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Abstract
A highly efficient route to a new class of organometallic polymers containing difunctional N-heterocyclic carbenes has been developed. Bis(imidazolium) halides and divalent group X metals were copolymerized to afford organometallic polymers in up to quantitative yields and with molecular weights up to 10(6) Da, depending on the structure of the N-heterocyclic carbene and the incorporated transition metal. Enhanced solubilities were demonstrated through post-polymerization ligation with phosphines. Finally, selective end-group functionalization and excellent molecular weight control was achieved through the inclusion of monofunctional chain transfer agents during the polymerization.
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... [28,48] Polymere mit NHC-Metall Einheiten zeigen interessante Selbstheilungs-Eigenschaften und sind potentielle elektrische Leiter (Abbildung 6b). [28,49] Weitere Anwendungen finden NHC-Metallkomplexe als ...
... Leuchtstoffe und photoaktive Materialien. [28,50] Abbildung 6: Anwendungen von NHC-Metallkomplexen in den Materialwissenschaften. [28,48,49] Die weitaus bedeutendste Anwendung von NHC-Metallkomplexen stellt die homogene Katalyse dar. Unter anderem werden Iridium-und Rutheniumkatalysierte Hydrogenierungen und Wasserstoff-Transfer-Reaktionen, Goldkatalysierte Aktivierungen von π-Bindungen und Rhodium-und Platinkatalysierte Hydrosilylierungen durch NHC-Metallkomplexen katalysiert. ...
... Schema 10 Dimamino-(49)(50)(51)(52)(53)(54), Amino-amido-(55)(56), Diamido-(57)(58)(59) und Malo-NHCs (60) mit unterschiedlicher Ringgröße. Das Grundgerüst der NHCs hat maßgebenden Einfluss auf die elektronischen Eigenschaften. ...
Im Rahmen dieser Arbeit wurde die Selbstmetathese verschiedener Styrol-Derivate in Wasser durchgeführt und dabei der Einfluss von Substraten und Katalysatoren auf den Umsatz untersucht. Des Weiteren wurden Syntheserouten für maßgeschneiderte Katalysatoren für verschiedene Anwendungen in der wässrigen Olefinmetathese entwickelt. Ein Piperidin-modifizierter Hoveyda-Grubbs II Komplex für den Einsatz in lebenden Zellen, wurde erfolgreich synthetisiert. Ebenso gelang die Darstellung von verschiedenen rückgratmodifizierten NHC Vorstufen über die Zyklisierung mit Formamidinen. Ausgehend von diesen Vorstufen können Grubbs-Typ Komplexe synthetisiert werden, die für die Untersuchung von supramolekularen Wechselwirkungen mit makrozyklischen Wirtsmolekülen von Interesse sind. Ein weiterer Teil dieser Arbeit beschäftigt sich mit neuen Klassen von NHC-Liganden und der Darstellung entsprechender Ruthenium(II)- und Palladium(II)-Komplexe mit Potential für den Einsatz in der wässrigen Katalyse.
... The benzobis(imidazolylidene) (BBI) ligand was firstly developed by Bielawski and co-workers in 2005. 104 From then, a series of BBI ligand-based Ag I /Ag I , 132 Au I /Au I , 133 Ni II /Ni II , 134 Pt II /Pt II , 135 Rh I /Rh I , 132 Fe II /Fe II136 and Ru II /Ru II136 homobimetallic complexes have been reported ( Figure 33). ...
... For example, when they are used as ligands to synthesize the bis(NHC) metallic complexes, the two facially opposed NHC moieties can inhibit chelation with one metal, thus the bis (NHC) ligands are able to bind to two metals.98 The most so far reported ditopic NHCs are summarized inFigure 29, A-C and I-K were explored mainly by the group of Peris,[99][100][101][102][103] and E-H were developed by the group of Bielawski.[104][105][106] The other new types of bis(NHC) ligands D, 107 L, 108 M, 109 N,110 and O-P111,112 were reported in recent years by several groups. ...
Cyclodextrin (CD)-N-heterocyclic carbene (NHC)-copper complexes, (α/β-ICyD)BnCuCl, previously developed by our group, were used as catalysts in carboboration of alkynes. The (α/β-ICyD)BnCuCl both gave the linear (L) (E)-vinyl boron isomers with high regioselectivity in the copper-catalyzed methyl boration of terminal alkynes. Furthermore, the intramolecular carboboration reaction with terminal alkynes functionalized by alkyl halides, (α/β-ICyD)BnCuCl both led to exocyclic vinyl boronate species as the major isomer, and an unexpected endocyclic (Z)-isomer was also observed.We have developed two novel bis(azolium)-precursors, with one azolium moiety inside the cavity of α-CD, and another one outside the cavity. These two azolium moieties are connected by an ethynyl or phenyl linker. Taking advantage of the steric hindrance of the α-CD cavity, a series of mono-, homo- and heterobimetallic complexes were developed, including AgI, CuI, AuI, AuIII, and PdII species. The electrochemical properties of corresponding Cu and Au complexes were investigated by cyclic voltammetry. And the electronic absorption properties of all complexes and precursors were studied by ultraviolet−visible (UV-Vis) spectroscopy.
... Since the seminal studies from Boydston et al. [88], NHC-based polymers have been regarded as promising structures for the development of novel catalysts. Among the most widely employed are coordination polymers 47 [89], porous organic polymers (POPs) 48 [90] and, more recently, the covalent organic frameworks (COFs) 49 [91]. ...
N‐Heterocyclic carbenes (NHCs) are molecules with exceptional catalytic properties in a wide range of reactions. Recently, there has been a growing interest in using them as catalysts for biomass conversion. In this review, we examine the current state of glycerol valorisation (GV) using several catalytic systems, including metallic complexes bearing NHCs as ligands, polymeric building blocks and organocatalysts. The study covers literature published in the last 15 years, detailing the catalyst scope, reaction conditions and products of GV. This analysis emphasises the importance of varying the steric and electronic properties of NHCs within the reviewed catalytic systems. Furthermore, a critical evaluation of the proposed mechanistic pathways complements these results. A comprehensive understanding of the challenges of using such catalytic systems will emerge by compiling literature on the use of NHCs for the studied reactions. Additionally, this will promote new opportunities for further research in both academic and industrial fields.
... In addition to these monodentate ligands, several multi-dentate ligands have been synthesized and used for various applications. The rigid bidentate benzimidazole-based N-heterocyclic carbene was used in the synthesis of conjugated organometallic polymers which show interesting electronic and mechanical properties [32]. Another application of such bidentate NHC was the formation of stable chelate complexes. ...
