Search

With the aim to design new water-soluble organometallic Ru(II) complexes acting as anticancer agents catalysing transfer hydrogenation (TH) reactions with biomolecules, we have synthesized four Ru(II) monocarbonyl complexes (1-4) featuring the 1,4-bis(diphenylphosphino)butane (dppb) ligand and different bidentate nitrogen (N^N) ligands, of general formula [Ru(OAc)CO(dppb)(N^N)]n (n = +1, 0; OAc = acetate). The compounds have been characterised by different methods, including 1H and 31P NMR spectroscopy, electrochemistry as well as single crystals X-ray diffraction in the case of 1 and 4. The compounds have also been studied for their hydrolysis in aqueous environment, and for the catalytic regioselective reduction of NAD+ to 1,4-NADH in aqueous solution with sodium formate as hydride source. Moreover, the stoichiometric and catalytic oxidation of 1,4-NADH have also been investigated by UV-Visible spectrophotometry and NMR spectroscopy. Overall, initial structure-activity relationships could be inferred which point towards the influence of the extension of the aromatic N^N ligand in the cationic complexes 1-3 on the TH in both reduction/oxidation processes. The neutral complex 4, featuring a picolinamidate N^N ligand, stands out as the most active catalyst for the reduction of NAD+, while being completely inactive towards NADH oxidation. The compound can also convert pyruvate into lactate in the presence of formate, albeit with scarce efficiency. In any case, for all compounds, Ru(II) hydride intermediates could be observed and even isolated in the case of complexes 1-3. Together, insight from the kinetic and electrochemical characterization suggests that, in the case of Ru(II) complexes 1-3, catalytic NADH oxidation sees the H-transfer from 1,4-NADH as the rate limiting step, whereas for NAD+ hydrogenation with formate as the H-donor, the rate limiting step is the transfer of the ruthenium hydride to the NAD+ substrate. The latter is further modulated by the presence of di-cationic aquo- or mono-cationic hydroxo-species of complexes 1-3. Instead, compound 4, stable with respect to hydrolysis in aqueous solution, appears to operate via a different mechanism. Finally, the anticancer activity and ability to form reactive oxygen species (ROS) of complexes 1-3 have been studied in cancerous and non-tumorigenic cells in vitro. Noteworthy, the conversion of aldehydes to alcohols could be achieved by the three Ru(II) catalysts in living cells, as assessed by fluorescence microscopy. Furthermore, the formation of Ru(II) hydride intermediate upon treatment of cancer cell extracts with complex 3 has been detected by 1H NMR spectroscopy. Overall, this study paves the way to the application of non-arene based organometallic complexes as TH catalysts in biological environment.

The cationic achiral and chiral terpyridine diphosphine ruthenium complexes [RuCl(PP)(tpy)]Cl (PP=dppp (1), (R,R)-Skewphos (2) and (S,S)-Skewphos (3)) are easily obtained in 85-88 % yield through a one-pot synthesis from [RuCl

The diacetate complexes trans-[Ru(κ1-OAc)2(PPh3)2(NN)] (NN = ethylenediamine (en) (1), 2-(aminomethyl)pyridine (ampy) (2), 2-(aminomethyl)pyrimidine (ampyrim) (3)) have been isolated in 76-88% yield by reaction of [Ru(κ2-OAc)2(PPh3)2] with the corresponding nitrogen ligands. The ampy-type derivatives 2 and 3 undergo isomerization to the thermodynamically most stable cationic complexes [Ru(κ1-OAc)(PPh3)2(NN)]OAc (2a and 3a) and cis-[Ru(κ1-OAc)2(PPh3)2(NN)] (2b and 3b) in methanol at RT. The trans-[Ru(κ1-OAc)2(P2)2] (P2 = dppm (4), dppe (5)) compounds have been synthesized from [Ru(κ2-OAc)2(PPh3)2] by reaction with the suitable diphosphine in toluene at 95 °C. The complex cis-[Ru(κ1-OAc)2(dppm)(ampy)](6) has been obtained from [Ru(κ2-OAc)2(PPh3)2] and dppm in toluene at reflux and reaction with ampy. The derivatives trans-[Ru(κ1-OAc)2P2(NN)] (7-16; NN = en, ampy, ampyrim, 8-aminoquinoline; P2 = dppp, dppb, dppf, (R)-BINAP) can be easily synthesized from [Ru(κ2-OAc)2(PPh3)2] with a diphosphine and treatment with the NN ligands at RT. Alternatively these compounds have been prepared from trans-[Ru(OAc)2(PPh3)2(NN)] by reaction with the diphosphine in MEK at 50 °C. The use of (R)-BINAP affords trans-[Ru(κ1-OAc)2((R)-BINAP)(NN)] (NN = ampy (11), ampyrim (15)) isolated as single stereoisomers. Treatment of the ampy-type complexes 8-15 with methanol at RT leads to isomerization to the cationic derivatives [Ru(κ2-OAc)P2(NN)]OAc (8a-15a; NN = ampy, ampyrim; P2 = dppp, dppb, dppf, (R)-BINAP). Similarly to 2, the dipivalate trans-[Ru(κ1-OPiv)2(PPh3)2(ampy)] (18) is prepared from [Ru(κ2-OPiv)2(PPh3)2] (17) and ampy in CHCl3. The pincer acetate [Ru(κ1-OAc)(CNNOMe)(PPh3)2] (19) has been synthesized from [Ru(κ2-OAc)2(PPh3)2] and HCNNOMe ligand in 2-propanol with NEt3 at reflux. In addition, the dppb pincer complexes [Ru(κ1-OAc)(CNN)(dppb)] (CNN = AMTP (20), AMBQPh (21)) have been obtained from [Ru(κ2-OAc)2(PPh3)2], dppb, and HAMTP or HAMBQPh with NEt3, respectively. The acetate NN and pincer complexes are active in transfer hydrogenation with 2-propanol and hydrogenation with H2 of carbonyl compounds at S/C values of up to 10000 and with TOF values of up to 160000 h-1.

Several mononuclear ruthenium complexes containing bulky diphosphines have been easily prepared from acetate ruthenium precursors. The cyclohexyl complex [Ru(η2-OAc)2(DCyPF)] (1-cy; DCyPF = 1,1′-bi...

Preparation of bis-aNHC Ru catalysts, identification of formation intermediates and application in transfer hydrogenation and Oppenauer-type oxidation, observing an inversion in activity appearing to be dependent on steric hindrance.

