Catalytic and coordination facets of single-site non-metallocene organometallic catalysts with N-heterocyclic scaffolds employed in olefin polymerization

This review discusses the principles underlying mononucleating N-heterocyclic ligand design, selectivity of metal centers, preparation of organometallic catalysts with a N-heterocyclic backbone, and their catalytic activity in olefin oligo/polymerization. A vast number of N-heterocyclic organometallic compounds have been applied for the polymerization on account of their modest cost, low toxicity, and the large availability of transition metals in stable and variable oxidation states, which makes them versatile precursors for these reactions. The main points of focus in this review are the key advances made over more the past 25 years in the design and development of non-metallocene single-site organometallic catalysts bearing different N-heterocyclic scaffolds as a backbone. These catalysts are applied as precursors for the transformation of ethylene, higher α-olefins, and cyclic olefins into oligo/polymers. Emphasis is placed on the architecture of ligand peripheries for tuning the formed polymer properties and the consequences on product formation of different alkyl or aryl substituents directly attached to the metal center in a N-heterocyclic ligand system.     

What difference one double bond makes: Electronic structure of saturated and unsaturated n-heterocyclic carbene ligands in Grubbs 2nd generation-type catalysts

N-heterocyclic carbene (NHC) ligands are a versatile and useful class of ligands that have enjoyed much success over the past few decades in organometallic chemistry. This fact is exemplified most convincingly in Grubbs 2nd generation olefin metathesis catalysts. We explore the electronic impact of the NHC-ligand by decoupling electronic and steric effects through simplified model N-heterocyclic carbenes. Saturated and unsaturated N-heterocyclic carbene ligands give rise to fundamentally different frontier orbitals in these catalysts, suggesting a need to classify them as two electronically distinct ligand classes.     

Addition Polymerization of 5-Ethylidene-2-Norbornene in the Presence of Pd N-Heterocyclic Carbene Complexes

New high-performance catalytic systems based on Pd N-heterocyclic carbene complexes for the selective addition polymerization of 5-ethylidene-2-norbornene (ENB) were proposed. With these catalysts, polymerization can be conducted at unprecedentedly high monomer/catalyst ratio (up to 5 × 10⁵/1) and gives high-molecular-weight soluble polymers with good film-forming properties. Varying the polymerization conditions (reaction temperature, monomer and catalyst concentrations, monomer/Pd ratio) makes it possible to prepare soluble ENB-based addition polymers with specified molecular weights in reasonable yields.     

Generation of carbanions through stibine-metal and bismuthine-metal exchange reactions and its applications to precision synthesis of ω-end-functionalized polymers

Generation of carbanions from organostibines and organobismuthines through heteroatom-metal exchange reactions was examined from synthetic and mechanistic viewpoints. The exchange reaction proceeded spontaneously upon treatment with various organometallic reagents, such as alkyl lithiums, tetraalkyl zincates, and alkyl magnesium halides to afford the corresponding carbanions quantitatively. Due to the high reactivity of these heteroatom compounds, the exchange reactions took place exclusively even in the presence of various polar functional groups, which potentially react with organometallic species. The advantage of this method was exemplified by the end-group transformation of living polymers that bear these heteroatom species at the ω-polymer end, prepared by using organostibine and bismuthine-mediated living radical polymerizations. Various polymers that bear polar functional groups and acidic hydrogen-for example, poly(methyl methacrylate), poly(butyl acrylate), poly(N-isopropyl acrylamide), and poly(2-hydroxyethyl methacrylate)-could be used in the exchange reactions, and subsequent trapping with electrophiles afforded the corresponding polymers with controlled molecular weights, molecular weight distributions, and end-group functionalities. Competition experiments showed that organostibines and organobismuthines were among the most reactive heteroatom compounds towards organometallic reagents and that their high reactivity was responsible for the high chemoselectivity in the exchange reaction.     