Carbenes are highly reactive intermediates in organic synthesis. These divalent carbon species are generally transient in nature and cannot be isolated. However, they can form stabile metal complexes. Later on, the development of N-heterocyclic carbene (NHC) and other stable carbene led to the application of these carbon (II) donor ligands in the synthesis of complex natural products, transition metal catalysis, organo-catalysis and several other synthetic methodologies. Here in this short review, we will discuss the brief history of the development of carbenes, synthesis of stable carbenes (NHC in particular), and their applications in natural products synthesis transition metal chemistry/organometallics. In addition to synthesis and application, the chapter will consist of a detailed structural analysis of carbenes and exciting photophysics of this class of compounds. Special emphasis will be given to electronic structure. The role of carbene in the development of luminescent NHC transition metal complexes, the tuning of emission properties as well as their active role as photocatalysts in the reduction of CO2 will also be discussed.
... While this dimerization reaction is often considered as deleterious for the isolation of carbenes, it eventually offers a powerful way to construct C=C double bonds. Surprisingly, in the context of polymer chemistry, dimerization of bis-carbenes has received little attention so far [25][26][27][28] . Bielawski et al. reported that some bis-benzimidazol-2-ylidenes, generated by deprotonation of the corresponding bis-benzimidazoliums, could undergo such a dimerization/polymerization, forming a class of thermally reversible dynamic covalent conjugated polymers [29][30][31] , though showing a low conjugation across the aryl spacer 32 in addition to be rather air-sensitive. ...
Despite the ubiquity of singlet carbenes in chemistry, their utility as true monomeric building blocks for the synthesis of functional organic polymers has been underexplored. In this work, we exploit the capability of purposely designed mono- and bis-acyclic amino(aryl)carbenes to selectively dimerize as a general strategy to access diaminoalkenes and hitherto unknown amino-containing poly(p-phenylene vinylene)s (N-PPV’s). The unique selectivity of the dimerization of singlet amino(aryl)carbenes, relative to putative C-H insertion pathways, is rationalized by DFT calculations. Of particular interest, unlike classical PPV’s, the presence of amino groups in α-position of C=C double bonds in N-PPV’s allows their physico-chemical properties to be manipulated in different ways by a simple protonation reaction. Hence, depending on the nature of the amino group (iPr2N vs. piperidine), either a complete loss of conjugation or a blue-shift of the maximum of absorption is observed, as a result of the protonation at different sites (nitrogen vs. carbon). Overall, this study highlights that singlet bis-amino(aryl)carbenes hold great promise to access functional polymeric materials with switchable properties, through a proper selection of their substitution pattern.
... For self-healing investigations, organometallic polymers are effectively used which is made up of N-heterocyclic carbenes (NHCs) and different metal salts (M¼Ni, Cu, Pt, Pd, Rh and Ag). To make the carbene moiety to be more secure numerous NHCs were arranged from the various compositions of R¼ alkyl, aryl [49][50][51][52]. Structured organometallic polymers were made ready for use with the featuring conductivities in the order of 10 −3 S/cm with several amalgamation of metallic salts. ...
The conductivity is always given the prime importance of any circuit to work properly, even minor loose connection pave path to catastrophe of the complete system. The circuit failure deemed to have human intervention to get rid out this problem. The self-healable conductive materials are unique in nature such that they regain the working state of the system without human assistance. As a matter of fact, self-healing means heal by itself but in other case it is not quite complex, when assessed, this technique for circuit connections it was found very hard to establish one such connections. The connection also could be established with the involvement of human beings, as how the soldering, brazing works are being carried out. The connection is very thin layer which could provide the complete flow of electricity. The growth in the field of automation, starts doing the complex works without human interventions. Very complex connections are done by the programmed machines which requires human skills in programming. In these case, one way or other humans are involved, but healing by themselves is the better option. In this technique human brain is also involved but only at the initial phase. This kindle the interest among the researchers to develop such novel materials. So, the curiosity put in various forms to attain the conductivity over the circuits. In this chapter, the discussion is all about the various methods by which the self-healing is achieved for conductive materials. The complex conductivity is established even as a coating, so that is also touched upon a bit.
... The most widely studied polytopic NHCs are Janus bis-NHCs where the two binding sites are on the opposite faces of the ligand. Some of the important examples are listed in Scheme 59 [89,[205][206][207][208][209][210][211][212][213]. ...
N-Heterocyclic carbenes (NHCs) have emerged as one of the most useful class of ligands for metal complexes primarily due to their relatively easy synthesis, broad structural and stereoelectronic diversity, and their ability to form stable compounds with different metal ions. Among a vast number of variants known, annellated systems which feature fused ring(s) on the basic framework of NHC have drawn wide attention for their unique structural and stereoelectronic properties. The annellation has been attributed to have a significant effect on the ligating behavior of NHCs. The actual effect depends on the nature of the annellated system and the position in the parent scaffold where annellation takes place. The transition metal complexes bearing annellated NHCs have the potential to exhibit superior activity and/or selectivity in catalytic transformations. Besides, the fusion of ring(s) provides further scope for the functionalization of NHC. The NHCs annellated to suitably functionalized scaffolds have been used to design the stimuli-modulable systems. The use of NHCs fused to extended aromatic systems has paved the way for improving the efficacies of the catalytic systems using non-covalent interactions. Metal complexes containing annellated NHCs have afforded compounds with interesting photophysical and electronic properties. Herein we classify annellated NHCs based on their structural attributes. The effects of annellation on the ligand behavior of NHCs and the catalytic utility of metal complexes featuring these annellated carbenes, along with a host of other promising applications, are described.
... N-heterocyclic carbenes (NHC), since the first isolation of a metal-free imidazol-2-ylidene by Arduengo and coworkers in 1991 [1], have become one of the most extensively investigated ligands for transition metal complexes [2][3][4][5][6][7][8][9][10][11][12]. Far from being just a curiosity, NHC have also played ubiquitous and indispensable roles in a wider range of fields such as homogeneous catalysis [13][14][15][16][17], materials science [18][19][20][21][22], organocatalysis [23,24], and even medicinal chemistry [25][26][27][28]. NHC as ancillary ligands can been widely applied in transition metal chemistry, and homogeneous catalysis, presumably due to forming the highly robust bonds between the soft carbon donors of NHC with soft metals [3,[13][14][15][16]29,30]. Compared to the corresponding phosphine ligands, NHC-complexes have stronger metal−carbon σ-bonds and exhibit higher activities [2,17,[31][32][33][34][35][36][37][38][39]. ...
