Ru Unit Research Articles

Air-Stable Coordinatively Unsaturated Ruthenium(II) Complex for Ligand Binding and Catalytic Transfer Hydrogenation of Ketones from Ethanol.

Coordinatively unsaturated complexes are interesting from a fundamental level for their formally empty coordination site and, in particular, from a catalytic perspective as they provide opportunities for substrate binding and transformation. Here, we describe the synthesis of a novel underligated ruthenium complex [Ru(cym)(N,N')]+, 3, featuring an amide-functionalized pyridylidene amide (PYA) as the N,N'-bidentate coordinating ligand. In contrast to previously investigated underligated complexes, complex 3 offers potential for dynamic modifications, thanks to the flexible donor properties of the PYA ligand. Specifically, they allow both for stabilizing the formally underligated metal center in complex 3 through nitrogen π-donation and for facilitating through π-acidic bonding properties the coordination of a further ligand L to the ruthenium center to yield the formal 18 e- complexes [Ru(cym)(N,N')(L)]+ (4: L = P(OMe)3; 5: L = PPh3; 6: L = N-methylimidazole; 7: L = pyridine) and neutral complex [RuCl(cym)(N,N')] 8. Analysis by 1H NMR and UV-vis spectroscopies reveals an increasing Ru-L bond strength along the sequence pyridine <1-methylimidazole < PPh3 < P(OMe)3 with binding constants varying over 3 orders of magnitude with log(Keq) values between 2.8 and 5.7. The flexibility of the Ru(PYA) unit and the ensuing accessibility of saturated and unsaturated species with one and the same ligand are attractive from a fundamental point of view and also for catalytic applications, as catalytic transformations rely on the availability of transiently vacant coordination sites. Thus, while complex 3 does not form stable adducts with O-donors such as ketones or alcohols, it transiently binds these species, as evidenced by the considerable catalytic activity in the transfer hydrogenation of ketones. Notably, and as one of only a few catalysts, complex 3 is compatible with EtOH as a hydrogen source. Complex 3 shows excellent performance in the transfer hydrogenation of pyridyl-containing substrates, in agreement with the poor coordination strength of this functional group to the ruthenium center in 3.

Folding-Induced Promotion of Proton-Coupled Electron Transfers via Proximal Base for Light-Driven Water Oxidation.

Proton-coupled electron-transfer (PCET) processes play a key role in biocatalytic energy conversion and storage, for example, photosynthesis or nitrogen fixation. Here, we report a series of bipyridine-containing di- to tetranuclear Ru(bda) macrocycles 2 C-4 C (bda: 2,2'-bipyridine-6,6'-dicarboxylate) to promote O-O bond formation. In photocatalytic water oxidation under neutral conditions, all complexes 2 C-4 C prevail in a folded conformation that support the water nucleophilic attack (WNA) pathway with remarkable turnover frequencies of up to 15.5 s-1 per Ru unit respectively. Single-crystal X-ray analysis revealed an increased tendency for intramolecular π-π stacking and preorganization of the proximal bases close to the active centers for the larger macrocycles. H/D kinetic isotope effect studies and electrochemical data demonstrate the key role of the proximal bipyridines as proton acceptors in lowering the activation barrier for the crucial nucleophilic attack of H2 O in the WNA mechanism.

Folding‐Induced Promotion of Proton‐Coupled Electron Transfers via Proximal Base for Light‐Driven Water Oxidation

AbstractProton‐coupled electron‐transfer (PCET) processes play a key role in biocatalytic energy conversion and storage, for example, photosynthesis or nitrogen fixation. Here, we report a series of bipyridine‐containing di‐ to tetranuclear Ru(bda) macrocycles 2 C–4 C (bda: 2,2′‐bipyridine‐6,6′‐dicarboxylate) to promote O−O bond formation. In photocatalytic water oxidation under neutral conditions, all complexes 2 C–4 C prevail in a folded conformation that support the water nucleophilic attack (WNA) pathway with remarkable turnover frequencies of up to 15.5 s−1 per Ru unit respectively. Single‐crystal X‐ray analysis revealed an increased tendency for intramolecular π‐π stacking and preorganization of the proximal bases close to the active centers for the larger macrocycles. H/D kinetic isotope effect studies and electrochemical data demonstrate the key role of the proximal bipyridines as proton acceptors in lowering the activation barrier for the crucial nucleophilic attack of H2O in the WNA mechanism.

