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Reactions of the triruthenium clusters {Cp*Ru(μ-H)}3(μ3-H)2 (1) and (Cp*Ru)3(μ3-H)(μ-H)2(μ-CO) (12) (Cp* = η5-C5Me5) with benzonitrile were investigated in relation to the selective hydrogenation of benzonitrile to benzylamine. Benzonitrile undergoes consecutive transformations into μ3-η2:η2(⊥)-nitrile, μ3-η²:η²(⊥)-imidoyl, μ3-η²(∥)-imidoyl, μ3-η²-alkylideneamido, μ3-imido, and μ-amido ligands on the Ru3 plane accompanied by the uptake of dihydrogen. The reactions are analogues of nitrile hydrogenation on a metal surface. The complexes are structural models of chemisorbed species and catalyze the hydrogenation of benzonitrile. Complex 1 catalyzes benzonitrile hydrogenation without additives but exhibits only moderate selectivity toward benzylamine. Although the μ3-benzylimido complex {Cp*Ru(μ-H)}3(μ3-NCH2Ph) (4) was obtained by reaction of 1 with benzonitrile, it was readily transformed into to the μ3-imidoyl complexes (Cp*Ru)3(μ-H)2{μ3-η²:η²(⊥)-PhCNH} (3) and (Cp*Ru)3(H)4{μ3-η²(∥∥)-PhCNH} (4), which are key intermediates in secondary imine formation. Two benzonitrile molecules were incorporated on the Ru3 plane under the reaction conditions, which decreases the selectivity of primary amine formation. In contrast, μ-carbonyl complex 12 suppresses the incorporation of additional benzonitrile ligands and the formation of μ3-imidoyl species due to the presence of CO. These features of 12 bring about significant improvement in the selectivity of benzonitrile hydrogenation and produce benzylamine in 92% yield. [ABSTRACT FROM AUTHOR]

The electronic structures of the prototypical bimetallic buta-1,3-diyn-1,4-diyl-bridged radical cation complexes [{M(dppe)Cp'}2(μ-C=CC=C)]+ (M= Fe, Cp' = Cp* (1a), Cp (1b); M = Ru, Cp' = Cp* (2a), Cp (2b)) have been (re)investigated using a combination of UV-vis-NIR and IR spectroelectrochemistry, and quantum chemical calculations based on both dispersion-corrected global (BLYP35-D3) and local (Lh-SsirPW92-D3) hybrid functionals. Following analysis of new and existing data, including the IR-active ν(C=C) bands, the iron compounds [1]+ are reclassified as valence-trapped (Robin and Day Class II) mixed-valence complexes, in contrast to the ruthenium complexes [2]+, which are delocalized (Robin and Day Class III) systems. All members of the series exist as a thermally populated distribution of conformers in solution, and the overlapping spectroscopic profiles make the accurate extraction of the parameters necessary for the analysis of [1]+ and [2]+ within the framework of the Marcus-Hush model extremely challenging. Analysis of the spin-density distributions from a range of conformational minima provides an alternative representation of the degree of charge localization, and a comparison between members of the series is presented. [ABSTRACT FROM AUTHOR]

