Juan C. Flores

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.

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.

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)

Polymerizations of olefins and diolefins catalyzed by monocyclopentadienyltitanium complexes containing a (dimethylamino)ethyl substituent and comparison with ansa‐zirconocene systems

{[2-(dimethylamino)ethyl]cyclopentadienyl}titanium trichloride (Cp N TiCl 3 , 1) was activated with methylaluminoxane (MAO) to catalyze polymerizations of ethylene (E), propylene (P), ethylidene norbornene (ENB), vinylcyclohexene (VCH), and 1,4-hexadiene (HD). The dependence of homopolymerization activity (A) of 1/MAO on olefin concentration ([M] n ) is n = 2.0 ± 0.5 for E and n = 1.8 ± 0.2 for P. The value of n is 2.4 ± 0.2 for CpTiCl 3 /MAO catalysis of ethylene polymerization; this system does not polymerize propylene. 1/MAO catalyzes HD polymerization at one-tenth of A H for 1-hexene, probably because of chelation effects in the HD case. The copolymerization of E and P has reactivity ratios of r E = 6.4 and r P = 0.29 at 20°C, and r E r P = 1.9, which suggests 1/MAO may be a multisite catalyst. The copolymerization activity of CpTiCl 3 /MAO is 50 times smaller than that of Cp N TiCl 3 /MAO. Terpolymerization of E/P/ENB has A of 10 5 g of polymer/(mol of Ti h), incorporates up to 14 mol % (∼40 wt %) of ENB, and high MWs of 1 to 3 x 10 5 . All of these parameters are surprisingly insensitive to the ENB concentration. The E/P/VCH terpolymerization has comparable A value of (1.3 ± 0.3) x 10 5 g/(mol of Ti h). The incorporation of VCH in terpolymer increases with increasing [VCH]. Terpolymerization with HD occurs at about one-third of the A of either ENB or VCH; the product HD-EPDM is low in molecular weight and contains less than 4% of HD. These terpolymerization results are compared with those obtained previously for three zirconocene precursors: rac-ethylenebis(1-η 5 -indenyl) dichlorozirconium (6), rac-(dimethylsilylene)bis(1-η 5 -indenyl) dichlorozirconium (7), and ethylenebis(9-η 5 -fluorenyl) dichlorozirconium (8). The last compound is a particularly poor terpolymerization catalyst; it incorporates very little VCH or HD and no ENB at all. 7/MAO is a better catalyst for E/P/VCH terpolymerization, while 6/MAO is superior in E/P/HD terpolymerization.

Effects of cerium oxide nanoparticles to fish and mammalian cell lines: An assessment of cytotoxicity and methodology

Two cerium oxide nanoparticles (CeO(2) NPs) and one micro-sized CeO(2) particle were thoroughly characterized in their pristine form, in water and in cell culture medium. The particles were tested for cytotoxicity to the H4IIE rat hepatoma cell line or the RTG-2 rainbow trout gonadal cell line by means of four standard cytotoxicity assays. Nominal concentrations were verified by inductively coupled plasma mass spectrometry (ICP-MS) and methods were assessed for their suitability to detect reliably adverse effects due to particle exposure. All three particles showed aggregation in water and media. In the H4IIE cell line, the MTT cytotoxicity test revealed that negative effects could be observed for the CeO(2) NPs after 24h and for all particles after 72h of exposure, making the effects size, concentration and time dependent. No negative effect for the concentrations tested was detected for the remaining three assays and the RTG-2 cell line, making the MTT assay and the H4IIE cell line an appropriate system to assess adverse effects of CeO(2) NPs. A verification of the nominal concentration through ICP-MS revealed that there was a discrepancy between nominal and measured concentration depending on concentration and particle tested. Interferences of particles with assays were found to be present and need to be taken into consideration.

Binuclear monoindenyl–titanium(IV) complexes. Synthesis and styrene polymerization catalysis

Abstract The first binuclear monoindenyl–titanium(IV) complexes have been synthesized by the stepwise reaction of [1,2-C 2 H 4 (1-Ind) 2 ]Li 2 with 2 equiv of ClTi(O- i -Pr) 3 , giving a 1:1 rac and meso mixture of 1,2-C 2 H 4 [1-IndTi(O- i -Pr) 3 ] 2 ( 2 ), followed by an in situ chlorination reaction with CH 3 C(O)Cl in ethyl ether to form 1,2-C 2 H 4 [1-IndTi(O- i -Pr)Cl 2 ] 2 ( 3 ). Further reaction of 3 with CH 3 C(O)Cl in CH 2 Cl 2 afforded 1,2-C 2 H 4 [1-IndTiCl 3 ] ( 4 ). Styrene polymerization studies with methylaluminoxane (MAO) as cocatalyst demonstrated 3 and 4 to be highly syndiospecific catalyst precursors, although their activities were one order of magnitude lower than for the reference mononuclear compound IndTiCl 3 ( 1 ).

Dicyclopentadienyl-titanium and -zirconium complexes as catalysts for hydrogenation of olefins

We thank the Comision Interministerial de Ciencia y Tecnologia (Project PB89-0208) for financial support and Repsol Quimica for a Fellowship to J.C.F.

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.

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.