Electron-rich Ru Research Articles

CO and CO2 fixation by Se-Ru-CO hydride clusters.

The selective insertion of CO and CO2 into the C-O and O-H bonds of alcohols by the Se-Ru-CO hydride clusters [(μ-H)Ru4(CO)10Se2](-) (1) and [(μ3-H)Ru5(CO)14Se](-) (2) was demonstrated by a cooperative effect of the protonic hydride, the electron-rich Ru atom, and the electronegative Se atom as well as the symmetry of the clusters. These reactions generated the first examples of Se-containing ruthenium carboxylate and alkylcarbonate clusters [{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4(μ-η(1):η(1)-OOCR)}](3-) (R = Me, 3; Et, 4) and [{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4(μ-η(1):η(1)-OOCOR)}](3-) (R = Me, 5; Et, 6), respectively. These results disclosed herein provide a new avenue for the capture and storage of CO and CO2 and useful synthetic routes to novel RCOO(-)- and ROCOO(-)-bridged ruthenium selenide clusters.

Synthesis, Characterization, and Reactivity of Ruthenium Hydride Complexes of N-Centered Triphosphine Ligands

The reactivity of the novel tridentate phosphine ligand N(CH2PCyp2)3 (N-triphos(Cyp), 2; Cyp = cyclopentyl) with various ruthenium complexes was investigated and compared that of to the less sterically bulky and less electron donating phenyl derivative N(CH2PPh2)3 (N-triphos(Ph), 1). One of these complexes was subsequently investigated for reactivity toward levulinic acid, a potentially important biorenewable feedstock. Reaction of ligands 1 and 2 with the precursors [Ru(COD)(methylallyl)2] (COD = 1,5-cycloocatadiene) and [RuH2(PPh3)4] gave the tridentate coordination complexes [Ru(tmm){N(CH2PR2)3-κ(3)P}] (R = Ph (3), Cyp (4); tmm = trimethylenemethane) and [RuH2(PPh3){N(CH2PR2)3-κ(3)P}] (R = Ph (5), Cyp (6)), respectively. Ligands 1 and 2 displayed different reactivities with [Ru3(CO)12]. Ligand 1 gave the tridentate dicarbonyl complex [Ru(CO)2{N(CH2PPh2)3-κ(3)P}] (7), while 2 gave the bidentate, tricarbonyl [Ru(CO)3{N(CH2PCyp2)3-κ(2)P}] (8). This was attributed to the greater electron-donating characteristics of 2, requiring further stabilization on coordination to the electron-rich Ru(0) center by more CO ligands. Complex 7 was activated via oxidation using AgOTf and O2, giving the Ru(II) complexes [Ru(CO)2(OTf){N(CH2PPh2)3-κ(3)P}](OTf) (9) and [Ru(CO3)(CO){N(CH2PPh2)3-κ(3)P}] (11), respectively. Hydrogenation of these complexes under hydrogen pressures of 3-15 bar gave the monohydride and dihydride complexes [RuH(CO)2{N(CH2PPh2)3-κ(3)P}] (10) and [RuH2(CO){N(CH2PPh2)3-κ(3)P}] (12), respectively. Complex 12 was found to be unreactive toward levulinic acid (LA) unless activated by reaction with NH4PF6 in acetonitrile, forming [RuH(CO)(MeCN){N(CH2PPh2)3-κ(3)P}](PF6) (13), which reacted cleanly with LA to form [Ru(CO){N(CH2PPh2)3-κ(3)P}{CH3CO(CH2)2CO2H-κ(2)O}](PF6) (14). Complexes 3, 5, 7, 8, 11, and 12 were characterized by single-crystal X-ray crystallography.

Cubic nonlinear optical properties of new zinc tetraphenyl porphyrins peripherally functionalized with electron-rich Ru(II) alkynyl substituents

We report the synthesis and characterization of new organometallic assemblies consisting of a central Zn(II) tetraphenylporphyrin (ZnTPP) core surrounded by four Ru(II) alkynyl complexes (trans-Ru(dppe)2R) appended at the para phenyl positions and acting as peripheral donor groups. The synthesis of the tetra-chlorido derivative 2 (R=Cl) is reported first, followed by that of three extended homologues (3–X) obtained from this building block by substitution of the chlorido ligands by functionalized arylacetylenes (R=CC(4-C6H4X); X=NO2, H, OMe). Measurement of the nonlinear absorption properties of the pentametallic derivatives 3–X reveals that these extended π-networks incorporating polarizable organometallic units behave as good two-photon absorbers in the visible and near-IR range.