Spectroscopic and Electrochemical Investigation of Peripherally Diruthenated Complexes of octakis(benzylthio) and (methylthio)tetraazaporphyrinmagnesium(II)

Four new trinuclear complexes were successfully synthesized through the dative binding of one unit of (bpy)2RuII and one unit of (phen)2RuII to the periphery of octakis(benzylthio)tetraazaporphyrinmagnesium(II) [Mg(OBTTAP)] and octakis(methylthio)tetraazaporphyrin- magnesium(II) ([Mg(OMTTAP)]). Spectroscopic techniques were used to perform an extensive characterization of the complexes, including an analysis of their fluorescence excitation/emission spectra and redox behaviours. Effective electronic interaction between the peripheral (bpy)2RuII/(phen)2RuII and [Mg(OBTTAP)] units is evidenced by the presence of these complexes. In cyclic voltammetry, the oxidation waves were slightly altered to a higher positive potential upon attaching a second Ru(II) unit in complexes 6, 7, 8 and 9.

Structure-Activity Relationship for Di- up to Tetranuclear Macrocyclic Ruthenium Catalysts in Homogeneous Water Oxidation.

Two di‐ and tetranuclear Ru(bda) (bda: 2,2′‐bipyridine‐6,6′‐dicarboxylate) macrocyclic complexes were synthesized and their catalytic activities in chemical and photochemical water oxidation investigated in a comparative manner to our previously reported trinuclear congener. Our studies have shown that the catalytic activities of this homologous series of multinuclear Ru(bda) macrocycles in homogeneous water oxidation are dependent on their size, exhibiting highest efficiencies for the largest tetranuclear catalyst. The turnover frequencies (TOFs) have increased from di‐ to tetranuclear macrocycles not only per catalyst molecule but more importantly also per Ru unit with TOF of 6 s−1 to 8.7 s−1 and 10.5 s−1 in chemical and 0.6 s−1 to 3.3 s−1 and 5.8 s−1 in photochemical water oxidation per Ru unit, respectively. Thus, for the first time, a clear structure–activity relationship could be established for this novel class of macrocyclic water oxidation catalysts.

A photoredox catalysed Heck reaction via hole transfer from a Ru(ii)-bis(terpyridine) complex to graphene oxide.

The attachment of homoleptic Ru bis-terpy complexes on graphene oxide significantly improved the photocatalytic activity of the complexes. These straightforward complexes were applied as photocatalysts in a Heck reaction. Due to covalent functionalization on graphene oxide, which functions as an electron reservoir, excellent yields were obtained. DFT investigations of the charge redistribution revealed efficient hole transfer from the excited Ru unit towards the graphene oxide.

Synthesis of a Novel Re(I)-Ru(II)-Re(I) Trinuclear Complex as an Effective Photocatalyst for CO2 Reduction

Abstract Supramolecular photocatalysts, which consist of redox photosensitizer and catalyst units, have recently attracted attention in the field of artificial photosynthesis. Aiming to construct a durable supramolecular photocatalyst for CO2 reduction, a new Re–Ru–Re trinuclear complex was successfully developed, where one Ru unit, acting as photosensitizer, was linked to two Re units, acting as catalysts, through ethylene chains. The novel complex was synthesized by creating two chelate moieties on the ligand of a Ru complex using the Negishi-coupling reaction, which were then coordinated to Re complexes. This trinuclear complex selectively photocatalyzed the reduction of CO2 to CO under visible light irradiation, with high durability compared with the corresponding Ru–Re binuclear complex and a mixed system of model mononuclear complexes. Absorption spectra and mass spectroscopic data of the reaction solutions revealed a rapid consumption of the electrons on the one-electron reduced species of the Ru unit, which led to suppression of the Ru unit decomposition, thereby maintaining its photosensitizing ability and product selectivity.

