Donor-Flexible Bis(pyridylidene amide) Ligands for Highly Efficient Ruthenium-Catalyzed Olefin Oxidation

An exceptionally efficient ruthenium-based catalyst for olefin oxidation has been designed by exploiting N,N′-bis(pyridylidene)oxalamide (bisPYA) as a donor-flexible ligand. The dynamic donor ability of the bisPYA ligand, imparted by variable zwitterionic and neutral resonance structure contributions, paired with the redox activity of ruthenium provided catalytic activity for Lemieux–Johnson-type oxidative cleavage of olefins to efficiently prepare ketones and aldehydes. The ruthenium bisPYA complex significantly outperforms state-of-the-art systems and displays extraordinary catalytic activity in this oxidation, reaching turnover frequencies of 650 000 h⁻¹ and turnover numbers of several millions.     

Highly Active,Chemo‐ and Regioselective YbII and SmII Catalysts for the Hydrophosphination of Styrene with Phenylphosphine

Stable heteroleptic amido Yb^II and Sm^II complexes bearing aminoether–phenolate ligands and devoid of coordinated solvent have been structurally characterized. They afford highly active, chemoselective and, in the case of monoadditions, 100 % anti‐Markovnikov regiospecific catalysts (down to 0.04 mol % loading) for the hydrophosphination of styrene with PhPH₂ under mild conditions.     

Phosphorus Heterocycles: From Laboratory Curiosities to Ligands in Highly Efficient Catalysts

Phosphabenzenes and phosphaferrocenes were among the first compounds with P−C multiple bonds. For nearly 30 years the chemistry of these molecules was essentially a domain left to basic researchers. Recently, however, it was reported that transition metal complexes with phosphabenzene and phosphaferrocene ligands exhibit remarkable potential as catalysts. Catalysts based on rhodium (I ) and various phosphabenzenes appear to be superior to classical systems in the hydroformylation of terminal and internal alkenes. In addition planar‐chiral phosphaferrocene species display an excellent performance as directing ligands in a series of enantioselective asymmetric syntheses.     

Relative Lability and Chemoselective Transmetallation of NHC in Hybrid Phosphine‐NHC Ligands: Access to Heterometallic Complexes

The relative lability and transmetallation aptitude of trialkylphosphine and NHC donors, integrated in semi‐rigid hybrid ligands attached to [Ag₄Br₄] pseudo‐cubanes, lies in favor of the NHC and is used to selectively access unprecedented NHC complexes with heterobimetallic cores, such as Ag–Cu ( 4^Cy ) and Ag–Ir ( 5 ^{t Bu} ). These can be viewed as an arrested state before the full transmetallation of both donors, which gives the homodinuclear Cu ( 3^Cy ) and Ir ( 6^Cy ) complexes. The observed NHC transmetallation aptitude and reactivity urges caution in the common notion that views the NHC as a universal spectator.     

Donor‐Flexible Bis(pyridylidene amide) Ligands for Highly Efficient Ruthenium‐Catalyzed Olefin Oxidation

An exceptionally efficient ruthenium‐based catalyst for olefin oxidation has been designed by exploiting N,N′‐bis(pyridylidene)oxalamide (bisPYA) as a donor‐flexible ligand. The dynamic donor ability of the bisPYA ligand, imparted by variable zwitterionic and neutral resonance structure contributions, paired with the redox activity of ruthenium provided catalytic activity for Lemieux–Johnson‐type oxidative cleavage of olefins to efficiently prepare ketones and aldehydes. The ruthenium bisPYA complex significantly outperforms state‐of‐the‐art systems and displays extraordinary catalytic activity in this oxidation, reaching turnover frequencies of 650 000 h^?1 and turnover numbers of several millions.     