The cationic abnormal N-heterocyclic carbene complex [Ru(OAc)(dppe)(PC)]Br (2b) (PC = bidentate phosphine-carbene ligand) has been obtained by treatment of the neutral derivative RuBr(OAc)(PPh3)(PC) (2a) with dppe in THF. Reaction of 2b with Ag2O afforded the heterobimetallic complex Ru(OAc)(dppe)(PC)AgBr (2c) which can be easily transmetallated with [Ir(cod)Cl]2 giving Ru(OAc)(dppe)(PC)IrCl(cod) (2d). Similarly, [{Ru(OAc)(PC)(PC′)}2Ag][AgBr2] (1b) reacts with [Ir(cod)Cl]2 with formation of Ru(OAc)(PC)(PC′)IrCl(cod) (1e). All complexes were characterized by NMR in solution and by single crystal X-ray diffraction studies in solid state. For the cationic complexes the 1H NMR resonance of the carbene-proton is considerably down-field shifted (δ > 9 ppm) with respect to that of the neutral derivatives. Cyclic voltammetry and differential pulse voltammetry studies show that the Ru-centered redox process occurs at lower potential (ΔE > 100 mV) after transmetallation with iridium, suggesting an interaction of the two metals along the π-system of the heterocycle ligand. These complexes display catalytic activity in transfer hydrogenation of acetophenone in 2-propanol with a significant influence of Ir and Ag centers on the Ru moiety. The synthesis of the imidazolyl-based N-heterocyclic dicarbene Ru–Ir complexes, which entails the easy metallation of the reactive cationic abnormal N-heterocyclic carbene Ru complexes with Ag2O, represents a suitable pathway for the preparation of heterometallic ruthenium complexes.

The dinuclear ruthenium(II) phosphine complexes Ru2Cl(O2CCHxF3−x)3(PPh3)4(μ-H2O) (x = 0, 1, 2), containing fluoroacetate ligands, were prepared from RuCl2(PPh3)3 and NaO2CCHxF3−x in tBuOH. The X-ray characterization of these complexes reveals a bridging water molecule, stabilized by hydrogen bonds with the fluoroacetate ligands. The isolation of the complex Ru(O2CCF3)2(PPh3)2 is described, starting from RuCl2(PPh3)3 or Ru2Cl(O2CCF3)3(PPh3)4(μ-H2O) and TlO2CCF3, correcting the reported preparation in which Ru2Cl(O2CCF3)3(PPh3)4(μ-H2O) was obtained. Ru(O2CCF3)2(PPh3)2 easily reacts with CO, affording Ru(O2CCF3)2(CO)2(PPh3)2. The protonation of Ru(OAc)2(dppb) with trifluoroacetic acid in the presence of bidentate O and N donor ligands affords the complexes Ru(O2CCF3)2(dppb)(LL) (LL = ethyleneglycol, ethylenediamine), which are catalytically active in the transfer hydrogenation of ketones with 2-propanol. In the reduction of cyclohexanone, the glycol derivative displays a higher catalytic activity than the diamine complex, reaching a TOF of 22 000 h−1.

For the first time, NHC-Au-X [NHC = (1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene, X− = BF4−, OTf−, OTs−, TFA−] catalysts were tested in the cycloisomerization of N-(prop-2-yn-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline in a wide set of green solvents. We found that most of these solvents (cyclohexanone, isopropyl acetate, MIBK, ethyl lactate, furfuryl alcohol, γ-valerolactone, propylene carbonate and propionic acid) are comparable or better solvents with respect to traditional VOS. Furthermore, the catalyst activity is related to the basic strength of the anion and the performances of the catalysts decrease gradually with increasing basicity and hydrogen-bond acceptor power of X−. Kinetic experiments and DFT calculations seem to indicate that both the characteristic of the solvent and counterion should be taken into account.

The transfer hydrogenation of flavanones and ortho-hydroxychalcones catalyzed by ruthenium pincer complexes RuCl(CNNPh)(disphosphine) has allowed the synthesis of ortho-hydroxy 1,3-diarypropanols in 80–88 % yield, under mild reaction conditions and short reaction times (1 h) in 2-propanol. The amount of the co-catalyst NaOiPr has been found crucial for the selective reduction of flavanones to ortho-hydroxy 1,3-diarypropanols vs. flavan-4-ols. Preliminary results show that with pincer catalysts bearing (S,R)-Josiphos, flavanone is reduced to the corresponding (S)-alcohol in moderate conversion (36 %) and up to 92 % ee., This work was supported by the Spanish Ministry of Science and Innovation (PID2019-109253RB-I00) and from the Ministero dell’Università e della Ricerca (MIUR).

The employment of easily affordable ruthenium(II)-complexes as pre-catalysts in the transfer hydrogenation of carbonyl compounds in deep eutectic media is described for the first time. The eutectic mixture tetrabutylammonium bromide/formic acid = 1/1 (TBABr/HCOOH = 1/1) acts both as reaction medium and hydrogen source. The addition of a base is required for the process to occur. An extensive optimization of the reaction conditions has been carried out, in terms of catalyst loading, type of complexes, H2-donors, reaction temperature and time. The combination of the dimeric complex [RuCl(p-cymene)-μ-Cl]2 (0.01–0.05 eq.) and the ligand dppf (1,1′-ferrocenediyl-bis(diphenylphosphine)ferrocene) in 1/1 molar ratio has proven to be a suitable catalytic system for the reduction of several and diverse aldehydes and ketones to their corresponding alcohols under mild conditions (40–60 °C) in air, showing from moderate to excellent tolerability towards different functional groups (halogen, cyano, nitro, phenol). The reduction of imine compounds to their corresponding amine derivatives was also studied. In addition, the comparison between the results obtained in TBABr/HCOOH and in organic solvents suggests a non-innocent effect of the DES medium during the process.

The cationic acetate ruthenium complex [Ru(η1-OAc)(CO)(dppb)(phen)]OAc (1) is easily prepared in 83% yield from [Ru(η1-OAc)(η2-OAc)(CO)(dppb)] (dppb = 1,4-bis(diphenylphosphino)butane) and 1,10-phenanthroline (phen) in MeOH. The derivative 1 undergoes easy substitution of the coordinated acetate by reaction with NaOPiv, KSAc, and KSCN in MeOH, affording the corresponding complexes [RuX(CO)(dppb)(phen)]X (X = OPiv, 2; SAc, 3; and NCS, 4), whereas its reaction with NaCl and NH4PF6 affords [RuCl(CO)(dppb)(phen)]PF6 (5). Carboxylate complexes 1 and 2 show high solubility in water, enabling easy exchange of the coordinated carboxylate by water and other ligands (CH3CN, glutathione). Cationic complexes 1–5, compared to Cisplatin, display a strong cell viability decrease in two human anaplastic thyroid cancer cell lines (SW1736 and 8505C), ranging from 3.10 μM to 0.09 μM EC50 values. The most active compounds 1–3 show a marked increment of apoptosis and decrease of cancer cell aggressiveness, making them promising candidates for further evaluation studies.

