Metal as precursors

Note that the equilibrium of the following type of reaction was postulated by Dislich (13) in 1971 to explain the ultrahomogeneity of the final ceramic product obtained in the sol-gel process that employed a mixture of alkoxides with different metals as precursors ... 

Glassy Metals as Precursors of Catalytically Active Materials... 

Carbon nanotubes can also be produced in thermal plasma by dissociation of carbon compounds, for example by tetrachloroethylene (TCE Harbec et al., 2005a,b). The process has been conducted by TCE injection into the nozzle of a 100 kW DC non-transferred thermal plasma torch. Metal catalysts stimulate the process of formation of nanotubes. In particular, erosion of tungsten electrodes provides nanoparticles of the metal as precursors for the carbon nanotubes (Harbec et al., 2005a,b). A catalytic stimulation of the process can also be achieved by using electrodes made of a nickel alloy and by using ferrocene dissolved in the... 

Niobium dioxyfluoride, Nb02F, and tantalum dioxyfluoride, Ta02F, can be successfully used as precursors for the synthesis of many oxyfluoride compounds of niobium and tantalum. Systematic investigations performed on MeC>2F - M2CO3 systems, in which Me = Nb or Ta and M = alkali metal, provided necessary information on optimal synthesis procedures and imparted some conformity on the mechanism of the chemical interaction between the components. 

Even acetophenone reacts with the magnesium compound 17 (R1 = R2 = H) to yield the w-diastereomer 18 with 90 % de 22 24. The structure of the metal-organic precursor and, as well, of the major product was determined by an X-ray crystal structure analysis23. 

Metal carbonyls form a large and important group of compounds which are used widely in the chemical industry, particularly in the preparation of heterogeneous catalysts and as precursors in CVD and metallo-organic CVD (MOCVD). 

The chemistry has also been extended to related formamidinato metal hydrides of aluminum and gallium. In the case of aluminum, LiAlH4 and AlH3(NMe3) served as metal hydride precursors. Scheme 39 summarizes the synthetic routes. The complexes thus formed are thermally very stable. °... 

In certain cases, free guanidines can also serve as precursors to Group 4 metal guanidinate complexes. The bis(guanidinato) bis(benzyl)zirconium complex [Pr NHC(NPr )2]2Zr(CH2Ph)2 was obtained by addition of 2 equivalents of... 

Several patents dealing with the use of volatile metal amidinate complexes in MOCVD or ALD processes have appeared in the literature.The use of volatile amidinato complexes of Al, Ga, and In in the chemical vapor deposition of the respective nitrides has been reported. For example, [PhC(NPh)2]2GaMe was prepared in 68% yield from GaMes and N,N -diphenylbenzamidine in toluene. Various samples of this and related complexes could be heated to 600 °C in N2 to give GaN. A series of homoleptic metal amidinates of the general type [MIRCfNROilnl (R = Me, Bu R = Pr, BuO has been prepared for the transition metals Ti, V, Mn, Fe, Co, Ni, Cu, Ag, and La. The types of products are summarized in Scheme 226. The new compounds were found to have properties well-suited for use as precursors for atomic layer deposition (ALD) of thin films. 

Fig. 29. The pseudo-cryptand 110 can be transformed with boric acid and a metal hydroxide to the corresponding charge neutral pseudocryptates 111-116. 117 serves as precursor ligand for Li ...

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

The molybdenum and tunsten diphenylacetylene compounds have been chemically useful primarily as precursors to the quadruple metal-metal bonded dimers [M(Por)]2, formed by solid-state vacuum pyrolysis reactions. However. Mo(TTP)()/"-PhC CPh) is also a useful substrate in atom-transfer reactions, reacting with Sx or Cp2TiS i to form Mo(TTP)=S. The reaction can be reversed by treatment of Mo(TTP)=S with PPh (which removes sulfur as PhxP=S) and PhC CPh. The order of preference for ligand binding to molybdenum 11) has been established to be PPh < PhC CPh < 4-picoline. ... 

The Rh complexes (cod)Rh[ jt-OSi(0 Bu)3] 2 and (nbd)Rh[ jt-OSi (0 Bu)3] 2 thermally decompose primarily via loss of HOSi(O Bu)3 and formation of Rh metal particles, rendering them ineffective for use as precursors to Rh/Si/0 materials however, these complexes are potentially useful as sources of Rh nanoparticles or site-isolated Rh species via grafting methods [99]. 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

Supported metal carbonyl clusters are alternatively formed from mononuclear metal complexes by surface-mediated synthesis [5,13] examples are [HIr4(CO)ii] formed from Ir(CO)2(acac) on MgO and Rh CCOlie formed from Rh(CO)2(acac) on y-Al203 [5,12,13]. These syntheses are carried out in the presence of gas-phase CO and in the absence of solvents. Synthesis of metal carbonyl clusters on oxide supports apparently often involves hydroxyl groups or water on the support surface analogous chemistry occurs in solution [ 14]. A synthesis from a mononuclear metal complex precursor is usually characterized by a yield less than that attained as a result of simple adsorption of a preformed metal cluster, and consequently the latter precursors are preferred when the goal is a high yield of the cluster on the support an exception is made when the clusters do not fit into the pores of the support (e.g., a zeolite), and a smaller precursor is needed. 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

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. 

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