Trisphosphine‐Chelate‐Substituted Molybdenum and Tungsten Nitrosyl Hydrides as Highly Active Catalysts for Olefin Hydrogenations

Reaction of [M(NO)Cl₃(NCMe)₂] (M=Mo, W) with (iPr₂PCH₂CH₂)₂PPh (etpⁱp) at room temperature afforded the syn/anti‐[M(NO)Cl₃(mer‐etpⁱp)] complexes (M=Mo, a ; W, b ; 3 a,b (syn,anti); syn and anti refer to the relative position of Ph(etpⁱp) and NO). Reduction of 3 a,b (syn,anti) produced [M(NO)Cl₂(mer‐etpⁱp)] ( 4 a,b (syn)), [M(NO)Cl(NCMe)(mer‐etpⁱp)] ( 5 a,b (syn,anti)), and [M(NO)Cl(η²‐ethylene)(mer‐etpⁱp)] ( 6 a,b (syn,anti)) complexes. The hydrides [M(NO)H(η²‐ethylene)(mer‐etpⁱp)] ( 7 a,b (syn,anti)) were obtained from 6 a,b (syn,anti) using NaHBEt₃ (75 °C, THF) or LiBH₄ (80 °C, Et₃N), respectively. 7 a,b (syn,anti) were probed in olefin hydrogenations in the absence or presence of a hydrosilane/B(C₆F₅)₃ mixture. The 7 a,b (syn,anti)/Et₃SiH/B(C₆F₅)₃ co‐catalytic systems were highly active in various olefin hydrogenations (60 bar H₂, 140 °C), with maximum TOFs of 5250 h^?1 ( 7 a (syn,anti)) and 8200 h^?1 ( 7 b (syn,anti)) for 1‐hexene hydrogenation. The Et₃SiH/(B(C₆F₅)₃ co‐catalyst is anticipated to generate a [Et₃Si]⁺ cation attaching to the O_NO atom. This facilitates NO bending and accelerates catalysis by providing a vacant site. Inverse DKIE effects were observed for the 7 a (syn,anti)/Et₃SiH/(B(C₆F₅)₃ (k_H/k_D=0.55) and the 7 b (syn,anti)/Et₃SiH/(B(C₆F₅)₃ (k_H/k_D=0.65) co‐catalytic mixtures (20 bar H₂/D₂, 140 °C).     

Hydrogenation of Imines Catalyzed by Trisphosphine‐Substituted Molybdenum and Tungsten Nitrosyl Hydrides and Co‐Catalytic Acid

Hydride complexes Mo,W(CO)(NO)H(mer‐etpⁱp) (iPr₂PCH₂CH₂)₂PPh=etpⁱp) ( 2 a,b(syn) , syn and anti of NO and Ph(etpⁱp) orientions) were prepared and probed in imine hydrogenations together with co‐catalytic [H(Et₂O)₂][B(C₆F₅)₄] (140 °C, 60 bar H₂). 2 a,b(syn) were obtained via reduction of syn/anti‐Mo,W(NO)Cl₃(mer‐etpⁱp) and syn,anti‐Mo,W(NO)(CO)Cl(mer‐etpⁱp). [H(Et₂O)₂][B(C₆F₅)₄] in THF converted the hydrides into THF complexes syn‐[Mo,W(NO)(CO)(etpⁱp)(THF)][B(C₆F₅)₄]. Combinations of the p‐substituents of aryl imines p‐R¹C₆H₄CH=N‐p‐C₆H₄R² (R¹,R²=H,F,Cl,OMe,α‐Np) were hydrogenated to amines (maximum initial TOFs of 1960 h^?1 ( 2 a(syn) ) and 740 h^?1 ( 2 b(syn) ) for N‐(4‐methoxybenzylidene)aniline). An ‘ionic hydrogenation’ mechanism based on linear Hammett plots (ρ=?10.5, p‐substitution on the C‐side and ρ=0.86, p‐substitution on the N‐side), iminium intermediates, linear P(H₂) dependence, and DKIE=1.38 is proposed. Heterolytic splitting of H₂ followed by ‘proton before hydride’ transfers are the steps in the ionic mechanism where H₂ ligand addition is rate limiting.     