The ligand HCNNOMe (6-(4-methoxyphenyl)-2-aminomethylpyridine) is easily prepared from the commercially available 6-(4-methoxyphenyl)pyridine-2-carbaldehyde by the reaction of hydroxylamine and hydrogenation (H2, 1 atm) with Pd/C. The pincer complexes cis-[RuCl(CNNOMe)(PPh3)2] (1) and [RuCl(CNNOMe)(PP)] (PP = dppb, 2; and dppf, 3) are synthesized from [RuCl2(PPh3)3], HCNNOMe and PP (for 2 and 3) in 2-propanol with NEt3 at reflux and are isolated in 85–93% yield. Carbonylation of 1 (CO, 1 atm) gives [RuCl(CNNOMe)(CO)(PPh3)] (4) (79% yield) which cleanly reacts with Na[BArf4] and PCy3, affording the cationic trans-[Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] (5) (92% yield). These robust pincer complexes display remarkably high catalytic activity in the transfer hydrogenation (TH) of lignocellulosic biomass carbonyl compounds, using 2-propanol at reflux in a basic medium (NaOiPr or K2CO3). Thus, furfural, 5-(hydroxymethyl)furfural and Cyrene are reduced to the corresponding alcohols with 2 and 3, at S/C in the range of 10 000–100 000, within minutes or hours (TOF up to 1 500 000 h−1). The monocarbonyl complex 5 was found to be extremely active in the TH of cinnamaldehyde, vanillin derivatives and ethyl levulinate at S/C in the range of 10 000–50 000. Vanillyl alcohol is also obtained by the TH of vanillin with 5 (S/C = 500) in 2-propanol in the presence of K2CO3.

The dinuclear ruthenium(ii) phosphine complexes Ru2Cl(O2CCHxF3-x)3(PPh3)4(μ-H2O) (x = 0, 1, 2), containing fluoroacetate ligands, were prepared from RuCl2(PPh3)3 and NaO2CCHxF3-x in tBuOH. The X-ray characterization of these complexes reveals a bridging water molecule, stabilized by hydrogen bonds with the fluoroacetate ligands. The isolation of the complex Ru(O2CCF3)2(PPh3)2 is described, starting from RuCl2(PPh3)3 or Ru2Cl(O2CCF3)3(PPh3)4(μ-H2O) and TlO2CCF3, correcting the reported preparation in which Ru2Cl(O2CCF3)3(PPh3)4(μ-H2O) was obtained. Ru(O2CCF3)2(PPh3)2 easily reacts with CO, affording Ru(O2CCF3)2(CO)2(PPh3)2. The protonation of Ru(OAc)2(dppb) with trifluoroacetic acid in the presence of bidentate O and N donor ligands affords the complexes Ru(O2CCF3)2(dppb)(LL) (LL = ethyleneglycol, ethylenediamine), which are catalytically active in the transfer hydrogenation of ketones with 2-propanol. In the reduction of cyclohexanone, the glycol derivative displays a higher catalytic activity than the diamine complex, reaching a TOF of 22 000 h-1.

The mitochondrial F-ATP synthase is the principal energy-conserving nanomotor of cells that harnesses the proton motive force generated by the respiratory chain to make ATP from ADP and phosphate in a process known as oxidative phosphorylation. In the energy-converting membranes, F-ATP synthase is a multisubunit complex organized into a membrane-extrinsic F1sector and a membrane-intrinsic FOdomain, linked by central and peripheral stalks. Due to its essential role in the cellular metabolism, malfunction of F-ATP synthase has been associated with a variety of pathological conditions, and the enzyme is now considered as a promising drug target for multiple disease conditions and for the regulation of energy metabolism. We discuss structural and functional features of mitochondrial F-ATP synthase as well as several conditions that partially or fully inhibit the coupling between the F1catalytic activities and the FOproton translocation, thus decreasing the cellular metabolic efficiency and transforming the enzyme into an energy-dissipating structure through molecular mechanisms that still remain to be defined.

The ruthenium complex [Ru(OAc)(a-PC)2]Br (3) containing two abnormal NHC ligands is obtained by reaction of Ru(OAc)2(PPh3)2 (1) with 1-(2-diphenylphosphinoethyl)-3-mesitylimidazolium bromide in the...

Hydration and alkoxylation of alkynes are key processes for the industrial production of carbonyl derivatives. In this review, the pivotal role of ion pairing in the mechanism of hydration and alkoxylation of alkynes promoted by gold(I) catalysts L-Au-X is deeply analyzed and discussed, that has been elucidated by means of experimental findings supported by some theoretical calculations. In particular, the crucial role of the counterion X − is fully described in chapter 2. The catalytic performances in the alkoxylation and hydration of alkynes promoted by gold(I) are influenced by the coordination ability and basicity (proton affinity) of the counterion (paragraph 2.3) and the anion/cation relative orientation (paragraph 2.3). Also the appropriate matching of X − and the neutral ligand L must be taken into account to improve the catalytic performance of gold catalysts (paragraph 2.4). A survey of other non-covalent interactions, which however play a kinetic important role (hydrogen bonds, those triggered by suitable functionalities present on the ligand L, formation of supramolecular catalytic systems or micelles, and others), is presented in chapter 3. The progress in the development of sustainable methodologies for the gold(I)-promoted hydration of alkynes is discussed in chapter 4. In particular, paragraph 4.1 focuses on the unique role played by the anion in the L-Au-X catalyzed hydration of alkynes conducted in solvent-, silver-, and acid-free conditions. To conclude this critical review, in paragraph 4.2 it is highlighted how the amount of ion pairing, combined with the presence of suitable functionalities in neoteric solvent, may allow the development of green protocols for gold(I) catalyzed hydration of alkynes.