Combining Very Large Quadratic and Cubic Nonlinear Optical Responses in Extended, Tris-Chelate Metallochromophores with Six π-Conjugated Pyridinium Substituents

We describe a series of nine new complex salts in which electron-rich Ru(II) or Fe(II) centers are connected via pi-conjugated bridges to six electron-accepting N-methyl-/N-arylpyridinium groups. This work builds upon our previous preliminary studies (Coe , B. J. J. Am. Chem. Soc. 2005, 127, 13399-13410; J. Phys. Chem. A 2007, 111, 472-478), with the aims of achieving greatly enhanced NLO properties and also combining large quadratic and cubic effects in potentially redox-switchable molecules. Characterization has involved various techniques, including electronic absorption spectroscopy and cyclic voltammetry. The complexes display intense, visible d --> pi* metal-to-ligand charge-transfer (MLCT) bands, and their pi --> pi* intraligand charge-transfer (ILCT) absorptions in the near-UV region show molar extinction coefficients as high as ca. 3.5 x 10(5) M(-1) cm(-1). Molecular quadratic nonlinear optical (NLO) responses beta have been determined by using hyper-Rayleigh scattering at 800 and 1064 nm and also via Stark (electroabsorption) spectroscopic studies. The directly and indirectly derived beta values are very large, with the Stark-based static first hyperpolarizabilities beta(0) reaching as high as ca. 10(-27) esu, and generally increase on extending the pi-conjugation and enhancing the electron-accepting strength of the ligands. Cubic NLO properties have also been measured by using the Z-scan technique, revealing relatively high two-photon absorption cross sections of up to 2500 GM at 750 nm.

Mixed-Valent Metals Bridged by a Radical Ligand: Fact or Fiction Based on Structure-Oxidation State Correlations

Electron-rich Ru(acac)2 (acac- = 2,4-pentanedionato) binds to the pi electron-deficient bis-chelate ligands L, L = 2,2'-azobispyridine (abpy) or azobis(5-chloropyrimidine) (abcp), with considerable transfer of negative charge. The compounds studied, (abpy)Ru(acac)2 (1), meso-(mu-abpy)[Ru(acac)2]2 (2), rac-(mu-abpy)[Ru(acac)2]2 (3), and (mu-abcp)[Ru(acac)2]2 (4), were calculated by DFT to assess the degree of this metal-to-ligand electron shift. The calculated and experimental structures of 2 and 3 both yield about 1.35 A for the length of the central N-N bond which suggests a monoanion character of the bridging ligand. The NBO analysis confirms this interpretation, and TD-DFT calculations reproduce the observed intense long-wavelength absorptions. While mononuclear 1 is calculated with a lower net ruthenium-to-abpy charge shift as illustrated by the computed 1.30 A for d(N-N), compound 4 with the stronger pi accepting abcp bridge is calculated with a slightly lengthened N-N distance relative to that of 2. The formulation of the dinuclear systems with monoanionic bridging ligands implies an obviously valence-averaged Ru(III)Ru(II) mixed-valent state for the neutral molecules. Mixed valency in conjunction with an anion radical bridging ligand had been discussed before in the discussion of MLCT excited states of symmetrically dinuclear coordination compounds. Whereas 1 still exhibits a conventional electrochemical and spectroelectrochemical behavior with metal centered oxidation and two ligand-based one-electron reduction waves, the two one-electron oxidation and two one-electron reduction processes for each of the dinuclear compounds Ru2.5(L*-)Ru2.5 reveal more unusual features via EPR and UV-vis-NIR spectroelectrochemistry. In spite of intense near-infrared absorptions, the EPR results show that the first reduction leads to Ru(II)(L*-)Ru(II) species, with an increased metal contribution for system 4*-. The second reduction to Ru(II)(L2-)Ru(II) causes the disappearance of the NIR band. One-electron oxidation of the Ru2.5(L*-)Ru2.5 species produces a metal-centered spin for which the alternatives RuIII(L0)Ru(II) or Ru(III)(L*-)Ru(III) can be formulated. The absence of NIR bands as common for mixed-valent species with intervalence charge transfer (IVCT) absorption favors the second alternative. The second one-electron oxidation is likely to produce a dication with Ru(III)(L0)Ru(III) formulation. The usefulness and limitations of the increasingly popular structure/oxidation state correlations for complexes with noninnocent ligands is being discussed.