Indole thiosemicarbazone ligands were prepared from indole-3-carboxaldehyde and N-(un)substituted thiosemicarbazide. The Ru(η6-p-cymene) complexes [Ru(η6-p-cymene)(HL1)Cl]Cl (1) and [Ru(η6-p-cymene)(L2)]2Cl2 (2*) were exclusively synthesized from thiosemicarbazone (TSC) ligands HL1 and HL2, and [RuCl2(p-cymene)]2. The compounds were characterized by analytical and various spectroscopic (electronic, FT-IR, 1D/2D NMR, and mass) tools. The exact structures of the compounds (HL1, HL2, 1, and 2*) were confirmed by single-crystal X-ray diffraction technique. In complexes 1 and 2*, the ligand coordinated in a bidentate neutral (1)/monobasic (2*) fashion to form a five-membered ring. The complexes showed a piano-stool geometry around the Ru ion. While 2* existed as a dimer, 1 existed as a monomer, and this was well explained through free energy, bond parameter, and charge values computed at the B3LYP/SDD level. The intercalative binding mode of the complexes with calf thymus DNA (CT DNA) was revealed by spectroscopic and viscometric studies. The DNA (pUC19 and pBR322 DNA) cleavage ability of these complexes evaluated by an agarose gel electrophoresis method confirmed significant DNA cleavage activity. Further, the interaction of the complexes with bovine serum albumin (BSA) was investigated using spectroscopic methods, which disclosed that the complexes could bind strongly with BSA. A hemolysis study with human erythrocytes revealed blood biocompatibility of the complexes. The in vitro anticancer activity of the compounds (HL1, HL2, 1, and 2*) was screened against two cancer cell lines (A549 and HepG-2) and one normal cell line (L929). Interestingly, the binuclear complex 2* showed superior activity with IC50 = 11.5 μM, which was lower than that of cisplatin against the A549 cancer cell line. The activity of the same complex (IC50 = 35.3 μM) was inferior to that of cisplatin in the HepG-2 cancer cell line. Further, the apoptosis mode of cell death in the cancer cell line was confirmed by using confocal microscopy and DNA fragmentation analysis. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of a series of arene ruthenium complexes bearing either (3,5-cycloheptadienyl)diphenylphosphine or (cycloheptyl)diphenylphosphine are reported. Upon irradiation or heating, all these complexes lose their arene ligand but then exhibit a different behavior depending on the nature of the phosphine ligand. (Cycloheptadienyl)phosphine complexes 1 and 3 give a cationic dinuclear Ru complex 5 for which the two Ru atoms are bridged by three chlorido ligands and flanked by two tridendate (cycloheptadienyl)phosphines. (Cycloheptyl)diphenylphosphine complexes 2 and 4 undergo arene exchange when toluene is used as solvent or degrade in dichloromethane. ATRA catalytic trials conducted in parallel with these complexes using CCl4 and styrene as standard substrates, highlighted the deep impact of the dienyl moiety on the results. Under smooth conditions (UV irradiation or moderate heating), only (cycloheptyl)phosphine derivatives give Karasch adduct in satisfactory yields. Their performance was considerably improved by combining irradiation and heating. At higher temperature, cationic dinuclear complex 5 was revealed as active and robust, giving turnover numbers as high as 9700 when tetradecene and CCl4 were used as substrates. [ABSTRACT FROM AUTHOR]

We describe the synthesis of mono- and bis([6]helicene-≡-Ru(dppe)2-≡)-DTE complexes 1o,c and 2o,c. The photochromic and electrochromic properties were studied, and attractive isomerization processes were observed, i.e., spontaneous reopening of [1c]•+ to [1o]•+ and ring closure of [2o]2+ to [2c]2+. Due to strong chiroptical responses, all of these chiral complexes correspond to new types of chiroptical switches (triggered by light and/or redox stimuli) and could be described as either "NOR" or "OR" logic gates. [ABSTRACT FROM AUTHOR]

The cyclometalated rhodium, iridium, and ruthenium complexes of N-methoxy-4-nitrobenzamide were synthesized by treatment of N-methoxy-4-nitrobenzamide with [Cp*MCl2]2] (M = Rh and Ir) or [(p-cymene)RuCl2]2 via C-H bond activation. The molecular structures of all these complexes were determined by single-crystal X-ray diffraction analysis. When the cyclometalated rhodium and ruthenium complexes were used as catalysts in the C-H bond functionalization reactions reported before, the desired products were obtained in high yields, proving that these cyclometalated complexes were probably the key catalytic intermediates of the C-H bond functionalization reactions of N-methoxybenzamide derivatives. The stoichiometric reactions of the cyclometalated rhodium, iridium, and ruthenium complexes with diphenylacetylene afforded the corresponding seven-membered metallacycles with insertion of two molecules of diphenylacetylene in 24, 6, and 20% yields, respectively, through the tandem reaction. However, the cyclometalated iridium complex could not catalyze the reaction of N-methoxy-4-nitrobenzamide with diphenylacetylene. This work provides a further understanding for the mechanisms of rhodium- and ruthenium-catalyzed C-H bond functionalization reactions of N-methoxybenzamide derivatives. [ABSTRACT FROM AUTHOR]

