Klaus Seevogel

Nanoscale colloidal metals and alloys stabilized by solvents and surfactants Preparation and use as catalyst precursors

Solvent-stabilized organosols of the early transition metal series, e.g. Ti, Zr, Nb, and Mn, may be prepared by the reduction of the THF adducts or thioether solutions of the corresponding metal halides with K[BEt3H]. Mono- and bimetallic organosols of Group 6–11 metals stabilized by tetraalkylammonium halides may be formed either by the reduction of the metal salts using NR4 hydrotriorganoborates or conventional agents, e.g. H2 or HCO2H, after the pretreatment of the metal salts with NR4X. The chemical reduction of transition metal salts in the presence of hydrophilic surfactants provides straightforward access to nanostructured mono- and bimetallic hydrosols. This synthesis can be performed even in water. Mono- and bimetallic nanoparticles stabilized by lipophilic or hydrophilic surfactants of the cationic, anionic or nonionic type serve as precursors for heterogeneous metal colloid catalysts effective for the hydrogenation and oxidation of organic substrates. Bimetallic precursors, e.g. PtRh, have a synergic effect on the catalytic activity. A comparison of catalytic results and CO chemisorption experiments has revealed that the protecting surfactants still cover the nanoparticle surface after adsorption on supports, which markedly improves the lifetime of the catalysts. Chiral protecting agents may induce enantioselectivity in metal colloid catalysts.

Substituenteneinflüsse auf die basizität von phosphorliganden in R3P-Ni(CO)3-komplexen

Abstract According to Tolman, the χ values of 157 P ligands can be determined exactly by FT-IR spectroscopy ( FT χ values). By replacing the phenyl groups of triphenylphosphine step-by-step by alkyl- or alkoxy-substituents, we found that Tolmans additivity rule is not strictly valid. The deviations determined from additivity give a pattern. Its shape is controlled by the DO/ACC-character of the atoms in the α-position of phosphorus. By this pattern, substituents with nearly the same FT χ i , parameters (-OCH 3 /-SPh; -t-Bu/-SiMe 3 ) but different influence on reactivity (activation/inhibition) can be recognized.

Transition metal allyls : IV. The (η3-allyl)2M complexes of nickel, palladium and platinum: reaction with tertiary phosphines

Abstract A series of 1 : 1 adducts have been prepared by treating the bis-η3-allyl complexes of nickel, palladium and platinum with tertiary phosphines. Investigations of their structure in solution as well as in the crystal have shown that both 18-electron (η3-allyl)2ML complexes as well as 16-electron (η1-allyl)-(η3-allyl)ML complexes may be formed.

Transition metal allyls ☆: III. The (η3-allyl)2M complexes of nickel, palladium and platinum: structural considerations

Abstract The structures of a series of (η 3 -allyl) 2 M complexes of nickel, palladium and platinum have been investigated with the help of 13 C NMR, 1 H NMR and Raman spectroscopy. The crystal structure of (η 3 -cyclooctatrienyl) 2 Ni has been determined by X-ray methods; the two nickel-bonded η 3 -allyl groups are mutually trans .

Ir- und 1H-NMR-spektroskopische untersuchungen an zirkon und hafniumallylen : I. Übergangsmetallallyle

Abstract A general scheme, utilizing both energetic and kinetic data, has been derived for the classification of allylmetals. On the basis of IR spectral data, three categories 1–3 may be formulated. These may be subdivided into a and b on the basis of their 1 H NMR spectra and hence 6 different types may be distinguished π-, σ-, π- and σ-dynamic or static. The spectra of allyl-zirconium and -hafnium compounds (MA 4 , COTMA 2 , COTMMet 2 , COTMCrot 2 , M = Zr, Hf; A = allyl; Met = methylallyl; Crot = crotyl; COT = cyclooctatetraene) are discussed and the compounds classified according to the above scheme.

SELECTIVE OXIDATION OF GLUCOSE ON BISMUTH-PROMOTED PD-PT/C CATALYSTS PREPARED FROM NOCT4CL-STABILIZED PD-PT COLLOIDS

Charcoal-supported Pd-Pt catalysts based on Pd-Pt/NOct4Cl colloidal alloys have superior activity and selectivity in the oxidation of glucose to gluconic acid compared with industrial heterogeneous Pd-Pt catalysts. According to transmission electron microscopy, X-ray diffraction/Debye function analysis, X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure analysis the chemical coreduction of PdCl2 and PtCl2 in the appropriate ratio with NOct4BEt3H yielded the alloyed Pd-Pt colloids in organic solvents. They are screened by the lipophilic NOct4Cl surfactant layer from coagulation and poisoning. TEM showed colloids of particle sizes in the range from 1.5 to 3 nm.

Dynamics of metal carbonyls on the infrared time scale : coalescence of CO stretching vibrational bands.

Abstract The variable-temperature IR spectra of (η 4 -norbornadiene)Fe(CO) 3 and related complexes reveal a coalescence phenomenon involving the CO stretching vibrational bands. This is interpreted in terms of fast CO site exchange, rapid enough to equalize the CO force field parameters. A preliminary line shape analysis yields E a ≅ 1.5 kcal mol −1 .

η3-Allyl complexes of tungsten—I. Preparation and reactions of [η3-C3H5)3WCl]2

Abstract An improved synthesis of (η 3 -C 3 H 5 ) 4 W from W(OPh) 6 , and allylmagnesium bromide is reported. The further reaction with HX (X = Cl, Br) or iodine leads to [(η 3 -C 3 H 5 ) 3 WX] 2 compounds, which in turn react with donor ligands to form the adducts (η 3 -C 3 H 5 ) 3 W(L)X. Treatment of (η 3 -C 3 H 5 ) 3 W(PMe 3 )Cl with RMgX (R = Me, Et) produces the tungsten-alkyl compounds (η 3 -C 3 H 5 ) 3 W(PMe 3 )R.

Tetramethylcyclobutadien-palladiumdichlorid durch ligandentransfer aus dem [(CH3)4C4]AlCl3-σ-komplex

Abstract The [(CH3)4C4]AlCl3 σ-complex (II), which is formed by cyclodimerisation of 2-butyne and AlCl3, reacts with (C6H5CN)2PdCl2 (I) by ligand transfer, giving the [(CH3)4C4]PdCl2 π-complex (III) are reported. The structure determination is based on chemical degradations and in particular on spectroscopic analyses.

Synthese und Struktur von (tmeda)Ni(H2C=CHCOOCH3)2

Abstract Ni(C 2 H 4 ) 3 , Ni(cdt), or Ni(cod) 2 react with tmeda and acrylic acid methyl ester, methyl vinyl ether, and acrylonitrile in ether to afford (tmeda)Ni(H 2 CCHCOOCH 3 ) 2 ( 3 , orange-red crystals), (tmeda)Ni(H 2 CCHCOCH 3 ) 2 ( 4 , red crystals), and (tmeda)Ni(H 2 CCHCN) 2 ( 5 , yellow precipitate) in up to 90% yield. These complexes are also formed by interaction of the homoleptic bis(alkene) nickel(0) complexes Ni(H 2 CCHCOOCH 3 2 , Ni(H 2 CCHCOCH 3 2 , and Ni(H 2 CCHCN) 2 with tmeda, and by the reaction of (tmeda)Ni(CH 3 ) 2 with the alkenes in ether at −30°C or above, under reductive elimination of ethane. Solid 3 is stable to about 110°C, whereas 4 decomposes slowly at 20°C; decomposition of 5 occurs at 136°C. The IR, 1 H and 13 C NMR spectra are reported. In addition, 3 has been characterized by an X-ray diffraction study.