The bidentate HCNN dicarbonyl ruthenium complexes trans,cis-[RuCl2(HCNN)(CO)2] (1–3) and trans,cis-[RuCl2(ampy)(CO)2] (1a) were prepared by reaction of [RuCl2(CO)2]n with 1-[6-(4′-methylphenyl)pyridin-2-yl]methanamine, benzo[h]quinoline (HCNN), and 2-(aminomethyl)pyridine (ampy) ligands. Alternatively, the derivatives 1–3 were obtained from the reaction of RuCl3 hydrate with HCO2H and HCNN. The pincer CNN cis-[RuCl(CNN)(CO)2] (4) was isolated from 1 by reaction with NEt3. The monocarbonyl complexes trans-[RuCl2(HCNN)(PPh3)(CO)] (5–7) were synthesized from [RuCl2(dmf)(PPh3)2(CO)] and HCNN ligands, while the diacetate trans-[Ru(OAc)2(HCNN)(PPh3)(CO)] (8) was obtained from [Ru(OAc)2(PPh3)2(CO)]. Carbonylation of cis-[RuCl(CNN)(PPh3)2] with CO afforded the pincer derivatives [RuCl(CNN)(PPh3)(CO)] (9–11). Treatment of 9 with Na[BArf]4 and PPh3 gave the cationic complex trans-[Ru(CNN)(PPh3)2(CO)][BArf4] (12). The dicarbonyl derivatives 1–4, in the presence of PPh3 or PCy3, and the monocarbonyl complexes 5–12 ca...

Aldehydes were chemoselectively reduced to primary alcohols by using HCOONH4 as the hydrogen donor through transfer hydrogenation catalyzed by benzo[h]quinoline pincer complexes RuCl(CNNPh)(PP) at substrate to catalyst molar ratios of 2000 to 20 000. This practical reaction performed with aldehydes of commercial-grade purity in a water/toluene biphasic system afforded alcohols without the formation of condensation or amination side products.

This review provides a brief overview of advances on ruthenium(II) N-heterocyclic carbene complexes (NHCs) applied for hydrogenation reactions undertaken during the last five years. Several structural motifs, containing mono-, bi-, tri- and tetradentate binding modes of the NHCs are discussed in combination with a variety of different wingtip substituents to provide active catalysts for hydrogenation reactions. While bidentate ligands afford the more active catalysts than their monodentate analogues, pincer ligands must be chosen carefully to enable the formation of a free coordination site in catalysis. Transfer hydrogenation and direct hydrogenation of ketones and aldehydes, olefins, nitriles, imines and esters are summarized, showing the trend towards hydrogen transfer from other sources than hydrogen gas. Recently developed chiral NHCs offer the opportunity for asymmetric transformations as a possible pathway to access natural products.

The NHC–Au–OTf [NHC = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] catalyst was tested in the alkoxylation and hydration of alkynes in a wide set of neoteric solvents for the first time. We found that most of these solvents (ethyl lactate, glycerol, propylene carbonate, D-limonene, γ-valerolactone, and p-cymene) are comparable or better solvents with respect to traditional VOSs. An important beneficial effect of γ-valerolactone was found in the hydration of inactive diphenylacetylene and one order of magnitude higher TOF was obtained with respect to the literature values. Kinetic experiments and DFT calculations seem to indicate that the polarity of the solvent and the presence of suitable proton acceptor/donor functionalities on the solvent itself could be taken into account.

The dicarbonyl complex RuCl2(L)2(CO)2 (1) was easily prepared by reaction of ruthenium chloride hydrate with formic acid and L (L = (2,6-Me2C6H3)PPh2) in ethanol at reflux, via the [RuCl2(CO)2]n intermediate. Alternatively, 1 was obtained from [RuCl2(CO)3]2 and L by CO elimination. Reaction of 1 with NEt3 in toluene at reflux afforded the cyclometalated derivative RuCl{(2-CH2-6-MeC6H3)PPh2}(L)(CO)2 (2). A simple one-pot synthesis of 2 was achieved by treatment of RuCl3 hydrate with formic acid, L, and NEt3. The cyclometalated dicarbonyl complexes [Ru{(2-CH2-6-MeC6H3)PPh2}(NN)(CO)2]Cl (NN = ethylenediamine, 3; 2-(aminomethyl)pyridine, 4; (R,R)-1,2-diphenylethane-1,2-diamine, 5) were isolated by reaction of 2 with the corresponding dinitrogen ligand in methanol at reflux. Complexes 1–4 catalyze the transfer hydrogenation (TH) of acetophenone in 2-propanol at reflux (S/C = 1000 and TOF up to 30 000 h–1) with alkali base (1–5 mol %), whereas 5 leads to (S)-1-phenylethanol with 68% ee. The derivatives 1–5 cata...

The osmium complex trans-[OsCl2(PPh3)2(en)] (2) was prepared by reaction of [OsCl2(PPh3)3] (1a) with ethylenediamine (en), whereas the diphosphine derivatives trans-[OsCl2(dppf)(NN)] (NN = en (3), bn (4; bn = 1,4-butanediamine)) and trans-[OsCl2(dpbp)(en)] (5) were obtained from 1a, dppf or dpbp, and the corresponding NN ligand in CH2Cl2 or toluene. An X-ray diffraction study has been provided for 3. The isolation of the chiral derivatives trans-[OsCl2(diphosphine)((R,R)-dpen)] (diphosphine = dppf (6), dpbp (7), (R,R)-skewphos (8)) was achieved by reacting 1a with the diphosphine and (R,R)-dpen in toluene. Treatment of the precursor [Os2Cl4(P(m-tolyl)3)5] (1b) with en afforded [OsCl2(P(m-tolyl)3)2(en)] (9), while reaction of 1b with dppb and N,N-dmen gave [OsCl2(dppb)(N,N-dmen)] (10). The chiral derivatives [OsCl2(diphosphine)(NN)] (11–21; diphosphine = (S)-MeObiphep, (R)-MeObiphep, (R)-xylMeObiphep, (R)-binap, (S)-xylbinap, (R)-xylbinap, (R,S)-Josiphos*; NN = en, (R,R)-dpen, (R)-daipen, (R,R)-dppn) were ...

This account focuses on the application in catalysis of ruthenium and osmium complexes containing 2-(aminomethyl)pyridine (Ampy)-based ligands. The combination of these aminoalkylpyridine ligands with appropriate phosphines affords ruthenium and osmium systems displaying unprecedented high catalytic activity and productivity in a variety of organic transformations such as hydrogenation by hydrogen transfer and dihydrogen, dehydrogenation, racemization, and alkylation.

