Moran Feller

Catalytic coupling of nitriles with amines to selectively form imines under mild hydrogen pressure

Imines are selectively formed by coupling of nitriles and amines under mild hydrogen pressure. The reaction is catalyzed by a bipyridine-based PNN Ru(II) pincer complex and proceeds under mild, neutral conditions at 4 bar of H(2).

Direct observation of reductive elimination of MeX (X = Cl, Br, I) from RhIII complexes: Mechanistic insight and the importance of sterics

Rare cases of directly observed reductive elimination (RE) of methyl halides from Rh(III) complexes are described. Treatment of the coordinatively unsaturated complexes [((t)BuPNP)Rh(CH3)X][BF4] (1-3, X = I, Br, and Cl; (t)BuPNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) with coordinating and noncoordinating compounds results in the formation of the corresponding free methyl halides and Rh(I) complexes. The rate increase of CH3I and CH3Br RE in the presence of polar aprotic solvents argues in favor of an SN2 RE mechanism. However, the RE of CH3Cl is faster in polar protic solvents, which argues in favor of a concerted C-Cl RE. The RE of methyl halides from complexes 1-3 is induced by steric factors, as treatment of the less bulky complexes [((i)PrPNP)Rh(CH3)X][BF4] (19-21; X = I, Br, Cl, respectively) with coordinating compounds leads to the formation of the adducts complexes rather than RE of the methyl halides. The accumulated evidence suggests that the RE process is nonassociative.

Competitive C-I versus C-CN reductive elimination from a Rh(III) complex. Selectivity is controlled by the solvent.

The RhIII complex [(PNP)Rh(CN)(CH3)][I] 5, obtained by oxidative addition of methyl iodide to [(PNP)Rh(CN)] 2, reacts selectively in two pathways: In aprotic solvents C-I reductive elimination of methyl iodide followed by its electrophilic attack on the cyano ligand takes place, giving the methyl isonitrile RhI complex [(PNP)Rh(CNCH3)][I] 3, while in protic solvents C-C reductive elimination of acetonitrile takes place forming an iodo RhI complex [(PNP)RhI] 9. Reaction of 2 with ethyl iodide in aprotic solvents gave the corresponding isonitrile complex, while in protic solvents no reactivity was observed. The selectivity of this reaction is likely due to a hydrogen bond between the cyano ligand and the protic solvent, as observed by X-ray diffraction, which retards electrophilic attack on this ligand.

Reductive Cleavage of CO2 by Metal-Ligand-Cooperation Mediated by an Iridium Pincer Complex.

A unique mode of stoichiometric CO2 activation and reductive splitting based on metal-ligand-cooperation is described. The novel Ir hydride complexes [((t)Bu-PNP*)Ir(H)2] (2) ((t)Bu-PNP*, deprotonated (t)Bu-PNP ligand) and [((t)Bu-PNP)Ir(H)] (3) react with CO2 to give the dearomatized complex [((t)Bu-PNP*)Ir(CO)] (4) and water. Mechanistic studies have identified an adduct in which CO2 is bound to the ligand and metal, [((t)Bu-PNP-COO)Ir(H)2] (5), and a di-CO2 iridacycle [((t)Bu-PNP)Ir(H)(C2O4-κC,O)] (6). DFT calculations confirm the formation of 5 and 6 as reversibly formed side products, and suggest an η(1)-CO2 intermediate leading to the thermodynamic product 4. The calculations support a metal-ligand-cooperation pathway in which an internal deprotonation of the benzylic position by the η(1)-CO2 ligand leads to a carboxylate intermediate, which further reacts with the hydride ligand to give complex 4 and water.

O2 Activation by Metal–Ligand Cooperation with IrI PNP Pincer Complexes

A unique mode of molecular oxygen activation, involving metal-ligand cooperation, is described. Ir pincer complexes [((t)BuPNP)Ir(R)] (R = C6H5 (1), CH2COCH3 (2)) react with O2 to form the dearomatized hydroxo complexes [((t)BuPNP*)Ir(R)(OH)] ((t)BuPNP* = deprotonated (t)BuPNP ligand), in a process which utilizes both O-atoms. Experimental evidence, including NMR, EPR, and mass analyses, indicates a binuclear mechanism involving an O-atom transfer by a peroxo intermediate.

