Metal zerovalent form

Mercury(II) forms a series of compounds in which it is bonded to a transition metal to form heteronuclear Hg—M bonds.540,541 The most widely used synthetic routes have been reviewed elsewhere.542-549 Besides heteronuclear bonds M—Hg there are structural elements M—Hg— M, 550 e.g. (31), or cyclic arrangements as in Os3(CO)uHg 3415 or (1/5-MeC5H4)Mn(CO)2Hg 4.416 A trigonal prismatic coordination of mercury has been reported in the green zerovalent mixed metal cluster [Hg Pt(2,6-Me2C6H3NC)fi ]. a... 

Two main synthetic routes are available for the preparation of dithiolenes in the first and most frequently used method, either the free ethylenedithiol or an appropriate salt of the ethylene-dithiolato ligand dianion is reacted with a metal salt to produce anions of the dithiolenes, which may or may not be subsequently oxidized to the neutral species the second one, applied so far only to transition metal dithiolenes, converts vicinal diketones into dithiodiketones and reacts these either with zerovalent metals to form dithiolenes directly or uses metal salts to arrive at cationic species, which are reduced to the neutral dithiolenes either during the reaction or in a subsequent step. 

A fairly efficient synthesis of 1-chloro-l-fluorocyclopropanes under mild conditions is based on the reduction of trichlorofluoromethane by a zerovalent titanium species in the presence of an alkene (Table 6). The metal is formed in the reaction system from titaniura(IV) chloride and lithium aluminum hydride. ... 

During the last 2-3 decades a vast amount of knowledge regarding the preparation and characterization of nanostructured transition metal colloids in the zerovalent form has accumulated [1, 2]. The emphasis has been largely on the development of methods for the control of size and, more recently, even of shape [3]. In the majority of cases application in catalysis was not pursued systematically, i.e., generally only a simple model reaction such as the hydrogenation of cyclohexene was studied. 

In contrast to nanoscaled transition metal colloids in the zerovalent form, less is known regarding the analogous metal oxides, i.e., colloidal MO (M=metal n= 1, 2, 3, etc.). Of course, such species in solid bulk form or as immobilizates on surfaces or in solid carriers have been known for a long time in heterogeneous catalysis [4], in semiconductor technology and other areas [3, 5]. More recently special methods have been developed for their preparation in constrained environments [6], in microemulsions [7], or as one- or two-dimensional nanostructures [5d, 8]. 

Size- and Shape-selective Preparation of Metal Nanopartides in the Zerovalent Form... 

As delineated in the Introduction, numerous research groups have focused on the preparation and application of transition metal nanoparticles in the zerovalent form during the last two decades but, parallel to this development, research in the area of nanosized metal oxides has also increased. The areas of application of metal oxide nanoparticles range from catalysis [4] to semiconductors (e.g., ZnO, ZnS, CdSe) [5]. In the area of heterogeneous catalysis, many different preparative methods have been described [4], Other methods are based on the hydrolysis of transition metal salts in microemulsions [3-8], These and other approaches have been reviewed and will not be specifically treated here. Rather, the main focus is on the author s own research. 

Evidence for the intermediate formation of nitrone species during the carbonylation of nitroarenes in e s-cyclooctene as solvent catalysed by Ru3(CO)i2 have been obtained [14], Moreover, zerovalent palladium species with nitrogen donor ligands have been shown to be active catalysts in the reductive carbonylation of organic nitro derivatives [41]. The hypothesis that an intermediate having the olefinic double bond coordinated to the metal is formed during the catalytic cycle is supported by the steric effect that has been observed in the case of p,p -dimethyl-o-nitrostyrene (7f) as substrate. Moreover, such an intermediate could explain why a pentaatomic indole nucleus is preferentially formed, even when a conjugated double bond is present in the olefinic chain ... 

CFg-PFa and iron pentacarbonyl. Metallic nickel (formed by decarboxylation of nickel oxalate) has been shown to react directly at 60 °C with several fluorophosphines, including CFs-PF and (CF8)2PF, to give the corresponding zerovalent nickel complexes, e.g. Ni(CFj PF2)4 analogous complexes of zerovalent platinum, e.g. Pt[(CF3)jPF]4 (which can also be obtained from potassium tetrachloroplatinite and fluorobistrifluoromethylphosphine ), have been prepared from platinum(n) chloride and PFj, CFj-PFj, and (CF3)2PF at 60°C. = ... 

Because they have low vapor pressures, transition metals cannot be loaded by direct adsorption, but their adsorption can be mediated by transient organo-metallic complexes formed between zerovalent metal atoms and solvent molecules. This is the basis of the solvated metal atom dispersion (SMAD) method developed by Klabunde and Tanaka [72]. Metal vapors condensed in Hquid hydrocarbons at low temperatures form weak complexes that are easily decomposed even below room temperature. Microporous supports impregnated with solutions of metal complexes at low temperatures are warmed up to decompose the complex and liberate zerovalent metal atoms which nucleate into clusters. Preparation of Ni- and Co-clusters in HY and HZSM-5 was reported [72]. In the same way, Nazar et al. [64] condensed iron and cobalt vapors in a slurry of dehydrated NaY zeolite in toluene at -120 °C, then the mixture was rotated at-78°C. The bis-toluene complex thus formed and adsorbed in the zeoUte was decomposed by warming to room temperature yielding clusters small enough to fit into supercages. 

This study could be extended to the synthesis of iron nanoparticles. Using Fe[N(SiMe3)2]2 as precursor and a mixture of HDA and oleic acid, spherical nanoparticles are initially formed as in the case of cobalt. However, a thermal treatment at 150 °C in the presence of H2 leads to coalescence of the particles into cubic particles of 7 nm side length. Furthermore, these particles self-organize into cubic super-structures (cubes of cubes Fig. ) [79]. The nanoparticles are very air-sensitive but consist of zerovalent iron as evidenced by Mossbauer spectroscopy. The fact that the spherical particles present at the early stage of the reaction coalesce into rods in the case of cobalt and cubes in the case of iron is attributed to the crystal structure of the metal particles hep for cobalt, bcc for iron. 

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

A3-Pyrrolinones have also been obtained from metal-mediated cyclooligomerization processes in which concomitant hydrolytic or carbonyl insertion occurs. For example, tert-butyl isocyanide is converted in aqueous methanol by zerovalent nickel compounds e.g., Ni(t-BuNC)4, Ni(CO)4, into a di(alkylamino)-A3-pyrrolinone in moderate yield (Scheme 34). The reaction takes a different course in anhydrous methanol in which a di-tert-butylamino)ethylene derivative is formed, albeit in poor yield (Scheme 34).62... 

The metal may be reduced to its zerovalent state and form small metallic particles on the support. These metal particles may take part in the reaction, especially in catalytic hydrogenation. 

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