Selective dehydrogenation of ammonia borane to polycondensated BN rings catalysed by ruthenium olefin complexes.

Dehydrogenation of ammonia borane to well-defined products is an important but challenging reaction. A dinuclear ruthenium complex with a Ru-Ru bond bearing a diazadiene (dad) unit and olefins as non-innocent ligands catalyzes the highly selective formation of conjugated polycondensed borazine oligomers (BxNxHy), predominantly B21N21H18, the BN analogue of superbenzene.

Reduction of Nitrous Oxide by Light Alcohols Catalysed by a Low-Valent Ruthenium Diazadiene Complex.

Decomposition of the environmentally harmful gas nitrous oxide (N2 O) is usually performed thermally or catalytically. Selective catalytic reduction (SCR) is currently the most promising technology for N2 O mitigation, a multicomponent heterogeneous catalytic system that employs reducing agents such as ammonia, hydrogen, hydrocarbons, or a combination thereof. This study reports the first homogenous catalyst that performs the reduction of nitrous oxide employing readily available and cheap light alcohols such as methanol, ethanol or ethylene glycol derivatives. During the reaction, these alcohols are transformed in a dehydrogenative coupling reaction to carboxylate derivatives, while N2 O is converted to N2 and H2 O, later entering the reaction as substrate. The reaction is catalysed by the low-valent dinuclear ruthenium complex [Ru2 H(µ-H)(Me2 dad)(dbcot)2 ] that carries a diazabutadiene, Me2 dad, and two rigid dienes, dbcot, as ligands. The reduction of nitrous oxide proceeds with low catalyst loadings under relatively mild conditions (65-80 °C, 1.4 bar N2 O) achieving turnover numbers of up to 480 and turnover frequencies of up to 56 h-1 .

Computational mechanistic studies of ruthenium catalysed methanol dehydrogenation.

Homogeneous ruthenium catalysed methanol dehydrogenation could become a key reaction for hydrogen production in liquid fuel cells. In order to improve existing catalytic systems, mechanistic insight is paramount in directing future studies. Herein, we describe what computational mechanistic research has taught us so far about ruthenium catalysed dehydrogenation reactions. In general, two mechanistic pathways can be operative in these reactions: a metal-centered or a metal-ligand cooperative (Noyori-Morris type) minimum energy reaction pathway (MERP). Discerning between these mechanisms on the basis of computational studies has proven to be highly input dependent, and to circumvent pitfalls it is important to consider several factors, such as solvent effects, metal-ligand cooperativity, alternative geometries, and complex electronic structures of metal centres. This Frontiers article summarizes the reported computational research performed on ruthenium catalyzed dehydrogenation reactions performed in the past decade, and serves as a guide for future research.

Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)-Platinum(II) Olefin Complexes as Catalysts.

The nitrogen oxides NO2 , NO, and N2 O are among the most potent air pollutants of the 21st century. A bimetallic RhI -PtII complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3-4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N2 O) to water and N2 was recorded, making these RhI -PtII complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON=38) or nitrogen dioxide (NO2 , TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NOx were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO2 proceeds stepwise, to first give NO and H2 O, followed by the generation of N2 O and H2 O, which is then further converted to N2 and H2 O. The nitrogen-nitrogen bond-forming step takes place in the conversion from NO to N2 O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N2 O2 2- ) complex, which was detected as an intermediate.

Hydrogenolysis of Polysilanes Catalyzed by Low-Valent Nickel Complexes.

The dehydrogenation of organosilanes (Rx SiH4-x ) under the formation of Si-Si bonds is an intensively investigated process leading to oligo- or polysilanes. The reverse reaction is little studied. To date, the hydrogenolysis of Si-Si bonds requires very harsh conditions and is very unselective, leading to multiple side products. Herein, we describe a new catalytic hydrogenation of oligo- and polysilanes that is highly selective and proceeds under mild conditions. New low-valent nickel hydride complexes are used as catalysts and secondary silanes, RR'SiH2 , are obtained as products in high purity.

DFT Provides Insight into the Additive-Free Conversion of Aqueous Methanol to Dihydrogen Catalyzed by [Ru(trop2dad)]: Importance of the (Electronic) Flexibility of the Diazadiene Moiety.

