Tuning the activity and selectivity of polymerised ionic liquid-stabilised ruthenium nanoparticles through anion exchange reactions.

The development of highly active and selective heterogeneous-based catalysts with tailorable properties is not only a fundamental challenge, but is also crucial in the context of energy savings and sustainable chemistry. Here, we show that ruthenium nanoparticles (RuNPs) stabilised with simple polymerised ionic liquids (PILs) based on N-vinyl imidazolium led to highly active and robust nano-catalysts in hydrogenation reactions, both in water and organic media. Of particular interest, their activity and selectivity could simply be manipulated through counter-anion exchange reactions. Hence, as a proof of concept, the activity of RuNPs could be reversibly turned on and off in the hydrogenation of toluene, while in the case of styrene, the hydrogenation could be selectively switched from ethylbenzene to ethylcyclohexane upon anion metathesis. According to X-ray photoelectron spectroscopy (XPS) and dynamic light scattering (DLS) analyses, these effects could originate not only from the relative hydrophobicity and solvation of the PIL corona but also from the nature and strength of the PIL-Ru interactions.

Direct and selective access to amino-poly(phenylene vinylenes)s with switchable properties by dimerizing polymerization of aminoaryl carbenes.

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.

The organocatalytic ring-opening polymerization of N-tosyl aziridines by an N-heterocyclic carbene.

The ring-opening polymerization of N-tosyl aziridines, in the presence of 1,3-bis(isopropyl)-4,5(dimethyl)imidazol-2-ylidene as an organocatalyst and an N-tosyl secondary amine as initiator mimicking the growing chain, provides the first metal-free route to well defined poly(aziridine)s (PAz) and related PAz-based block copolymers.

Imidazolium-Based Poly(Ionic Liquid)s Featuring Acetate Counter Anions: Thermally Latent and Recyclable Precursors of Polymer-Supported N-Heterocyclic Carbenes for Organocatalysis.

Statistical copoly(ionic liquid)s (coPILs), namely, poly(styrene)-co-poly(4-vinylbenzylethylimidazolium acetate) are synthesized by free-radical copolymerization in methanolic solution. These coPILs serve to in situ generate polymer-supported N-heterocyclic carbenes (NHCs), referred to as polyNHCs, due to the noninnocent role of the weakly basic acetate counter-anion interacting with the proton in C2-position of pendant imidazolium rings. Formation of polyNHCs is first evidenced through the quantitative formation of NHC-CS2 units by chemical postmodification of acetate-containing coPILs, in the presence of CS2 as electrophilic reagent (= stoichiometric functionalization of polyNHCs). The same coPILs are also employed as masked precursors of polyNHCs in organocatalyzed reactions, including a one-pot two-step sequential reaction involving benzoin condensation followed by addition of methyl acrylate, cyanosilylation, and transesterification reactions. The catalytic activity can be switched on and off successively upon thermal activation, thanks to the deprotonation/reprotonation equilibrium in C2-position. NHC species are thus in situ released upon heating at 80 °C (deprotonation), while regeneration of the coPIL precursor occurs at room temperature (reprotonation), triggering its precipitation in tetrahydrofuran. This also allows recycling the coPIL precatalyst by simple filtration, and reusing it for further catalytic cycles. The different organocatalyzed reactions tested can thus be performed with excellent yields after several cycles.

From the N-Heterocyclic Carbene-Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates.

Among various N-heterocyclic carbenes (NHCs) tested, only 1,3-bis(tert-butyl)imidazol-2-ylidene (NHC(tBu) ) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC-catalyzed 1,4-addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well-defined α-alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]0/[ROH]0 molar ratio, were ultimately obtained in N,N-dimethylformamide at 25 °C. A hydroxyl-terminated poly(ethylene oxide) (PEO-OH) macro-initiator was also employed to directly access PEO-b-PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHC(tBu) .

Cyclodimerization versus polymerization of methyl methacrylate induced by N-heterocyclic carbenes: a combined experimental and theoretical study.