Recent rapid development in homogeneous gold catalysis affords an alternative and particularly thriving strategy for the generation of gold carbenes through gold-catalyzed oxidation/amination/cycloaddition of alkynes, while it avoids the employment of hazardous and potentially explosive diazo compounds as starting materials for carbene generation. In addition to facile and secure operation, gold carbenes generated in this strategy display good chemoselectivity distinct from other metal carbenes produced from the related diazo approach. N-heterocyclic carbene (NHC) gold is a special metal complex that can be used as ancillary ligands, which provides enhanced stability and can also act as an efficient chiral directing group. In this review, we will present an overview of these recent advances in alkyne oxidation/amination/cycloaddition by highlighting their specificity and applicability, aiming to facilitate progress in this very exciting area of research.
Metal–N-heterocyclic carbene (M–NHC) complexes are well-known as an important class of organometallic compounds widely used in transition-metal catalysis. Taking into account that the steric hindrance around the metal center is one of the major effects in M–NHC catalysis, the development of new, sterically hindered M–NHC complexes is an ongoing interest in this field of research. Herein, we report the synthesis and characterization of exceedingly sterically hindered, well-defined, air- and moisture-stable Cu(I) and Ag(I) complexes, [Cu(NHC)Cl] and [Ag(NHC)Cl], in the recently discovered IPr# family of ligands that hinge upon modular peralkylation of anilines. The complexes in both the BIAN and IPr families of ligands are reported. X-ray crystallographic analyses and computational studies were conducted to determine steric effects, Frontier molecular orbitals, and bond orders. The complexes were evaluated in the model hydroboration of the alkynes. We identified [Cu(BIAN–IPr#)Cl] and [Ag(BIAN–IPr#)Cl] as highly reactive catalysts with the reactivity outperforming the classical IPr and IPr*. Considering the attractive features of well-defined Cu(I)–NHC and Ag(I)–NHC complexes, this class of sterically bulky yet wingtip-flexible complexes will be of interest for catalytic processes in various areas of organic synthesis and catalysis.
The first doubly P-bridged rigid, bent bis(NHCs) were generated and shown to be good building blocks in main group adducts and metal complexes with selective thermal retro-[4 + 2] cycloaddition chemistry.
Complexes of N‐heterocyclic carbenes have become an irrefutable class of molecules for the researchers in the field of organocatalysis. They rank as one of the potent tools in organic chemistry, which have a wide range of uses in various commercially important processes. Palladium(II) mesoionic carbene (MIC) complexes embedded with PPh3 as an ancillary ligand appeared to be efficient catalysts for the aforementioned purpose. Two new triazolylidene‐based palladium(II) complexes bearing either Py or PPh3 as an ancillary ligand were synthesized, with chirality present at one of the nitrogen atoms of the triazolylidene moiety. The MIC unit in these complexes possesses a mesityl group at the C‐wingtip and 1‐(1‐naphthyl)ethyl substitution at the N‐wingtip. NMR spectroscopy and ESI mass spectrometry were utilized to analyze these complexes. For the determination of the structure of the palladium(II) complex possessing mixed MIC and PPh3 donor ligands, single crystal X‐ray diffraction was utilized. The new complexes were employed in α‐arylation reaction and Sonogashira coupling reactions under copper‐free conditions. While studying their catalytic activity, it was brought to the conclusion that the PdII MIC complex having PPh3 as an ancillary ligand surpassed the complex with Py as an ancillary ligand.
This work includes the synthesis of a new series of palladium‐based complexes containing both morpholine and N ‐heterocyclic carbene (NHC) ligands. The new complexes were characterized using NMR ( ¹ H and ¹³ C), FTIR spectroscopic, and elemental analysis techniques. The crystal structure of complex 1b was obtained by utilizing the single‐crystal X‐ray diffraction method. X‐ray studies show that the coordination environment of palladium atom is completed by the carbene carbon atom of the NHC ligand, the nitrogen atom of the morpholine ring, and a pair of bromide ligand, resulting in the formation of slightly distorted square planar geometry. All complexes were determined for some metabolic enzyme activities. Results indicated that all the synthetic complexes exhibited powerful inhibitory actions against all aims as compared to the control molecules. K i values of new morpholine‐liganded complexes bearing 4‐hydroxyphenylethyl group 1a‐e for hCA I, hCA II, AChE, BChE, and α ‐glycosidase enzymes were obtained in the ranges 0.93–2.14, 1.01–2.03, 4.58–10.27, 7.02–13.75, and 73.86–102.65 µM, respectively. Designing of reported complexes is impacted by molecular docking study, and interaction with the current enzymes also proclaimed that compounds 1e (–12.25 kcal/mol for AChE and –11.63 kcal/mol for BChE), 1c (–10.77 kcal/mol and –9.26 kcal/mol for α‐Gly and hCA II, respectively), and 1a (–8.31 kcal/mol for hCA I) are showing binding affinity and interaction from the synthesized five novel complexes.
N-heterocyclic carbenes (NHCs) have emerged as a major direction in ancillary ligand development for stabilization of reactive metal centers in inorganic and organometallic chemistry. In particular, wingtip-flexible NHCs have attracted significant attention due to their unique ability to provide a sterically-demanding environment for transition metals in various oxidation states. Herein, we report a new class of sterically-hindered, wingtip-flexible NHC ligands that feature N,C-chelating oxazole donors. These ligands are readily accessible through a modular arylation of oxazole derivatives. We report their synthesis and complete structural and electronic characterization. The evaluation of steric, electron-donating and π-accepting properties and coordination chemistry to Ag(I), Pd(II) and Rh(I) is described. Preliminary studies of catalytic activity in Ag, Pd and Rh-catalyzed coupling and hydrosilylation reactions are presented. This study establishes the fluxional behavior of a freely-rotatable oxazole unit, wherein the oxazolyl ring adjusts to the steric and electronic environment of the metal center. Considering the tremendous impact of sterically-hindered NHCs and their potential to stabilize reactive metals by N-chelation, we expect that this class of NHC ligands will be of broad interest in inorganic and organometallic chemistry.
Cationic Au(I)–NHC (NHC = N-heterocyclic carbene) complexes have become an important class of catalysts for alkyne pi-activation reactions in organic synthesis. In particular, these complexes are characterized by high stability...
N-Heterocyclic carbenes are regarded as neutral compounds containing a divalent carbon atom with a six-electron valence shell, carbenes are an intriguing class of carbon-containing compounds. The successful isolation and characterization of an N-heterocyclic carbene in 1991 opened up a new class of organic compounds for investigation. They are cyclic molecules that contain one carbene and at least one nitrogen atom within the carbene-containing ring structure. These molecules are widely used as ancillary ligands for the preparation of transition-metal-based catalysts and also possess a unique characteristic feature i.e. Umpolung nature. The underlying principle of many of the NHC-catalyzed reactions is the remarkable ability of NHCs to reverse the normal mode of reactivity of aldehydes (umpolung). NHCs can catalyze transformations proceeding via the umpolung or nonumpolung mode for the synthesis of various functionalized molecules.
