) compound was present in the deprotonated carboxylate form when anchored to the TiO(2) surface. The MLCT excited states of both compounds injected electrons into TiO(2) with quantum yields of 0.70 in 0.1 M LiClO(4) CH(3)CN. Micro- to milli-second charge recombination yielded ground state products. In regenerative solar cells with 0.5 M LiI/0.05 M I(2) in CH(3)CN, the Ru(bpy)(2)(dfm)/TiO(2) displayed incident photon-to-current efficiencies of 0.7 at the absorption maximum. Under the same conditions, the diethylester compound was found to rapidly desorb from the TiO(2) surface.     

Surprising photochemical reactivity and visible light-driven energy transfer in heterodimetallic complexes

The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been used for the synthesis of a series of novel heterodimetallic complexes starting from [Ru(bpy)(2)(dppcb)]X(2) (1; X = PF(6), SbF(6)), so-called dyads, showing surprising photochemical reactivity. They consist of [Ru(bpy)(2)](2+)"antenna" sites absorbing light combined with reactive square-planar metal centres. Thus, irradiating [Ru(bpy)(2)(dppcb)MCl(2)]X(2) (M = Pt, 2; Pd, 3; X = PF(6), SbF(6)) dissolved in CH(3)CN with visible light, produces the unique heterodimetallic compounds [Ru(bpy)(CH(3)CN)(2)(dppcb)MCl(2)]X(2) (M = Pt, 7; Pd, 8; X = PF(6), SbF(6)). In an analogous reaction the separable diastereoisomers (ΔΛ/ΛΔ)- and (ΔΔ/ΛΛ)-[Ru(bpy)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (5/6) lead to [Ru(bpy)(CH(3)CN)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (9), where only the RuP(2)N(4) moiety of 5/6 is photochemically reactive. By contrast, in the case of [Ru(bpy)(2)(dppcb)NiCl(2)]X(2) (4; X = PF(6), SbF(6)) no clean photoreaction is observed. Interestingly, this difference in photochemical behaviour is completely in line with the related photophysical parameters, where 2, 3, and 5/6, but not 4, show long-lived excited states at ambient temperature necessary for this type of photoreaction. Furthermore, the photochemical as well as the photophysical properties of 2-4 are also in accordance with their single crystal X-ray structures presented in this work. It seems likely that differences in "steric pressure" play a major role for these properties. The unique complexes 7-9 are also fully characterized by single-crystal X-ray structure analyses, clearly showing that the stretching vibration modes of the ligand CH(3)CN, present only in 7-9, cannot be directly influenced by "steric pressure". This has dramatic consequences for their photophysical parameters. The trans-[Ru(bpy)(CH(3)CN)(2)](2+) chromophore of 9 acts as efficient "antenna" for visible light-driven energy transfer to the Os-centred "trap" site, resulting in k(en) ≥ 2 × 10(9) s(-1) for the energy transfer. Since electron transfer is made possible by the use of this intervening energy transfer, in dyads like 2-4 highly reactive M(0) species (M = Pt, Pd, Ni) could be generated. These species are not stable in water and M(II) hydride intermediates are usually formed, further reacting with H(+) to give H(2). Thus, derivatives of 3, namely [M(bpy)(2)(dppcb)Pd(bpy)](PF(6))(4) (M = Os, Ru) dissolved in 1:1 (v/v) H(2)O-CH(3)CN produce H(2) during photolysis with visible light.     

A regenerable ruthenium tetraammine nitrosyl complex immobilized on a modified silica gel surface: preparation and studies of nitric oxide release and nitrite-to-NO conversion

Silica gel bearing isonicotinamide groups was prepared by further modification of 3-aminopropyl-functionalized silica by a reaction with isonicotinic acid and 1,3-dicyclohexylcarbodiimide to yield 3-isonicotinamidepropyl-functionalized silica gel (ISNPS). This support was characterized by means of infrared spectroscopy, elemental analysis, and specific surface area. The ISNPS was used to immobilize the [Ru(NH(3))(4)SO(3)] moiety by reaction with trans-[Ru(NH(3))(4)(SO(2))Cl]Cl, yielding [Si(CH(2))(3)(isn)Ru(NH(3))(4)(SO(3))]. The related immobilized [Si(CH(2))(3)(isn)Ru(NH(3))(4)(L)](3+/2+) (L=SO(2), SO(2-)(4), OH(2), and NO) complexes were prepared and characterized by means of UV-vis and IR spectroscopy, as well as by cyclic voltammetry. Syntheses of the nitrosyl complex were performed by reaction of the immobilized ruthenium ammine [Si(CH(2))(3)(isn)Ru(NH(3))(4)(OH(2))](2+) with nitrite in acid or neutral (pH 7.4) solution. The similar results obtained in both ways indicate that the aqua complex was able to convert nitrite into coordinated nitrosyl. The reactivity of [Si(CH(2))(3)(isn)Ru(NH(3))(4)(NO)](3+) was investigated in order to evaluate the nitric oxide (NO) release. It was found that, upon light irradiation or chemical reduction, the immobilized nitrosyl complex was able to release NO, generating the corresponding Ru(III) or Ru(II) aqua complexes, respectively. The NO material could be regenerated from these NO-depleted materials obtained photochemically or by reduction. Regeneration was done by reaction with nitrite in aqueous solution (pH 7.4). Reduction-regeneration cycles were performed up to three times with no significant leaching of the ruthenium complex.     

