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The importance of ligand structural diversity in palladium catalyzed polymerization
No full textLo scopo principale della presente tesi di Dottorato è stato lo sviluppo di nuovi catalizzatori omogenei per la copolimerizzazione dell'etilene con monomeri vinilici polari, come gli esteri acrilici, al fine di ottenere poliolefine funzionalizzate contenenti una quantità controllata di monomero polare.
Il progetto di ricerca ha riguardato la sintesi e caratterizzazione di complessi di palladio(II) con nuovi leganti azotati non simmetrici di complessità via via crescente, appartenenti alla classe delle alfa-diimmine o a quella delle piridilimmine. Il comportamento catalitico dei complessi di palladio così ottenuti è stato studiato approfonditamente per la copolimerizzazione etilene/metil acrilato. Alcuni dei complessi sono stati testati quali catalizzatori anche per altre reazioni, come la copolimerizzazione CO/vinil arene e la doppia alcossicarbonilazione di alcheni, dimostrando la versatilità dei catalizzatori di palladio con leganti azotati.The main goal of the PhD research was the development of novel homogeneous catalysts for copolymerization of ethylene with polar vinyl monomers, such as acrylic esters, in order to obtain functionalized polyolefins with a controlled content of the inserted polar monomer.
The project focused on the synthesis and characterization of palladium(II) complexes with novel non symmetric nitrogen-donor ligands of increasing complexity, and belonging either to the class of alpha-diimines or to the group of the pyridylimines. The catalytic behaviour of the synthesized palladium(II) complexes in the ethylene/methyl acrylate copolymerization was investigated in detail. Some of the catalysts were also applied to other reactions such as the CO/vinyl arene copolymerization and the bis-alkoxycarbonylation of alkenes, demonstrating the versatility of palladium catalysts with N-donor ligands
Olefin Dimerization and Isomerization Catalyzed by Pyridylidene Amide Palladium Complexes
Full text linkA series of cationic palladium complexes [Pd(N^N')Me(NCMe)]+ was synthesized, comprising three different N^N'-bidentate coordinating pyridyl-pyridylidene amide (PYA) ligands with different electronic and structural properties depending on the PYA position (ortho-, meta-, and para-PYA). Structural investigation in solution revealed cis/trans isomeric ratios that correlate with the donor properties of the PYA ligand, with highest cis ratios for the complex having the most donating ortho-PYA ligand and lowest ratios for that with the weakest donor para-PYA system. The catalytic activity of the cationic complexes [Pd(N^N')Me(NCMe)]+ in alkene insertion and dimerization showed a strong correlation with the ligand setting. While complexes bearing more electron-donating meta- and ortho-PYA ligands produced butenes within 60 and 30 min respectively, the para-PYA complex was much slower and only reached 50% conversion of ethylene within 2 h. Likewise, insertion of methyl acrylate as polar monomer was more efficient with stronger donor PYA units, reaching a 32% ratio of methyl acrylate vs ethylene insertion. Mechanistic investigations about the ethylene insertion allowed to detect, for the first time, by NMR spectroscopy both cis- and trans-Pd-ethyl intermediates and, furthermore, revealed a trans-to-cis isomerization of the Pd–ethyl resting state as the rate-limiting step for inducing ethylene conversion. These PYA palladium complexes induce rapid double bond isomerization of terminal to internal alkenes through a chain walking process, which prevents both polymerization and also the conversion of higher olefins, leading selectively to ethylene dimerization
In situ generated Pd(0) nanoparticles stabilized by bis(aryl)acenaphthenequinone diimines as catalysts for aminocarbonylation reactions in water