The preparation of diimidazolium salt HBDIM 1, a precursor for a di‐NHCs ligand, from cheap and easily available agent hexabenzylhexaazaisowurtzitane (HBIW) is reported. Under basic conditions, HBDIM undergoes facile deprotonation to in situ generate CageCarbene, which could efficiently coordinate to transition‐metals, such as, Au, Cu or Pd, to give the corresponding bimetallic complexes 2–4. These complexes were isolated and fully characterized, including X‐ray diffraction of their single crystals. It was found that the steric hinderance of CageCarbene is similar to that of SIMes but smaller than that of IPr, and electronically, CageCarbene is a strong σ‐donator similar to SIMes and a stronger σ‐donator than IPr. Further studies showed that complexes 2–4 were highly reactive to catalyze up to 17 reactions. Control experiments utilizing a N‐benzyl‐substituted monoimidazolium salt showed much lower catalytic reactivity when it was bound to Au or Cu, but exhibited similar reactivity for the Pd complex. Kinetic studies showed that the low reactivity of the monodentate carbene‐ligated Au or Cu complex was due to the low stability of the complex under the reaction conditions.
A new C3‐symmetric tris‐imidazolium tribromide salt 3, featuring 1,3,5‐substituted triethynylbenzene, was used for the preparation of a trinuclear PdII pyridine‐enhanced precatalyst preparation stabilization and initiation‐type (PEPPSI) complex by triple C2 deprotonation followed by the addition of PdCl2. Trinuclear PdII complex possessing a combination of NHC and PPh3 ligands has also been synthesized. The corresponding mononuclear palladium(II) complexes have also been synthesized for the comparison purpose. All these complexes have been characterized by using NMR spectroscopy and ESI mass spectrometry. The molecular structure of the trinuclear palladium(II) complex bearing mixed carbene and pyridine donor ligands has been established by using single crystal XRD. All the palladium(II) complexes have been used as pre‐catalysts, which gave good to excellent yields in intermolecular α‐arylation of 1‐methyl‐2‐oxindole and Sonogashira coupling reaction. Catalytic studies indicate an enhanced activity of the trinuclear PdII complex in comparison to the corresponding mononuclear PdII complex for both catalytic transformations. The better performance of the trinuclear complex has also been further supported by preliminary electrochemical measurements. A negative mercury poison test was observed for both the aforementioned catalyses and therefore, it is likely that these organic transformations proceed homogeneously.
Self-assembled discrete molecular architectures that show selective molecular recognition within their internal cavities are highly desirable. Such hosts often show guest recognition through several noncovalent interactions. This emulates the activity of naturally occurring enzymes and proteins. Research in the formation of 3D cages of different shapes and sizes has progressed rapidly since the development of coordination-driven self-assembly and dynamic covalent chemistry. Such molecular cages find applications in catalysis, stabilization of metastable molecules, purification of isomeric mixtures via selective encapsulation, and even in biomedical applications. Most of these applications stem from the ability of the host cages to bind guests strongly in a selective fashion, providing a suitable environment for the guests to perform their functions. Molecular cages having closed architectures with small windows either show poor encapsulation or inhibit easy guest release while those with wide open structures fail to form stable host-guest complexes. In this context, molecular barrels obtained by dynamic metal-ligand/covalent bond formation techniques possess optimized architectures. With a hollow-walled cavity and two large openings, molecular barrels satisfy the structural requirements for many applications. In this perspective, we will discuss in detail the synthetic strategies for obtaining barrels or barrel-like architectures employing dynamic coordination and covalent interactions, their structure-based classification, and their applications in catalysis, storing transient molecules, separation of chemicals, and photoinduced antibacterial activity. We aim to highlight the structural advantages of molecular barrels over other architectures for efficiently carrying out several functions and for the development of new applications.
Herein we present the synthesis of a tetratopic bisphosphine ligand derived from 4,6-disubstituted pyrimidine scaffold. This new ligand (2) was doubly alkylated using iodomethane to give bisphosphonium dication (3) as an iodide salt, and was also coordinated to both gold(I) chloride to give the metal complex (4) and to silver(I) triflate to give a coordination polymer (5), respectively. All compounds have been fully characterized using multinuclear NMR spectroscopy as well as single crystal X-ray diffraction. Gold complex 4 was also found to exhibit weak emission at λmax = 530 nm due to Au(I)–Au(I) aurophilic interactions. Additionally we demonstrated that bisphosphine ligand 2 can be used as a tetratopic ligand, as evidenced by the formation of the coordination polymer 5 in which both phosphine→silver and nitrogen→silver interactions were observed.
A new synthetic approach to π-extended imidazolium salts is developed based on 1,3-dipolar cycloaddition of polycyclic aromatic azomethine ylides with imidoyl chlorides, where the key to the successful conversion is the addition of cesium fluoride as an additive. Experimental and theoretical studies revealed that this cycloaddition proceeds via a stepwise mechanism rather than a concerted mechanism. This present method provides an efficient method to synthesize a variety of imidazolium-containing polycyclic aromatic compounds.
The field of asymmetric catalysis is rapidly developing and the chiral ligands play a key role in enantioselective transition‐metal catalysis. The electron‐rich chiral N‐heterocyclic carbenes (NHCs) have established themselves as a popular class of stereodirecting ancillary ligands to catalyze enantioselective organic transformations in more efficient ways. Several novel transition‐metal complexes in combination with tailored ligand design have emerged during last few decades in asymmetric catalysis. The tailor‐made NHCs can easily be accessed due to the modular synthesis of their parent azolium salt precursors. Their donor capability and the molecular shape can easily be tuned by changing substituent at N‐atom or by changing the cyclic backbone framework. This review article aims to describe the recent advances in this rapidly evolving research area of enantioselective catalysis using well‐defined transition‐metal complexes possessing chiral NHC donor ligands.
Here, we show for the first time that main-chain organometallic polymers (MCOPs) can be prepared from Janus N-heterocyclic carbene (NHC) linkers and polynuclear cluster nodes. The crosslinked framework Co4S4-MCOP is synthesized via ligand displacement reactions and undergoes reversible electron transfer in the solid state. Discrete molecular cluster species can be excised from the framework by digesting the solid in solutions of excess monocarbene. Finally, we demonstrate a synthetic route to monodisperse framework particles via coordination modulation.