First example of N-heterocyclic carbenes as catalysts for living polymerization: organocatalytic ring-opening polymerization of cyclic esters

A novel metal-free, organocatalytic approach to living polymerization is presented. N-heterocyclic carbenes were employed as nucleophilic catalysts for the ring-opening polymerization (ROP) of cyclic ester monomers. The catalysts is used in combination with an initiator, such as an alcohol, which generates an alpha-end group bearing the ester from the initiating alcohol upon ring-opening and a hydroxyl functional omega-chain end that propagates the chain. This class of catalyst proved to be more reactive than tertiary amine and phosphine nucleophiles, producing narrowly dispersed polymers of predictable molecular weights at room temperature in 1-2 h. Catalysis with respect to both initiating alcohol and monomer was observed. Control of the alpha and omega end-groups was demonstrated with a pyrene-labeled initiator, allowing the preparation of well-defined macromolecular architectures. Analogous to the ROP of cyclic esters using biocatalysts, the polymeriztion pathway using the N-heterocyclic carbenes is believed to ensue through a monomer-activated mechanism.     

A general and versatile approach to thermally generated N-heterocyclic carbenes

The synthesis of N-heterocyclic carbene (NHC) adducts by condensation of diamines with appropriately substituted benzaldehydes is described. This simplified approach provides the NHC adduct without first having to generate the carbene followed by its protection. These adducts undergo thermal deprotection to generate N-heterocyclic carbene in situ. Adduct decomposition temperatures were investigated as a function of catalyst structure by using thermal analysis and spectroscopic techniques. Importantly, unlike adducts derived from chloroform, the new pentafluorobenzene-based adducts are more readily prepared and are stable at room temperature. The utility of these adducts as organic catalyst precursors for living ring-opening polymerization (ROP) of lactide, transesterification reactions, and the synthesis of N-heterocyclic carbene ligated organometallic complexes is also described.     

Metallorganic side group liquid crystalline polymers containing pi-allylpalladium(II) groups

The preparation and phase properties of some LC pi-allyl and pi-crotyl Pd(II) side group metallorganic polymers and some related salicylaldiminates are described. The LC acrylate functionalized monomers are easily prepared, but their use to obtain the corresponding polymers by radical reaction failed because extensive decomposition of the complexes occurs with the formation of Pd metal. The synthesis of the metallated polymers was therefore performed by reacting the dimeric chloro-bridged organometallic pi-allyl or pi-crotyl Pd(II) complexes with the appropriate ligand polymer which is prepared without difficulties. The organometallic polymers show a nematic mesophase, while the ligand polymer exhibits a smectic A or C phase. Both metallated low molecular mass model compounds and the polymers give stable mesophases, although at lower temperatures compared with the parent ligand compounds.     

Dynamic covalent polymers based upon carbene dimerization

Thermally-reversible covalent polymers featuring dynamic carbon-carbon double bonds and tunable molecular weights were prepared from difunctional carbenes; addition of transition metal complexes to these materials afforded the respective main-chain organometallic polymers.     

Conjugated organometallic polymers containing Vollhardt's cyclobutadiene complex: aggregation and morphologies

Organometallic polymers were prepared by acyclic diyne metathesis (ADIMET) or by Pd-catalyzed coupling of 1,3-diethynylcyclobutadiene(cyclopentadienyl)cobalt with a suitably substituted diiodobenzene. The polymers obtained by Heck coupling show a degree of polymerization (Pn) of 20-60. The monomers for ADIMET were made by the Pd-catalyzed coupling of [1,3-bis(trimethylsilylethynyl)-2,4-bis(trimethylsilyl)cyclobutadiene](cyclopentadienyl)cobalt to 1-bromo-2,5-dialkyl-4-propynylbenzenes in the presence of KOH in yields of 40-48%. The monomers carry hexyl, ethylhexyl, and (S)-3,7-dimethyloctyl side chains. Polymerization of the propynylated monomers furnishes organometallic polymers with a Pn of up to 230 arylene-ethynylene units. The polymers were fully characterized by polarizing microscopy, transmission electron microscopy, circular dichroism, differential scanning calorimetry, and X-ray diffraction (XRD). They show nematic, lyotropic liquid crystalline phases as well as chiroptical properties from which aggregation in poor solvents and in the solid state can be concluded. Lamellar or irregular honeycomb-shaped morphologies in these organometallic polymers can be detected by electron microscopy.     