Full text linkAminocarbonylation of aryl iodides with aromatic and aliphatic amines, leading to formation of the corresponding amides, was efficiently carried out in water under 1 atm of CO using palladium nanoparticles (Pd NPs) formed in situ from [PdCl2(Ar2-BIAN)] complexes. The role of Ar2-BIAN ligands in the stabilization of Pd NPs was evidenced. The nature of the catalytically active species was confirmed by poisoning experiments, which highlighted that the catalyst is actually in the form of Pd NPs rather than soluble palladium complexes. In the aminocarbonylation of iodobenzene with substituted anilines good yields of amides were obtained, although the activity was depleted by the presence of substituents in the ortho positions of the aniline. On the other hand, in the reaction with aliphatic amines α-ketoamides were formed in addition to the amides. The selectivity towards α-ketoamides was increased by increasing the CO pressure to 10 atm, at equimolar amounts of PhI and amine. Pd NPs were successfully recovered after the catalytic reaction and recycled in five subsequent runs with only a marginal loss of activity after the fourth cycle
Coordination chemistry to palladium(II) of pyridylbenzamidine ligands and the related reactivity with ethylene
No full textThe coordination chemistry to palladium of three pyridylbenzamidines (N-N') was investigated in detail. The studied pyridylbenzamidines are featured by the bulky 2,6-diisopropylphenyl substituent at the azomethine nitrogen atom of the amidine unit, and differ in the substitution either at the amino atom, which bears a 2-pyridyl or a 6-methyl-2-pyridyl group, or at the bridging N-atom that, in one case, is substituted by a methyl group, leading to a molecule reported herein for the first time. The accurate NMR characterization of the free ligands points out the presence of dynamic phenomena in solution, due to the interconversion of several possible isomers, including tautomers. The coordination chemistry to Pd(II) is studied using both [Pd(cod)(CH3)Cl] and [Pd(cod)(CH3)(CH3CN)][PF6] as metal precursor. Depending on the palladium precursor and on the pyridylbenzamidine, different coordination compounds are obtained, demonstrating the capability of these molecules to act both as mono- and bidentate ligands. For the pyridylbenzamidine substituted with the methyl group on the bridging N-atom, the C-H activation of one of the isopropyl groups is observed with the formation of a six-membered palladacycle and methane. None of the isolated complexes generates active catalysts either for ethylene homopolymerization or for ethylene/methyl acrylate copolymerization. When reacting with ethylene, the complexes lead to the formation of propylene and the inactive dicationic [Pd(N-N')2][PF6]2 complex
Nonsymmetric alpha-diimines in Pd-catalyzed ehtene/polar monomer copolymerization
No full textOne of the major unsolved problems in polymer chemistry is represented by the synthesis of functionalized polyolefines, that are currently produced through radical processes, a technology that suffers from high energy consumption, low cost efficiency and poor control over the macromolecule structure.1 The direct, controlled, homogeneously catalyzed copolymerization of ethylene with polar vinyl monomers represents a sustainable technology to overcome these limits. The main catalytic systems reported in the literature are based on Pd(II) compounds with either -diimines or phosphino-sulfonate ligands.2 Their productivity is thus far low from the values required for an industrial application, and better performing catalysts are strongly needed.
With this aim we are studying Pd(II) complexes with new nonsymmetric -diimines (Ar,Ar'-BIAN) featured by an acenaphthene skeleton and one aryl ring substituted in meta positions with electron-withdrawing groups, while the other ring bears electron-releasing substituents on the ortho positions (Figure up). The related monocationic complexes, [Pd(CH3)(Ar,Ar'-BIAN)(L)][PF6], that include both acetonitrile and dimethyl sulfoxide derivatives, have been applied as precatalysts in the ethylene/methyl acrylate copolymerization under mild conditions of temperature and pressure showing a remarkably different catalytic behaviour depending on the nature of the L ligand (Figure below).3 While the catalyst originated from the acetonitrile precursor was found to be less active than the dmso counterpart and to deactivate within 16 h of reaction, that obtained from the dmso derivative was still active after 48 h achieving a productivity of 350 g P/g Pd. The catalysts with the nonsymmetric Ar,Ar'-BIAN showed a productivity twice as high as that of the catalysts with the corresponding symmetric ligands together with a higher incorporation of the polar monomer into the catalytic product. The latter is a mixture of ethylene oligomers and ethylene/methyl acrylate cooligomers, having the polar monomer at the end of the branches. Detailed investigation on the studied catalytic system will be presented.