(P,C) Cyclometalated Gold(III) Complexes: Highly Active Catalysts for the Hydroarylation of Alkynes

The first catalytic application of well‐defined (P,C) cyclometalated gold(III) complexes is reported. The bench‐stable bis(trifluoroacetyl) complexes 2 a , b perform very well in the intermolecular hydroarylation of alkynes. The reaction is broad in scope, it proceeds within few hours at 25 °C at catalytic loadings of 0.1–5 mol %. The electron‐rich arene adds across the C≡C bond with complete regio‐ and stereo‐selectivity. The significance of well‐defined gold(III) complexes and ligand design are highlighted in a powerful but challenging catalytic transformation.     

Heteroleptic and Homoleptic Iron(III) Complexes with a Tris(N-Heterocyclic Carbene) Borate Ligand: Synthesis,Characterization, and Catalytic Application

Heteroleptic and homoleptic iron(III) complexes supported by a tris(N-heterocyclic carbene) borate ligand have been prepared and crystallographically characterized. The strong electron-donating character of the tris(carbene) donor was revealed by UV-vis absorption spectroscopy and electrochemical measurements combined with quantum chemical calculations. The catalytic activity of each complex was evaluated in cyclohexane oxidation reaction using meta-chloroperoxybenzoic acid (=mCPBA) as an oxidant, and both complexes show high catalytic activity and selectivity with TON=∼350 and A/(K+L)=8–10. Mechanistic studies suggested that radical-chain processes are involved in the reaction due to mCPBA acting as a one-electron oxidant, concomitant with the pathway of metal-based reactive species. Moreover, it was found that the homoleptic and heteroleptic complexes differed significantly in the involvement of metal-based active species, with the homoleptic complex exhibiting more metal-based reactions.     

Sc3+ Chloro and Alkyl Complexes Coordinated by Pincer NHC-Tethered Bis(phenolate) Ligands

Bis(phenolate) ligands with benzimidazole-2-ylidene ( L¹) and tetrahydropyrimidine-2-ylidene ( L² ) linkers proved to be suitable coordination environments for the synthesis of isolable Sc³⁺ chloro and alkyl complexes. The treatment of Sc(CH₂SiMe₃)₃(THF)₂ with equimolar amounts of [ L^1,2H₃ ] Cl afforded chloro complexes L^1,2ScCl ( solv ) ₂ (solv=THF, Py) in 76–85 % yields. L^1,2ScCl ( THF ) ₂ were also prepared by the salt metathesis reactions of ScCl₃ with [ L^1,2 ] Na₂ generated from [ L^1,2H₃ ] Cl and 3 equiv. of NaN(SiMe₃)₂ (−40 °C, THF) and isolated in somewhat lower yields (68–73 %). L²ScCl ( THF ) ₂ was subjected to the alkylation reaction with LiCH₂SiMe₃ affording alkyl derivative [ L²Sc ( CH₂SiMe₃ )] ₂ . This compound can be alternatively prepared by the subsequent reactions of [ L²H₃ ] Cl with equimolar amount of NaN(SiMe₃)₂ and Sc(CH₂SiMe₃)₃(THF)₂. In the dimeric alkyl compound [ L²Sc ( CH₂SiMe₃ )] ₂ , one of the phenoxide groups of the dianionic ligand is coordinated to one scandium center, while the second one features μ-bridging coordination with two metal centers.     

Highly basic alkyl-substituted bis(benzhydryl) CaII and YbII complexes with β-CH–M agostic interactions

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Synthesis and Reactivity of Bis(silylene)-Coordinated Calcium and Divalent Lanthanide Complexes

Divalent lanthanide complexes of Eu ( 1 ) and Yb ( 2 ) coordinated by a chelating pyridine-based bis(silylene) ligand were isolated and fully characterized. Compared to the Eu^II complex 1 , the Yb^II complex 2 presents a lower thermal stability, resulting in the activation of one Si^II−N bond and formation of an Yb^III complex ( 3 ), which features a unique silylene-pyridyl-amido ligand. The different thermal stability of 1 and 2 points towards reduction-induced cleavage of one Si^II−N bond of the bis(silylene) ligand. Successful isolation of the corresponding redox-inert bis(silylene) Ca^II complex ( 5 ) was achieved at low temperature and thermal decomposition into a Ca^II complex ( 4 ) bearing the same silylene-pyridyl-amido ligand was identified. In this case, the thermolysis reaction proceeds through another, non-redox induced, mechanism. An alternative higher yielding route to 4 was developed through an in situ generated silylene-pyridyl-amine proligand.     