Heteronuclear d-d and d-f Ru(ii)/M complexes [M = Gd(iii), Yb(iii), Nd(iii), Zn(ii) or Mn(ii)] of ligands combining phenanthroline and aminocarboxylate binding sites: combined relaxivity, cell imaging and photophysical studies.

A ligand skeleton combining a 1,10-phenanthroline (phen) binding site and one or two heptadentate N3O4 aminocarboxylate binding sites, connected via alkyne spacers to the phen C3 or C3/C8 positions, has been used to prepare a range of heteronuclear Ru·M and Ru·M2 complexes which have been evaluated for their cell imaging, relaxivity, and photophysical properties. In all cases the phen unit is bound to a {Ru(bipy)2}2+ unit to give a phosphorescent {Ru(bipy)2(phen)}2+ luminophore, and the pendant aminocarboxylate sites are occupied by a secondary metal ion M which is either a lanthanide [Gd(iii), Nd(iii), Yb(iii)] or another d-block ion [Zn(ii), Mn(ii)]. When M = Gd(iii) or Mn(ii) these ions provide the complexes with a high relaxivity for water; in the case of Ru·Gd and Ru·Gd2 the combination of high water relaxivity and 3MLCT phosphorescence from the Ru(ii) unit provides the possibility of two different types of imaging modality in a single molecular probe. In the case of Ru·Mn and Ru·Mn2 the Ru(ii)-based phosphorescence is substantially reduced compared to the control complexes Ru·Zn and Ru·Zn2 due to the quenching effect of the Mn(ii) centres. Ultrafast transient absorption spectroscopy studies on Ru·Mn (and Ru·Zn as a non-quenched control) reveal the occurrence of fast (<1 ns) PET in Ru·Mn, from the Mn(ii) ion to the Ru(ii)-based 3MLCT state, i.e. MnII-(phen˙-)-RuIII → MnIII-(phen˙-)-RuII; the resulting MnIII-(phen˙-) state decays with τ ≈ 5 ns and is non-luminescent. This occurs in conformers when an ET pathway is facilitated by a planar, conjugated bridging ligand conformation connecting the two units across the alkyne bridge but does not occur in conformers where the two units are electronically decoupled by a twisted conformation of the bridging ligand. Computational studies (DFT) on Ru·Mn confirmed both the occurrence of the PET quenching pathway and its dependence on molecular conformation. In the complexes Ru·Ln and Ru·Ln2 (Ln = Nd, Yb), sensitised near-infrared luminescence from Nd(iii) or Yb(iii) is observed following photoinduced energy-transfer from the Ru(ii) core, with Ru → Nd energy-transfer being faster than Ru → Yb energy-transfer due to the higher density of energy-accepting states on Nd(iii).

(Invited) Development of Molecular Photocathodes Based on Metal Complex Photocatalyst and Their Application for Photoelectrochemical CO2 Reduction in Aqueous Electrolyte

