Piano-Stool Ruthenium N-Heterocyclic Carbene Complexes for gem-Specific Catalytic Dimerization of Terminal Alkynes

A series of piano-stool Ru−NHC (NHC=N-heterocyclic carbene) complexes have been prepared and characterized. The NHC ligands used herein have varying wingtip groups, showing the impact of steric congestion on the selectivity for the catalytic dimerization of terminal alkynes.     

Gold(I) Complexes Stabilized by Nine- and Ten-Membered N-Heterocyclic Carbene Ligands

Nine- and ten-membered N-heterocyclic carbene (NHC) ligands have been developed and for the first time their gold(I) complexes were synthesized. The protonated NHC pro-ligands 2 a – h were prepared by the reaction of readily available N,N′-diarylformamidines with bis-electrophilic building blocks, followed by anion exchange. In situ deprotonation of the tetrafluoroborates 2 a – h with tBuOK in the presence of AuCl(SMe₂) provided fast access to NHC-gold(I) complexes 3 – 10 . These new NHC-gold(I) complexes show very good catalytic activity in a cycloisomerization reaction (0.1 mol % catalyst loading, up to 100 % conversion) and their solid-state structures reveal high steric hindrance around the metal atom (%V_bur up to 53.0) which is caused by their expanded-ring architecture.     

Preparation of Mixed Bis-N-Heterocyclic Carbene Rhodium(I) Complexes

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(η²-olefin)(NHC)(NHC’) have been synthesized by a stepwise reaction of [Rh(μ-Cl)(η²-olefin)₂]₂ with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the η²-olefin is critical. Similarly, new mixed coumarin-functionalized bis-NHC rhodium complexes have been prepared by a reaction of mono NHC complexes of type RhCl(NHC-coumarin)(η²,η²-cod) with the corresponding azolium salt in the presence of an external base. Both synthetic procedures proceed selectively and allow the preparation of mixed bis-NHC rhodium complexes in good yields.     

Dimerization of Propylene by Nickel (Ⅱ) and Cobalt (Ⅱ) Catalysts Based on Bidentate Nitrogen-phosphino Chelating Ligands

The catalytic property of propylene dimerization by several nickel (Ⅱ), cobalt (Ⅱ) complexes containing N-P bidentate ligands was studied in combination with organoaluminum co-catalysts. The effects of the type of aluminum co-catalysts and its relative amount, the nature of precursors in terms of ligand backbone and metal center were investigated. The results indicated that precursor I (N,N-dimethyl-2-(diphenylphosphino)aniline nickel (Ⅱ) dichloride) exhibited high activity in propylene dimerization in the presence of the strong Lewis acid Et3Al2Cl3, whereas low productivity by its cobalt analogues was observed under identical reaction conditions.     

Selective Oxidation of Glycerol via Acceptorless Dehydrogenation Driven by Ir(I)-NHC Catalysts

Iridium(I) compounds featuring bridge-functionalized bis-NHC ligands (NHC = N-heterocyclic carbene), [Ir(cod)(bis-NHC)] and [Ir(CO)₂(bis-NHC)], have been prepared from the appropriate carboxylate- or hydroxy-functionalized bis-imidazolium salts. The related complexes [Ir(cod)(NHC)₂]⁺ and [IrCl(cod)(NHC)(cod)] have been synthesized from a 3-hydroxypropyl functionalized imidazolium salt. These complexes have been shown to be robust catalysts in the oxidative dehydrogenation of glycerol to lactate (LA) with dihydrogen release. High activity and selectivity to LA were achieved in an open system under low catalyst loadings using KOH as a base. The hydroxy-functionalized bis-NHC catalysts are much more active than both the carboxylate-functionalized ones and the unbridged bis-NHC iridium(I) catalyst with hydroxyalkyl-functionalized NHC ligands. In general, carbonyl complexes are more active than the related 1,5-cyclooctadiene ones. The catalyst [Ir(CO)₂{(MeImCH₂)₂CHOH}]Br exhibits the highest productivity affording TONs to LA up to 15,000 at very low catalyst loadings.     