The acetate complex Ru(OAc)

The acetate complex Ru(OAc)2(DiPPF) (2) obtained from Ru(OAc)2(PPh3)2 (1) and 1,1′-bis(diisopropylphosphino)ferrocene (DiPPF) reacts cleanly with formaldehyde affording Ru(OAc)2(CO)(DiPPF) (3) in high yield. The monocarbonyl complex 3 (0.4-2 mol %) efficiently catalyzes the N-alkylation of primary and secondary alkyl and aromatic amines using primary alcohols ROH (R=Et, nPr, nBu, PhCH2) under mild reaction conditions (30–100 °C) with an alcohol/amine molar ratio of 10-100. Formation of the monohydride RuH(OAc)(CO)(DiPPF) (4) has been observed by reaction of 3 with iPrOH in the presence of NEt3 at RT through an equilibrium reaction.

A cationic phthalimido-functionalized N-heterocyclic carbene (NHC) palladium(II) complex has been synthesized from [3-methyl-1-(2′-phthalimidoethyl)imidazolium] hexafluorophosphate ([NHCMe,PhtH]PF6) by transmetalation and isolated in 67 % yield. The title complex has been applied as catalyst in the Suzuki–Miyaura cross-coupling reaction under benign aqueous conditions. The catalyst is active without any observable initiation period. High average turnover frequencies (TOFs) of up to 55000 h–1 have been reached with catalyst concentrations as low as 0.01 mol-%.

The bifunctional phosphine–abnormal N-heterocyclic carbene ruthenium(II) complex RuBr(OAc)(PPh3)(P–aNHC) (1) has been synthesized in high yield by reaction of Ru(OAc)2(PPh3)2 with a phosphine imidazolium bromide (P–NHC × HBr) and characterized by X-ray diffraction. This compound shows high catalytic activity for the transfer hydrogenation of ketones to alcohols in 2-propanol. Rate and efficiency of 1 can be enhanced by the addition of ethylenediamine or benzylamine, affording TOFs up to 140 000 h–1. Reaction of 1 with ethylenediamine leads to the Ru carbene/amine complex [Ru(OAc)(PPh3)(P–aNHC)(H2NCH2CH2NH2)]Br (2), which displays the same activity of the in situ generated species.

The cationic [RuCl(η6-p-cymene)(HCNNa)]Cl (1a) (HCNNa = 1-(6-arylpyridin-2-yl)methanamine) and the neutral RuCl2(η6-p-cymene)(HCNNb) (1b) (HCNNb = 2-aminomethylbenzo[h]quinoline) complexes have been obtained by reaction of the precursor [RuCl(μ-Cl)(η6-p-cymene)]2 with the corresponding nitrogen ligand (HCNNa and HCNNb) in THF. Complex 1a reacts cleanly with monodentate (P = PPh3) and bidentate phosphines (PP = dppb, dppf) in ethanol in the presence of NEt3, affording the pincer catalysts RuCl(CNNa)(PPh3)2 (2) and RuCl(CNNa)(PP) (PP = dppb 3, dppf 4). Similarly, the benzo[h]quinoline pincer derivative RuCl(CNNb)(dppb) (5) is obtained from 1b and dppb. Complex 3 has also been prepared in a one-pot reaction from [RuCl(μ-Cl)(η6-p-cymene)]2, HCNNa, and dppb in ethanol. Similarly, the chiral complex RuCl(CNNa)((R,S)-Josiphos) was isolated as a single stereoisomer by treatment of [RuCl(μ-Cl)(η6-p-cymene)]2 with HCNNa and (R,S)-Josiphos in 1-butanol. Reaction of 1a and 1b with dppb affords cymene diphosphine spec...

CNN pincer ruthenium(II) and osmium(II) complexes of general formula RuCl(CNN)(dppb) (1 a-c) and OsCl(CNN)(dppb) (2 a,b) (dppb=Ph2P(CH2)4PPh2, HCNN=[6-(p-tolyl)-2-pyridyl]methylamine (4), N-methyl-[6-(p-tolyl)-2-pyridyl]methylamine (7) and N,N-dimethyl-[6-(p-tolyl)-2-pyridyl]methylamine (8)) were prepared and assessed as catalysts in the hydrogenation of a variety of imines. Among the examined complexes, the Ru-complex 1 a, RuCl(4)(dppb), showed the best activity, reducing N-aryl and N-alkyl imines to the related amines in high yields (up to 100 %) with a catalyst loading of 0.1-1 mol% and under mild conditions (5 atm H2, 40-70 °C). The Os-complexes exhibited a lower activity than their ruthenium analogues. Despite this, the Os-complex 2 a, OsCl(4)(dppb), was successful in the reduction of N-benzylideneaniline at 70 °C, affording a rare example of application of osmium complexes in the catalytic hydrogenation of imines.

Two novel di(N-heterocyclic carbene) complexes of formula (μ-PyrIm-CH2-ImPyr)[IrCp*Cl]2(PF6)2 (1) and μ-MeIm-CH2(p-C6H2)CH2-ImMe[IrCp*Cl]2 (2) (Im = imidazol-2-ylidene) have been synthesised by transmetallation of the dicarbene ligand from the corresponding dicarbene silver complex, using [IrCp*(μ-Cl)Cl]2 as an iridium precursor. The structure of complex 2 has been determined by X-ray diffraction and is characterized by a double ortho-metallation of the p-xylylene bridge between the carbene units. Both complexes show good activity in the transfer hydrogenation of ketones to alcohols in 2-propanol.

The chemoselective reduction of commercial-grade aldehydes (97–99 %) to primary alcohols is achieved with cis-[RuCl2(ampy)(PP)] [ampy=2-(aminomethyl)pyridine; PP=1,4-bis(diphenylphosphino)butane, 1,1′-ferrocenediyl-bis(diphenylphosphine)] and pincer [RuCl(CNNR)(PP)] [PP=1,3-bis(diphenylphosphino)propane, 1,4-bis(diphenylphosphino)butane, 1,1′-ferrocenediyl-bis(diphenylphosphine); HCNNR=4-substituted-2-aminomethyl-benzo[h]quinoline; R=Me, Ph] complexes by transfer hydrogenation and hydrogenation reactions. Aromatic, conjugated, and aliphatic aldehydes are converted quantitatively to the corresponding alcohols using 2-propanol with potassium carbonate at substrate/catalyst ratios up to 100 000 by transfer hydrogenation, whereas aldehyde hydrogenation (5–20 atm of H2) is achieved efficiently in MeOH in the presence of KOtBu at substrate/catalyst ratios up to 40 000.