Active Molybdenum‐Based Anode for Dehydrogenative Coupling Reactions

A new and powerful active anode system that can be operated in 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP) has been discovered. In HFIP the molybdenum anode forms a compact, conductive, and electroactive layer of higher‐valent molybdenum species. This system can replace powerful but stoichiometrically required Mo^V reagents for the dehydrogenative coupling of aryls. This electrolytic reaction is more sustainable and allows the conversion of a broad scope of activated arenes.     

Highly Active Phosphine‐Free Bifunctional Iron Complex for Hydrogenation of Bicarbonate and Reductive Amination

Based on a “transition metal frustrated Lewis pair” approach, a cyclopentadienone iron tricarbonyl complex has been designed and applied in the reductive amination and hydrogenation of bicarbonate. This well‐defined phosphine‐free complex displays the best activities reported to date for an iron complex in the reduction of bicarbonate into formate and in reductive amination.     

Switch in Catalyst State: Single Bifunctional Bi‐state Catalyst for Two Different Reactions

Disclosed here is a molecular switch which responds to acid‐base stimuli and serves as a bi‐state catalyst for two different reactions. The two states of the switch serve as a highly active and poorly active catalyst for two catalytic reactions (namely a hydrogenation and a dehydrogenative coupling) but in a complementary manner. The system was used in an assisted tandem catalysis set‐up involving dehydrogenative coupling of an amine and then hydrogenation of the resulting imine product by switching between the respective states of the catalyst.     

Asymmetric Transfer and Pressure Hydrogenation with Earth‐Abundant Transition Metal Catalysts

The asymmetric transfer and pressure hydrogenation of various unsaturated substrates provides a succinct pathway to important chiral intermediates and products such as chiral alcohols, amines, and alkanes. The use of earth‐abundant transition metals such as Fe, Co, Ni, and Cu in hydrogenation reactions provides an attractive alternative to traditionally used metals such as Ru, Rh, Ir, and Pd because they are comparatively inexpensive, less toxic, and as their name suggests, more abundant in nature. Earth‐abundant transition metal‐catalyzed asymmetric hydrogenation is rapidly becoming an important area of research. This review summarizes advances in the asymmetric hydrogenation of unsaturated bonds (ketones, imines, and alkenes) with earth‐abundant transition metals.     

Bifunctional Rhenium Complexes for the Catalytic Transfer‐Hydrogenation Reactions of Ketones and Imines

The silyloxycyclopentadienyl hydride complexes [Re(H)(NO)(PR₃)(C₅H₄OSiMe₂tBu)] (R=iPr ( 3 a ), Cy ( 3 b )) were obtained by the reaction of [Re(H)(Br)(NO)(PR₃)₂] (R=iPr, Cy) with Li[C₅H₄OSiMe₂tBu]. The ligand–metal bifunctional rhenium catalysts [Re(H)(NO)(PR₃)(C₅H₄OH)] (R=iPr ( 5 a ), Cy ( 5 b )) were prepared from compounds 3 a and 3 b by silyl deprotection with TBAF and subsequent acidification of the intermediate salts [Re(H)(NO)(PR₃)(C₅H₄O)][NBu₄] (R=iPr ( 4 a ), Cy ( 4 b )) with NH₄Br. In nonpolar solvents, compounds 5 a and 5 b formed an equilibrium with the isomerized trans‐dihydride cyclopentadienone species [Re(H)₂(NO)(PR₃)(C₅H₄O)] ( 6 a,b ). Deuterium‐labeling studies of compounds 5 a and 5 b with D₂ and D₂O showed H/D exchange at the H_Re and H_O positions. Compounds 5 a and 5 b were active catalysts in the transfer hydrogenation reactions of ketones and imines with 2‐propanol as both the solvent and H₂ source. The mechanism of the transfer hydrogenation and isomerization reactions was supported by DFT calculations, which suggested a secondary‐coordination‐sphere mechanism for the transfer hydrogenation of ketones.     

