Proton-responsive Ruthenium(II) Catalysts for the Solvolysis of Ammonia-Borane

Photocatalytic Conversion of CO2 to Formate/CO by an (η6-para-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO2 Immobilization on the Catalytic Mechanism and Product Selectivity

The catalytic efficacy of the monobipyridyl (η6-para-Cymene)Ru(II) half-metallocene, [(p-Cym)Ru(bpy)Cl]+ was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO2/Catalyst) for photochemical CO2 reduction. A series of homogeneous photolysis experiments revealed that the (p-Cym)Ru(II) catalyst engages in two competitive routes for CO2 reduction (CO2 to formate conversion via RuII-hydride vs CO2 to CO conversion through a RuII-COOH intermediate). The conversion activity and product selectivity were notably impacted by the pKa value and the concentration of the proton source added. When a more acidic TEOA additive was introduced, the half-metallocene Ru(II) catalyst leaned toward producing formate through the RuII-H mechanism, with a formate selectivity of 86%. On the other hand, in homogeneous catalysis with TFE additive, the CO2-to-formate conversion through RuII-H was less effective, yielding a more efficient CO2-to-CO conversion with a selectivity of >80% (TONformate of 140 and TONCO of 626 over 48 h). The preference between the two pathways was elucidated through an electrochemical mechanistic study, monitoring the fate of the metal-hydride intermediate. Compared to the homogeneous system, the TiO2-heterogenized (p-Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO2-to-CO and 665 for CO2-to-formate.

Selective N-Alkylation of Aminobenzenesulfonamides with Alcohols for the Synthesis of Amino-(N-alkyl)benzenesulfonamides Catalyzed by a Metal-Ligand Bifunctional Ruthenium Catalyst

[(p-Cymene)Ru(2,2'-bpyO)(H2O)] was proven to be an efficient catalyst for the synthesis of amino-(N-alkyl)benzenesulfonamides via selective N-alkylation of aminobenzenesulfonamides with alcohols. It was confirmed that functional groups in the bpy ligand are crucial for the activity of catalysts. Furthermore, the utilization of this catalytic system for the preparation of a biologically active compound was presented.

Reducible tungsten(VI) oxide-supported ruthenium(0) nanoparticles: highly active catalyst for hydrolytic dehydrogenation of ammonia borane

Reducible WO3 powder with a mean diameter of 100 nm is used as support to stabilize ruthenium(0) nanoparticles. Ruthenium(0) nanoparticles are obtained by NaBH4 reduction of ruthenium(III) precursor on the surface of WO3 support at room temperature. Ruthenium(0) nanoparticles are uniformly dispersed on the surface of tungsten(VI) oxide. The obtained Ru0/WO3 nanoparticles are found to be active catalysts in hydrolytic dehydrogenation of ammonia borane. The turnover frequency (TOF) values of the Ru0/WO3 nanocatalysts with the metal loading of 1.0%, 2.0%, and 3.0% wt. Ru are 122, 106, and 83 min-1, respectively, in releasing hydrogen gas from the hydrolysis of ammonia borane at 25.0 °C. As the Ru0/WO3 (1.0% wt. Ru) nanocatalyst with an average particle size of 2.6 nm provides the highest activity among them, it is extensively investigated. Although the Ru0/WO3 (1.0% wt. Ru) nanocatalyst is not magnetically separable, it has extremely high reusability in the hydrolysis reaction as it preserves 100% of initial catalytic activity even after the 5th run of hydrolysis. The high activity and reusability of Ru0/WO3 (1.0% wt. Ru) nanocatalyst are attributed to the favorable metal-support interaction between the ruthenium(0) nanoparticles and the reducible tungsten(VI) oxide. The high catalytic activity and high stability of Ru0/WO3 nanoparticles increase the catalytic efficiency of precious ruthenium in hydrolytic dehydrogenation of ammonia borane.