Perforated organometallic nanotubes prepared from a Rh N-heterocyclic carbene using a porous alumina membrane

Fabrication of perforated organometallic nanotubes using a di-rhodium bis(N-heterocyclic carbene) complex by a simple nanoporous template wetting technique is described along with characterization data from scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), proton NMR and Mass spectroscopy.     

High charge carrier mobility in conjugated organometallic polymer networks

The improvement of charge transport in conjugated polymers is a focal point of current research. It is shown here that the carrier mobility can be substantially increased through the introduction of conjugated cross-links between the conjugated chains. Novel organometallic polymer networks based on a poly(p-phenylene ethynylene) (PPE) derivative and Pt0 were synthesized by ligand-exchange reactions between the linear PPE and a low-molecular Pt complex. Time-of-flight measurements revealed ambipolar charge carrier mobilities of up to 1.6 x 10-2 cm2 V-1 s-1 for these materials, which are an order of magnitude higher than those of the neat polymer and represent the highest mobilities yet observed in disordered conjugated polymers.     

Living Radical Polymerization of Styrene Derivatives Containing Trialkylmetal Groups

Fourteenth group oganometallic styrene derivatives were synthesised by nitroxide-mediated controlled radical polymerization using di-tert butyl nitroxide (A-T) as initiator. This is the first time that nitroxide-controlled radical polymerization has been successfully adapted for the synthesis of new polystyrenes bearing organometallic species with controlled molecular weights and narrow polydispersities. Monomer reactivity ratios were determined in controlled nitroxide-mediated radical copolymerization between styrene and substituted styrene. All experiments permitted the synthesis of new organometallic polymers that will be used for the development of a polymer capsule for Inertial Confinement Fusion Experiments.     

N-杂环卡宾: 一种多用途有机小分子催化剂

近年来, N-杂环卡宾化学引起了化学家们广泛的研究兴趣, 这一切主要归功于Arduengo等的杰出工作, 在1991年他们首次成功分离得到了第一个稳定的N-杂环卡宾——咪唑-2-碳烯. 这推动了N-杂环卡宾化学的飞速发展. 尽管N-杂环卡宾作为配体在金属有机化学中已经被广泛研究, 然而, 对N-杂环卡宾作为反应底物和有机小分子催化剂的研究还有待进一步深入. N-杂环卡宾作为有机小分子催化剂在有机反应中的应用及其研究进展, 引起有机化学家对N-杂环卡宾化学这一热点领域的更多关注.     

Drug-loaded, bivalent-bottle-brush polymers by graft-through ROMP

Graft-through ring-opening metathesis polymerization (ROMP) using ruthenium N-heterocyclic carbene catalysts has enabled the synthesis of bottle-brush polymers with unprecedented ease and control. Here we report the first bivalent-brush polymers; these materials were prepared by graft-through ROMP of drug-loaded polyethylene-glycol (PEG) based macromonomers (MMs). Anticancer drugs doxorubicin (DOX) and camptothecin (CT) were attached to a norbornene-alkyne-PEG MM via a photocleavable linker. ROMP of either or both drug-loaded MMs generated brush homo- and co-polymers with low polydispersities and defined molecular weights. Release of free DOX and CT from these materials was initiated by exposure to 365 nm light. All of the CT and DOX polymers were at least 10-fold more toxic to human cancer cells after photoinitiated drug release while a copolymer carrying both CT and DOX displayed 30-fold increased toxicity upon irradiation. Graft-through ROMP of drug-loaded macromonomers provides a general method for the systematic study of structure-function relationships for stimuli-responsive polymers in biological systems.     