The easily accessible complexes [MCl2(dppf)(ampy)] (M = Ru (cis-1), Os (trans-2); dppf = 1,1′-bis(diphenylphosphino)ferrocene; ampy = 2-aminomethylpyridine) in the presence of base (NaOiPr, KOtBu) are efficient catalysts for several reactions involving carbonyl compounds and alcohols. The derivatives 1 and 2 catalyze the selective transfer hydrogenation of aldehydes and ketones to alcohols with 2-propanol using 0.1–0.005 mol % of catalyst at 82 °C with TOF values up to 3.0 × 105 h–1. These compounds (0.1–0.02 mol %) promote the hydrogenation of aldehydes and ketones in EtOH or a MeOH/EtOH mixture at 30–90 °C (5 atm of H2) and the acceptorless dehydrogenation of alcohols to ketones in tBuOH at 130–145 °C (0.4 mol %). Complexes 1 and 2 (0.5 mol %) catalyze the racemization of chiral alcohols at 70 °C in 2-propanol and the isomerization of allylic alcohols to ketones in tBuOH at 70–120 °C (1 mol %). In addition, 1 and 2 (0.5 mol %) promote the α alkylation of α-tetralone with primary alcohols (EtOH, nPrOH, a...

This account focuses on the application in catalysis of ruthenium and osmium complexes containing 2-(aminomethyl)pyridine (Ampy)-based ligands. The combination of these aminoalkylpyridine ligands with appropriate phosphines affords ruthenium and osmium systems displaying unprecedented high catalytic activity and productivity in a variety of organic transformations such as hydrogenation by hydrogen transfer and dihydrogen, dehydrogenation, racemization, and alkylation.

The osmium compound trans,cis-[OsCl 2 (PPh 3 ) 2 (Pyme)] (1) (Pyme = 1-(pyridin-2-yl)methanamine), obtained from [OsCl 2 (PPh 3 ) 3 ] and Pyme, thermally isomerizes to cis,cis-[OsCl 2 (PPh 3 ) 2 (Pyme)] (2) in mesitylene at 150°C. Reaction of [OsCl 2 (PPh 3 ) 3 ] with Ph 2 P(CH 2 ) 4 PPh 2 (dppb) and Pyme in mesitylene (150 °C, 4 h) leads to a mixture of trans-[OsCl2(dppb)(Pyme)] (3) and cis-[OsCl2(dppb)(Pyme)] (4) in about an 1:3 molar ratio. The complex trans-(OsCl 2 (dppb)(Pyet)] (5) (Pyet= 2-(pyridin-2-yl)ethanamine) is formed by reaction of [OsCl 2 (PPh 3 ) 3 ] with dppb and Pyet in toluene at reflux. Compounds 1, 2, 5 and the mixture of isomers 3/4 efficiently catalyze the transfer hydrogenation (TH) of different ketones in refluxing 2-propanol and in the presence of NaOiPr (2.0 mol%). Interestingly, 3/4 has been proven to reduce different ketones (even bulky) by means of TH with a remarkably high turnover frequency (TOF up to 5.7x10 5 h -1 ) and at very low loading (0.05-0.001 mol%). The system 3/4 also efficiently catalyzes the hydrogenation of many ketones (H 2 , 5.0 atm) in ethanol with KOtBu (2.0 mol%) at 70°C (TOF up to 1.5× 10 4 h -1 ). The in-situ-generated catalysts prepared by the reaction of [OsCl 2 -(PPh 3 ) 3 ] with Josiphos diphosphanes and (±)-1-alkyl-substituted Pyme ligands, promote the enantioselective TH of different ketones with 91-96% ee (ee=enantiomeric excess) and with a TOF of up to 1.9×10 4 h -1 at 60°C.

The catalytic activity of the terdentate complex [RuCl(CNN)(dppb)] (A) [dppb=Ph(2)P(CH(2))(4)PPh(2); HCNN=6-(4'-methylphenyl)-2-pyridylmethylamine] in the transfer hydrogenation of acetophenone (S) with 2-propanol has been found to be dependent on the base concentration. The limit rate has been observed when NaOiPr is used in high excess (A/base molar ratio10). The amino-isopropoxide species [Ru(OiPr)(CNN)(dppb)] (B), which forms by reaction of A with sodium isopropoxide via displacement of the chloride, is catalytically active. The rate of conversion of acetophenone obeys second-order kinetics v=k[S][B] with the rate constants in the range 218+/-8 (40 degrees C) to 3000+/-70 M(-1) s(-1) (80 degrees C). The activation parameters, evaluated from the Eyring equation are DeltaH(++)=14.0+/-0.2 kcal mol(-1) and DeltaS(++)=-3.2 +/-0.5 eu. In a pre-equilibrium reaction with 2-propanol complex B gives the cationic species [Ru(CNN)(dppb)(HOiPr)](+)[OiPr](-) (C) with K approximately 2x10(-5) M. The hydride species [RuH(CNN)(dppb)] (H), which forms from B via beta-hydrogen elimination process, catalyzes the reduction of S and, importantly, its activity increases by addition of base. The catalytic behavior of the hydride H has been compared to that of the system A/NaOiPr (1:1 molar ratio) and indicates that the two systems are equivalent.

The Vaska-type iridium(I) complex [IrCl(CO){PPh(2)(2-MeC(6)H(4))}(2)] (1), characterized by an X-ray diffraction study, was obtained from iridium(III) chloride hydrate and PPh(2)(2,6-MeRC(6)H(3)) with R=H in DMF, whereas for R=Me, activation of two ortho-methyl groups resulted in the biscyclometalated iridium(III) compound [IrCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)] (2). Conversely, for R=Me the iridium(I) compound [IrCl(CO){PPh(2)(2,6-Me(2)C(6)H(3))}(2)] (3) can be obtained by treatment of [IrCl(COE)(2)](2) (COE=cyclooctene) with carbon monoxide and the phosphane in acetonitrile. Compound 3 in CH(2)Cl(2) undergoes intramolecular C-H oxidative addition, affording the cyclometalated hydride iridium(III) species [IrHCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}] (4). Treatment of 2 with Na[BAr(f) (4)] (Ar(f)=3,5-C(6)H(3)(CF(3))(2)) gives the fluxional cationic 16-electron complex [Ir(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)][BAr(f) (4)] (5), which reversibly reacts with dihydrogen to afford the delta-agostic complex [IrH(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}][BAr(f)(4)] (6), through cleavage of an Ir-C bond. This species can also be formed by treatment of 4 with Na[BAr(f)(4)] or of 2 with Na[BAr(f)(4)] through C-H oxidative addition of one ortho-methyl group, via a transient 14-electron iridium(I) complex. Heating of the coordinatively unsaturated biscyclometalated species 5 in toluene gives the trans-dihydride iridium(III) complex [IrH(2)(CO){PPh(2)(2,6-MeC(6)H(3)CH=CHC(6)H(3)Me-2,6)PPh(2)}][BAr(f) (4)] (7), containing a trans-stilbene-type terdentate ligand, as result of a dehydrogenative carbon-carbon double bond coupling reaction, possibly through an iridium carbene species.