Asymmetric Hydrogenation with Highly Active IndolPhos–Rh Catalysts: Kinetics and Reaction Mechanism

The mechanism of the IndolPhos–Rh‐catalyzed asymmetric hydrogenation of prochiral olefins has been investigated by means of X‐ray crystal structure determination, kinetic measurements, high‐pressure NMR spectroscopy, and DFT calculations. The mechanistic study indicates that the reaction follows an unsaturate/dihydride mechanism according to Michaelis–Menten kinetics. A large value of K_M (K_M=5.01±0.16 M ) is obtained, which indicates that the Rh–solvate complex is the catalyst resting state, which has been observed by high‐pressure NMR spectroscopy. DFT calculations on the substrate–catalyst complexes, which are undetectable by experimental means, suggest that the major substrate–catalyst complex leads to the product. Such a mechanism is in accordance with previous studies on the mechanism of asymmetric hydrogenation reactions with C₁‐symmetric heteroditopic and monodentate ligands.     

茂钛配合物—纳米氢化钠双组分高活性加氢催化剂的研究

提出了带有不同取代基的茂钛配合物与纳米氢化钠（ＮａＨ）组成的高活性加氢催化剂，在常温常压下，取代茂钛配合物ＴｉＣｌ２／ＮａＨ对１－己烯的加氢反应有极高的初始活性，ＴＯＦｍａｘ达到１１０ｍｏｌＨ２／（ｍｏｌＴｉ．ｓ），催化转换数达到２２２００ｍｏｌＨ２／（ｍｏｌＴｉ）。该催化体系对底物有明显的专一选择性，只有端烃才能发生加氢反应，且无民构化副反应发生，纳米氢化钠的助剂作用是该催化体系高活性关键因素。     

Highly Active Multidentate Ligand‐Based Alkyne Metathesis Catalysts

Alkyne metathesis catalysts composed of molybdenum(VI) propylidyne and multidentate tris(2‐hydroxylbenzyl)methane ligands have been developed, which exhibit excellent stability (remains active in solution for months at room temperature), high activity, and broad functional‐group tolerance. The homodimerization and cyclooligomerization of monopropynyl or dipropynyl substrates, including challenging heterocycle substrates (e.g., pyridine), proceed efficiently at 40–55 °C in a closed system. The ligand structure and catalytic activity relationship has been investigated, which shows that the ortho groups of the multidentate phenol ligands are critical to the stability and activity of such a catalyst system.     

Cationic NHC-Phosphine Iridium Complexes: Highly Active Catalysts for Base-Free Hydrogenation of Ketones

Novel bidentate N-heterocyclic carbene-phosphine iridium complexes have been synthesized and evaluated in the hydrogenation of ketones. Reported catalytic systems require base additives and, if excluded, need elevated temperature or high pressure of hydrogen gas to achieve satisfactory reactivity. The developed catalysts showed extremely high reactivity and good enantioselectivity under base-free and mild conditions. In the presence of 1 mol % catalyst under 1 bar hydrogen pressure at room temperature, hydrogenation was complete in 30 minutes giving up to 96 % ee. Again, this high reactivity was achieved in additive-free conditions. Mechanistic experiments demonstrated that balloon pressure of hydrogen was sufficient to form the activate species by reducing and eliminating the 1,5-cyclooctadiene ligand. The pre-activated catalyst was able to hydrogenate acetophenone with 89 % conversion in 5 min.     

Towards a Zinc‐Catalyzed Asymmetric Hydrogenation/Transfer Hydrogenation of Imines

The first asymmetric hydrogenation/transfer hydrogenation of imines to amines using zinc(II) triflate in combination with chiral ligands is described. The monodentate binaphthophosphepine ligand ( 3g ) provided the highest enantioselectivities. Using different imines, the corresponding amines were obtained in moderate yields and enantioselectivities.     