Monohydride-Dichloro Rhodium(III) Complexes with Chiral Diphosphine Ligands as Catalysts for Asymmetric Hydrogenation of Olefinic Substrates

We report full details of the synthesis and characterization of monohydride-dichloro rhodium(III) complexes bearing chiral diphosphine ligands, such as (S)-BINAP, (S)-DM-SEGPHOS, and (S)-DTBM-SEGPHOS, producing cationic triply chloride bridged dinuclear rhodium(III) complexes ( 1 a : (S)-BINAP; 1 b : (S)-DM-SEGPHOS) and a neutral mononuclear monohydride-dichloro rhodium(III) complex ( 1 c : (S)-DTBM-SEGPHOS) in high yield and high purity. Their solid state structure and solution behavior were determined by crystallographic studies as well as full spectral data, including DOSY NMR spectroscopy. Among these three complexes, 1 c has a rigid pocket surrounded by two chloride atoms bound to the rhodium atom together with one tBu group of (S)-DTBM-SEGPHOS for fitting to simple olefins without any coordinating functional groups. Complex 1 c exhibited superior catalytic activity and enantioselectivity for asymmetric hydrogenation of exo-olefins and olefinic substrates. The catalytic activity of 1 c was compared with that of well-demonstrated dihydride species derived in situ from rhodium(I) precursors such as [Rh(cod)Cl]₂ and [Rh(cod)₂]⁺[BF₄]⁻ upon mixing with (S)-DTBM-SEGPHOS under dihydrogen.     

四种N-杂环卡宾钯配合物的电喷雾质谱研究

利用电喷雾质谱技术研究了四种N-杂环钯卡宾配合物,优化出较适宜检测的电喷雾质谱条件极性较高的溶剂(如乙腈),较低的离子源温度,样品浓度以1.0×10-4mol/L左右为宜,使用较低锥孔电压(5~35V).在此实验条件下,在全扫描电喷雾正离子谱中都会出现[M-I CH3CN] 及[M-I] 的质谱峰.通过源内碰撞诱导电离(CID)技术进一步分析样品在溶液中的性质.实验说明ESI不仅可作为一种分析工具,也可作为获得更多信息的一种方法,来研究有机金属基团在溶液中的性质.     

Pseudotetrahedral Rhodium(I) Complexes

A combination of four‐electron donors, such as alkynes, with strongly donating and strong‐field scorpionate ligands is appropriate to create pseudotetrahedral rhodium(I) environments, as found in [Rh(PhBP₃)(HC?CPh)], which promotes H?C bond activation and C?C coupling reactions under very smooth conditions.     

Asymmetric Hydrogenation of Aryl Perfluoroalkyl Ketones Catalyzed by Rhodium(III) Monohydride Complexes Bearing Josiphos Ligands

The asymmetric hydrogenation of 2,2,2-trifluoroacetophenones and aryl perfluoroalkyl ketones was developed using a unique, well-defined chloride-bridged dinuclear rhodium(III) complex bearing Josiphos-type diphosphine ligands. These complexes were prepared from [RhCl(cod)]₂, Josiphos ligands, and hydrochloric acid. As catalyst precursors, they allow for the efficient and enantioselective synthesis (up to 99 % ee) of chiral secondary alcohols with perfluoroalkyl groups. This system does not require an activating base for the hydrogenation of 2,2,2-trifluoroacetophenones. Additionally, the enantioselective C=O hydrogenations of 2-phenyl-3-(haloacetyl)-indoles, a class of privileged structures in medicinal chemistry, is reported for the first time.     