We report the synthesis and the reactivity of 1,2,3‐triazolin‐5‐imine type mesoionic imines (MIIs). The MIIs are accessible by a base‐mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O‐stretching frequencies in MII‐CO2 and Rh(CO)2Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N‐heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent‐induced fluorine‐specific interactions and undergo C‐H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO‐LUMO gap compared to other related ligands. MIIs are potentially able to act as both π‐donor and π‐acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.
We report the synthesis and the reactivity of 1,2,3‐triazolin‐5‐imine type mesoionic imines (MIIs). The MIIs are accessible by a base‐mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O‐stretching frequencies in MII−CO2 and −Rh(CO)2Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N‐heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent‐induced fluorine‐specific interactions and undergo C−H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO–LUMO gap compared to other related ligands. MIIs are potentially able to act as both π‐donor and π‐acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.
Bis-azolium salts with one azolium capping a perbenzylated α-cyclodextrin have been designed to generate Janus-type bimetallic complexes with various combinations of copper, silver, gold or palladium salts. Encapsulation of one metal center inside the cavity allowed (trans)metalation and oxidation reactions to be controlled at selected positions. In particular, it was possible to oxidize AuI into AuIII selectively on the position outside the cavity of the cyclodextrin on the bis-AuI Janus complex.
The synthesis of bisnitrile derivatives of benzobisimidazole and bibenzimidazole in a good yield is described in detail for the first time. Nucleophilic substitution of 1,5‐difluoro‐2,4‐dinitrobenzene using different amines produced the intermediate diamines that were reduced using sodium borohydride/Pd(C) to produce the tetramines. These tetramines were allowed to couple with different aldehydes to produce the final benzimidazoles. In a different investigation, these bisnitrile derivatives will be used to make benzimidazole diamidines that could be used as potential mixed sequence minor groove binders.
This article describes an overview on the organometallic chemistry of N-heterocyclic carbene ligands and their isovalent analogues with other p-block elements as coordinating atoms. The contents are organized in terms of their major categories, synthetic methods, electronic and steric properties, and representative applications in synthetic organometallic chemistry, catalysis, medicinal chemistry, and materials science.
Janus di-N-heterocyclic carbene (NHC) ligands are a subclass of poly-NHCs that feature coordination to two transition metals in a facially opposed manner. The combination of the structural features of Janus type ligands, with the properties conferred by the NHC ligands, has conferred Janus-di-NHCs with privileged attributes for their use in diverse areas of research, such as homogeneous catalysis, materials chemistry and supramolecular chemistry. In molecular chemistry, Janus di-NHCs constitute one of the most useful chemical platforms for constructing dimetallic structures, and this includes both homo- and hetero-dimetallic compounds. This review aims to cover the most relevant advances in the use of Janus-di-NHCs during the last 15 years, by classifying them according to their specific structural features.
There is a very high demand and search for unique material that possesses self‐healing attributes. Several materials have been utilized as synthetic self‐healing materials majorly from thermoplastic, elastic, shape memory polymers, polymer composites, nanocomposites and coatings. However, many of these materials still need drastic improvement in order to enhance their self‐healing attributes such as strength, resistance to corrosion and conductivity. This will go a long way in improving and increasing the life span of manufactured products with self‐healing characteristics. Therefore, this chapter intends to provide a general overview on numerous materials with self‐healing attributes. More emphasis was also made on autonomic and non‐autonomic types of self‐healing materials. The mechanisms of action of these self‐healing materials were also highlighted.
Triazolate-based pillarplexes, supramolecular organometallic complexes (SOCs) featuring a tubular pore, are introduced. Following a macrocycle-templated synthesis strategy, a hybrid imidazolium/triazole cyclophane of defined ring size was used as ligand precursor,...
Over the last fifteen years, N-heterocyclic carbenes (NHCs) have mostly been used as ancillary ligands for the preparation of transition metal-based catalysts. Compared to phosphorus-containing ligands, NHCs tend to bind more strongly to metal centres, avoiding the necessity for the use of excess ligand in catalytic reactions. The corresponding complexes are often less sensitive to air and moisture, and have proven remarkably resistant to oxidation. Recent developments in catalysis applications have been facilitated by the availability of carbenes stable enough to be bottled, particularly for their use as organocatalysts. This book shows how N-heterocyclic carbenes can be useful in various fields of chemistry and not merely laboratory curiosities or simple phosphine mimics. NHCs are best known for their contribution to ruthenium and palladium-catalysed reactions but the scope of this book is much broader. The synthesis of NHC ligands and their corresponding metal complexes are covered in depth. Moreover, the biological activity of NHC-containing complexes, as well as an overview of their theoretical aspects are included. Such metal species are further examined, not only in terms of their catalytic applications, but also of their stereoelectronic parameters and reactivity/stability. Finally, special attention is given to the hot topic of organocatalysis. The book will be of interest to postgraduates, academic researchers and those working in industry.
Due to the presence of metal centers in a supramolecular polymer, material properties can be realized that are hardly accessible on the basis of covalent polymers or “organic” supramolecular polymers: redox activity, conductivity, light emission, magnetism, etc. Metal‐to‐ligand interactions are highly directional and, thus, predestined for the formation of linear supramolecular polymers. The stability and reversibility of the assemblies are mainly dictated via the nature of the organic ligand sites and the type of metal center incorporated. Thus, an enormous range of stabilities can be found, from highly labile to almost covalently inert. The chemistry and physics of these metallo‐supramolecular materials are discussed in the chapter.
This chapter encompasses a survey on various sustainable synthetic strategies to construct important classes of heterocycles – benzimidazoles, quinoxalines, and their congeners, primarily developed over the period of last two decades. It highlights on various protocols, their scopes, limitations, the greener aspects and also discusses plausible mechanisms, where ever appropriate. Special emphasis is given to sustainable processes that involve solid-phase synthesis, supported metal-catalyzed synthesis, photocatalysis, and other catalytic methods and reactions promoted by different solid surfaces, ionic liquids, carbonaceous graphene-based materials, etc., under room-temperature, solvent-free, microwave irradiation, or on-water conditions.
In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.
