Bidentate Donor-Functionalized N-Heterocyclic Carbenes: Valuable Ligands for Ruthenium-Catalyzed Transfer Hydrogenation

Ruthenium complexes are by far the most studied compounds that catalyze hydrogen transfer reactions. In this review, we describe the use in this field of ruthenium complexes bearing bidentate donor-functionalized N-heterocyclic carbene ligands. The review specifically covers the application in transfer hydrogenations of (κ²-C_NHC,Y)-ruthenacyclic compounds where the Y donor atom is a N, P, O, or S atom, and where the N-heterocyclic carbene ligand is a classical imidazol-2-ylidene, a benzimidazol-2-ylidene, a mesoionic 1,2,3-triazolylidene, or an imidazol-4-ylidene ligand. Tridentate donor-functionalized N-heterocyclic carbene complexes thus fall outside the scope of the review. Applications in (asymmetric) transfer hydrogenation of ketones, aldehydes, imines, alkenes, and nitrobenzene are discussed.

Tandem transfer hydrogenation–epoxidation of ketone substrates catalysed by alkene-tethered Ru(ii)–NHC complexes

Eight novel alkene-tethered Ru(ii)–NHC complexes were employed as catalysts in tandem transfer hydrogenation–epoxidation reactions using phenacyl bromide derivatives as substrates.

Effects of Coordinating a Hemilabile Ligand to 14e Cp*M(NO) Scaffolds (M = Mo, W)

This article describes the differing chemical properties imparted by the two ligands, hemilabile 2-[(diisopropylphosphino)methyl]-3-methylpyridine (ⁱPr₂PN) and the related 1,2-bis(dimethylphosphino)ethane (dmpe), when attached to the 14e Cp*M(NO) scaffolds (Cp* = η⁵-C₅Me₅; M = W, Mo). For instance, the treatment of [Cp*W(NO)Cl₂]₂ with 2 or 1 equiv of dmpe in C₆H₆ affords excellent yields of [Cp*W(NO)(κ²-dmpe)Cl]Cl (1) or [Cp*W(NO)Cl₂]₂[μ-dmpe] (2). In contrast, the treatment of [Cp*W(NO)Cl₂]₂ with 1 equiv of ⁱPr₂PN in C₆H₆ does not produce the complex analogous to 1 but rather affords orange [Cp*W(NO)(κ²-P-N-ⁱPr₂PN)Cl][Cp*W(NO)Cl₃] (3) in 90% yield. Furthermore, subsequent reduction of 1 or 2 with 2 or 4 equiv of Cp₂Co in tetrahydrofuran (THF), respectively, results in the production of orange Cp*W(NO)(κ²-dmpe) (4) in good yields. However, a similar treatment of 3 with 1 equiv of Cp₂Co in THF does not result in the production of Cp*W(NO)(κ²-P,N-ⁱPr₂PN), the analogue of 4, but rather generates a 1:1 mixture of the novel complexes Cp*W(NO)(H)(κ¹-P-ⁱPr₂PN)Cl (5) and Cp*W(NO)(κ²-P,N-ⁱPr₂PCH-2-(3-Me-C₅H₃N))Cl (6), which are separable by crystallization from pentane and diethyl ether solutions, respectively. The divergent reactivity imparted by the dmpe and ⁱPr₂PN proligands is a unique demonstration of the unusual properties of a mixed-donor ligand. In the case of molybdenum, the reaction of [Cp*Mo(NO)Cl₂]₂ with 2 equiv of ⁱPr₂PN in C₆H₆ first forms Cp*Mo(NO)(κ¹-P-ⁱPr₂PN)Cl₂, which then converts to [Cp*Mo(NO)(κ²-P,N-ⁱPr₂PN)Cl][Cp*Mo(NO)Cl₃], the analogue of 3. Reduction of the Cp*Mo(NO)(κ¹-P-ⁱPr₂PN)Cl₂ intermediate complex with 2 equiv of Cp₂Co affords dark-green Cp*Mo(NO)(κ²-P,N-ⁱPr₂PN) (7). All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Gold(III) NHC Complexes for Catalyzing Dihydroalkoxylation and Hydroamination Reactions

A gold(III) complex of an N-heterocyclic carbene based hemilabile ligand with two pendant pyrazole arms (1,3-bis((1H-pyrazol-3-yl)methyl)-2,3-dihydro-1H-imidazole, LH) was synthesized. Complex [LAu(III)Cl₃] is an excellent catalyst for promoting dihydroalkoxylation at room temperature, even catalyzing this reaction at 0 °C. [LAu(III)Cl₃] is one of the most efficient catalysts reported to date for the spirocyclization of alkynyl diols. Furthermore, [LAu(III)Cl₃] catalyzed intra- and intermolecular hydroamination reactions, achieving good to excellent conversions. [LAu(III)Cl₃] is a more efficient catalyst than a gold(I) analogue, [LAu(I)Cl]. The dependence of the quantity of weakly coordinating anion [BAr^F₄]^- ((3,5-trifluoromethyl)phenyl borate) present on catalysis efficiency was probed for the dihydroalkoxylation reaction. X-ray diffraction analysis of single crystals demonstrated the solid-state structure of gold complexes [LAu(III)Cl₃] and [LAu(I)Cl], which displayed the expected square-planar and linear coordination geometries, respectively.

Catalytic Activity of Gold(I) Complexes with Hemilabile P,N Ligands

Two new cationic dinuclear gold(I) complexes, [Au₂ {μ(P,N)-5}₂ ]X₂ -in which X=NTf₂ (7; Tf=trifluoromethanesulfonate) or SbF₆ (8) and 2-(diphenylphosphanyl)benzonitrile (5) is a P,N-bridging donor-have been synthesized and structurally characterized. These air-stable species and their dimeric and polymeric analogues possessing 1'-(diphenylphosphanyl)-1-cyanoferrocene (1) as the bridging ligand, [Au₂ {μ(P,N)-1}₂ ](NTf₂ )₂ and [Au{μ(P,N)-1}]_n [SbF₆ ]_n , were used as precatalysts in various Au-mediated C-C and C-O bond-forming reactions. The reactivity of these complexes revealed the hemilabile nature of their P,N ligands. In the series of tested precatalysts, complex 8 exerted particularly high catalytic activity at low Au loading, even in reactions that usually require high amounts of gold catalyst to proceed efficiently under standard reaction conditions.

Aryl appended neutral and cationic half-sandwich ruthenium(ii)–NHC complexes: synthesis, characterisation and catalytic applications

Aryl appended half-sandwich Ru(ii)–NHC complexes were synthesised and their use as selective catalysts for transfer hydrogenation of aromatic ketones was demonstrated.