The activation behavior of two N-heterocyclic carbenes (NHCs), namely, 1,3-bis(isopropyl)imidazol-2-ylidene(NHCiPr) and 1,3-bis(tert-butyl) imidazol-2-ylidene (NHCtBu), as organic nucleophiles in the reaction with methyl methacrylate (MMA) is described. NHCtBu allows the polymerization of MMA in DMF at room temperature and in toluene at 50 °C, whereas NHCiPr reacts with two molecules of MMA, forming an unprecedented imidazolium-enolate cyclodimer (NHCiPr/MMA=1:2). It is proposed that the reaction mechanism occurs by initial 1,4-nucleophilic addition of NHCiPr to MMA, generating a zwitterionic enolate 2, followed by addition of 2 to a second MMA molecule, forming a linear imidazolium-enolate 3 (NHCiPr/MMA=1:2). Proton transfer, generating intermediate 5, followed by cyclization and release of methanol yielded the aforementioned zwitterionic cyclodimer 1:2 adduct 7, the molecular structure of which has been established by NMR spectroscopy, X-ray diffraction, and mass spectrometry. This unexpected difference between NHCtBu and NHCiPr in the reaction with MMA (polymerization and cyclodimerization, respectively) can be rationalized by using DFT calculations. In particular, the nature of the NHC strongly influences the cyclodimerization pathway, the cyclization of 5 and the release of methanol are the discriminating step and limiting step, respectively. In the case of NHCtBu, both steps are strongly disfavoured compared with that of NHCiPr (energetic difference of around 14 and 9 kcal mol(-1), respectively), preventing the cyclization mechanism from a kinetic viewpoint. Moreover, addition of a third molecule of MMA in the polymerization pathway results in a lower activation barrier than that of the limiting step in the cyclodimerization pathway (difference of around 14 kcal mol(-1)), in agreement with the formation of polymethyl methacrylate (PMMA) by using NHCtBu as nucleophile.

Precision synthesis of poly(ionic liquid)-based block copolymers by cobalt-mediated radical polymerization and preliminary study of their self-assembling properties.

A poly(ionic liquid)-based block copolymer (PIL BCP), namely, poly(vinyl acetate)-b-poly(N-vinyl-3-butylimidazolium bromide), PVAc-b-PVBuImBr, is synthesized by sequential cobalt-mediated radical polymerization (CMRP). A PVAc precursor is first prepared at 30 °C in bulk by CMRP of VAc, using bis(acetylacetonato)cobalt(II), Co(acac)2, and a radical source (V-70). Growth of PVBuImBr from PVAc-Co(acac)2 is accomplished by CMRP in DMF/MeOH (2:1, v/v). This PIL BCP self-assembles in the sub-micron size range into aggregated core-shell micelles in THF, whereas polymeric vesicles are observed in water, as evidenced by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Thin-solid sample cut from raw materials analyzed by TEM shows an ordered lamellar organization by temperature-dependent synchrotron small-angle X-ray scattering (SAXS). Anion exchange can be accomplished to achieve the corresponding PIL BCP with bis(trifluorosulfonyl)imide (Tf2 N(-)) anions, which also gives rise to an ordered lamellar phase in bulk samples. A complete suppression of SAXS second-order reflection suggests that this compound has a symmetric volume fraction (f ≈ 0.5). SAXS characterization of both di- and triblock PIL BCP analogues previously reported also shows a lamellar phase of very similar behavior, with only an increase of the period by about 8% at 60 °C.

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis.

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.

Imidazolium hydrogen carbonates versus imidazolium carboxylates as organic precatalysts for N-heterocyclic carbene catalyzed reactions.

Imidazolium-2-carboxylates (NHC-CO(2) adducts, 3) and (benz)imidazolium hydrogen carbonates ([NHC(H)][HCO(3)], 4) were independently employed as organic precatalysts for various molecular N-heterocyclic carbene (NHC) catalyzed reactions. NHC-CO(2) adducts were obtained by carboxylation in THF of related free NHCs (2), while the synthesis of [NHC(H)][HCO(3)] precursors was directly achieved by anion metathesis of imidazolium halides (1) using potassium hydrogen carbonate (KHCO(3)) in methanolic solution, without the need for the prior preparation of free carbenes. Thermogravimetric analysis (TGA) and TGA coupled with mass spectrometry (TGA-MS) of most [NHC(H)][HCO(3)] precursors 4 showed a degradation profile in stages, with either a concomitant or a stepwise release of H(2)O and CO(2), between 108 and 280 °C, depending on the nature of the azolium and substituents. In solution, NHC generation from both [NHC(H)][HCO(3)] salts and NHC-CO(2) adducts could be achieved at room temperature, most likely by a simple solvation effect. Both types of precursors proved efficient for organocatalyzed molecular reactions, including cyanosilylation, benzoin condensation, and transesterification reactions. The catalytic efficiencies of NHC-CO(2) adducts 3 were found to be approximately 3 times higher than those of their [NHC(H)][HCO(3)] counterparts 4.