Three-dimensional (3D) triply interlocked catenanes are a family of chemical topologies that consist of two identical, mechanically interlocked coordination cage components with intriguingly complex structures. Although only a few successful constructions of 3D interlocked catenanes have been achieved to date via metal-mediated assembly, these complex structures have thus far only been targeted by metal-nitrogen/oxygen coordination techniques. Here, taking advantage of rational ligand design, we report the efficient construction of a series of 3D triply interlocked [2]catenanes of the formula [Ag3L2]2, wherein the metal ions exclusively form bonds to N-heterocyclic carbene (NHC) units, and their subsequent transmetalation to the corresponding [Au3L2]2 gold analogues. The formation and transmetalation reactions proceed under mild conditions and are generally applicable. A series of characterization techniques were applied to confirm the formation and structure of the desired 3D triply interlocked architectures: multinuclear NMR spectroscopy, ESI-MS, and single-crystal X-ray diffraction analysis. The solid-state structure of [Ag3(1a)2]2(PF6)6 unambiguously confirms the existence of a 3D catenane that consists of two identical, mechanically interlocked trinuclear hexacarbene cage components. The interlocking of two 3D cages into a [2]catenane is driven by the efficient π•••π stacking of triazine-triazine stacks with cooperative interactions between imidazo[1,5-a]pyridine subunits. Notably, the triply interlocked organometallic cages exhibit good stability toward various organic solvents, concentrations, temperatures, and no disassembly occurred in the presence of coronene or pyrene. The future construction of mechanically interlocked architectures using metal-carbene bonds rather than metal-nitrogen bonds may provide assemblies with interesting properties for as-yet-unimagined applications in fields such as sensors and molecular electrical conductors.
ConspectusThe field of metallosupramolecular chemistry is clearly dominated by the use of O-, N-, and P-donor Werner-type polydentate ligands. These molecular architectures are of high interest because of their wide range of applications, which include molecular encapsulation, stabilization of reactive species, supramolecular catalysis, and drug delivery, among others. Only recently, organometallic ligands have allowed the preparation of a variety of supramolecular coordination complexes, and the term supramolecular organometallic complexes (SOCs) is gaining space within the field of metallosupramolecular chemistry. While the early examples of SOCs referred to supramolecular architectures mostly containing bisalkenyl, diphenyl, or bisalkynyl linkers, the development of SOCs during the past decade has been boosted by the parallel development of multidentate N-heterocyclic carbene (NHC) ligands. The first examples of NHC-based SOCs referred to supramolecular assemblies based on polydentate NHC ligands bound to group 11 metals. However, during the last 10 years, several planar poly-NHC ligands containing extended π-conjugated systems have facilitated the formation of a large variety of architectures in which the supramolecular assemblies can contain metals other than Cu, Ag, and Au. Such ligands are Janus di-NHCs and trigonal-planar tris-NHCs-most of them prepared by our research group-which have allowed the preparation of a vast range of NHC-based metallosupramolecular compounds with interesting host-guest chemistry properties. Although the number of SOCs has increased in the past few years, their use for host-guest chemistry purposes is still in its earliest infancy. In this Account, we describe the achievements that we have made during the last 4 years toward broadening the applications of planar extended π-conjugated NHC ligands for the preparation of organometallic-based supramolecular structures, including their use as hosts for some selected organic and inorganic guests, together with the catalytic properties displayed by some selected host-guest inclusion complexes. Our contribution describes the design of several Ni-, Pd-, and Au-based metallorectangles and metalloprisms, which we used for the encapsulation of several organic substrates, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The large binding affinities found are ascribed to the incorporation of two cofacial panels with large π-conjugated systems, which provide the optimum conditions for guest recognition by π-π-stacking interactions. We also describe a series of digold(I) metallotweezers for the recognition of organic and inorganic substrates. These metallotweezers were used for the recognition of "naked" metal cations and polycyclic aromatic hydrocarbons. The recognition properties of these metallotweezers are highly dependent on the nature of the rigid connector and of the ancillary ligands that constitute the arms of the tweezer. A peculiar balance between the self-aggregation properties of the tweezer and its ability to encapsulate organic guests is observed.
Since the first isolation of an N-heterocyclic carbene (NHC) in the early 1990s, these species have been identified as valuable tools for the stabilization of main-group elements in unusual oxidations states and allowed for the isolation of novel types of compounds. While bis(NHC)s, i.e., compounds in which two NHC units are framed within one molecule by using suitable linker groups, found numerous application in rare-earth and transition-metal chemistry, this does not hold true for main-group elements. Although examples are found throughout the s- and p-block, their potential still needs to be realized. This review summarizes the syntheses of the related bis(NHC)s and gives an overview of mono-, di- and polynuclear complexes incorporating main-group elements and bis(NHC)s.
A family of palladium complexes that combine two types of aromatic N‐heterocyclic carbene (NHC) ligands has been prepared starting from dinuclear palladium complexes. Two of the complexes are mono‐metallic and contain a pyrene‐based mono‐NHC and a benzimidazolylidene ligand. The other two are dimetallic and combine a bridging pyrene‐based bis‐NHC ligand with benzimidazolylidene ligands. All new complexes therefore contain two types of NHC ligands that differ in the size of their aromatic backbones coordinated to the same metal center. All complexes have been characterized and the molecular structure of two of them has been confirmed by X‐ray diffraction studies.
The first trisanionic Cerberus-type maloNHC has been generated and coordinated to two different coinage metals (Au(I) and Ag(I)). The resulting triszwitterionic metal complexes have been characterized by NMR spectroscopy and...
Homo (Au2)- and heterodinuclear coinage metal complexes (AuAg) ligated by a six-membered Janus-type ditopic N-heterocyclic carbene (NHC) have been prepared by deprotonation of aNHC (abnormal NHC) gold complex followed by complexation. The two ditopic NHC coinage metal complexes were structurally characterized by single crystal X-ray diffraction. The carbene character for the C2 carbon and C5 carbon of the ditopic NHC ligand in 3 and 4 was confirmed by the NMR and structural data.
N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.
We describe new developments in hybrid conjugated materials containing metal complexes or nanoparticles. Phosphino-oligothiophenes are useful building blocks for metal-containing polymers and oligomers, and as cross-linkable surface passivating groups for metal nanoparticles. These materials may find application as new chemical sensors, switches, or transistors in molecular electronics.
Functional conjugated polymers prepared by electrochemical polymerization are reviewed. Various classes of functionalized polymers are considered such as polymers for energy storage and sensor applications, small band gap polymers and novel polymeric materials with original macromolecular structure. Particular emphasis is placed on the thiophene-based polymers and on the relationships between the structures of the various kinds of derivatized precursors, their aptitude for electropolymerisation and the electronic properties of the resulting polymers.
