N,N'-dialkyl- and N-alkyl-N-mesityl-substituted N-heterocyclic carbenes as ligands in Grubbs catalysts

Various symmetrically and asymmetrically substituted N-heterocyclic carbene (NHC) ligands bearing aliphatic nitrogen-containing side groups have been synthesised. In our attempts to isolate the corresponding second-generation Grubbs catalysts, we were unsuccessful when using the symmetrical aliphatic NHC ligands. For the asymmetrical ligands bearing an aliphatic moiety on one side and an aromatic mesityl group on the other side, substitution of a phosphine ligand was achieved. The performance of a so-formed series of Ru-based metathesis initiators has been evaluated for the ring-opening metathesis polymerisation (ROMP) of cycloocta-1,5-diene and the ring-closing metathesis (RCM) of diethyl diallylmalonate.     

Pyrazole-3-carboxylates assisted N-heterocyclic carbene palladium complexes: synthesis,characterization, and catalytic activities towards arylation of azoles with arylsulfonyl hydrazides

Four mononuclear and dinuclear pyrazole-3-carboxylates assisted NHC–Pd complexes have been synthesized and characterized. Notably, the bridge-cleavage reactions of [Pd(μ-Cl)(Cl)(NHC)]₂ with 1H-pyrazole-3-carboxylic acid afforded dinuclear complexes [(NHC)Pd(μ-1H-pyrazolato-3-carboxylate)]₂, in which the 1H-pyrazolato-3-carboxylate was employed as a N^N^O dianionic chelating and bridging ligand. To further explore the structural features and catalytic properties of the complexes, 1-methyl-1H-pyrazole-3-carboxylic acid was introduced into the coordination with [Pd(μ-Cl)(Cl)(NHC)]₂ and the corresponding mononuclear complexes (NHC)PdCl(1-methyl-1H-pyrazole-3-carboxylate) were obtained. The catalytic properties of the complexes in desulfitative arylation of azoles with arylsulfonyl hydrazides were initially investigated.     

Ruthenium complexes bearing bidentate Schiff base ligands as efficient catalysts for organic and polymer syntheses

A concise overview is given on mononuclear and dinuclear, bidentate Schiff base ruthenium complexes with different additional ligands and on their applications in various chemical transformations such as Kharasch addition, enol-ester synthesis, alkyne dimerization, olefin metathesis and atom transfer radical polymerization. These new ruthenium complexes, conveniently prepared from commonly available ruthenium compounds, are very stable, exhibit a good tolerance towards organic functionalities, air and moisture and display high activity and chemoselectivity in chemical transformations. Relevant features of coordination chemistry connected with the reaction mechanism and chemoselectivity are also fully described. Since the nature of Schiff bases can be changed in a variety of ways, appealing routes for designing and preparing novel ruthenium complexes can be foreseen in the future.     

N-杂环卡宾Ru配合物的合成及其在Suzuki偶联反应中的催化应用

合成了两个新的氮杂环卡宾金属钌配合物1和2,通过核磁共振氢谱、核磁共振碳谱、红外光谱和元素分析对其结构进行了表征,同时,X射线单晶衍射确证了配合物2的结构为cis(I)顺式构型.化合物1和2均能在温和的反应条件下有效催化卤代芳烃和苯硼酸的Suzuki偶联反应,并表现出较高的催化活性.     

Synthesis and characterisation of nonclassical ruthenium hydride complexes containing chelating bidentate and tridentate phosphine ligands

The synthesis and characterisation of nonclassical ruthenium hydride complexes containing bidentate PP and tridentate PCP and PNP pincer-type ligands are described. The mononuclear and dinuclear ruthenium complexes presented have been synthesised in moderate to high yields by the direct hydrogenation route (one-pot synthesis) or in a two-step procedure. In both cases [Ru(cod)(metallyl)(2)] served as a readily available precursor. The influences of the coordination geometry and the ligand framework on the structure, binding, and chemical properties of the M--H(2) fragments were studied by X-ray crystal structure analysis, spectroscopic methods, and reactivity towards N(2), D(2), and deuterated solvents.     