Ruthenium Nanoparticles on Nano‐Level‐Controlled Carbon Supports as Highly Effective Catalysts for Arene Hydrogenation

The reaction of three types of carbon nanofibers (CNFs; platelet: CNF‐P, tubular: CNF‐T, herringbone: CNF‐H) with [Ru₃(CO)₁₂] in toluene heated at reflux provided the corresponding CNF‐supported ruthenium nanoparticles, Ru/CNFs (Ru content=1.1–3.8 wt %). TEM studies of these Ru/CNFs revealed that size‐controlled Ru nanoparticles (2–4 nm) exist on the CNFs, and that their location was dependent on the surface nanostructures of the CNFs: on the edge of the graphite layers (CNF‐P), in the tubes and on the surface (CNF‐T), and between the layers and on the edge (CNF‐H). Among these Ru/CNFs, Ru/CNF‐P showed excellent catalytic activity towards hydrogenation of toluene with high reproducibility; the reaction proceeded without leaching of the Ru species, and the catalyst was reusable. The total turnover number of the five recycling experiments for toluene hydrogenation reached over 180 000 (mol toluene) (mol Ru)^?1. Ru/CNF‐P was also effective for the hydrogenation of functionalized benzene derivatives and pyridine. Hydrogenolysis of benzylic C? O and C? N bonds has not yet been observed. Use of poly(ethylene glycol)s (PEGs) as a solvent made possible the biphasic catalytic hydrogenation of toluene. After the reaction, the methylcyclohexane formed was separated by decantation without contamination of the ruthenium species and PEG. The insoluble PEG phase containing all of the Ru/CNF was recoverable and reusable as the catalyst without loss of activity.     

Highly Active Superbulky Alkaline Earth Metal Amide Catalysts for Hydrogenation of Challenging Alkenes and Aromatic Rings

Two series of bulky alkaline earth (Ae) metal amide complexes have been prepared: Ae[N(TRIP)₂]₂ ( 1 ‐Ae) and Ae[N(TRIP)(DIPP)]₂ ( 2 ‐Ae) (Ae=Mg, Ca, Sr, Ba; TRIP=SiiPr₃, DIPP=2,6‐diisopropylphenyl). While monomeric 1 ‐Ca was already known, the new complexes have been structurally characterized. Monomers 1 ‐Ae are highly linear while the monomers 2 ‐Ae are slightly bent. The bulkier amide complexes 1 ‐Ae are by far the most active catalysts in alkene hydrogenation with activities increasing from Mg to Ba. Catalyst 1 ‐Ba can reduce internal alkenes like cyclohexene or 3‐hexene and highly challenging substrates like 1‐Me‐cyclohexene or tetraphenylethylene. It is also active in arene hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as biphenyl. Benzene could be reduced to cyclohexane but full conversion was not reached. The first step in catalytic hydrogenation is formation of an (amide)AeH species, which can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate size but it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has a noticeable influence on the thermodynamics for formation of the (amide)AeH species. Complex 1 ‐Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts, the substrate scope in hydrogenation catalysis could be extended to challenging multi‐substituted unactivated alkenes and even to arenes among which benzene.     

Catalytic Carbanion Reactions: Formation and Reaction of Carbanions from Ester or Amide Equivalents Using Catalytic Amounts of Bases

We describe here carbanion reactions using catalytic amounts of bases. The carbanions formed are different from conventional carbanions in which stoichiometric amounts of bases are needed for the formation. Two types of reactions using such carbanions from amide (imido) and ester equivalents are discussed.     

Porous Molybdenum‐Based Hybrid Catalysts for Highly Efficient Hydrogen Evolution

We have synthesized a porous Mo‐based composite obtained from a polyoxometalate‐based metal–organic framework and graphene oxide (POMOFs/GO) using a simple one‐pot method. The MoO₂@PC‐RGO hybrid material derived from the POMOFs/GO composite is prepared at a relatively low carbonization temperature, which presents a superior activity for the hydrogen‐evolution reaction (HER) in acidic media owing to the synergistic effects among highly dispersive MoO₂ particles, phosphorus‐doped porous carbon, and RGO substrates. MoO₂@PC‐RGO exhibits a very positive onset potential close to that of 20 % Pt/C, low Tafel slope of 41 mV dec^?1, high exchange current density of 4.8×10^?4 A cm^?2, and remarkable long‐term cycle stability. It is one of the best high‐performance catalysts among the reported nonprecious metal catalysts for HER to date.