A personal account is given of the “early days” in the development of metallocene addition polymers in order to mark the 50th anniversary of the first report of a metallocene addition polymer, poly(vinylferrocene), by Arimoto and Haven at DuPont Inc. in 1955. Sometimes looking back, it seems like the early work was quaint and even obvious by today’s standards. However, this was not the case in the 1960s and ‘70s. These polymers seemed a strange departure from the “classic” world of polystyrene, polyacrylates, polyvinyl acetate, polyvinyl chloride, polybutadiene etc. The Polymer Handbook gave us no clue about how vinyl metallocenes might behave. Only through the (then) tedious synthesis of new monomers and a careful study of their homopolymerization kinetics and mechanism, their copolymerization reactivities and the characterization of the contributions the metal makes to their properties, was the stage set for the wider inclusion of organometallic addition polymers into mainstream macromolecular and materials science. Starting in the 1990s, and continuing today, an explosive growth of metal-containing macromolecular science and its extension to materials science has occurred. Herein, the early work in our laboratory is revisited. In the 1960s, metallocenes and organometal carbonyl compounds were a new exotic thrust of chemistry, merging inorganic and organic disciplines. However, few chemists had any thoughts of adapting this young field to polymers and materials. This is easily forgotten now, when practically every issue of numerous journals are filled with articles about polymers, films, multilayered devices etc. containing organic and inorganic hybrid substances (including metals) prepared by designer synthetic chemistry, self-assembly or biologically inspired routes. We have certainly come a long way!
Hybrid materials which combine the electronic conductivity of conjugated polymers and the redox and optical properties of metal complexes are being developed to take advantage of synergistic electronic interactions. Many systems show a splitting of the metal redox wave analogous to that in dinuclear complexes with through-ligand metal-metal interactions (superexchange). The influence of the conjugated backbone on electron transport between metal centres has been the focus of much work, and the existence of a superexchange pathway has been demonstrated. Several systems have been shown to exhibit catalytic and photocatalytic activities, and a number of applications in sensors have been described.
The synthesis of bis(N-heterocyclic carbene) chelates of formula [cis-E{N(H)C=C(H)N(R)C}(2)PdX2] (E = spacer linkage; R = alkyl, aryl; X = halide) is presented via improved high-yielding, air-stable procedures (for E = CH2). A detailed study of the intermediates involved in the preparation of [cis-CH2{N(H)C=C(H)N(t-Bu)C}(2)PdX2] (X = Br, I) provide evidence for the difficulty in extending our synthetic procedures to include chelates of different ring sizes and also in preparing the analogous nickel complexes.
Mononuclear and binuclear carbonylruthenium(II) complexes with N2O2 Schiff base ligands based on 3,5-di-tert-butylsalicylaldehyde and three different ortho-diamines have been prepared. The mononuclear Ru(BSP)(CO) [BSP = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-phenylenediamine] complex 4 acts as a versatile supramolecular synthon, as illustrated by the fact that it spontaneously forms linear and three-dimensional assemblies through axial coordination with pyridyl Lewis bases. Using this motif, neutral and charged assemblies 6, 9, and 12 were prepared. The versatility of the salophen ligand was highlighted by the preparation of bimetallic carbonylruthenium(II) compounds 14−17 from 1,2,4,5-tetraaminobenzene and 2,3,7,8-tetraaminodibenzo[1,4]dioxin. The bimetallic complexes were isolated as a mixture of cis and trans diastereomers with respect to the spatial relationship between the two axially bound carbon monoxide ligands. The electronic spectral and electrochemical properties of the pyridyl adducts 5, 15, and 17 were compared. The properties of 17 closely resembled 5 due to the insulating effect of the extended central tetraamino fragment, while 15 behaved as a single, novel chromophore. The electrochemical studies revealed that the central tetraamino linker regulates the communication between the two metal centers of 15 and 17. It was found that the two metal atoms of 15 sense each other to a larger extent than those of 17.
Un certain nombre de sels quaternaires du méthylene-1,1′ diimidazole et du méthylène-1,1′ dibenzimidazole ont été préparés et étudiés par résonance magnétique du proton comparativement aux sels quaternaires de méthyl-1 imidazole et méthyl-1 benzimidazole. L'effet des chélates d'europium ainsi que des expériences d'effet overhauser nucléaire ont été utilisées pour l'attribution des protons.Serval quaternary salts dervied from 1,1′-methylemediimisazole and 1,1′-methylemediimisazole have been prepared. A comparative study with the salts dervied from 1-methylimidazole and 1-methylimidazole has been carried out using paoton magnetic resonance. Nuclear Overhauser effect and shift reagents have been used for the assignment of heterocyclic protons.
The first 9- and 11-membered cis and trans C2-symmetric benzimidazol-2-ylidene palladium(II) complexes based on a trans-2,2-dimethyl-1,3-dioxalane backbone were synthesised and the configuration of the complexes was elucidated via NMR and X-ray crystallography.
Reactions of the ethylene- and methylene-bridged bis(imidazolium) salts with an equivalent amount of silver oxide in dichloromethane at room temperature produced readily the silver NHC compounds [Ag2LBr2]. These compounds are partially soluble in DMF. The X-ray structure determination on 3d (L = 1,1′-dibenzyl-3,3′-ethylenediimidazolin-2,2′-diylidene) reveals the formation of bromide bibridged (Ag2LBr2)n chains and a unique supramolecular motif with weak Ag⋯Ag interactions of 3.429 Å. Similar to monomeric silver(I) NHC complexes, the silver coordination polymers can also act as carbene transfer reagents for the formation of chelating palladium NHC complexes in excellent yields.
This review describes the chemistry of alkynylated cyclobutadiene complexes and alkynylated ferrocenes. These complexes are made by a combination of metalation, formylation and Ohira alkynylation. The transformation of a formyl group into an alkyne is a critical step in these syntheses. The use of a diazophosphonate as the source of a carbon atom is convenient and has allowed for the synthesis of a host of alkynylated p-complexes. Multiply alkynylated ferrocenes and cyclobutadiene(cyclopentadienyl)cobalt complexes are utilized as stepping stone for complex carbon-rich organometallics. Half-wheel, butterfly, and similar structures have been accessed and characterized by single crystal X-ray methods. A ferrocene-fused dehydro[18]annulene decomposes explosively when heated above 200 8C with formation of carbon nanostructures that are bagel-or onion-shaped and that have been characterized by electron microscopy. This account as well describes the synthesis and characterization of organometallic dendrimers and conjugated organometallic polymers.
The use of N-heterocyclic carbenes as ligands for transition metals has increased dramatically in the last few years, spurred on by their remarkable successes in the areas of metathesis chemistry and coupling reactions. The stability of the transition metal complexes of N-heterocyclic carbenes is often cited as one of the key advances of these ligands versus their phosphine counterparts. However, to quote Danopoulos, “recent reports are questioning the belief that the NHC–metal bond is inert” [Chem. Commun. (2003) 756]. This review describes some of the reactions that N-heterocyclic carbenes undergo in the metal co-ordination sphere that lead to decomposition of the NHC–metal complex. Implications for catalysis using these species are described.