Ruthenium complexes of Schiff base ligands as efficient catalysts for catechol–hydrogen peroxide reaction

Zeolite Y-encapsulated ruthenium(III) complexes of Schiff bases derived from 3-hydroxyquinoxaline-2-carboxaldehyde and 1,2-phenylenediamine, 2-aminophenol, or 2-aminobenzimidazole (RuYqpd, RuYqap and RuYqab, respectively) and the Schiff bases derived from salicylaldehyde and 1,2-phenylenediamine, 2-aminophenol, or 2-aminobenzimidazole (RuYsalpd, RuYsalap and RuYsalab, respectively) have been prepared and characterized. These complexes, except RuYqpd, catalyze catechol oxidation by H₂O₂ selectively to 1,2,4-trihydroxybenzene. RuYqpd is inactive. A comparative study of the initial rates and percentage conversion of the reaction was done in all cases. Turn over frequency of the catalysts was also calculated. The catalytic activity of the complexes is in the order RuYqap > RuYqab for quinoxaline-based complexes and RuYsalap > RuYsalpd > RuYsalab for salicylidene-based complexes. The reaction is believed to proceed through the formation of a Ru(V) species.     

Phosphoraneiminato tungsten alkylidyne complexes as highly efficient alkyne metathesis catalysts

The tungsten neopentylidyne complexes [Me₃CCW(NPR₃){OCMe(CF₃)₂}₂] (3a, R = Cy; 3b, R = iPr) were prepared in excellent yields by the reaction of the solvate complex [Me₃CCW{OCMe(CF₃)₂}₃(DME)] (DME = 1,2-dimethoxyethane) with equimolar amounts of the corresponding phosphoraneiminato lithium species R₃PNLi (R = Cy, iPr) at room temperature. The products were characterized by ¹H, ¹³C{¹H}, ¹⁹F{¹H} and ³¹P{¹H} NMR spectroscopy and elemental analysis. Single crystals of both complexes were obtained from diethyl ether solutions at −35 °C, and the molecular structures were determined by X-ray diffraction analysis. Complexes 3a and 3b are able to efficiently catalyse alkyne cross-metathesis (ACM) of 3-pentynyl benzyl ether (4) and ring-closing alkyne metathesis (RCAM) of bis(3-pentynyl)adipate (6) at room temperature and with low catalyst loadings (1 or 2 mol%) to afford the diether 5 and the cyclic diester 7 in virtually quantitative yields. The reactions were carried out in the presence of molecular sieve 5 Å to adsorb 2-butyne during the metathesis.     

Ruthenium half-sandwich complexes with tautomerized pyrazolyl-pyridazine ligands: Synthesis, spectroscopic and molecular structural studies

Condensation of 1,4-dichloropyridazine with pyrazole, 3,5-dimethylpyrazole and 3-methylpyrazole yielded two types of pyrazolyl-pyridazine ligands, viz., (i) products of substitution on one side of the pyridazine as 3-chloro-6-(pyrazolyl)pyridazine (Cl-L1), 3-chloro-6-(3,5-dimethylpyrazolyl)pyridazine (Cl-L2) and 3-chloro-6-(3-methylpyrazolyl)pyridazine (Cl-L3), and (ii) products of substitution on both sides such as 3,6-bis(pyrazolyl)pyridazine (L1), 3,6-bis(3,5-dimethylpyrazolyl)pyridazine (L2) and tautomers of 3,6-bis(3-methylpyrazolyl)pyridazine (L3). The reactions of η⁶-areneruthenium complexes in methanol with the above mentioned pyrazolyl-pyridazine ligands form mononuclear complexes of the type [(η⁶-arene)Ru(Cl-L)(Cl)]⁺ and [(η⁶-arene)Ru(L)(Cl)]⁺; (arene = benzene and p-cymene; Cl-L = Cl-L1, Cl-L2, Cl-L3; L = L1, L2, L3). All these complexes are characterized by IR, NMR, mass spectrometry and UV-vis spectroscopy. The structures of some representative complexes are established by single crystal X-ray diffraction studies.     