A diruthenium μ-phosphane complex, {Cp‡Ru(μ-H)}2(μ-η²-HPtBu2) (2a; Cp‡ = 1,2,4-C5tBu3H2), which features an agostic Ru-H-P interaction at one ruthenium center, was synthesized by the reaction of Cp‡Ru(μ-H)4RuCp‡ (1) with PtBu2H. The agostic Ru-H-P interaction was stabilized by the bulky Cp‡ and tBu groups. The agostic Ru-H-P interaction in 2a was unambiguously confirmed by X-ray diffraction. The rapid site exchange at the P-H and Ru-H-Ru positions indicated that the η²-P-H bond underwent oxidative addition in solution, yielding a μ-phospanido isomer, {Cp‡Ru(μ-H)}2(H)(μ-PtBu2) (3a). The population of the μ-phosphanido isomer increased when the size of the substituent at the phosphorus atom was reduced, and μ-phosphanido complex 3b was exclusively obtained by the reaction of 1 with PPh3. [ABSTRACT FROM AUTHOR]

The pH-dependent intramolecular chelation of a tethered sulfonamide ligand in ruthenium(II) arene complexes is demonstrated, a process shown to modulate metal-centered reactivity toward the model ligand guanosine 5'-monophosphate within the physiologically relevant pH region. [ABSTRACT FROM AUTHOR]

We present a C-H activation protocol for aromatic compounds that overcomes the current limitations of the need for a directing group or covalently bound activating groups, by exploiting the increase in C-H acidity of aromatic compounds on π-coordination to a Ru(II) center. The increased acidity facilitates catalytic concerted metalation-deprotonation and subsequent arylation reactions. We present the development and optimization of the C-H activation protocol and show the applicability of the reaction to a range of aromatic substrates, including the simplest of substrates (benzene). Furthermore, we demonstrate the recyclability of the activating Ru(II) fragment using photolysis and give a mechanistic study, which provides strong evidence that this reaction occurs via a silver-mediated C-H bond activation. This is the first time Ru complexes have been shown to allow C-H activation of arenes by a π-coordination mechanism. [ABSTRACT FROM AUTHOR]

A stereoselective metal-mediated cycloaromatization of chiral conjugated dienynes and enediynes is described. For dienyne cycloaromatization, placement of the carbon stereocenter in the allylic position gives the highest diastereomeric ratios (dr). The observed stereoselectivity depends on the steric bulk of the alkyne substituent, as replacing a propargylic methyl for trimethylsilyl increases the dr from 56:44 to 80:20. For both enediyne and dienyne substrates, [(η5-C5Me5)Ru(NCMe)3]PF6 exhibits greater diastereoselectivity than does [(η5-C5H5)Ru(NCMe)3]PF6. For the same chiral enediyne substrate, [(η5-C5Me5)Ru(NCMe)3]PF6 generates a 4:1 ratio of diastereomeric arene products, whereas both [(η5-C5Me4CF3)Ru(NCMe)3]PF6 and [(η5-C5H5)Ru(NCMe)3]PF6 generate a 1:1 product mixture, indicative of a significant electronic influence of the ancillary ligand on diastereoselectivity. X-ray structure determination of several isolated ruthenium arene diastereomers confirms the assigned relative stereochemistry for the major and minor stereoisomeric metal arene products. Arene-binding experiments demonstrate that the observed stereoselectivity does not involve complexation of free arene by ruthenium. [ABSTRACT FROM AUTHOR]

Pincer-type ruthenium(II)-NNN complex catalysts bearing a chiral bis(NHTs)-substituted imidazolyl-oxazolinyl-pyridine ligand were synthesized and structurally characterized by NMR, IR, elemental analysis, and X-ray single-crystal crystallographic determinations. The two NHTs groups substituted on the imidazolyl moiety of the chiral NNN ligand exhibited a remarkable effect on the enantioselectivity of the Ru(II)-NNN complexes for the asymmetric transfer hydrogenation (ATH) of ketones. The Ru(II)-NNN complex bearing a chiral (NHTs)2-substituted imidazolyl-(isopropyl)oxazolinyl-pyridine ligand exhibited excellent catalytic activity, reaching an enantioselectivity up to 99.9% ee for the target alcohol products. [ABSTRACT FROM AUTHOR]

Ruthenium benzylidene complexes containing a carbodicarbene (CDC) ligand are reported. Mechanistic studies indicate that the CDC ligand can dissociate under relatively mild conditions to afford active olefin metathesis catalysts. These catalysts were found to be effective at ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions [ABSTRACT FROM AUTHOR]

