Ernesto de Jesús

Catalysts based on palladium dendrimers

This report reviews the role of palladium dendrimers in catalysis as (a) soluble macromolecules for the support of catalysts, that are separable by nanofiltration techniques; (b) ligand-modifiers that can create specific metal nanoenvironments and tune the solubility of the catalyst; (c) spacers for the support of molecular catalysts that can proportionate a more homogeneous-like environment to the supported catalytic sites; and (d) precursors for the synthesis of mono- and bimetallic nanoparticles of controlled size and narrow size distribution. Some examples of catalysis with related metal systems, such as star-shaped molecules or hyperbranched polymers, are also given.

Dendrimer-encapsulated Pd nanoparticles versus palladium acetate as catalytic precursors in the Stille reaction in water

The performance of several palladium precatalysts, namely, palladium(II) acetate, palladium(0) nanoparticles encapsulated into poly(amidoamine) (PAMAM) dendrimers (Pd DENs), and palladium(II)-PAMAM complexes, in the Stille reaction between trichloro(phenyl)stannane and iodoarenes in water is compared. The reactivity of Pd DENs is similar or inferior to that of palladium(II) acetate, although the presence of the dendrimer suppresses the formation of homocoupling products and allows catalyst recycling. It is suggested that the reaction catalyzed by Pd DENs occurs via palladium species which are leached from the nanoparticle but which remain coordinated to the dendritic macromolecule.

Turnover Numbers and Soluble Metal Nanoparticles

The catalytic activity expressed by turnover number (TON) and turnover frequency (TOF) in different fields of catalysis (enzymatic, homogeneous (single‐site), heterogeneous (multi‐site), and nanocatalysis (oligo‐site)) are usually estimated in slightly different ways and with slightly different, yet important meanings. For soluble metal nanoparticles, the ideal is to determine the TON by using the titrated number of active catalytic sites before the catalyst is inactivated. However, in the absence of reliable titration methods it is suggested that TON figures should always be reported as the number of moles of reactants consumed per mol of soluble metal catalyst, and that they should also be corrected by the number of exposed surface atoms by using the metal atom’s magic number approach. Moreover, it is strongly recommended that the TOF should be determined from the slope of plots of turnover numbers versus time, because in various cases the size and shape of the soluble nanoparticles might change dramatically during the reaction. As in organometallic catalysis, in the absence of TON vs. time data, the TOF should be estimated for low substrate conversions.

Highly Stable Water‐Soluble Platinum Nanoparticles Stabilized by Hydrophilic N‐Heterocyclic Carbenes

Controlling the synthesis of stable metal nanoparticles in water is a current challenge in nanochemistry. The strategy presented herein uses sulfonated N-heterocyclic carbene (NHC) ligands to stabilize platinum nanoparticles (PtNPs) in water, under air, for an indefinite time period. The particles were prepared by thermal decomposition of a preformed molecular Pt complex containing the NHC ligand and were then purified by dialysis and characterized by TEM, high-resolution TEM, and spectroscopic techniques. Solid-state NMR studies showed coordination of the carbene ligands to the nanoparticle surface and allowed the determination of a (13)C-(195)Pt coupling constant for the first time in a nanosystem (940 Hz). Additionally, in one case a novel structure was formed in which platinum(II) NHC complexes form a second coordination sphere around the nanoparticle.

Mechanistic Studies on the Pd-Catalyzed Vinylation of Aryl Halides with Vinylalkoxysilanes in Water: The Effect of the Solvent and NaOH Promoter

The mechanism of the Pd-catalyzed vinylation of aryl halides with vinylalkoxysilanes in water has been studied using different catalytic precursors. The NaOH promoter converts the initial vinylalkoxysilane into a highly reactive water-soluble vinylsilanolate species. Similarly, deuterium-labeling experiments have shown that, irrespective of the catalytic precursor used, vinylation occurs exclusively at the CH vinylic functionality via a Heck reaction and not at the C-Si bond via a Hiyama cross-coupling. The involvement of a Heck mechanism is interpreted in terms of the reduced nucleophilicity of the base in water, which disfavors the transmetalation step. The Heck product (β-silylvinylarene) undergoes partial desilylation, with formation of a vinylarene, by three different routes: (a) hydrolytic desilylation by the aqueous solvent (only at high temperature); (b) transmetalation of the silyl olefin on the PdH Heck intermediate followed by reductive elimination of vinylarene; (c) reinsertion of the silyl olefin into the PdH bond of the Heck intermediate followed by β-Si syn-elimination. Both the Hiyama and Heck catalytic cycles and desilylation mechanisms b and c have been computationally evaluated for the [Pd(en)Cl2] precursor in water as solvent. The calculated Gibbs energy barriers support the reinsertion route proposed on the basis of the experimental results.