Application of Ni(II) complexes of air stable Schiff base functionalized N-heterocyclic carbene ligands as catalysts for the transfer hydrogenation of aliphatic ketones

Abstract

New air stable N-heterocyclic carbene functionalized Schiff base ligands (L) of the type 2-[-2-[3-(R)imidazol-1-yl]ethyliminomethyl]phenol [R?=?methyl (2), 2-pyridylmethyl (3)] were synthesized and characterized by NMR, IR, MS, and CHN analysis. Single crystal X-ray structural analysis of their Ni(II) complexes revealed square planar arrangement of the chelating ligands coordinated in tridentate (2, C^N^O) and tetradentate (3, N^C^N^O) modes around the metal. The three new isolated and fully characterized complexes were utilized as catalysts for the catalytic transfer hydrogenation of aliphatic ketones in 2-propanol as solvent and source of hydrogen. Based on 0.2?mol% catalyst concentration, the complexes showed activity for aliphatic ketones and 100% conversion (turnover number of 500) for cyclohexanone and all the aromatic ketones tested.     

Syntheses and structural studies of mononuclear arene ruthenium complexes with nitrogen-based chelating ligands

Dinuclear arene ruthenium complexes [(η⁶-arene)Ru(μ-Cl)Cl]₂ (arene?=?C₆H₆; p ⁱPrC₆H₄Me; C₆Me₆) and monomeric cyclopentadienyl complexes [(η⁵-Cp)Ru(PPh₃)₂Cl] (Cp?=?cyclopentadienyl) react with polypyridyl nitrogen ligands L¹ (3-(pyridin-2-yl)-1H-1,2,4-triazole) and L² (1,3-bis(di-2-pyridylaminomethyl)benzene) in methanol to afford cationic mononuclear compounds [(η⁶-arene)Ru(L¹)Cl]⁺ (arene?=?C₆H₆, 1; p ⁱPrC₆H₄Me, 2; C₆Me₆, 3), [(η⁶arene)Ru(L²)Cl]⁺ (arene?=?C₆H₆, 4; p ⁱPrC₆H₄Me, 5; C₆Me₆, 6), [(η⁵-Cp)Ru(L¹)(PPh₃)]⁺ (7), and [(η⁵Cp)Ru(L²)(PPh₃)]⁺ (8). All cationic mononuclear compounds were isolated as their hexafluorophosphate salts and characterized by elemental analyses, NMR, and IR spectroscopic methods and some representative complexes by UV-Vis spectroscopy. The solid state structures of two derivatives, [6]PF₆ and [7]PF₆, have been determined by the X-ray structure analysis.     

Nickel(II) complexes of bidentate N-heterocyclic carbene/phosphine ligands: efficient catalysts for Suzuki coupling of aryl chlorides

Nickel(II) complexes of bidentate N-heterocyclic carbene (NHC)/phosphane ligand L were prepared and structurally characterized. Unlike palladium, which forms [PdCl(2)(L)], the stable nickel product isolated is the ionic [Ni(L)(2)]Cl(2). These Ni(II) complexes are highly robust in air. Among different N-substituents on the ligand framework, the nickel complex of ligand L bearing N-1-naphthylmethyl groups (2 a) is a highly effective catalyst for Suzuki cross-coupling between phenylboronic acid and a range of aryl halides, including unreactive aryl chlorides. The activities of 2 a are largely superior to those of other reported nickel NHC complexes and their palladium counterparts. Unlike the previously reported [NiCl(2)(dppe)] (dppe=1,2-bis(diphenylphosphino)ethane), 2 a can effectively catalyze the cross-coupling reaction without the need for a catalytic amount of PPh(3), and this suggests that the PPh(2) functionality of hybrid NHC ligand L can partially take on the role of free PPh(3). However, for unreactive aryl chlorides at low catalyst loading, the presence of PPh(3) accelerates the reaction.     