Metal complex photocatalysts are promising candidates in CO2 photo-reduction systems in terms of its high efficiency and selectivity under visible light even in aqueous solution.[1] These systems, however, need the sacrificial reductive additives as electron donor due to the low oxidation power of photosensitizer unit. In this study, we developed novel “molecular” photocathodes consisting of Ru(II)-Re(I) metal complex photocatalyst (RuRe) and p-type semiconductor electrodes for visible-light-driven CO2 reduction (Figure). The application of the molecular photocathodes for photoelectrochemical CO2 reduction in aqueous solution was also examined. RuRe consists of tris-diimine Ru(II) unit as a photosensitizer and tricarbonyl diimine Re(I) unit as a catalyst, and has methylphosphonate anchoring group to adsorb on the electrode surface. p-type NiO electrode was synthesized and hybridized with RuRe to obtain molecular photocathode (RuRe/NiO).[2] This RuRe/NiO photocathode displayed photocathodic responses under the visible light (λex > 460 nm), which selectively photoexcites Ru(II) photosensitizer in RuRe, in a CO2-purged aqueous electrolyte without the need for any sacrificial additives. It reflects that the photo-excited RuRe can receive electrons from an external circuit through the NiO electrode. The onset potential for cathodic photocurrent was approx. -0.1 V vs. Ag/AgCl in a CO2-purged 50 mM NaHCO3 aqueous solution. Photoelectrochemical CO2 reduction using the RuRe/NiO photocathode was examined under the continuous irradiation at the potential of -0.7 V vs. Ag/AgCl, resulting that the catalytic amount of CO was observed. Its turnover number for CO formation, which was based on the amount of RuRe on the NiO, was 32 for 12 h irradiation. These results clearly suggest that the immobilized metal complex photocatalyst (RuRe) functions to drive photoreduction of CO2 with using electrons supplied from the NiO electrode. We next developed p-type CuGaO2 electrode as an alternative of NiO for such molecular photocathode with RuRe.[3] The resulting RuRe/CuGaO2 photocathode also shows photoelectrochemical activity for CO2 reduction under visible light irradiation. The photocathodic responses was obtained from approx. +0.3 V vs. Ag/AgCl, which is equivalent to +0.9 V vs. RHE. This value of onset potential is around 0.4 V positive than that for RuRe/NiO, which means that the utilization of CuGaO2 as an electrode material enlarged the region of working potential for CO2 reduction to positive direction. This tendency agreed well to the flat band potentials of these semiconductor electrodes obtained from the electrochemical impedance spectroscopy. The turnover number of CO formation using the RuRe/CuGaO2 photocathode reached to 125 for 15 h irradiation, while H2 was also generated as byproduct. These results indicate the advantage of CuGaO2 for usage of an electrode material for molecular photocathode from the aspect of efficient interfacial electron injection. In addition, these photocathodes were investigated to combine with a CoO x /TaON photoanode for the oxidation of water.[4] The constructed photoelectrochemical cells consisting of these molecular photocathodes and the CoO x /TaON photoanode enabled the visible-light-driven catalytic reduction of CO2 with water oxidation to obtain CO and O2 as the products. These systems successfully demonstrated CO2 reduction using water as an electron donor by means of combined photocatalytic abilities of both the molecular metal complex (RuRe) and the semiconductor material (TaON). To the best of our knowledge, these are the sole examples of visible-light-driven CO2 reduction systems using water as the reductant, based on molecular photocatalysts with hybridizing semiconductor. [1] Y. Yamazaki, H. Takeda and O. Ishitani, J. Photochem. Photobiol. C, 2015, 25, 106–137.; A. Nakada, K. Koike, K. Maeda and O. Ishitani, Green Chem., 2016, 18, 139–143. [2] G. Sahara, R. Abe, M. Higashi, T. Morikawa, K. Maeda, Y. Ueda and O. Ishitani, Chem. Commun., 2015, 51, 10722–10725.; G. Sahara, H. Kumagai, K. Maeda, N. Kaeffer, V. Artero, M. Higashi, R. Abe and O. Ishitani, J. Am. Chem. Soc., 2016, 138, 14152–14158. [3] H. Kumagai, G. Sahara, K. Maeda, M. Higashi, R. Abe and O. Ishitani, Chem. Sci., 2017, 8, 4242–4249. [4] M. Higashi, K. Domen and R. Abe, J. Am. Chem. Soc., 2012, 134, 6968–6971. Figure 1

Investigation of excited state, reductive quenching, and intramolecular electron transfer of Ru(ii)-Re(i) supramolecular photocatalysts for CO2 reduction using time-resolved IR measurements.