Triazine Chalcogenones from Thiocyanate or Selenocyanate Addition to Tetrazine Ligands in Ruthenium Arene Complexes

The chemistry of 1,2,4,5-tetrazines has attracted considerable interest both from a synthetic and applicative standpoint. Recently, regioselective reactions with alkynes and alkenes have been reported to be favored once the tetrazine ring is coordinated to Re(I), Ru(II), and Ir(III) centers. Aiming to further explore the effects of metal coordination, herein, we unveil the unexplored reactivity of tetrazines with chalcogenocyanate anions. Thus, ruthenium(II) tetrazine complexes, [RuCl{κ²N-3-(2-pyridyl)-6-R-1,2,4,5-tetrazine}(η⁶-arene)]⁺ (arene = p-cymene, R = H, [1a]⁺, R = Me, [1b]⁺, R = 2-pyridyl, [1c]⁺; arene = C₆Me₆, R = H, [1d]⁺, R = Me, [1e]⁺; PF₆^- salts), reacted quantitatively and in mild conditions with M(ECN) salts (M = Na, K, Bu₄N; E = O, S, Se). The addition of thiocyanate or selenocyanate to the tetrazine ligand is regioselective and afforded, via N₂ release, 1,2,4-triazine-5-chalcogenone heterocycles, the one with selenium being unprecedented. The novel ruthenium complexes [RuCl{κ²N-(2-pyridyl)}{triazine chalcogenone}(η⁶-arene)] 2a-e (sulfur), 3b, 3d, and 3e (selenium) were characterized by analytical (CHNS analyses, conductivity), spectroscopic (IR, multinuclear and two-dimensional (2D) NMR), and spectrometric (electrospray ionization mass spectrometry (ESI-MS)) techniques. According to density functional theory (DFT) calculations, the nucleophilic attack of SCN^- on the tetrazine ring is kinetically driven. Compound 2b is selectively and reversibly mono-protonated on the triazine ring by HCl or other strong acids, affording a single tautomer. When reactions of chalcogenocyanates were performed on the 2,2'-bipyridine (bpy) complex [RuCl(bpy)(η⁶-p-cymene)]⁺, the chloride substitution products [Ru(ECN)(bpy)(η⁶-p-cymene)]⁺ (E = O, [4]⁺; E = S, [5]⁺; E = Se, [6]⁺) were obtained in 82-90% yields (PF₆^- salts). Combined spectroscopic data (IR, ¹H/¹³C/⁷⁷Se NMR) was revealed to be a useful tool to study the linkage isomerism of the chalcogenocyanate ligand in [4-6]⁺.

Formation of Irida-β-ketoimines and PCNamine-Ir(III) Complexes by Reacting Irida-β-diketones with Aliphatic Diamines: Catalytic Activity in Hydrogen Release by Methanolysis of H3N-BH3

Aliphatic diamines [(H2N(CH2)nNHR) (a-d) n = 2: R = H (a), R = CH3 (b), R = C2H5 (c), n = 3, R = H (d) or rac-2-(aminomethyl)piperidine (e)] react with [IrH(Cl){(PPh2(o-C6H4CO))2H}] in THF to afford ketoimine complexes [IrH(Cl){(PPh2(o-C6H4CO))(PPh2(o-C6H4CN(CH2)nNHR))H}] (2a-2d) or [IrH(Cl){(PPh2(o-C6H4CO))(PPh2(o-C6H4CNCH2(C5H9NH)))H}] (2e), containing a bridging N-H···O hydrogen bond and a dangling amine. Complex 2e consists of an almost equimolar mixture of two diastereomers. In protic solvents, the dangling amine in complexes 2 displaces chloride to afford cationic acyl-iminium compounds, [IrH(PPh2(o-C6H4CO))(PPh2(o-C6H4CNH(CH2)nNHR))]X (3a-3d, X = Cl) or [IrH(PPh2(o-C6H4CO))(PPh2(o-C6H4CNHCH2(C5H9NH)))]Cl (3e) and (4a-4b, X = ClO4), with new hemilabile terdentate PCNamine ligands adopting a facial disposition. Complexes 3 contain the corresponding phosphorus atom trans to hydride and the amine fragment trans to acyl, while complexes 4 contain the amine trans to hydride. 3b and 4b consist of 80:20 and 95:5 mixtures of diastereomers, respectively, while 3e contains a 65:35 mixture. In the presence of KOH, intermediate cationic acyl-iminium complexes 3 transform into neutral acyl-imine [IrH(PPh2(o-C6H4CO))(PPh2(o-C6H4CN(CH2)nNHR))] derivatives (5) with retention of the stereochemistry. Single-crystal X-ray diffraction analysis was performed on 2a, [3a]Cl, [3b]Cl, [4a]ClO4, and 5b. Complexes 2, 3, and 5 catalyze the methanolysis of ammonia-borane under air to release hydrogen. The highest activity is observed for ketoimine complexes 2.