Bis(amido)ruthenium(IV) Complexes with 2,3-Diamino-2,3-dimethylbutane. Crystal Structure and Reversible Ru(IV)-Amide/Ru(III)-Amine and Ru(IV)-Amide/Ru(II)- Amine Redox Couples in Aqueous Solution

Two bis(amido)ruthenium(IV) complexes, [Ru(IV)(bpy)(L-H)(2)](2+) and [Ru(IV)(L)(L-H)(2)](2+) (bpy = 2,2'-bipyridine, L = 2,3-diamino-2,3-dimethylbutane, L-H = (H(2)NCMe(2)CMe(2)NH)(-)), were prepared by chemical oxidation of [Ru(II)(bpy)(L)(2)](2+) and the reaction of [(n-Bu)(4)N][Ru(VI)NCl(4)] with L, respectively. The structures of [Ru(bpy)(L-H)(2)][ZnBr(4)].CH(3)CN and [Ru(L)(L-H)(2)]Cl(2).2H(2)O were determined by X-ray crystal analysis. [Ru(bpy)(L-H)(2)][ZnBr(4)].CH(3)CN crystallizes in the monoclinic space group P2(1)/n with a = 12.597(2) ?, b = 15.909(2) ?, c = 16.785(2) ?, beta = 91.74(1) degrees, and Z = 4. [Ru(L)(L-H)(2)]Cl(2).2H(2)O crystallizes in the tetragonal space group I4(1)/a with a = 31.892(6) ?, c = 10.819(3) ?, and Z = 16. In both complexes, the two Ru-N(amide) bonds are cis to each other with bond distances ranging from 1.835(7) to 1.856(7) ?. The N(amide)-Ru-N(amide) angles are about 110 degrees. The two Ru(IV) complexes are diamagnetic, and the chemical shifts of the amide protons occur at around 13 ppm. Both complexes display reversible metal-amide/metal-amine redox couples in aqueous solution with a pyrolytic graphite electrode. Depending on the pH of the media, reversible/quasireversible 1e(-)-2H(+) Ru(IV)-amide/Ru(III)-amine and 2e(-)-2H(+) Ru(IV)-amide/Ru(II)-amine redox couples have been observed. At pH = 1.0, the E degrees is 0.46 V for [Ru(IV)(bpy)(L-H)(2)](2+)/[Ru(III)(bpy)(L)(2)](3+) and 0.29 V vs SCE for [Ru(IV)(L)(L-H)(2)](2+)/[Ru(III)(L)(3)](3+). The difference in the E degrees values for the two Ru(IV)-amide complexes has been attributed to the fact that the chelating saturated diamine ligand is a better sigma-donor than 2,2'-bipyridine.     

Synthesis, crystal structures, and magnetic properties of a new family of heterometallic cyanide-bridged Fe(III)2M(II)2 (M=Mn, Ni, and Co) square complexes

New heterobimetallic tetranuclear complexes of formula [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Mn(II)(bpy)(2)](2)(ClO(4))(2)·CH(3)CN (1), [Fe(III){HB(pz)(3)}(CN)(2)(μ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2a), [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2b), [Fe(III){HB(pz)(3)}(CN)(2)(μ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3a), and [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3b), [HB(pz)(3)(-) = hydrotris(1-pyrazolyl)borate, B(Pz)(4)(-) = tetrakis(1-pyrazolyl)borate, dmphen = 2,9-dimethyl-1,10-phenanthroline, bpy = 2,2'-bipyridine] have been synthesized and structurally and magnetically characterized. Complexes 1-3b have been prepared by following a rational route based on the self-assembly of the tricyanometalate precursor fac-[Fe(III)(L)(CN)(3)](-) (L = tridentate anionic ligand) and cationic preformed complexes [M(II)(L')(2)(H(2)O)(2)](2+) (L' = bidentate α-diimine type ligand), this last species having four blocked coordination sites and two labile ones located in cis positions. The structures of 1-3b consist of cationic tetranuclear Fe(III)(2)M(II)(2) square complexes [M = Mn (1), Ni (2a and 2b), Co (3a and 3b)] where corners are defined by the metal ions and the edges by the Fe-CN-M units. The charge is balanced by free perchlorate anions. The [Fe(L)(CN)(3)](-) complex in 1-3b acts as a ligand through two cyanide groups toward two divalent metal complexes. The magnetic properties of 1-3b have been investigated in the temperature range 2-300 K. A moderately strong antiferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Mn(II) (S = 5/2) ions has been found for 1 leading to an S = 4 ground state (J(1) = -6.2 and J(2) = -2.7 cm(-1)), whereas a moderately strong ferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Ni(II) (S = 1) and Co(II) (S = 3/2) ions has been found for complexes 2a-3b with S = 3 (2a and 2b) and S = 4 (3a and 3b) ground spin states [J(1) = +21.4 cm(-1) and J(2) = +19.4 cm(-1) (2a); J(1) = +17.0 cm(-1) and J(2) = +12.5 cm(-1) (2b); J(1) = +5.4 cm(-1) and J(2) = +11.1 cm(-1) (3a); J(1) = +8.1 cm(-1) and J(2) = +11.0 cm(-1) (3b)] [the exchange Hamiltonian being of the type H? = -J(S?(i)·S?(j))]. Density functional theory (DFT) calculations have been used to substantiate the nature and magnitude of the exchange magnetic coupling observed in 1-3b and also to analyze the dependence of the exchange magnetic coupling on the structural parameters of the Fe-C-N-M skeleton.