Titanocene and Zirconocene Complexes containing Dendrimer-Substituted Cyclopentadienyl Ligands − Synthesis and Ethylene Polymerization

This paper describes the synthesis of a series of titanium and zirconium metallocenes bearing one or two first-generation silane dendritic wedges as bulky substituents at their cyclopentadienyl rings. Wedges (R2R′SiCH2CH2)3SiCl [R = R′ = Et (1); R = Ph, R′ = Me (2)] were prepared by hydrosilylation of chlorotrivinylsilane with R2R′SiH. They were reacted with K(C5H5) and, subsequently, with KH to give K[(R2R′SiCH2CH2)3Si(C5H4)] [R = R′ = Et (3); R = Ph, R′ = Me (4)]. The dendronized cyclopentadienides 3 and 4 were the starting materials for preparation of the mixed-ring titanocenes [{(R2R′SiCH2CH2)3SiC5H4}(C5R′′5)TiCl2] [R = R′ = Et, R′′ = H (5), R′′ = Me (6); R = Ph, R′ = Me, R′′ = H (7), R′′ = Me (8)] or the symmetrically substituted metallocenes [{(Ph2MeSiCH2CH2)3SiC5H4}2MCl2] [M = Ti (9), Zr (10)]. Cyclic voltammograms and catalytic behavior of all the new metallocenes in ethylene polymerization, using MAO as a cocatalyst, have been studied and compared to that of related non-dendritic complexes. Polyethylene polydispersities increase with the number of dendritic wedges in the catalyst, while activities decrease. Bimodal molecular weight distributions were clearly observed for the bis-dendritic titanocene 9. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Silane dendrimers containing titanium complexes on their periphery

Abstract Hydrosilylation of 4-allyl-2-methoxyphenol (eugenol) with silane dendrimers of first or second generation and SiMe 2 H ending groups affords new dendrimers with SiMe 2 (CH 2 ) 3 {C 6 H 3 (OMe)(OH)} terminating groups. The reaction of the hidroxy functionalities of these dendrimers with [CpTiCl 3 ] (Cp=η 5 -C 5 H 5 ) provides an effective route for the attachment of cyclopentadienyl titanium complexes to the dendritic periphery. The yields of the prepared dendrimers with SiMe 2 (CH 2 ) 3 {C 6 H 3 (OMe)(OTiCl 2 Cp)} terminating groups are near quantitative. All the dendrimers are characterised by 1 H-, 13 C{ 1 H}-, and 29 Si{ 1 H}-NMR spectroscopy, elemental analysis, and (except for Ti dendrimers) MALDI-TOF mass spectroscopy.

Synthesis and 1H NMR studies of paramagnetic nickel(II) complexes containing bis(pyrazolyl)methane ligands with dendritic substituents.

The substituted bis(pyrazolyl)methane ligands RCH(3,5-Me2pz)2(R=SiMe3, CH2Ph, G1, G2, and G3; Gn=Fréchet-type dendritic wedges of generation n) have been prepared starting from H2C(3,5-Me2pz)2. Reaction of these didentate ligands with [NiBr2(DME)] is a straightforward procedure that allows the synthesis of the nickel(II) complexes [NiBr2{RCH(3,5-Me2pz)2}]. The molecular structure of compound (R=CH2Ph) has been determined by X-ray diffraction studies. The nickel centre coordinates two bromine and two nitrogen atoms in a tetrahedral environment, and the metallacycle Ni(NN)2C adopts a boat conformation with the benzyl group in an axial position. 1H NMR studies have been carried out to characterize these paramagnetic nickel compounds in solution. Valuable information about the disposition of the ligands and dendritic wedges in solution has been obtained thanks to the influence of the paramagnetic centre on the proton resonances.

On the Pd-catalyzed vinylation of aryl halides with tris(alkoxy)vinylsilanes in water.

Pd-catalyzed vinylation of aryl halides with tris(alkoxy)vinylsilanes occurs in aqueous NaOH solution through Heck coupling of the aryl halide with the silyl olefin followed by desilylation at the Pd center or, at high temperatures, hydrolysis of the C-Si bond. Pd-mediated desilylation does not occur via beta-Si elimination under these conditions.

Arylimido niobium(V) complexes: mononuclear and dendritic derivatives

Abstract The compound 4-LiC6H4N(SiMe3)2 reacted in tetrahydrofuran with SiMe3Cl or with silicon-chloride groups located at the periphery of carbosilane dendrimers of first, second, and third generation to afford 4-SiMe3C6H4N(SiMe3)2 (3) and functionalized dendrimers Si(CH2CH2CH2SiMe2C6H4N(SiMe3)2)4 (6), Si(CH2CH2CH2SiMe(CH2CH2CH2SiMe2C6H4N(SiMe3)2)2)4 (7), and Si(CH2CH2CH2SiMe(CH2CH2CH2SiMe(CH2CH2CH2SiMe2C6H4N(SiMe3)2)2)2)4 (8). The N,N-bis(trimethylsilyl)aniline groups of 3 reacted with niobium pentachloride in acetonitrile or [NbCp′Cl4] (Cp′=η5-C5H4SiMe3) in dichloromethane to give imido mononuclear complexes [Nb(NC6H4SiMe3-4)Cl3(CH3CN)2] (4) and [NbCp′(NC6H4SiMe3-4)Cl2] (5), respectively. Reaction of dendrimers 6–8 and [NbCp′Cl4] in CH2Cl2 afforded metallodendrimers [Si(CH2CH2CH2SiMe2C6H4NNbCp′Cl2)4] (9), [Si(CH2CH2CH2SiMe(CH2CH2CH2SiMe2C6H4NNbCp′Cl2)2)4] (10), and [Si(CH2CH2CH2SiMe(CH2CH2CH2SiMe(CH2CH2CH2SiMe2C6H4NNbCp′Cl2)2)2)4] (11), in which the metal moieties are linked to the carbosilane framework through imidometal bonds. The structure of complexes 4 and 5 has been determined by X-ray diffraction methods.