Activation of Si–H and B–H bonds by Lewis acidic transition metals and p-block elements: same, but different

In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si–H and B–H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E–H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η¹ or η²) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.

In this Perspective we compare the ability of transition metals and p-block Lewis acids to activate electrophilically hydrosilanes and hydroboranes. The mechanistic similarities and dissimilarities in different catalytic transformations are analyzed.

Reduction of Carbon Dioxide with Ammonia-Borane under Ambient Conditions: Maneuvering a Catalytic Way

In the development of alternatives to the traditional catalytic hydrogenation of CO₂ with gaseous H₂, employing nongaseous H₂ storage compounds as potential reductants for catalytic transfer hydrogenation of CO₂ is promising. Ammonia-borane, due to its high hydrogen storage capacity (19.6 wt %), has been used for catalytic transfer hydrogenation of several organic unsaturated compounds. However, a similar protocol involving catalytic transfer hydrogenation of less reactive CO₂ with NH₃BH₃ is yet to be realized experimentally. Herein, we demonstrate the first catalytic CO₂ transfer hydrogenation process for generating formate salt with NH₃BH₃ under ambient conditions (1 atm and 30 °C) employing a cationic "Ir(III)-abnormal NHC" catalyst via an electrophilic NH₃BH₃ activation route. It exhibited an initial turnover frequency of 686 h^-1 and a high turnover number (TON) of ≈1300 in just 4 h. Most significantly, the catalyst was durable enough to maintain long-term activity, and upon only periodic recharging of NH₃BH₃, it furnished a total TON of >4200 in 10 h.

An Amine-Borane System Featuring Room-Temperature Dehydrogenation and Regeneration

Amine-borane complexes have been extensively studied as hydrogen storage materials. Herein, we report a new amine-borane system featuring a reversible dehydrogenation and regeneration at room temperature. In addition to high purity H₂ , the reaction between ethylenediamine bisborane (EDAB) and ethylenediamine (ED) leads to unique boron-carbon-nitrogen 5-membered rings in the dehydrogenation product where one boron is tricoordinated by three nitrogen atoms. Owing to the unique cyclic structure, the dehydrogenation product can be efficiently converted back to EDAB by NaBH₄ and H₂ O at room temperature. This finding could lead to the discovery of new amine boranes with potential usage as hydrogen storage materials.

N-Methylation of Amines with Methanol in the Presence of Carbonate Salt Catalyzed by a Metal-Ligand Bifunctional Ruthenium Catalyst [(p-cymene)Ru(2,2'-bpyO)(H2O)]

A ruthenium complex [(p-cymene)Ru(2,2'-bpyO)(H₂O)] was found to be a general and efficient catalyst for the N-methylation of amines with methanol in the presence of carbonate salt. Moreover, a series of sensitive substituents, such as nitro, ester, cyano, and vinyl groups, were tolerated under present conditions. It was confirmed that OH units in the ligand are crucial for the catalytic activity. Notably, this research exhibited the potential of metal-ligand bifunctional ruthenium catalysts for the hydrogen autotransfer process.