The mechanism for complete dehydrogenation of aqueous methanol to CO2 and three equivalents of H2 catalyzed by [Ru(trop2dad)] was investigated with DFT (trop2dad = 1,4-bis(5 H-dibenzo[ a, d]cyclohepten-5-yl)-1,4-diazabuta-1,3-diene). To date, this is the only catalyst that promotes the acceptorless dehydrogenation of aqueous methanol in homogeneous phase under mild conditions without the addition of an additive (base, acid, or a secondary catalyst). A detailed understanding of the mechanism of this transformation may therefore be of significant importance for the conversion of liquid organic fuels. Previous computational studies using simplified models of the catalyst suggested entirely ligand-centered reaction pathways with rather high-energy barriers for complete dehydrogenation of aqueous methanol. These are, however, not consistent with the experimental data. In the present paper, we reveal a different reaction mechanism for aqueous methanol dehydrogenation that involves metal-ligand cooperativity involving the diazadiene (dad) ligand and has substantially lower barriers, in good agreement with the experimental data. The dad moiety of the ligand actively participates in the alcohol activation mechanism. In the first step of the reaction, the dad ligand rearranges from a σ- to a π-bound coordination mode. This adjusts the electronic structure of both the metal and the ligand, leading to an enhanced Brønsted basicity of the nitrogen centers and higher Lewis acidity of the ruthenium center. As a result, concerted proton-hydride transfer to/from metal-hydride and N-protonated dad-ligand moieties becomes possible, leading to low-barrier metal-ligand cooperative elementary steps for alcohol activation and H2 elimination.

Highly Efficient Base-Free Dehydrogenation of Formic Acid at Low Temperature.

The ruthenium complex [RuH2 (PPh3 )4 ] is a competent catalyst for the selective dehydrogenation of formic acid (FA) at low temperature. It tolerates water and shows excellent performance (TOF up to 36 000 h-1 at 60 °C). Remarkably, no basic additives are necessary to obtain such high activity and the defined complex is stable for up to 120 days, making this system one of the most effective formic acid dehydrogenation catalysts known to date.

Ligand- and Metal-Based Reactivity of a Neutral Ruthenium Diolefin Diazadiene Complex: The Innocent, the Guilty and the Suspicious.

Coordination of the diazadiene diolefin ligand (trop2 dad) to ruthenium leads to various complexes of composition [Ru(trop2 dad)(L)]. DFT studies indicate that the closed-shell singlet (CSS), open-shell singlet (OSS), and triplet electronic structures of this species are close in energy, with the OSS spin configuration being the lowest in energy for all tested functionals. Singlet-state CASSCF calculations revealed a significant multireference character for these complexes. The closed-shell singlet wavefunction dominates, but these complexes have a significant (≈8-16 %) open-shell singlet [d7 -RuI (L)(trop2 dad.- )] contribution mixed into the ground state. In agreement with their ambivalent electronic structure, these complexes reveal both metal- and ligand-centered reactivity. Most notable are the reactions with AdN3 , diazomethane, and a phosphaalkyne leading to scission of the C-C bond of the diazadiene (dad) moiety of the trop2 dad ligand, resulting in net (formal) nitrene, carbene, or P≡C insertion in the dad C-C bond, respectively. Supporting DFT studies revealed that several of the ligand-based reactions proceed via low-barrier radical-type pathways, involving the dad.- ligand radical character of the OSS or triplet species.

A Stable Aminyl Radical Coordinated to Cobalt.

A family of cobalt complexes bearing the trop2 NH [bis(5-H-dibenzo[a,d]cyclohepten-5-yl)-amine] and 2,2'-bpy (2,2'-bipyridine) chelate ligands were prepared and fully characterized. The compounds [Co(trop2 N)(bpy)], [Co(trop2 NH)(bpy)]+ , and [Co(trop2 N)(bpy)]+ are cobalt complexes interrelated by one-electron redox processes and/or proton transfer. Two limiting resonance structures can be used to describe the paramagnetic complex [Co(trop2 N)(bpy)]+ : [CoII (trop2 N- )(bpy)]+ (CoII amido) and [CoI (trop2 N⋅ )(bpy)]+ (CoI -aminyl radical). Structural data, DFT calculations, and reactivity toward H-abstraction indicate a slightly higher contribution of the aminyl radical form to the ground state of [Co(trop2 N)(bpy)]+ . The results described here complete the series of Group 9 metal aminyl radical complexes bearing the diolefin amine ligand trop2 NH.

Zero-Valent Amino-Olefin Cobalt Complexes as Catalysts for Oxygen Atom Transfer Reactions from Nitrous Oxide.