Synthesis and Reversible H2 Activation by Coordinatively Unsaturated Rhodium NHC Complexes

We report the synthesis of coordinatively unsaturated cationic rhodium complexes bearing the sterically encumbered electron-rich NHC ligand IPr*^OMe. The COD (1,5-cyclooctadiene) complex [Rh(IPr*^OMe)(COD)]BF₄ adopts a tilted, pseudo-square planar coordination geometry, where bonding to the ipso-carbon of the NHC aryl substituent was observed in the solid state. Hydrogenation of this complex afforded a metastable dihydride complex [Rh(IPr*^OMe)(H)₂]BF₄ with an unusual internal coordination to an arene of the ligand. In the absence of a hydrogen atmosphere, spontaneous reductive elimination of H₂ afforded a rhodium complex [Rh(IPr*^OMe)]BF₄ with a single chelating ligand that stabilizes the highly unsaturated metal by two-fold π-face donation as suggested by NMR spectroscopy and computational studies. This unusual complex might serve as a versatile precatalyst for a variety of transformations.     

A Chemical–Biological Evaluation of Rhodium(I) N‐Heterocyclic Carbene Complexes as Prospective Anticancer Drugs

Rhodium(I) complexes bearing N‐heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry, but there are very few reports of biological properties of these organometallics. A series of Rh^I‐NHC derivatives with 1,5‐cyclooctadiene and CO as secondary ligands were synthesized, characterized, and biologically investigated as prospective antitumor drug candidates. Pronounced antiproliferative effects were noted for all complexes, along with moderate inhibitory activity of thioredoxin reductase (TrxR) and efficient binding to biomolecules (DNA, albumin). Biodistribution studies showed that the presence of albumin lowered the cellular uptake and confirmed the transport of rhodium into the nuclei. Changes in the mitochondrial membrane potential (MMP) were observed as well as DNA fragmentation in wild‐type and daunorubicin‐ or vincristine‐resistant Nalm‐6 leukemia cells. Overall, these studies indicated that Rh^I‐NHC fragments could be used as partial structures of new antitumor agents, in particular in those drugs designed to address resistant malignant tissues.     

Nickel Complexes Bearing Bidentate Schiff Base as Ethylene Oligomerization Catalysts

Several nickel complexes [N,N]NiBr2, in which IN,N] indicates bidentate nitrogen-containing ligands (1: [N,N]=N-(2,6-diisopropylphenyl)pyridine-2-carboxaldimine (Cl8H22N2); 2: N-(2,6-diisopropylphenyl)-6-methylpyridine-2-carboxaldimine (C19H24N2); 3: N-(2,4,6-trimethylphenyl)pyridine-2-carboxaldimine(Cl5Hl6N2); 4: N-(2,4,6-trimethylphenyl)-6-methylpyridine-2-carboxaldlmlne (Cl6Hl8N2) were synthesized. Some of the nickel complexes exhibit high activity for ethylene oligomerization in the presence of an organoaluminum activator. The main factor affecting the activity and the structure of oligomers is the steric effect of substituents on [N,N] ligands. Methylaluminoxane (MAO) -activated catalysts showed higher activities and produced oligomers with higher molecular weight than Et2AlCl-activated ones. The oligomerization in toluene rather than hexane results in much higher activity, and the oligomers produced in toluene have relatively high molecular weight. With activation of MAO or Et2AlCl,the [N,N]NiBr2 system tended to produce highly branched oligomers with low α-olefin content, but the α-olefin content could be increased by changing the reaction conditions.     

Binuclear C^C* Cyclometalated Platinum(II) NHC Complexes with Bridging Amidinate Ligands

Binuclear C^C* cyclometalated NHC platinum(II) compounds with bridging amidinate ligands were synthesized to evaluate their photophysical properties. Their three‐dimensional structures were determined by a combination of 2D NMR experiments, mass spectrometry, DFT calculations, and solid‐state structure analysis. The bridging amidinate ligands enforce short distances between the platinum centers of the two cyclometalated structures, which gives rise to extraordinary photophysical properties.     

