Román Andrés

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.

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)

Allyl derivatives of [{Ti(η5-C5Me5)(μ-O)Cl)}3]: X-ray crystal structure of [{Ti(η5-C5Me5)(μ-O)(CH2CH=CHMe)}3]

Abstract Reactions of [{Ti( η 5 -C 5 Me 5 )( μ -O)Cl} 3 ] ( 1 ) with Grignard reagents, RMgCl (R = allyl (CH 2 CHCH 2 ), crotyl (CH 2 CHCHMe)), in diverse ratios and conditions allow the characterization of the allyl oxotrimers [{Ti( η 5 -C 5 Me 5 )( μ -O)} 3 R n Cl 3− n ] ( n = 3, R = allyl, 2 ; n = 1, R = allyl, 4 ; n = 2, R = allyl, 5 ; n = 3, R = crotyl, 7 ). The complexes [{Ti( η 5 -C 5 Me 5 )( μ -O)} 3 ( π -C 3 H 5 ) 3 ] ( 2 ) and [{Ti( η 5 -C 5 Me 5 )( μ -O)} 3 ( π -C 3 H 5 ) 2 Cl] ( 5 ) undergo thermal rearrangements, in a regio- and stereoselective way, to give the derivatives [{Ti( η 5 -C 5 Me 5 )( μ -O)} 3 ( μ 2 -CH 2 CH(CH 2 CHCH 2 )CH 2 }X]X = allyl ( 3 ) and X = Cl ( 6 ) respectively. These processes involve the migration of one allyl group to the β-carbon of the adjacent allyl ligand and the formation of a 2-allyl-1,3-propanediyl unit bridging two titanium atoms. The crystal structure of [{Ti( η 5 -C 5 Me 5 )( μ -O)} 3 ( σ -CH 2 CHCHMe 3 ] ( 7 ) has been studied by X-ray crystallography and can be described as three Ti( η 5 -C 5 Me 5 )( σ -crotyl) units linked through oxygen bridges forming a nearly planar Ti 3 O 3 ring.

Organotitanium oxides as Lewis acidic supports of metal carbonyl species: [{Ti3(η5-C5Me5)3(µ-O)3Me}{(µ-OC)M(CO)2(η5-C5H5)}2](M = Mo, W)

Reaction of [{Ti(η5-C5Me5)(µ-O)}3(µ3-CH)]1 with [M(η5-C5H5)(CO)3H](M = Mo, W) leads to the formation of Lewis acid carbonyl adducts [{Ti3(η5-C5Me5)3(µ-O)3Me}{(µ-OC)M(CO)2(η5-C5H5)}2]4, 5 showing the transformation of the µ3-methylidyne group into a Me group through the µ-methylene intermediates [{Ti3(η5-C5Me5)3(µ-O)3(µ-CH2)}(µ-OC)M(CO)2(η5-C5H5)]2, 3.

Knight Shift in 13C NMR Resonances Confirms the Coordination of N-Heterocyclic Carbene Ligands to Water-Soluble Palladium Nanoparticles

The coordination of N-heterocyclic carbene (NHC) ligands to the surface of 3.7 nm palladium nanoparticles (PdNPs) can be unambiguously established by observation of Knight shift (KS) in the 13 C resonance of the carbenic carbon. In order to validate this coordination, PdNPs with sizes ranging from 1.3 to 4.8 nm were prepared by thermal decomposition or reduction with CO of a dimethyl NHC PdII complex. NMR studies after 13 CO adsorption established that the KS shifts the 13 C resonances of the chemisorbed molecules several hundreds of ppm to high frequencies only when the particle exceeds a critical size of around 2 nm. Finally, the resonance of a carbenic carbon is reported to be Knight-shifted to 600 ppm for 13 C-labelled NHCs bound to PdNPs of 3.7 nm. The observation of these very broad KS resonances was facilitated by using Car-Purcell-Meiboom-Gill (CPMG) echo train acquisition NMR experiments.

Monitoring the stereoselectivity of biodegradation of chiral polychlorinated biphenyls using electrokinetic chromatography

The authors thank the Comunidad Autonoma de Madrid (Spain) for projects 07 M/0640/1997 and 07 M/0049/1998.

Bifunctional carbosilane dendrons for the immobilization of zirconocene catalysts on silica

A method for the preparation of bifunctional carbosilane dendrons has been developed and used to synthesize first- and second-generation dendrons containing [Cp2MCl2] end-groups (M = Ti, Zr; Cp = cyclopentadienyl) and Si–Cl focal-point bonds. These dendrons have been fully characterized in solution and, in the case of the zirconium derivatives, immobilized on an amorphous silica surface and tested as ethylene polymerization catalysts. The immobilization was quantitative and quite stable under the conditions of the catalysis.

Synthesis of Water-Soluble Palladium Nanoparticles Stabilized by Sulfonated N-Heterocyclic Carbenes

A strategy involving the decomposition of palladium(II) organometallic complexes with sulfonated N-heterocyclic carbene ligands leads to the formation of stable and water-soluble Pd nanoparticles. Three different methodologies (thermal decomposition, reduction under 13 CO atmosphere, and reduction with H2 ) gave particles with different shapes and sizes, ranging from 1.5 to 7 nm. The structures of the organometallic intermediates and organic decomposition products were elucidated by NMR spectroscopy. To check the accessibility of the surface, the nanoparticles were tested as catalysts for the chemoselective hydrogenation of styrene in water. An effect of the particle size on the catalyst activity was observed. The aqueous phase was recycled up to ten times without any precipitation of metallic palladium.