Bonding and Substituent Effects in Electron-Rich Mononuclear Ruthenium σ-Arylacetylides of the Formula [(η2-dppe)(η5-C5Me5)Ru(C⋮C)-1,4-(C6H4)X][PF6]n(n= 0, 1; X = NO2, CN, F, H, OMe, NH2)

This study reports the isolation and the structural (X-ray), UV−vis, and NMR characterization of a series of electron-rich Ru(II) acetylide complexes of the formula (η2-dppe)(η5-C5Me5)Ru(C⋮C)-1,4-(C6H4)X (1a−f; X = NO2, CN, F, H, OMe, NH2) and (η2-dppe)(η5-C5Me5)Ru(C⋮C)-1,3-(C6H4)F (1c-m), as well as the spectroscopic (near-IR and ESR) in situ characterization of the corresponding elusive Ru(III) radical cations. The spectroscopic data are discussed in connection with DFT computations, and a consistent picture of the electronic structure of these Ru(II) and Ru(III) acetylide complexes is proposed. Notably, the strong reactivity of the Ru(III) radicals evidenced in this contribution constitutes a major difference with the relative stability of the known iron analogues.

Synthesis, characterization, and DNA binding properties of a series of Ru, Pt mixed-metal complexes.

A series of mixed-metal complexes coupling ruthenium light absorbers to platinum reactive metal sites through polyazine bridging ligands have been prepared of the form [(tpy)RuCl(BL)PtCl(2)](PF(6)) (BL = 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), 2,3-bis(2-pyridyl)benzoquinoxaline (dpb); tpy = 2,2':6',2' '-terpyridine). These systems possess electron-rich Ru metal centers bound to five polyazine nitrogens and one chloride ligand. This leads to complexes with low-energy Ru --> BL charge-transfer bands that are tunable with BL variation occurring at 544, 632, and 682 nm for dpp, dpq, and dpb, respectively. This tuning of the charge-transfer energy results from a stabilization of the BL(pi) orbitals in this series as evidenced by the cathodic shift in the first reduction of these complexes occurring at -0.50, -0.32, and -0.20 V vs Ag/AgCl, for dpp, dpq, and dpb, respectively. The chlorides bound to the Pt(II) center are substitutionally labile giving these complexes the ability to covalently bind to DNA. All three title bimetallics, [(tpy)RuCl(BL)PtCl(2)](PF(6)), avidly bind double-stranded DNA with t(1/2) = 1-2 min, substantially reducing the migration of DNA through an agarose gel. Details of the synthetic methods, FAB MS data, spectroscopic and electrochemical properties, and DNA binding studies are presented.

The addition of protons and metallic electrophiles to the electron-rich RuRu bond of [Ru 2(μ,-dan)(CO) 4(P iPr 3) 2]. X-ray structure of [Ru 2(μ-AgPPh 3)(μ-dan)(CO) 4(P iPr 3) 2][BF 4]·CH 2Cl 2 (dan=1,8-diamidonaphthalene)

The ruthenium(I) complex [Ru 2(μ-dan)(CO) 4(P iPr 3) 2] ( 1) (dan= 1,8-diamidonaphthalene) reacts with HBF 4·OEt 2, [AuCI(PPh 3)]/TIPF 6 and AgBF 4/PPh 3 to give the cationic complexes [Ru 2(μ-M)(μ- dan)(CO) 4(P iPr 3) 2] + (M=H (2), AuPPh 3 (3), AgPPh 3 ( 4)), while the reactions with [AuCl(tht)] (tht = tetrahydrothiophene) and AgO 2CCF 3 give the neutral derivatives [Ru 2(μ-M)(μ-dan)(CO) 4(p ipr 3) 2] (M=AuCI (5), AgO 2CCF 3 ( 6)). Complex 1 also reacts with SnCl 2 to give [Ru 2(μ-SnCle 2)(μ- dan)(CO) 4(P iPr 3) 2] (7), hut in solution it dissociates SnCI 2 unless a large excess of the latter is present. In all cases, the added electrophiles symmetrically bridge the RuRu bond of complex 1, as indicated by IR and NMR spectroscopies. The structure of complex 4 has been confirmed by X-ray diffraction methods.

Structure and reactivity of ammonia synthesis catalysts derived from CeRu 2 precursors: A study by in Situ X-ray absorption spectroscopy

The activation, steady-state performance, and poisoning of efficient ammonia synthesis catalysts derived from a CeRu 2 precursor have been studied by in situ X-ray absorption spectroscopy at 50 bar pressure and at temperatures up to 450°C. The EXAFS and near-edge data supported by earlier XRD studies under identical conditions indicate that the active phase consists of ultradispersed, electron-rich Ru entities with low metal-metal coordination and in intimate contact with the CeH 2+ x support. Oxidation of this material yields CeO 2/Ru; this system is catalytically inert and exhibits none of the unusual structural or electronic properties of the active CeH 2+ x /Ru catalyst.