Synthesis of heterocyclic carbene ligands via 1,2,3-diheterocyclization of allenylidene complexes with dinucleophiles

Heterocyclic carbene complexes are accessible from π-donor-substituted allenylidene complexes, [(CO)₅CrCCC(NMe₂)Ph] (1) and [(CO)₅CrCCC(O-endo-Bornyl)OEt] (4), and various dinucleophiles by 1,2,3-diheterocyclization. The reaction of 1 with 1,2-dimethylhydrazine gives the 1,2-dimethylpyrazolylidene complex (2) in high yield in addition to small amounts of the α,β-unsaturated carbene complex [(CO)₅CrC(NMe₂)-C(H)C(NMe₂)Ph] (3). The analogous reaction of 4 with 1,2-dimethylhydrazine affords the 1,2-dimethylpyrazolylidene complex (5) and, via displacement of the C_γ-bound ethoxy substituent, the hydrazinoallenylidene complex [(CO)₅CrCCC(O-endo-Bornyl){NMe-N(H)Me}] (6). Treatment of 6 with catalytic amounts of acids induces cyclization to 5. On addition of 1,1-dimethylhydrazine to 1 the zwitterionic pyrazolium-5-ylidene complex (7) is formed. The reaction of 1 with 1,2-diaminocyclohexane affords a octahydro-benzo[1,4]diazepinylidene complex (10) and, via intermolecular substitution, a binuclear bisallenylidene complex (11). Thiazepinylidene complexes (12-14), containing 7-membered N/S-heterocyclic carbene ligands, are formed highly selectively in the reaction of 1 with 2-aminoethanethiol or related cysteine derivatives by a substitution/cyclization sequence. The analogous reaction of 1 with homocysteine methylester yields a thiazocanylidene complex (15). All new heterocyclic carbene ligands are strong donors exhibiting σ-donor/π-acceptor ratios similar to those of the known imidazolylidene complexes. On photolysis of 2 and 12 in the presence of triphenylphosphine, the corresponding cis-carbene tetracarbonyl triphenylphosphine complexes (16 and 17) are formed. The solid state structure of complexes 2, 7, 14, 15, and 16 is established by X-ray structural analysis.     

Ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene ligands: synthesis, structure, and catalytic activity

A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized. These complexes are readily accessible in one or two steps from commercially available [(PCy(3))(2)Cl(2)Ru==CHPh]. All of the complexes reported herein promote the ring-closing of diethyldiallyl and diethylallylmethallyl malonate, the ring-opening metathesis polymerization of 1,5-cyclooctadiene, and the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, in some cases surpassing in efficiency the existing second-generation catalysts. Especially in the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, all new catalysts demonstrate similar or higher activity than the second-generation ruthenium catalysts and, most importantly, afford improved E/Z ratios of the desired cross-product at conversion above 60 %. The influence of the unsymmetrical NHC ligands on the initiation rate and the activation parameters for the irreversible reaction of these ruthenium complexes with butyl vinyl ether were also studied. Finally, the synthesis of the related chlorodicarbonyl(carbene) rhodium(I) complexes allowed for the study of the electronic properties of the new unsymmetrical NHC ligands that are discussed in detail.     

Pincer and diamine Ru and Os diphosphane complexes as efficient catalysts for the dehydrogenation of alcohols to ketones

The ruthenium and osmium complexes [MCl₂(diphosphane)(L)] (M=Ru, Os; L=bidentate amino ligand) and [MCl(CNN)(dppb)] (CNN=pincer ligand; dppb=1,4‐bis‐ (diphenylphosphino)butane), containing the N–H moiety, have been found to catalyze the acceptorless dehydrogenation of alcohols in tBuOH and in the presence of KOtBu. The compounds trans‐[MCl₂(dppf)(en)] (M=Ru 7 , Os 13 ; dppf=1,1′‐bis(diphenylphosphino)ferrocene; en=ethylenediamine) display very high activity and different substrates, including cyclic and linear alcohols, are efficiently oxidized to ketones by using 0.8–0.04 mol % of catalyst. The effect of the base and the comparison of the catalytic activity of the Ru versus Os complexes are reported. The ruthenium complex 7 generally leads to a faster conversion into ketones with respect to the osmium complex 13 , which displays better activity in the dehydrogenation of 5‐en‐3β‐hydroxy steroids. The synthesis of new Ru and Os complexes [MCl₂(PP)(L)] (PP=dppb, dppf; L=(±)‐trans‐1,2‐diaminocyclohexane, 2‐(aminomethyl)pyridine, and 2‐aminoethanol) of trans and cis configuration is also reported.     