The synthesis and characterization of several zero-valent cobalt complexes with a bis(olefin)-amino ligand is presented. Some of these complexes proved to be efficient catalysts for the selective oxidation of secondary and allylic phosphanes, as well as diphosphanes, even with a direct P-P bond. With 5 mol % catalyst loadings the oxidations proceed under mild conditions (25-70 °C, 7-22 h, 2 bar N2 O) and afford good to excellent yields (65-98 %). In this process, the greenhouse gas N2 O is catalytically converted into benign N2 and added-value organophosphorus compounds, some of which are difficult to obtain otherwise.

A homogeneous transition metal complex for clean hydrogen production from methanol-water mixtures.

The development of an efficient catalytic process that mimics the enzymatic function of alcohol dehydrogenase is critical for using biomass alcohols for both the production of H2 as a chemical energy carrier and fine chemicals under waste-free conditions. Dehydrogenation of alcohol-water mixtures into their corresponding acids with molecular hydrogen as the sole by-product from the reaction can be catalysed by a ruthenium complex with a chelating bis(olefin) diazadiene ligand. This complex, [K(dme)2][Ru(H)(trop2dad)], stores up to two equivalents of hydrogen intramolecularly, and catalyses the production of H2 from alcohols in the presence of water and a base under homogeneous conditions. The conversion of a MeOH-H2O mixture proceeds selectively to CO2/H2 gas formation under neutral conditions, thereby allowing the use of the entire hydrogen content (12% by weight). Isolation and characterization of the ruthenium complexes from these reactions suggested a mechanistic scenario in which the trop2dad ligand behaves as a chemically 'non-innocent' co-operative ligand.

Metal-ligand cooperation in the catalytic dehydrogenative coupling (DHC) of polyalcohols to carboxylic acid derivatives.

Several polyols, which are easily available from sugars through biochemical conversion or hydrogenolytic cleavage, are directly converted into carboxylic acids and amides. This efficient dehydrogenative coupling process, catalyzed by a rhodium(I) diolefin amido complex, is an attractive approach for the production of organic fine chemicals from renewable resources. This method tolerates the presence of several hydroxy groups and can be extended to the direct synthesis of lactams from the corresponding amino alcohols under mild conditions.

Domino rhodium/palladium-catalyzed dehydrogenation reactions of alcohols to acids by hydrogen transfer to inactivated alkenes.

The combination of the d(8) Rh(I) diolefin amide [Rh(trop(2)N)(PPh(3))] (trop(2)N=bis(5-H-dibenzo[a,d]cyclohepten-5-yl)amide) and a palladium heterogeneous catalyst results in the formation of a superior catalyst system for the dehydrogenative coupling of alcohols. The overall process represents a mild and direct method for the synthesis of aromatic and heteroaromatic carboxylic acids for which inactivated olefins can be used as hydrogen acceptors. Allyl alcohols are also applicable to this coupling reaction and provide the corresponding saturated aliphatic carboxylic acids. This transformation has been found to be very efficient in the presence of silica-supported palladium nanoparticles. The dehydrogenation of benzyl alcohol by the rhodium amide, [Rh]N, follows the well established mechanism of metal-ligand bifunctional catalysis. The resulting amino hydride complex, [RhH]NH, transfers a H(2) molecule to the Pd nanoparticles, which, in turn, deliver hydrogen to the inactivated alkene. Thus a domino catalytic reaction is developed which promotes the reaction R-CH(2)-OH+NaOH+2 alkene-->R-COONa+2 alkane.

Intramolecular iodoarylation reaction of alkynes: easy access to derivatives of benzofused heterocycles.

The iodoarylation reaction of heteroatom-tethered omega-arylalkynes offers an efficient and straightforward entry to heterocycles. As a result, both C-C ring-closing from readily available precursors, and concomitant selective iodination take place. The first related study conducted in water is presented.

Intramolecular arylation reactions of alkenes: a flexible approach to chromans and tetrahydroquinoline derivatives.

A metal-free approach to chroman derivatives by the reaction of different allylphenyl ethers with Ipy2BF4 is described. The access to this frame, by direct cyclization of the starting materials, takes place smoothly. In addition, an unusual and selective sequence that comprises initial rearrangement and further cyclization could be accomplished, with temperature as an excellent element of control. The process can be extended to the synthesis of tetrahydroquinolines when using allylamines. Also, two cascade reactions, leading to relevant oxygen and nitrogen-containing skeletons, have been devised.