Supramolecular photocatalysts in which Ru(ii) photosensitizer and Re(i) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO2 reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the 3MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.0-1.1) × 104 s-1 as a major component and (3.5-4.3) × 106 s-1 as a minor component, in acetonitrile. The produced charge-separated state has a long lifetime, with charge recombination rate constants of only (6.5-8.4) × 104 s-1. Thus, although it has a large driving force (-ΔG0CR ∼ 2.6 eV), this process is in the Marcus inverted region. On the other hand, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH), reductive quenching of the excited Ru unit proceeds much faster (kq[BNAH (0.2 M)] = (3.5-3.8) × 106 s-1) than the abovementioned intramolecular oxidative quenching, producing the one-electron-reduced species (OERS) of the Ru unit. Nanosecond TRIR data clearly show that intramolecular electron transfer from the OERS of the Ru unit to the Re unit (kET > 2 × 107 s-1) is much faster than from the excited state of the Ru unit, and that it is also faster than the reductive quenching process of the excited Ru unit by BNAH. To measure the exact value of kET, picosecond TRIR spectroscopy and a stronger reductant were used. Thus, in the case of the binuclear complex with tri(p-fluorophenyl)phosphine ligands (RuRe(FPh)), for which intramolecular electron transfer is expected to be the fastest among the three binuclear complexes, in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), kET was measured as kET = (1.4 ± 0.1) × 109 s-1. This clearly shows that intramolecular electron transfer in these RuRe binuclear supramolecular photocatalysts is not the rate-determining process in the photocatalytic reduction of CO2, which is one of the main reasons why they work so efficiently.

Highly Fluorescent and Stable Ruthenium Unit/Layered Double Hydroxide Composite with Sensitive Detection of Cr2O72−

AbstractA highly stable and intensely fluorescent composite was assembled by incorporating ruthenium functional unit (Ru unit) into layered double hydroxide matrices. This composite inherits the photoluminescence property of Ru unit but exhibits enhanced photoluminescence with a long lifetime. Importantly, it acts as an excellent chemosensor for sensitive detection on Cr2O72−. Such detection resulted in visible photoluminecence quenching. Furthermore, the composite is remarkably stable in the detection conditions without Ru unit leaking out. Therefore, these results provide us a simple, sensitive, and effective approach to detect Cr2O72−.

Trans-C versus Cis-C thermally induced isomerisation of a terpyridine adduct of cytotoxic cycloruthenated compound

Two novel cycloruthenated compounds, 2 and 3, derived from cycloruthenated 2-phenyl pyridine (Ru(PhPy)) have been synthesized by adding one equivalent of 2,2′;6′,2″-terpyridine (terpy) to [Ru(2-C6H4-2′-C5H4N-κC,N)(MeCN)4]PF6 in MeOH/MeCN (16:1) at reflux temperature as a mixture of 1:1 isomers. The structures of both compounds that were formed under kinetic and thermodynamic control respectively, differ by the position of the central pyridine unit of terpy that was found trans or cis to the C-Ru bond of the Ru(PhPy) unit for 2 and 3 respectively. Whereas 2 did not afford substitution reactions when treated with any ligand, the substitution by MeOH or H2O of the MeCN trans to C in 3 could be followed by UV-vis spectroscopy. Moreover, the reaction in MeOH between 3 and imidazoles afforded new cycloruthenated compounds, 6a and 6b whose cytotoxicities, together with that of 2 and 3, against HCT116 and AGS cancer cells were determined.

Light-Induced Activation of a Molybdenum Oxotransferase Model within a Ru(II)-Mo(VI) Dyad.