The mechanism of the dimerization of terminal aryl alkynes promoted by [{RuCl(μ-Cl)(η6-p-cymene)}2](1)/AcOH, under cooperative transition metal/Brønsted acid catalysis, has been investigated with regard to (i) the activation of the dinuclear ruthenium complex and (ii) the catalytic formation of the trans-1,4-diaryl-1,3-enyne products, by a detailed kinetic investigation of both processes. Complex 1 is subject to a slow solvolytic process in neat acetic acid or is transformed rapidly in the presence of sodium acetate to form the monomeric ruthenium(II) acetato complex [RuCl(η6-p-cymene)(OAc)]. The latter is the active catalytic species promoting the alkyne dimerization process, via initial π-alkyne coordination and intramolecular C-H abstraction by the acetate ligand, as key steps of the catalytic cycle. The presence of additive acetate salts allows for the reaction to proceed at room temperature with short reaction times and high trans/cis stereoselectivity, thus rendering this catalytic system among the most active and selective procedures for the dimerization of terminal alkynes in a protic medium. The linear coupling of three molecules of phenylacetylene affords an organometallic ruthenium complex featuring a butenynyl ligand which has been characterized by X-ray crystallography. [ABSTRACT FROM AUTHOR]

Dinuclear ruthenium(II)-NNN complexes were efficiently assembled by means of coordinatively unsaturated 16-electron mononuclear ruthenium(II)-pyrazolyl-imidazolyl-pyridine complex and 4,4'-linked bipyridine ligands. The diruthenium(II)-NNN complex assembled through 4,4'-(CH2)3-bipyridine exhibited exceptionally high catalytic activity for the transfer hydrogenation (TH) of ketones in refluxing 2-propanol and reached TOF values up to 1.4 × 107 h-1, demonstrating a remarkable cooperative effect from the ruthenium(II)-NNN functionalities. [ABSTRACT FROM AUTHOR]

A series of four mononuclear and one dinuclear ruthenium styryl complexes with redox active triarylmethylium ligands were prepared. All new compounds were characterized by NMR spectroscopy, MS spectrometry, cyclic voltammetry, and in various oxidation states by IR and UV/vis/NIR and EPR spectroscopy. The increase in conjugation length by the introduction of the vinyl ruthenium entity pushes the electronic absorption to low energy, almost into the near-infrared region. Electrochemical and spectroscopic properties are strongly influenced by the para substituents at the triarylmethylium ligands. The complexes were characterized in up to four different oxidation states up to the trication level and show pronounced electrochromism. The oxidized mixed-valent diruthenium complex 52+ shows a moderate degree of charge and spin delocalization over the styryl ruthenium sites. [ABSTRACT FROM AUTHOR]

The dehydrogenative oxidation of cyclic amines catalyzed by a diruthenium complex and mechanistic studies are described. Cyclic amines and water reacted in the presence of Cp‡Ru(μ-H)4RuCp‡ (1) (Cp‡ = 1,2,4-tri-tert-butylcyclopentadienyl) to afford lactams accompanied by elimination of the hydrogen gas. The reaction of hexamethylenimine with 1 at 160 °C afforded Cp‡Ru(μ-H)2(μ-C6H11N)RuCp‡ (2), having a novel μ-cyclic imine ligand through N-H and C-H bond cleavages. Further C-H bond cleavage of 2 proceeded at 180 °C to afford Cp‡Ru(μ-H)(μ-η²:η²-C6H10N)RuCp‡ (4), having a perpendicularly coordinated imidoyl ligand. Complex 4 readily reacted with water and liberated ε-caprolactam. The cooperative interaction of the two ruthenium atoms leading to N-H and double C-H bond cleavages was the key to the dehydrogenative oxidation of cyclic amines. [ABSTRACT FROM AUTHOR]

The preference for concave mode binding of the CpM unit with sumanene in CpM(η6-sumanene)+ (M = Fe, Ru, Os) over the convex mode is analyzed by various density functional theory based methods including (or not) dispersion and solvent effects. In the case of the iron complex, the concave-bound isomer becomes energetically more favorable than the convex form only after the proper inclusion of dispersion effects, highlighting the importance of such contributions to stabilize the former arrangement. For the ruthenium complex, both the dispersion and solvent effects should be taken into account to provide a correct trend. The noncovalent interaction index corroborates the role of dispersion in concave selectivity. Our computations also show that the presence of the counterion is not relevant for this selectivity, discarding the previously reported argument made by Okumura et al. [ABSTRACT FROM AUTHOR]