Ruthenium(II) pyridylamine complexes with diimine ligands showing reversible photochemical and thermal structural change

Ruthenium(II)-TPA-diimine complexes, [Ru(TPA)(diimine)]2+ (TPA=tris(2-pyridylmethyl)amine; diimine=2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpm), 1,10-phenanthroline (phen)) were synthesized and characterized by spectroscopic and crystallographic methods. Their crystal structures demonstrate severe steric hindrance between the TPA and diimine ligands. They exhibit drastic structural changes on heating and photoirradiation at their MLCT bands, which involve partial dissociation of the tetradentate TPA ligand to exhibit a facially tridentate mode accompanied by structural change and solvent coordination to give [Ru(TPA)(diimine)(solvent)]2+ (solvent=acetonitrile, pyridine). The incoming solvent molecules are required to have pi-acceptor character, since sigma-donating solvent molecules do not coordinate. The thermal process is irreversible dissociation to give the solvent-bound complexes, which takes place by an interchange associative mechanism with large negative activation entropies. The photochemical process is a reversible reaction reaching a photostationary state, probably by a dissociative mechanism involving a five-coordinate intermediate to afford the same product as obtained in the thermal reaction. Quantum yields of the forward reactions to give dissociated products were lower than those of the backward reactions to recover the starting complexes. In the photochemical process, the conversions of the forward and backward reactions depend on the absorption coefficients of the starting materials and those of the products at certain wavelength, as well as the quantum yields of those reactions. The reversibility of the motions can be regulated by heating and by photoirradiation at certain wavelength for the recovery process. In the bpm system, we could achieve about 90 % recovery in thermal/photochemical structural interconversion.     

Synthesis and small molecule reactivity of uranium(IV) alkoxide complexes with both bound and pendant N-heterocyclic carbene ligands

The syntheses of two tetravalent uranium alkoxide-carbene complexes are reported, [UIL3], and [UL4] where L = OCMe2CH2[1-C(NCHCHNiPr)]. The latter shows dynamic behaviour of the alkoxycarbene ligands in solution at room temperature, and the crystal structure of [UL4] shows that one carbene group remains uncoordinated. The unbound N-heterocyclic carbene group is trapped by a range of reagents such as 16-valence-electron metal carbonyl fragments and BH3 moieties, forming, for example, [UL3(mu-L)W(CO)5], [UL2(mu-L)2Mo(CO)4], and [UL(n)(L-BH3)(4-n)] (n = 1-4), demonstrating the potential for these hemilabile electropositive metal-carbene complexes to participate in the bifunctional activation of small molecules.     

Ruthenium catalysts bearing chelating carboxylate ligands: application to metathesis reactions in water

The novel catalytic system, composed of a ruthenium alkylidene containing a surfactant fragment in the catalysts molecule, is reported. Ring closing metathesis and cross metathesis reactions proceed efficiently in neat water at room temperature, in air, without need of adding an external surfactant.     

Ruthenium olefin metathesis catalysts with modified styrene ethers: influence of steric and electronic effects

A series of olefin metathesis catalysts with modified isopropoxybenzylidene ligands were synthesised, and the effects of ligands on the rate of metathesis investigated. Increased steric hinderance ortho to the isopropoxy group enhanced reaction rates. In the case of N-heterocyclic carbene complexes, decreasing electron density at both the chelating oxygen atom and the RuC bond accelerated reaction rates appreciably. Catalysts containing a tricyclohexylphosphane ligand, followed the same trend with regard to benzylidene electrophilicity, while higher electron density at oxygen enhanced reaction rates.