Dinuclear gold(I) Complexes with Bidentate NHC Ligands as Precursors for Alkynyl Complexes via Mechanochemistry

The use of alkynyl gold(I) complexes covers different research fields, such as bioinorganic chemistry, catalysis, and material science, considering the luminescent properties of the complexes. Regarding this last application, we report here the synthesis of three novel dinuclear gold(I) complexes of the general formula [(diNHC)(Au-C≡CPh)₂]: two Au-C≡CPh units are connected by a bridging di(N-heterocyclic carbene) ligand, which should favor the establishment of semi-supported aurophilic interactions. The complexes can be easily synthesized through mechanochemistry upon reacting the pristine dibromido complexes [(diNHC)(AuBr)₂] with phenylacetylene and KOH. Interestingly, we were also able to isolate the monosubstituted complex [(diNHC)(Au-C≡CPh)(AuBr)]. The gold(I) species were fully characterized by multinuclear NMR spectroscopy and mass spectrometry. The emission properties were also evaluated, and the salient data are comparable to those of analogous compounds reported in the literature.     

Divergent Reactivity of Rhodium(I) Carbenes Derived from Indole Annulations

Rhodium(I) carbenes were generated from propargylic alcohol derivatives as the result of a dehydrative indole annulation. Depending on the choice of the electron‐withdrawing group on the aniline nitrogen nucleophile, either a cyclopropanation product or dimerization product was obtained chemoselectively. Intramolecular hydroamidation occurred for the same type of propargylic alcohol derivatives when other transition‐metal catalysts were employed.     

Case Study of N-iPr versus N-Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC)2]

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)₂] (NHC=ⁱPr₂Im^Me 1^Me , Mes₂Im 2 ) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni₂(ⁱPr₂Im^Me)₄(μ-(η² : η²)-COD)] B / [Ni(ⁱPr₂Im^Me)₂(η⁴-COD)] B’ or [Ni(Mes₂Im)₂] 2 , respectively, with alkynes afforded complexes [Ni(NHC)₂(η²-alkyne)] (NHC=ⁱPr₂Im^Me: alkyne=MeC≡CMe 3 , H₇C₃C≡CC₃H₇ 4 , PhC≡CPh 5 , MeOOCC≡CCOOMe 6 , Me₃SiC≡CSiMe₃ 7 , PhC≡CMe 8 , HC≡CC₃H₇ 9 , HC≡CPh 10 , HC≡C(p-Tol) 11 , HC≡C(4-^tBu-C₆H₄) 12 , HC≡CCOOMe 13 ; NHC=Mes₂Im: alkyne=MeC≡CMe 14 , MeOOCC≡CCOOMe 15 , PhC≡CMe 16 , HC≡C(4-^tBu-C₆H₄) 17 , HC≡CCOOMe 18 ). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p-Tol) and HC≡C(4-^tBu-C₆H₄), 11 and 12 , which were formed by addition of a C−H bond of one of the NHC N-ⁱPr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2-butyne, 4-octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1-pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1^Me is not a good catalyst. The reaction of 2 with 2-butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)₂]. DFT calculations reveal that the differences between 1^{M e} and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2 , and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N-alkyl substituted NHC, to enhanced Ni-alkyne backbonding due to a smaller C_NHC−Ni−C_NHC bite angle, and to less steric repulsion of the smaller NHC ⁱPr₂Im^Me.     

Chloride‐Bridged Dinuclear Rhodium(III) Complexes Bearing Chiral Diphosphine Ligands: Catalyst Precursors for Asymmetric Hydrogenation of Simple Olefins

Efficient rhodium(III) catalysts were developed for asymmetric hydrogenation of simple olefins. A new series of chloride‐bridged dinuclear rhodium(III) complexes 1 were synthesized from the rhodium(I) precursor [RhCl(cod)]₂, chiral diphosphine ligands, and hydrochloric acid. Complexes from the series acted as efficient catalysts for asymmetric hydrogenation of (E)‐prop‐1‐ene‐1,2‐diyldibenzene and its derivatives without any directing groups, in sharp contrast to widely used rhodium(I) catalytic systems that require a directing group for high enantioselectivity. The catalytic system was applied to asymmetric hydrogenation of allylic alcohols, alkenylboranes, and unsaturated cyclic sulfones. Control experiments support the superiority of dinuclear rhodium(III) complexes 1 over typical rhodium(I) catalytic systems.