Controlling growth of self-propagating molecular assemblies

A series of six self-propagating molecular assemblies (SPMAs) were generated by alternative solution-deposition of ruthenium polypyridyl complexes and d8palladium and platinum salts on glass and silicon substrates. The d6 polypyridyl complexes have three pyridine units available for forming networks by coordination to the metal salts. This two-step film growth process is fast (15 min/step) and can be carried out conveniently under ambient conditions in air. The reactivity of the common metal salts (ML2X2: M = Pd, X = Cl, L = PhCN, ½ 1,5-cyclooctadiene (COD), SMe2 and M = Pt, X = Cl, Br, I, L = PhCN) is a dominant factor in the film growth. Although the assembly structures are comparable, their exponential growth can be controlled by varying the metals salts. The co-ligands, halides, and metal centers can be used to control the film thicknesses and light absorption intensities of the metal-to-ligand charge transfer (MLCT) bands by a factor of ∼3.5 for 13 deposition steps, whereas the surface morphologies and molecular densities of the SPMAs are similar. The surface-confined assemblies have been characterized using a combination of optical (UV/Vis, ellipsometry) spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray reflectivity (XRR).

Hydrogenation and Hydrosilylation of Nitrous Oxide Homogeneously Catalyzed by a Metal Complex

Due to its significant contribution to stratospheric ozone depletion and its potent greenhouse effect, nitrous oxide has stimulated much research interest regarding its reactivity modes and its transformations, which can lead to its abatement. We report the homogeneously catalyzed reaction of nitrous oxide (N2O) with H2. The reaction is catalyzed by a PNP pincer ruthenium complex, generating efficiently only dinitrogen and water, under mild conditions, thus providing a green, mild methodology for removal of nitrous oxide. The reaction proceeds through a sequence of dihydrogen activation, “O”-atom transfer, and dehydration, in which metal–ligand cooperation plays a central role. This approach was further developed to catalytic O-transfer from N2O to Si–H bonds.

Bottom-Up Construction of a CO2-Based Cycle for the Photocarbonylation of Benzene, Promoted by a Rhodium(I) Pincer Complex

The use of carbon dioxide for synthetic applications presents a major goal in modern homogeneous catalysis. Rhodium-hydride PNP pincer complex 1 is shown to add CO2 in two disparate pathways: one is the expected insertion of CO2 into the metal-hydride bond, and the other leads to reductive cleavage of CO2, involving metal-ligand cooperation. The resultant rhodium-carbonyl complex was found to be photoactive, enabling the activation of benzene and formation of a new benzoyl complex. Organometallic intermediate species were observed and characterized by NMR spectroscopy and X-ray crystallography. Based on the series of individual transformations, a sequence for the photocarbonylation of benzene using CO2 as the feedstock was constructed and demonstrated for the production of benzaldehyde from benzene.

Ketone hydrogenation catalyzed by a new iron(II)–PNN complex

The formal FeII–hydride complex [Fe(H)(CO)(MeCN)LPNN](BF4) (1) (LPNN = 2-[(di-tert-butylphosphino)methyl]-6-[1-(mesitylimino)ethyl]pyridine) catalyzes the hydrogenation of ketones under mild conditions (room temperature, p(H2) = 4 bar) and short reaction times (1–3 h) in the presence of catalytic amounts of KHMDS as a base. The reaction presumably proceeds via a dearomatization/rearomatization mechanism. However, in comparison with the reaction of related iron–PNP complexes, the reaction mechanism seems to be different, and an enolate formation step appears to precede catalysis. Moreover, the catalytic performance of the PNN system is inferior under similar conditions, and this observation is probably a consequence of an intramolecular deactivation pathway, which involves reductive proton migration within a dearomatized FeII–hydride complex to form a catalytically inactive Fe0 species. The weaker electron-donating properties of the PNN ligand system, when compared with analogous PNP-based ligands, cause the dearomatized PNN iron(II)–hydride intermediate to be less electron-rich and consequently more prone to the intramolecular reductive elimination pathway. This result is in line with the need for electron-rich metal hydrides for efficient hydrogenation catalysis to take place.

CO Oxidation by N2O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism

Both CO and N2O are important, environmentally harmful industrial gases. The reaction of CO and N2O to produce CO2 and N2 has stimulated much research interest aimed at degradation of these two gases in a single step. Herein, we report an efficient CO oxidation by N2O catalyzed by a (PNN)Ru–H pincer complex under mild conditions, even with no added base. The reaction is proposed to proceed through a sequence of O-atom transfer (OAT) from N2O to the Ru–H bond to form a Ru–OH intermediate, followed by intramolecular OH attack on an adjacent CO ligand, forming CO2 and N2. Thus, the Ru–H bond of the catalyst plays a central role in facilitating the OAT from N2O to CO, providing an efficient and novel protocol for CO oxidation.