Nature uses molybdenum-containing enzymes to catalyze oxygen atom transfer (OAT) from water to organic substrates. In these enzymes, the two electrons that are released during the reaction are rapidly removed, one at a time, by spatially separated electron transfer units. Inspired by this design, a Ru(II)-Mo(VI) dyad was synthesized and characterized, with the aim of accelerating the rate-determining step in the cis-dioxo molybdenum-catalyzed OAT cycle, the transfer of an oxo ligand to triphenyl phosphine, via a photo-oxidation process. The dyad consists of a photoactive bis(bipyridyl)-phenanthroline ruthenium moiety that is covalently linked to a bioinspired cis-dioxo molybdenum thiosemicarbazone complex. The quantum yield and luminescence lifetimes of the dyad [Ru(bpy)2(L2)MoO2(solv)]2+ were determined. The major component of the luminescence decay in MeCN solution (τ = 1149 ± 2 ns, 67%) corresponds closely to the lifetime of excited [Ru(bpy)2(phen-NH2)]2+, while the minor component (τ = 320 ± 1 ns, 31%) matches that of [Ru(bpy)2(H2-L2)]2+. In addition, the (spectro)electrochemical properties of the system were investigated. Catalytic tests showed that the dyad-catalyzed OAT from dimethyl sulfoxide to triphenyl phosphine proceeds significantly faster upon irradiation with visible light than in the dark. Methylviologen acts as a mediator in the photoredox cycle, but it is regenerated and hence only required in stoichiometric amounts with respect to the catalyst rather than sacrificial amounts. It is proposed that oxidative quenching of the photoexcited Ru unit, followed by intramolecular electron transfer, leads to the production of a reactive one-electron oxidized catalyst, which is not accessible by electrochemical methods. A significant, but less pronounced, rate enhancement was observed when an analogous bimolecular system was tested, indicating that intramolecular electron transfer between the photosensitizer and the catalytic center is more efficient than intermolecular electron transfer between the separate components.

Photocatalyses of Ru(II)–Re(I) binuclear complexes connected through two ethylene chains for CO2 reduction

We synthesized two novel supramolecular photocatalysts for CO2 reduction [Ru(BL1)Re(CO)3Cl]2+ (12+) and [Ru(BL1)Re(CO)2{P(p-F-C6H4)3}2]3+ (33+), where BL1 is two 2,2′-bipyridine moieties connected through two ethylene chains. Compared to the corresponding supramolecular photocatalysts (22+, 43+) with only one ethylene chain in the bridging ligand (BL2), these complexes had stronger electronic interactions between the Ru and Re units through the two ethylene chains, which strengthened the oxidation power of the excited Ru unit. Although BL1 compounds functioned as photocatalysts for CO2 reduction by selectively providing CO, their photocatalyses were very different, especially, when compared with BL2 compounds. In the case of 12+, the quantum yield of CO formation (ΦCO) was larger than that of the corresponding BL2 compound. Conversely, ΦCO of the system using 33+ was approximately 40% less than the corresponding BL2 compound. The reasons for these differences were successfully clarified through mechanistic studies of these photocatalytic CO2 reductions.

Efficient light harvesting via sequential two-step energy accumulation using a Ru–Re5 multinuclear complex incorporated into periodic mesoporous organosilica

An efficient artificial light harvesting (LH) system was developed via sequential two-step energy accumulation. A periodic mesoporous organosilica with bridging biphenyl groups in the framework (Bp–PMO) was used as an LH antenna. The center of a linear-shaped Re(I) pentanuclear complex was connected to a Ru(II) trisdiimine complex through a covalent bond, these served as the first and second energy acceptors, respectively, (Ru–Re5). Hybridization was achieved with the non-ionic surfactant C12H25(OCH2CH2)4OH in an acetonitrile solution, and in the hybrid (Ru–Re5–Bp–PMO), the Ru–Re5 molecules were adsorbed in an orderly fashion in the mesopores of the Bp–PMO. Photons absorbed by 437 ± 43 of the Bp units were first accumulated in the five Re units in Ru–Re5 and then transferred to only one Ru unit, which emitted the light strongly.