N-heterocyclic carbenes as organocatalysts

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.     

若干新型有机光电功能材料的分子设计、合成与性能 研究

介绍本研究组得到国家自然科学基金资助的四个方面工作的进展。研究了金属有机化合物的结构与非线性光学性质的关系,总结了从分子几何构型着手,根据不同用途,对金属有机非线性光学材料进行分子设计的经验规律;提出了利用“组合式共轭桥”进行有机非线性光学发色团分子设计的新思路,所合成的几个有机化合物既有很大的光学非线性,又有紫移的最大吸收峰;通过化学键将有机发色团分子张到各种高分子的侧链上,合成和表征了潜在的电光高分子和光折变高分子材料;采用无机-有机夹层复合的思路对兼有导电性和强磁性的分子材料进行了探索,将一些有机小分子和导电高分子分别插入了层状无机物MPS3的层间,得到了8个新的分子磁体,而另一夹层化合物则表现了较高的电导率。     

N-heterocyclic carbenes in gold catalysis

The appealing properties of N-heterocyclic carbenes (NHC) as ancillary ligands and the high potential of gold as an organometallic catalyst have made their encounter inevitable. Still in its infancy, NHC-gold catalysis is nevertheless growing rapidly. In this tutorial review, catalytic transformations involving NHC-containing gold(i) and gold(iii) complexes are presented. Particular attention is drawn to the versatility and selectivity of these catalysts.     

Macromolecular architecture of group 10 metal‐poly(yne)s: dendritic,hyperbranched, and helical macromolecules

Novel two‐ and three‐dimensional organometallic polymers of palladium and platinum have been prepared by choice of some acetylene linkages with characteristic structures. When o‐diethynylbenzene was used for a bridging ligand, a macrocyclic complex was selectively produced. Helical poly(yne) polymers were prepared from the reaction with chiral 1,1′‐bi(6‐ethynyl‐2‐naphthol). Platinum acetylide dendrimers, in which platinum atoms are linked by 1,3,5‐triethynylbenzene derivatives, have been prepared by a convergent method up to the third generation.     

Selective dimerization,oligomerization, homopolymerization and copolymerization of olefins with complex organometallic catalysts

Review embarrasses the problems of low molecular weight olefins (ethylene and propylene) selective oligomerization to butene-1, hexene-1, octene-1, 4-methylpentene-1; selective polymerization of olefins to obtain polymers with a given molecular mass, molecular mass distribution, branching (for the polyethylene), chain structure [atactic, iso-, syndio-, gemiisotactic, stereoblock type and containing terminal vinyl and vinylidene bonds (for polypropylene)]; “live” homo-and copolymerization of olefins, and alternating copolymerization of olefins in the presence of complex organometallic catalysts.     

N-heterocyclic carbene enabled synthesis of conjugated polymers

The Suzuki polycondensation of a dihalogenated 4,5-diphenylimidazole (2) with a fluorenyl diboronic acid diester followed by methylation afforded a conjugated poly(imidazolium) copolymer (P2) in 93% yield. Upon exposure to strong base, P2 was converted in situ to the corresponding poly(N-heterocyclic carbene) P3, as evidenced by ¹H NMR spectroscopy and a trapping experiment involving sulfur that afforded the corresponding poly(thiourea) P4. Similarly, treating a solution of P2 with KOtBu and [Ir(1,5-cyclooctadiene)Cl]₂ afforded a conjugated polymer bearing pendant Ir complexes (P5) in 63% yield. Thermal and photophysical analyses of the aforementioned polymers revealed that they were thermally stable with tunable fluorescence properties, features which poise them for use in various electronic and sensing applications. The presented methodology is expected to facilitate the synthesis of a broad range conjugated organometallic polymers from a common and readily accessible precursor.