4-Ethynyl-2,2'-bipyridyl-substituted ruthenium alkynyl complexes have been prepared and used to access a range of binuclear homometallic ruthenium and heterometallic ruthenium-rhenium complexes. These have been characterized by a variety of spectroscopic and single-crystal X-ray diffraction experiments. The IR spectra of a number of these ruthenium alkynyls display multiple v(C≡C) bands in the IR spectra, which are rationalized in terms of putative conformational isomers, whose calculated infrared stretching frequencies are comparable to those obtained experimentally. The mononuclear alkynyl ruthenium complexes undergo reversible one-electron oxidations centered largely on the alkynyl ligands, as inferred from the significant shift in v(C≡C) frequency on oxidation, while the binuclear complex [Ru{C≡C-4-bpy-κ²-N,N'-RuClCp}(dppe)Cp*]+ undergoes initial oxidation at the very electron rich {RuCl(bpy)Cp} fragment, causing only a small change in v(C≡C). A combination of IR and UV--vis spectroelectrochemical experiments, supported by quantum chemical calculations on a selected range of conformers, led to the classification of [Ru{C≡C-4-bpy-κ²-N,N'-RuClCp}(dppe)Cp*]+ as a weakly coupled class II mixed-valence compound (Hab = 306 cm-1). These results indicate that there is improved electronic communication through the 4-ethynyl-2,2'-bipyridyl ligand in comparison to the analogous 5-ethynyl-2,2'-bipyridyl complexes (Hab = 17 cm-1). [ABSTRACT FROM AUTHOR]

The reaction of the 1,3-bis(pyrazol-3-yl)benzenes 1 with [RuCl(OAc)(PPh3)3] resulted in selective C-H cleavage at the 2-position of 1 to give the protic NCN pincer-type ruthenium(II) complexes [RuCl(R-NCN-LH2)(PPh3)2] (2; R-NCN-LH2 = 4-R-2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)phenyl). Similar cyclometalation with iridium trichloride followed by addition of triphenylphosphine led to the formation of the iridium(III) analogue [IrCl(But-NCN-LH2)(PPh3)]Cl (5). Treatment of the ruthenium complexes 2 with carbon monoxide afforded the carbonyl complexes [Ru(CO)(R-NCN-LH2)(PPh3)2]Cl (4). On the other hand, the pyridine analogue of 1, 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine (NNN-LH2), reacted with iridium trichloride to yield the protic NNN pincer-type complex [IrCl3(NNN-LH3)] (7). The stronger σ donation of the NCN pincer-type ligand in comparison with the analogous NNN ligand was suggested by the CO stretching frequencies of the ruthenium carbonyl complexes 4 as well as the M-Cl distances. The catalytic activity of the ruthenium complexes 2a,b toward transfer hydrogenation of a ketone was also evaluated. [ABSTRACT FROM AUTHOR]

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6'-dihydroxybipyridine (6,6'-dhbp)) for both their proton-responsive features and for metal-ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-pyOR)Cl]OTf complexes where R = tBu (1), H (2), or Me (3). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir(6,6'-dxbp)Cl]OTf, where x = hydroxy (4Ir) or methoxy (5Ir); 4Ir was reported previously, but 5Ir is new. The analogous ruthenium complexes were also tested using [(η6-cymene)Ru(6,6'-dxbp)Cl]OTf, where x = hydroxy (4Ru) or methoxy (5Ru); 4Ru and 5Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5Ir, and for two [Ag(NHC-pyOR)2]OTf complexes 6 (R = tBu) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1-5. In general, NHC-pyOR complexes 1-3 were modest precatalysts for both reactions. NHC complexes 1-3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3SP, which we characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4Ir) is 5-8 times more active than x = methoxy (5Ir). Notably, ruthenium complex 4Ru showed 95% of the activity of 4Ir. For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4Ir ≫ 4Ru and 4Ir ≈ 5Ir. Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6'-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2 hydrogenation and formic acid dehydrogenation. [ABSTRACT FROM AUTHOR]