Preparation of tetrabutylammonium salt of a mono-Ru(iii)-substituted α-Keggin-type silicotungstate with a 4,4′-bipyridine ligand and its electrochemical behaviour in organic solvents

The tetrabutylammonium (TBA) salt of a mono-ruthenium(III)-substituted α-Keggin-type silicotungstate with a 4,4'-bipyridine (bipy) ligand, TBA5[α-SiW11O39Ru(III)(bipy)] (1), which is soluble in various organic solvents, was prepared by a cation exchange reaction of Cs5[α-SiW11O39Ru(III)(bipy)] with tetrabutylammonium bromide. Compound 1 was characterised using IR, (1)H-NMR, elemental analysis, single crystal X-ray analysis, X-ray absorption near-edge structure (XANES) analysis (Ru L3-edge), electron spin resonance (ESR), cyclic voltammetry (CV) and UV-Vis. Single crystal X-ray analysis of 1 revealed that the Ru(III) unit was incorporated into the α-Keggin-type silicotungstate framework and coordinated by a bipy molecule through a Ru-N bond. CV indicated that the incorporated Ru(III)-bipy was reversibly oxidised to the Ru(IV)-bipy derivative and reduced to the Ru(II)-bipy derivative in organic solvents. The redox potential of Ru(IV/III)-bipy was found to be affected by organic solvents. Moreover, the Ru(V)-bipy derivative was observed in acetonitrile.

Octacarbonyldi-μ2-hydrido-[μ3-(1,3,5-trimethylphenyl)phosphinidene](triphenylphosphane)-triangulo-triruthenium

In the crystal structure of the title compound, [Ru3(C9H11P)H2(C18H15P)(CO)8], the triangular Ru3 unit is capped with one mesitylphosphin­idene ligand. In the trigonal–pyramidal Ru3P core, one RuII atom is coordinated by a triphenyl­phosphane ligand in a terminal fashion. Two hydride ligands bridge over two Ru—Ru bonds. These Ru—Ru bonds [2.9400 (4) and 2.9432 (4) Å] are slightly longer than the nonhydride-bridged Ru—Ru bond [2.8146 (4) Å]. The terminal triphenyl­phosphane ligand coordinates to the RuII atom, which is involved in two hydride bridges.

Visible‐Light‐Driven Generation of High‐Valent Oxo‐Bridged Dinuclear and Tetranuclear Manganese Terpyridine Entities Linked to Photoactive Ruthenium Units of Relevance to Photosystem II

AbstractWe report on a new heterobimetallic manganese–ruthenium complex, in which a MnII bis(terpyridine) centre is covalently linked to two RuII tris(bipyridine) units. By electro‐ or photoinduced oxidation of this Mn centre, high‐valent multinuclear oxo‐bridged manganese species can be generated. These were unambiguously identified by electrochemistry and UV/Vis and electron paramagnetic resonance (EPR) spectroscopy. In a CH3CN solution of water (1 M), the dinuclear core, [(H2O)(terpy)MnIII(μ‐O)2MnIV(terpy)(H2O)]3+ (terpy = 2,2′:6′,2″‐terpyridine) is first obtained by dimerisation, and can be further oxidised to give rise to the tetranuclear core, [(H2O)(terpy)MnIV(μ‐O)2MnIV(terpy)(μ‐O)(terpy)MnIV(μ‐O)2MnIV(terpy)(H2O)]6+. These cores are connected to two and four [Ru(bpy)3]2+ units, respectively. The second dimerisation process is prevented when coordinating CF3CO2– anions are present in solution, as oxidation of the initially generated species, [(CF3CO2)(terpy)MnIII(μ‐O)2MnIV(terpy)(CF3CO2)]+, produces the stable dinuclear [(CF3CO2)(terpy)MnIV(μ‐O)2MnIV(terpy)(CF3CO2)]2+. All these species are generated in high yields either by exhaustive electrolysis or continuous visible‐light irradiation. A diazonium salt is required as the irreversible electron acceptor for photochemical experiments. Molecular mechanics calculations confirm that the formation of the higher nuclearity complex with a tetranuclear Mn central core linked to four Ru unit is feasible. The Ru–Ru distances are long enough to preclude possible steric hindrance of the Ru units.