1A. Nakamura, S. Ito and K. Nozaki, Chem. Rev., 2009, 109, 5215.
2 a) L.K. Johnson, S. Mecking and M. Brookhart, J. Am. Chem. Soc., 1996, 118, 267; b) E. Drent, R. van Dijk, R. van Ginkel B. van Oort and R.I. Pugh, Chem. Commun., 2002, 744.
3A. Meduri, T. Montini, F. Ragaini, P. Fornasiero, E. Zangrando and B. Milani, ChemCatChem published on the web, DOI: 10.1002/cctc.201200520
Palladium alkyl complexes with a formazanate ligand: synthesis, structure and reactivity
Full text linkPalladium(ii) complexes with a bidentate, anionic formazanate ligand are described. Attempts to prepare mono(formazanate) palladium alkyl complexes often leads to the homoleptic bis(formazanate) complex, which shows rich electrochemistry due to the redox-active nature of the ligands. Performing salt metathesis between the precursor [Pd(COD)(CH3)Cl] and the potassium salt of the ligand in the presence of tetrabutylammonium chloride yields a square planar mono(formazanate) palladate complex through coordination of chloride anion. Ligand exchange allows binding of unsaturated molecules and evaluation of the reactivity of the Pd-CH3 fragment. Using this approach, insertion reactions of CO, isocyanide and methyl acrylate into the Pd-CH3 bond are demonstrated
Pyridylbenzamidines: versatile ligands for palladium(II)
No full textRecently, we have demonstrated that in copolymerization reactions palladium complexes with nonsymmetric-diimines generate more productive catalysts than those having the corresponding symmetric counterparts, highlighting the positive effect of an unbalance in the steric and electronic properties of the N-donor atoms.1
With the aim to verify the more general validity of this principle, we have now studied bidentate N-donor ligands belonging to the family of pyridylbenzamidines (Figure). While the two molecules with R1 = H were reported in literature,2 the N-methyl substituted derivative is new. The NMR characterization of the molecules pointed out the presence of dynamic phenomena in solution, due to the interconversion of several isomers, including tautomers.
The coordination chemistry to Pd(II) was studied on [Pd(cod)(CH3)Cl] and [Pd(cod)(CH3)(CH3CN)][PF6]: depending on the precursor and on the ligand, different coordination compounds were obtained, indicating that these molecules can act both as mono- and bidentate ligands. None of the isolated complexes generated active catalysts either for ethylene homopolymerization or for ethylene/methyl acrylate copolymerization. The deactivation pathway was unraveled
Dinuclear Organometallic Pd(II) Complexes with Iminopyridines: the Importance of Methyl Groups
No full textIminopyridines have been used as ancillary ligands for polymerization catalysts based on late-transition-metal complexes1 and as scaffolds for polydentate ligands to obtain dinuclear complexes.2
For several years we have been studying palladium complexes as catalysts for both CO/vinyl arene and ethylene/methyl acrylate copolymerization reactions.3 With the aim to develop better performing catalysts, we have now started to investigate dinuclear palladium complexes based on polydentate ligands (N-N'-N'-N) featured by two iminopyridyl compartments held together by a phenyl or a tetramethyl-substituted phenyl ring and being relatively positioned in either para or ortho with respect to each other. The iminopyridyl moieties derive by the condensation reaction of the proper diamine with either pyridine-2-aldehyde or 2-acetyl pyridine.