During the last decade, the use of N-heterocyclic carbene ligands (NHCs) based on imidazolium ions and related heterocycles has emerged as an alternative to phosphines in the design of new organometallic catalysts. We review catalysts with chelate and pincer NHC ligands, including complexes of palladium, ruthenium, rhodium and iridium. Transfer hydrogenation and Heck chemistry are given special attention. Also discussed are Suzuki and Sonogashira coupling and immobilization on clay supports. Synthetic aspects are covered as well as a discussion of structural features, catalytic properties and catalyst recovery and recycling.
Ligand exchange reactions reveal unexpected lability of the carbene ligands in two coordinate palladium(0) N-heterocyclic carbene complexes; the latter are found to be very effective catalysts for amination of aryl chlorides.
Highly active but sterically demanding: Ambient-temperature Suzuki cross-coupling of aryl chlorides is possible with a palladium(o) complex bearing two bulky, N-heterocyclic carbene ligands (see structure). Nearly quantitative yields are obtained within two hours with some reagents, which makes this compound the most active catalyst known to date under these conditions.
N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C-H activation, C-C, C-H, C-O, and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.
The incorporation of organometallic moieties into polymer structures has attracted growing attention. However, unlike the situation in organic polymer chemistry where many different synthetic approaches are effective, the development of main chain metal-containing polymer offers a considerable synthetic challenge. To access the favorable characteristics of macromolecular materials, molecular weights with the consequent chain entanglement are necessary.
We show that imidazolium salts do not always give normal or even aromatic carbenes on metalation, and the chemistry of these ligands can be much more complicated than previously thought. N,N'-disubstituted imidazolium salts of type [(2-py)(CH(2))(n)(C(3)H(3)N(2))R]BF(4) react with IrH(5)(PPh(3))(2) to give N,C-chelated products (n = 0, 1; 2-py = 2-pyridyl; C(3)H(3)N(2) = imidazolium; R = mesityl, n-butyl, i-propyl, methyl). Depending on the circumstances, three types of kinetic products can be formed: in one, the imidazole metalation site is the normal C2 as expected; in another, the metalation occurs at the abnormal C4 site; and in the third, C4 metalation is accompanied by hydrogenation of the imidazolium ring. The bonding mode is confirmed by structural studies, and spectroscopic criteria can also distinguish the cases. Initial hydrogen transfer can take place from the metal to the carbene to give the imidazolium ring hydrogenation product, as shown by isotope labeling; this hydrogen transfer proves reversible on reflux when the abnormal aromatic carbene is obtained as final product. Care may therefore be needed in the future in verifying the structure(s) formed in cases where a catalyst is generated in situ from imidazolium salt and metal precursor.
High-resolution solid-state 31P cross-polarization magic angle spinning (CP/MAS) spectra of a series of Pd(II) complexes were obtained. All of these spectra exhibit low-intensity satellite peaks flanking the main resonances which are assigned to originate from a combination of direct (D) and indirect (J) spin-spin coupling between the 31P and the 105Pd spins. The parameter 1J(105Pd,31P) is found to be sensitive to the nature of the ligand in a trans position and thus of great value in assigning the configuration, i.e. cis or trans, in square-planar complexes of Pd(II). A linear relationship between 1J(105Pd,31P) and 1J(195Pt,31P) in analogous Pd(II) and Pt(II) complexes is suggested, the latter parameter being a factor of ca 14 larger. Two-dimensional exchange spectroscopy proved valuable in resolving overlapping resonances and relating pairs of inequivalent 31P spins within the same complex and spreading their satellite manifolds into two dimensions. These two spectral features are unrelated, being due to dipolar coupling among the phosphorus spins in the former and finite lifetimes of the spin states of the 105Pd isotope in the latter case.
Recent reports of highly conductive metallopolymers are reviewed. This literature is classified into one of two categories (inner or outer sphere) depending on the mode of interaction between the transition metal centers with each other and the conducting polymer backbone. The critical nature of charge transport is discussed in the context of the relative energies of the organic polymer-based and metal-centered redox processes. Also included are recent advances in the development of functional materials based on metal-containing conducting polymers.
Polyolefins are macromolecular alkanes and include the most familiar and most commercially produced plastic, polyethylene. The low cost of these materials combined with their diverse and desirable property profiles drive such large-scale production. One property that renders polyolefins so attractive is their resistance to harsh chemical environments. However, this attribute becomes a severe limitation when attempting to chemically convert these plastics into value-added materials. Functionalization of polymers is a useful methodology for the generation of new materials with wide ranging applications, and this tutorial review describes both new and established methods for the post-polymerization modification of polyolefins.
The imidazolium salts [3-R1-1-{2-Ar-imino)-2-R 2-ethyl}imidazolium] chloride (C-N; Ar = 2,6-iPr 2C6H3; R1/R2 = Me/Me (a), Me/Ph (b), Ph/Me (c), 2,4,6-Me3C6H2 (d), 2,6-iPr2C6H3 (e)) react with Ag 2O to give Ag(I) iminocarbene complexes (C-N)AgCl (4a-e) in which the iminocarbene ligand is bonded to Ag via the imidazoline-2-ylidene carbon atom. The solid-state structures of 4b and 4d were determined by X-ray crystallography and revealed the presence of monomeric (carbene)AgCl units with Z and E configurations at the imine C=N bonds, respectively. Carbene transfer to Pd occurs when compounds 4b-e are treated with (COD)PdCl2 to yield bis(carbene) complexes (C-N)2PdCl2 (6b-e) containing two κ1-C bonded iminocarbene moieties. NMR spectroscopic data indicated a trans coordination geometry at Pd. This conclusion was supported by an X-ray structure determination of 6b which clearly demonstrated the non-chelating nature of the iminocarbene ligand system. EXSY 1H NMR spectroscopy suggests that the non-chelating structures undergo E/Z isomerization at the imine C=N double bonds in solution. The preparative results contrast our earlier report that the reaction between 4a and (COD)PdCl 2 results in a chelating κ2-C,N bonded iminocarbene complex (C-N)PdCl2. The coordination mode and dynamic behavior of the iminocarbene ligand systems have been found to be dramatically affected by changes in the substitution pattern of the ligand system. Sterically unencumbered systems (a) favor the formation of κ2-C,N chelate structures containing one iminocarbene moiety per metal upon coordination at Pd(II); these complexes were demonstrated to engage in reversible, solvent-mediated chelate ring-opening reactions. Sterically encumbered systems (b-e) form non-chelating κ1-C iminocarbene Pd(II) complexes containing two iminocarbene ligands per metal. Transannular repulsions across the chelate ring are believed to be the origin of these structural differences.