Precursor cluster

Reactions of clusters with mononuclear or dinuclear metal complexes frequently provide a method of expanding the metal core nuclearity under controlled conditions. The majority of medium- and high-nuclearity homometallic clusters has been prepared from lower-nuclearity cluster precursors by thermolyses ("heat-it-and-hope ) reactions. This is less true of the heterometallic clusters in this... 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

The PtRu/MgO catalysts containing 1.0 wt% Pt and 1.0 wt% Ru were prepared by slurrying MgO power with a CH2CI2 solution of cluster A which was synthesized by a procedure described elsewhere [3] or a mixed solution of Pt(acac)2 and Ru(acac)3 in toluene. After solvent removal by evacuation, the ligands of adsorbed cluster precursor were eliminated by heating in He flow at 300°C for 2 h while acac ligands of the second precursor were removed in H2 flow at the same conditions. 

IR Evidence for strong adsorption between cluster precursor and MgO surface... 

Both PtRu/MgO catalysts prepared from cluster precursor and organometallic mixture were active for ethylene hydrogenation. The apparent activation energy of the former catalyst obtained from the Arrhenius plot during -40 to -25°C was 5.2 kcal/mol and that of the latter catalyst obtained during -50 to -30°C was 6.0 kcal/mol. The catalytic activity in terms of turn over frequency (TOP) was calculated on the assumption that all metal particles were accessible for reactant gas. Lower TOP of catalyst prepared from cluster A at -40°C, 57.3 x lO" s" was observed probably due to Pt-Ru contribution compared to that prepared from acac precursors. 

Nashner MS, Frenkel Al, Adler DL, Shapley JR, Nuzzo RG. 1997. Structural characterization of carbon supported platinum-ruthenium nanoparticles from the molecular cluster precursor PtRu5(CO)i6. J Am Chem Soc 119 7760. 

Et4N]2[Fe2lr2(CO)i2] cluster precursor, which exhibit a high activity in the synthesis of methanol from CO and H2, were studied by Ir and Fe Mossbauer spectroscopy. The study extends from the precursors via the fresh to the aged catalysts. The presence of iridium in the metallic state as well as the presence of trivalent, divalent and alloyed iron is detected. Representative Ir and Fe Mossbauer spectra are shown in Fig. 7.69. Information about the adsorption on the surface of MgO... 

Supported nanoparticles are related to the idea of starting with polynuclear cluster precursors. While there is no clear line that divides polynuclear clusters from nanoparticles, clusters are generally small, low nuclearity (MnL n = 3-20), structurally well-characterized species approximately 1-2 nm in size. Nanoparticles are larger (>2 nm) and frequently defined by a size distribution rather than a discrete number of atoms and ligands [41]. In the area of catalysis, gold nanopar-... 

Synthesis of Supported Bimetallic Catalysts from Molecular Cluster Precursors... 

Different mixed carbonyl cluster precursors of Ru and Pt having phenyl-containing tin Hgands have recently been proposed for the preparation of Ru PtySnz nanoparticles on mesoporous sihca support These systems show high performance for the production of cyclododecene and 1,4-cyclohexanedimethanol by selective hydrogenation reactions [117]. 

Figure 11. The [Cd1oSe4(SC6H5)16](NMe4)4 cluster precursor used in the synthesis of CdSe and Co2+ CdSe QDs. Note that the thiophenolates have been truncated for clarity, and the molecule contains no sulfides. [Adapted from (89).]...

Complex 2 is a potent cluster precursor. The isolobal relationship (194) between diarylalkynes and the alkylidynes CpW(CO)2(=CR) is now quite well known (195). Since 2 and 3 readily add alkynes, one might reason that alkylidynes CpW(CO)2(=CR) will also add across the M=M bonds of 2 or 3. In fact, this happens, and in quantitative yield for 2, when the product is 73. The yield of 74 in the tungsten reaction is less impressive, but it is obtained in higher yield in the reaction of Cp2W2(CO)4-(fi-rf-RCCR) with CpW(CO)2(=CR). The behavior of the chromium system is quite different, the product being the /x-alkyne species Cp2-W2(CO)4(jU-i72-RCCR). Remarkably, 1 is catalytic for this dimerization (196). 

It is possible to synthesize mixed-metal clusters containing Au(PR3) fragments by treating neutral as well as anionic precursors with either [Au(PR3)]+ or a compound that acts as a source of this cation. In many cases, a [Au(PR3)]+ unit is simply added to the cluster precursor to afford a cationic heteronuclear gold cluster (e.g., 59,62-66). Equation (27)... 

Non-aqueous synthetic methods have recently been used to assemble mesoporous transition metal oxides and sulfides. This approach may afford greater control over the condensation-polymerization chemistry of precursor species and lead to enhanced surface area materials and well ordered structures [38, 39], For the first time, a rational synthesis of mesostructured metal germanium sulfides from the co-assembly of adamantanoid [Ge4S ()]4 cluster precursors was reported [38], Formamide was used as a solvent to co-assemble surfactant and adamantanoid clusters, while M2+/1+ transition metal ions were used to link the clusters (see Fig. 2.2). This produced exceptionally well-ordered mesostructured metal germanium sulfide materials, which could find application in detoxification of heavy metals, sensing of sulfurous vapors and the formation of semiconductor quantum anti-dot devices. 

A facile route into Fe, Ru, Os, and Fe203 nanoparticles has been reported that involves thermolysis of cluster precursors in ionic liquids the nanoparticles are in the range 1.5-2.5 nm, and require no extra stabilizers or capping molecules.43... 

The emphasis of the first two sections was the generation of novel substances. Here we note that there are advantages to the utilization of cluster precursors for solid materials for which conventional syntheses are well established. 

The XPS technique provides identification of oxidation states of metals in supported catalysts, but the determinations are often inexact and require confirmation by other methods. XPS is especially useful for detecting changes in oxidation states of cluster precursors on various oxide supports. For example, the transformation of Rh4(CO)i2 on silica, alumina, MgO, ZnO, and TiOj to yield different surface species such as a raft of Rh(CO)2(OM) or Rh metal aggregates has been inferred from chemical shifts in XPS data (Fig. 5) 39-41). The XPS technique requires ultrahigh vacuum, and instability of the... 

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