Iridium(I) N‐Heterocyclic Carbene (NHC)/Phosphine Catalysts for Mild and Chemoselective Hydrogenation Processes

The directed chemoselective hydrogenation of olefins has been established by using iridium(I) catalysts, which feature a tuned NHC/phosphine ligand combination. This selective reduction process has been demonstrated in a wide array of solvents, including more environmentally acceptable media, also allowing further refinement of hydrogenation selectivity.     

Postmetallocene lanthanide-hydrido chemistry: A new family of complexes [{Ln{(Me3Si)2NC(NiPr)2}2(mu-H)}2] (Ln = Y, Nd, Sm, Gd, Yb) supported by guanidinate ligands-synthesis, structure, and catalytic activity in olefin polymerization

The new family of Lewis base free hydrido complexes of rare-earth metals supported by guanidinate ligands [{Ln{(Me3Si)2NC(NiPr)2}2(mu-H)}2] (Ln = Y, Nd, Sm, Gd, Yb) was synthesized and structurally characterized. Single-crystal X-ray and solution NMR studies revealed that these complexes are dimeric in both solid state and in [D6]benzene. The dimeric hydrido complexes can adopt eclipsed (Nd, Sm, Gd) or staggered (Y, Yb, Lu) conformations depending on the metal-atom size. Catalytic activity of these [{Ln{(Me3Si)2NC(NiPr)2}2(mu-H)}2] complexes in the polymerization of ethylene, propylene, and styrene has been investigated. Complexes of Sm and Y have high catalytic activity in ethylene polymerization (1268 and 442 g mmol(-1) atm(-1) h(-1), respectively).     

Propane Activation by Palladium Complexes with Chelating Bis(NHC) Ligands and Aerobic Cooxidation

The development of efficient aerobic oxidation methods remains a challenge for the selective functionalization of C H bonds in alkanes. Herein we report the development of a C H functionalization procedure for propane by using a palladium catalyst with chelating bis(N‐heterocyclic carbene) ligands in trifluoroacetic acid together with a vanadium co‐catalyst. Halides play a decisive role in the reaction. The experimental results are presented together with supporting kinetic data and an isotope effect. The reaction can be run with dioxygen as the oxidant if vanadium salts and halides are present in the reaction mixture. Experimental as well as computational results favor a mechanism involving C H activation by palladium(II), followed by oxidation to palladium(IV) by bromine.     

Facile,Catalytic Dehydrocoupling of Phosphines Using β‐Diketiminate Iron(II) Complexes

Catalytic dehydrocoupling of primary and secondary phosphines has been achieved for the first time using an iron pre‐catalyst. The reaction proceeds under mild reaction conditions and is successful with a range of diarylphosphines. A proton acceptor is not needed for the transformation to take place, but addition of 1‐hexene does allow for turnover at 50 °C. The catalytic system developed also facilitates the dehydrocoupling of phenylphosphane and dicyclohexylphosphane. A change in solvent switches off dehydrocoupling to allow hydrophosphination of alkenes.     