Imidazol(in)ium hydrogen carbonates as a genuine source of N-heterocyclic carbenes (NHCs): applications to the facile preparation of NHC metal complexes and to NHC-organocatalyzed molecular and macromolecular syntheses.

Anion metathesis of imidazol(in)ium chlorides with KHCO(3) afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO(3)]. In solution, these compounds were found to be in equilibrium with their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene (NHC)-CO(2) adducts. The [NHC(H)][HCO(3)] salts were next shown to behave as masked NHCs, allowing for the NHC moiety to be readily transferred to both organic and organometallic substrates, without the need for dry and oxygen-free conditions. In addition, such [NHC(H)][HCO(3)] precursors were successfully investigated as precatalysts in two selected organocatalyzed reactions of molecular chemistry and polymer synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of d,l-lactide, respectively. The generation of NHCs from [NHC(H)][HCO(3)] precursors occurred via the formal loss of H(2)CO(3)via a concerted low energy pathway, as substantiated by Density Functional Theory (DFT) calculations.

N-heterocyclic carbene-stabilized gold nanoparticles and their assembly into 3D superlattices.

The facile one-phase synthesis of N-heterocyclic carbene-stabilized gold nanoparticles (NHC-AuNP) by reduction of NHC-gold(I) complexes and their self-assembly into 3D superlattices is presented.

Perfluoropentaphenylborole: a new approach to Lewis acidic, electron-deficient compounds.

Zirconocene is the key: A new synthetic method, which utilizes zirconocene-mediated coupling of alkynes, has been developed for the preparation of a new class of highly Lewis acidic boroles (see scheme). Such compounds hold potential for applications in catalysis and the field of electron-deficient organic materials.

Rearrangement of biaryl monoaminocarbenes via concerted asynchronous insertion into aromatic C-H bonds.

The biphenyl and binaphthyl diisopropylaminocarbenes were found to be only transient species that spontaneously and quantitatively rearrange into the corresponding aminofluorenes. DFT calculations confirm that these insertion reactions of aminocarbenes into proximal aromatic C-H bonds require only a moderate energy barrier and support a concerted, strongly asynchronous, mechanism dominated by C arom-->C carbene proton transfer.

Transient palladadiphosphanylcarbenes: singlet carbenes with an "inverse" electronic configuration (p(pi)2 instead of sigma2) and unusual transannular metal-carbene interactions (piC-->Pd donation and sigmaPd-->C back-donation).

Upon treatment with [PdCl(allyl)]2, asymmetrically substituted alpha,alpha'-diphosphanyl diazo compounds eliminate dinitrogen to afford C-chlorodiphosphanylmethanide complexes in high yields. In the presence of a chloride-abstracting agent, such as sodium tetraphenylborate, the C-chlorodiphosphanylmethanide complexes react with pyridine and trimethylphosphine, readily affording the corresponding nitrogen and phosphorus ylides. The postulated intermediate in this process, namely palladadiphosphanylcarbenes, could not be spectroscopically characterized, but their transient formation was chemically supported further by a Lewis base exchange reaction between pyridine and 4-dimethylaminopyridine. This hypothesis has also been substantiated by computing the corresponding dissociation energy using two model systems featuring methyl groups at the phosphorus. Of particular interest, density functional theory calculations reveal that these palladadiphosphanylcarbenes have a singlet ground state with an "inverse" ppi2 electronic configuration and a distorted geometry associated with unusual transannular metal-carbene interactions (piC-->Pd donation and sigmaPd-->C back-donation).

An unusual norcaradiene/tropylium rearrangement from a persistent amino-phosphonio-carbene.

An amino-phosphonio-carbene featuring a bromobiphenyl backbone was prepared and spectroscopically characterized at low temperature. This carbene was found to readily rearrange upon warm up, affording an original tricyclic phospholium derivative, presumably via a norcaradiene/tropylium isomerization.

Cyclic C-amino phosphorus ylides as a source of bidentate heteroditopic ligands (phosphine/aminocarbene) for transition metals.

In contrast with most of their congeners, acyclic and cyclic C-amino phosphorus ylides behave as genuine carbene and phosphine transfer agents for transition metal centers. They allow the facile synthesis of a variety of metal complexes that feature heteroditopic ligands, such as 1,6-(phosphine)(aminocarbene) systems with a biphenyl backbone.