The complexes cis-[RuCl2(ampy){R1N(CH2PPh2)2}] [ampy = (2-aminomethyl)pyridine, R1 = C6H5 or (CH3CH2O)3Si(CH2)3] showed very high catalytic activity in the homogeneous transfer hydrogenation of acetophenone (TOF > 300000 h–1) with the use of 2-propanol as the hydrogen donor and in the presence of sodium isopropoxide. The ligand (CH3CH2O)3Si(CH2)3N(CH2PPh2)2 (ATM) was prepared in high yield by reacting (3-aminopropyl)triethoxysilane, paraformaldehyde, and diphenylphosphane in toluene heated at 80 °C. The –N(CH2PPh2)2 function was attached to the surface of three different kinds of silica by means of two alternative methods. Thus, MA-Si-150 and mesoporous MA-Si-MCM-41 were prepared by the reaction of the inorganic matrix with ATM, whereas MA-Si-60 was synthesized by reaction of the commercially available 3-aminopropyl-functionalized silica gel with HCHO and PHPh2 in refluxing toluene. The –(CH2)3N(CH2PPh2)2 functionalized inorganic materials were used to anchor the RuCl2(ampy) moiety; thus, three different silica-immobilized versions of the cis-[RuCl2(ampy)(ATM)] complex were obtained. The silica-anchored complexes were tested in the transfer hydrogenation of acetophenone, which was found to be fast and quantitative; the effect of the nature of the silica support on the activity of the catalyst was almost negligible. It was possible to reuse the catalytic system for a second cycle without a decrease in the activity, but the efficiency of the catalyst considerably diminished in successive reuses.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Reaction of [Ru(OAc)2(PPh3)2] with a P-functionalized imidazolium bromide easily affords a cationic abnormal carbene Ru system. Metalation with Ag2O yields a Ru-Ag complex containing an anionic dicarbene ligand, while subsequent transmetalation with Au(tht)Cl leads to the corresponding Ru-Au system. The bimetallic complexes were characterized by single crystal X-ray diffraction and are the first examples of complexes bearing anionic dicarbene ligands connecting two different d-block elements.

CONSPECTUS: A current issue in metal-catalyzed reactions is the search for highly efficient transition-metal complexes affording high productivity and selectivity in a variety of processes. Moreover, there is also a great interest in multitasking catalysts that are able to efficiently promote different organic transformations by careful switching of the reaction parameters, such as temperature, solvent, and cocatalyst. In this context, osmium complexes have shown the ability to catalyze efficiently different types of reactions involving hydrogen, proving at the same time high thermal stability and simple synthesis. In the catalytic reduction of C═X (X = O, N) bonds by both hydrogenation (HY) and transfer hydrogenation (TH) reactions, the most interest has been focused on homogeneous systems based on rhodium, iridium, and in particular ruthenium catalysts, which have proved to catalyze chemo- and stereoselective hydrogenations with remarkable efficiency. By contrast, osmium catalysts have received much less attention because they are considered less active on account of their slower ligand exchange kinetics. Thus, this area remained almost neglected until recent studies refuted these prejudices. The aim of this Account is to highlight the impressive developments achieved over the past few years by our and other groups on the design of new classes of osmium complexes and their applications in homogeneous catalytic reactions involving the hydrogenation of carbon-oxygen and carbon-nitrogen bonds by both HY and TH reactions as well as in alcohol deydrogenation (DHY) reactions. The work described in this Account demonstrates that osmium complexes are emerging as powerful catalysts for asymmetric and non-asymmetric syntheses, showing a remarkably high catalytic activity in HY and TH reactions of ketones, aldehydes, imines, and esters as well in DHY reactions of alcohols. Thus, for instance, the introduction of ligands with an NH function, possibly in combination with a pyridine ring, led to a new family of [OsCl2(PP)(NN)] (NN = diamine, 2-aminomethylpyridine; PP = diphosphine) and pincer [OsCl(CNN)(PP)] (HCNN = 6-aryl-2-aminomethylpyridine, 2-aminomethylbenzo[h]quinoline) complexes, which are outstanding catalysts for (asymmetric) HY and TH of carbonyl compounds and DHY of alcohols with turnover numbers and turnover frequencies up to 10(5) and 10(6) h(-1), respectively. In addition, PNN osmium complexes containing the 2-aminomethylpyridine motif have been found to be among the most active catalysts for HY of esters. These complexes have shown catalytic activities that are comparable and in some cases superior to those reported for analogous ruthenium systems. These results give an idea of the potential of Os complexes for the design of new highly productive and robust catalysts for the synthesis of chiral and nonchiral alcohols and amines as well as ketones from alcohols. Thus, we hope that this report will promote increased interest in the chemistry of these metal complexes, opening novel opportunities for new catalytic processes as well as the improvement of existing ones.