Palladium complexes of general formula [Pd(Me)Cl]2[N-N'-N'-N] and {[Pd(Me)(MeCN)]2[N-N'-N'-N]}{PF6}2 have been synthesized. Their characterization in solution by NMR spectroscopy is not so trivial and thanks to the methyl groups on both the phenylene linker and the iminopyridyl compartment it was possible to unambiguously recognize the different isomers present. The solid state structure was solved for a neutral complex having the bidentate moieties in para position (fig. left), and for a dicationic derivative with the bidentate compartments in ortho to each other (Fig. right). The latter was shown to have a peculiar structure with a methylene group bridging the two palladium ions, and resulting from the neutral derivative through the C-H activation of one Pd-Me group and the dissociation of the second methyl from the palladium coordination sphere as a methane molecule
Palladium-Catalyzed Ethylene/Methyl Acrylate Copolymerization: Moving from the Acenaphthene to the Phenanthrene Skeleton of α-Diimine Ligands
Full text linkThe development of efficient homogeneous catalysts for the synthesis of functionalized polyolefins is a challenging topic. Palladium(II) complexes with -diimine ligands having a phenanthrene skeleton and 2,6-disubstituted aryl rings (Ar-BIP) were synthesized, characterized and tested as precatalysts in the copolymerization of ethylene with methyl acrylate. The direct comparison with analogous complexes having the corresponding -diimines with an acenaphthene skeleton (Ar-BIAN) was performed. X-ray characterization in solid state and NMR analysis in solution of both neutral, [Pd(Ar-BIP)(CH3)Cl], and monocationic [Pd(Ar-BIP)(CH3)(NCCH3)][PF6] complexes, indicate that the Ar-BIP ligands have a higher Lewis basicity and are more strongly coordinated to the metal center than the Ar-BIAN counterparts. Therefore, the Pd-(Ar-BIP) cationic complexes can be regarded as electron-rich metal cations. In addition, they create a higher steric congestion around palladium than Ar-BIAN, regardless of the substituents on the aryl rings. The monocationic species generate active catalysts for the ethylene/methyl acrylate copolymerization leading to copolymers with Mn values up to 37000 and a content of polar monomer of 5.3 mol %. The detailed study of the catalytic behavior points out that Pd-(Ar-BIP) catalysts show a good affinity for the polar monomer, a good thermal stability and favor the cleavage of the catalyst resting state, leading to copolymer with Mw values higher than that of the macromolecules produced with the corresponding Pd-(Ar-BIAN) under the same reaction conditions. NMR characterization of the produced copolymers points out that the polar monomer is inserted both at the end of the branches and into the main chain, with a more selective enchainment than that achieved when the copolymerization is carried out in dichloromethane. In situ NMR investigations allowed us to detect relevant intermediates of the catalytic cycle and shed light on the nature of possible deactivation species
Pd-catalyzed ethylene/methyl acrylate cooligomerization: the effect of a new nonsymmetric alfa-diimine with the 1,4-diazabutadiene skeleton
Full text linkPalladium(II) complexes with a nonsymmetric bis(aryl-imino)diazabutadiene ligand (ArDAB) have been synthesized and characterized. The new ligand is featured by one aryl ring substituted in ortho positions with methyl groups and the other ring bearing a trifluoromethyl group on the meta positions, leading to a subtle steric and electronic difference on the two nitrogen-donor atoms. This peculiar substitution makes the direct synthesis of the ligand not feasible, and the relevant Pd(II) complex, [Pd(CH3)Cl(ArDAB)], is directly obtained through a template reaction. The corresponding cationic complexes with either acetonitrile or dimethyl sulfoxide have been synthesized and characterized. The X-ray crystal structure of a palladium complex with an -diimine and dimethyl sulfoxide is reported. The monocationic complexes have been tested as precatalysts in the ethylene/methyl acrylate cooligomerization under mild reaction conditions of temperature and pressure. The comparison with the catalytic behavior of the precatalysts with the corresponding nonsymmetric ArBIAN ligand indicated that the active species with the currently investigated ligand is more productive, leading to the formation of ethylene oligomers and ethylene/methyl acrylate cooligomers
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