Novel Unsymmetric α‐Diimine Nickel(II) Complexes: Suitable Catalysts for Copolymerization Reactions

We established a strategy to synthesize novel unsymmetric 2,3‐diaza‐1,4‐dithiane ligands. Reaction of [Ni(acac)₂] and trityl tetrakis(pentafluorophenyl)borate in the presence of these ligands afforded the corresponding salt‐type complexes. All new compounds were characterized by means of elemental analysis and NMR spectroscopy, and the complexes additionally by mass spectroscopy. NMR spectroscopic experiments on polymers generated by the symmetric ligand/trimethylaluminum catalyst system showed that all products were nearly linear, independent of the polymerization conditions. By contrast, polymers produced by the unsymmetric ligand/trimethylaluminum catalyst system under homopolymerization conditions were branched (15–24 ‰). Additionally, copolymerization experiments with propylene and 1‐hexene afforded copolymers with a branching level of up to 50 ‰.     

Highly Efficient Hydrophosphonylation of Aldehydes and Unactivated Ketones Catalyzed by Methylene‐Linked Pyrrolyl Rare Earth Metal Amido Complexes

A series of rare earth metal amido complexes bearing methylene‐linked pyrrolyl‐amido ligands were prepared through silylamine elimination reactions and displayed high catalytic activities in hydrophosphonylations of aldehydes and unactivated ketones under solvent‐free conditions for liquid substrates. Treatment of [(Me₃Si)₂N]₃Ln(μ‐Cl)Li(THF)₃ with 2‐(2,6‐Me₂C₆H₃NHCH₂)C₄H₃NH ( 1 , 1 equiv) in toluene afforded the corresponding trivalent rare earth metal amides of formula {(μ‐η⁵:η¹):η¹‐2‐[(2,6‐Me₂C₆H₃)NCH₂](C₄H₃N)LnN(SiMe₃)₂}₂ [Ln=Y ( 2 ), Nd ( 3 ), Sm ( 4 ), Dy ( 5 ), Yb ( 6 )] in moderate to good yields. All compounds were fully characterized by spectroscopic methods and elemental analyses. The yttrium complex was also characterized by ¹H NMR spectroscopic analyses. The structures of complexes 2 , 3 , 4 , and 6 were determined by single‐crystal X‐ray analyses. Study of the catalytic activities of the complexes showed that these rare earth metal amido complexes were excellent catalysts for hydrophosphonylations of aldehydes and unactivated ketones. The catalyzed reactions between diethyl phosphite and aldehydes in the presence of the rare earth metal amido complexes (0.1 mol %) afforded the products in high yields (up to 99 %) at room temperature in short times of 5 to 10 min. Furthermore, the catalytic addition of diethyl phosphite to unactivated ketones also afforded the products in high yields of up to 99 % with employment of low loadings (0.1 to 0.5 mol %) of the rare earth metal amido complexes at room temperature in short times of 20 min. The system works well for a wide range of unactivated aliphatic, aromatic or heteroaromatic ketones, especially for substituted benzophenones, giving the corresponding α‐hydroxy diaryl phosphonates in moderate to high yields.     

Ruthenium(II) Complexes Containing Lutidine‐Derived Pincer CNC Ligands: Synthesis,Structure, and Catalytic Hydrogenation of CN bonds

A series of Ru complexes containing lutidine‐derived pincer CNC ligands have been prepared by transmetalation with the corresponding silver‐carbene derivatives. Characterization of these derivatives shows both mer and fac coordination of the CNC ligands depending on the wingtips of the N‐heterocyclic carbene fragments. In the presence of tBuOK, the Ru‐CNC complexes are active in the hydrogenation of a series of imines. In addition, these complexes catalyze the reversible hydrogenation of phenantridine. Detailed NMR spectroscopic studies have shown the capability of the CNC ligand to be deprotonated and get involved in ligand‐assisted activation of dihydrogen. More interestingly, upon deprotonation, the Ru‐CNC complex 5 e (BF₄) is able to add aldimines to the metal–ligand framework to yield an amido complex. Finally, investigation of the mechanism of the hydrogenation of imines has been carried out by means of DFT calculations. The calculated mechanism involves outer‐sphere stepwise hydrogen transfer to the C?N bond assisted either by the pincer ligand or a second coordinated H₂ molecule.