CONSPECTUS: A current issue in metal-catalyzed reactions is the search for highly efficient transition-metal complexes affording high productivity and selectivity in a variety of processes. Moreover, there is also a great interest in multitasking catalysts that are able to efficiently promote different organic transformations by careful switching of the reaction parameters, such as temperature, solvent, and cocatalyst. In this context, osmium complexes have shown the ability to catalyze efficiently different types of reactions involving hydrogen, proving at the same time high thermal stability and simple synthesis. In the catalytic reduction of C═X (X = O, N) bonds by both hydrogenation (HY) and transfer hydrogenation (TH) reactions, the most interest has been focused on homogeneous systems based on rhodium, iridium, and in particular ruthenium catalysts, which have proved to catalyze chemo- and stereoselective hydrogenations with remarkable efficiency. By contrast, osmium catalysts have received much less attention because they are considered less active on account of their slower ligand exchange kinetics. Thus, this area remained almost neglected until recent studies refuted these prejudices. The aim of this Account is to highlight the impressive developments achieved over the past few years by our and other groups on the design of new classes of osmium complexes and their applications in homogeneous catalytic reactions involving the hydrogenation of carbon-oxygen and carbon-nitrogen bonds by both HY and TH reactions as well as in alcohol deydrogenation (DHY) reactions. The work described in this Account demonstrates that osmium complexes are emerging as powerful catalysts for asymmetric and non-asymmetric syntheses, showing a remarkably high catalytic activity in HY and TH reactions of ketones, aldehydes, imines, and esters as well in DHY reactions of alcohols. Thus, for instance, the introduction of ligands with an NH function, possibly in combination with a pyridine ring, led to a new family of [OsCl2(PP)(NN)] (NN = diamine, 2-aminomethylpyridine; PP = diphosphine) and pincer [OsCl(CNN)(PP)] (HCNN = 6-aryl-2-aminomethylpyridine, 2-aminomethylbenzo[h]quinoline) complexes, which are outstanding catalysts for (asymmetric) HY and TH of carbonyl compounds and DHY of alcohols with turnover numbers and turnover frequencies up to 10(5) and 10(6) h(-1), respectively. In addition, PNN osmium complexes containing the 2-aminomethylpyridine motif have been found to be among the most active catalysts for HY of esters. These complexes have shown catalytic activities that are comparable and in some cases superior to those reported for analogous ruthenium systems. These results give an idea of the potential of Os complexes for the design of new highly productive and robust catalysts for the synthesis of chiral and nonchiral alcohols and amines as well as ketones from alcohols. Thus, we hope that this report will promote increased interest in the chemistry of these metal complexes, opening novel opportunities for new catalytic processes as well as the improvement of existing ones.

Terdentate ruthenium(II) complexes of general formula RuX(CNN)(dppb) (X = chloride, hydride, alkoxide; dppb = Ph2P(CH2)4PPh2), where CNN is a deprotonated 2-aminomethyl-6-arylpyridine ligand, have been prepared. The orthometalated derivative RuCl(b)(dppb) (1) has been obtained by reaction of RuCl2(PPh3)(dppb) with N,N-dimethyl-2-aminomethyl-6-(4-methylphenyl)pyridine (Hb) in 2-propanol and in the presence of triethylamine by elimination of PPh3 and HCl. Similarly, RuCl(a)(dppb) (2) and the chiral analogue RuCl(c)(dppb) (3), containing primary amine ligands, have been isolated starting from 2-aminomethyl-6-(4-methylphenyl)pyridine (Ha) and (R)-2,2-dimethyl-1-(6-phenylpyridin-2-yl)propylamine (Hc), respectively. The synthesis of the functionalized pyridines Ha−Hc is here described, whereas the crystal structure of 3 has been determined through an X-ray diffraction experiment. Treatment of 1−3 with sodium or potassium isopropoxide gives the corresponding hydrides RuH(b)(dppb) (4), RuH(a)(dppb) (5), and RuH(c...

The complexes [Pd(acac)2] 1, [Pd(hfa)2] 2 (hfa = hexafluoroacetylacetonate), [Pd(CF3CO2)2] 3 and [Pd3(CH3CO2)6] 4 exhibit very different catalytic efficiency in the reaction between secondary amines and activated alkenes. Complexes 1 and 4 generally show an enhanced activity when catalytic amounts of NH4X salts (X− = low-coordinating anion) are added to the reaction mixtures. On the contrary, the activity of the perfluorurate analogues 2 and 3, which is much higher than that of 1 and 4, is generally scarcely affected by the presence of the NH4X additive. The cocatalytic effect of NH4X is comparable with that of strong acids such as CF3SO3H. The ammonium salts alone can behave as a catalyst giving an almost quantitative yield of the hydroamination product.

A neutron diffraction study of the complex RuCl(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (1) defines the precise nature of the delta agostic interactions between the unsaturated metal center and two o-methyl groups of the xylyl substituents. The CH(3) carbon atoms lie in the RuP(2) equatorial plane with Ru...C distances of 2.637(7) and 2.668(6) A, whereas four short Ru.H distances (from 2.113(11) to 2.507(11) A) indicate that each methyl group interacts with two C-H bonds. A survey of the X-ray structures with beta, gamma, delta, and epsilon M...H(3)C-C moieties (no neutron data have been previously reported) shows a linear correlation between the angle M.C-C and the torsion of the methyl group about the C-C bond. Thus, the agostic interactions span the range between the classical (M...eta(2)-HC) and the nonclassical (M...eta(3)-H(2)C) types. A solution study of 1 shows intramolecular rearrangement of each xylyl substituent that equilibrates the environments of its two ortho CH(3) groups. Activation parameters, evaluated from the analysis of (1)H NMR line shape as a function of temperature, are Delta H(++) = 9.6 +/- 0.2 kcal mol(-1) with Delta S(++) = -15.4 +/- 0.7 eu (CDCl(3)). The related 14-electron complexes RuX(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (X = I, 2; NCO, 3), prepared from 1 and NaX, show a similar dynamic process in solution, with the iodo derivative displaying the most hindered rotation of the xylyl group. A DFT optimization of the complex RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))](2) (1a) reproduces well the nonclassical Ru...eta(3)-H(2)C agostic mode, whereas the classical Ru...eta(2)-HC one corresponds to a transition state 1b, destabilized by 3.4 kcal mol(-1). A similar barrier (ca. 3.8 kcal mol(-1)) is calculated for the xylyl rotation in the further simplified model RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))][PH(2)CH[double bond]CHCH(3)] (1c), the absence of bulky phenyl substituents being largely responsible for the difference with respect to the experimental value. Finally, the MO analysis addresses the intrinsic stability of the 14-electron complex RuCl(2)(PH(3))(2) and, in agostic complexes, accounts for the different interactions between the methyl group and the metal atom in relation to the length of their interconnecting chain.

The reaction between primary amines (RNH2) and diaryl diazomethanes (Ar2CN2) in the presence of catalytic amounts of the complex [RuCl(η5-C5H5)(PPh3)2] (1), in chloroform at 60 °C, generally affords mixtures of imines Ar2CNR as main product and amines Ar2CHNHR. Whereas Ar2CHNHR are formed by the expected carbene insertion into the NH bond of the substrate, the generation of Ar2CNR is unprecedented. The carbene intermediate [RuCl(CAr2)(η5-C5H5)(PPh3)] seems to be the key-species involved in the formation of both products.