Electrochemiluminescent Monomers for Solid Support Syntheses of Ru(II)-PNA Bioconjugates: Multimodal Biosensing Tools with Enhanced Duplex Stability

The feasibility of devising a solid support mediated approach to multimodal Ru(II)-peptide nucleic acid (PNA) oligomers is explored. Three Ru(II)-PNA-like monomers, [Ru(bpy)(2)(Cpp-L-PNA-OH)](2+) (M1), [Ru(phen)(2)(Cpp-L-PNA-OH)](2+) (M2), and [Ru(dppz)(2)(Cpp-L-PNA-OH)](2+) (M3) (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, dppz = dipyrido[3,2-a:2',3'-c]phenazine, Cpp-L-PNA-OH = [2-(N-9-fluorenylmethoxycarbonyl)aminoethyl]-N-[6-(2-(pyridin-2yl)pyrimidine-4-carboxamido)hexanoyl]-glycine), have been synthesized as building blocks for Ru(II)-PNA oligomers and characterized by IR and (1)H NMR spectroscopy, mass spectrometry, electrochemistry and elemental analysis. As a proof of principle, M1 was incorporated on the solid phase within the PNA sequences H-g-c-a-a-t-a-a-a-a-Lys-NH(2) (PNA1) and H-P-K-K-K-R-K-V-g-c-a-a-t-a-a-a-a-lys-NH(2) (PNA4) to give PNA2 (H-g-c-a-a-t-a-a-a-a-M1-lys-NH(2)) and PNA3 (H-P-K-K-K-R-K-V-g-c-a-a-t-a-a-a-a-M1-lys-NH(2)), respectively. The two Ru(II)-PNA oligomers, PNA2 and PNA3, displayed a metal to ligand charge transfer (MLCT) transition band centered around 445 nm and an emission maximum at about 680 nm following 450 nm excitation in aqueous solutions (10 mM PBS, pH 7.4). The absorption and emission response of the duplexes formed with the cDNA strand (DNA: 5'-T-T-T-T-T-T-T-A-T-T-G-C-T-T-T-3') showed no major variations, suggesting that the electronic properties of the Ru(II) complexes are largely unaffected by hybridization. The thermal stability of the PNA·DNA duplexes, as evaluated from UV melting experiments, is enhanced compared to the corresponding nonmetalated duplexes. The melting temperature (T(m)) was almost 8 °C higher for PNA2·DNA duplex, and 4 °C for PNA3·DNA duplex, with the stabilization attributed to the electrostatic interaction between the cationic residues (Ru(II) unit and positively charged lysine/arginine) and the polyanionic DNA backbone. In presence of tripropylamine (TPA) as co-reactant, PNA2, PNA3, PNA2·DNA and PNA3·DNA displayed strong electrochemiluminescence (ECL) signals even at submicromolar concentrations. Importantly, the combination of spectrochemical, thermal and ECL properties possessed by the Ru(II)-PNA sequences offer an elegant approach for the design of highly sensitive multimodal biosensing tools.

Ru–Pt and Ru–Pd heterobimetallic complexes based on a new ligand with two distinct chelate sites

A new ligand p-[N-2-(2'-pyridyl)benzimidazolyl]-[N-2-(2'-pyridyl)indolyl]-benzene (L1) has been synthesized and fully characterized. L1 has two distinct chelating sites: one N,N-chelate site and one N,C-chelate site. This ligand has been found to be very effective in selective binding to two different metal ions. Two new heterobimetallic complexes Ru-Pt and Ru-Pd using L1 as the bridging ligand have been successfully synthesized and fully characterized. To understand the mutual influence of the two metal centers on electronic and photophysical properties, the corresponding monometallic Ru(II), Pt(II) and Pd(II) compounds have also been synthesized and investigated. All Ru(II)-containing complexes have been found to be luminescent. Electronic communication between the two different metal centers in the heterobimetallic compounds was found to be weak. The Pt(II) moiety appears to enhance the phosphorescent efficiency of the Ru(II) unit while the Pd(II) analogue has little influence.