Complexes precursor metal

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. 

Syntheses in which a reaction of a mononuclear metal complex precursor gives a tethered metal cluster are rare an early example is the formation of a tetrairidium carbonyl on a phosphine-fimctionaUzed polymer [17]. 

Based on the molecular design of precursor metal complexes, the solvent-free controlled thermolysis of metal complexes may cause the thermal reduction and simultaneous attachment of organic moiety on the growing metal nuclei and give us a solution of the defects of ordinary... 

According to the general migratory-insertion mechanism proposed by Cossee,58 chain propagation (4.105) is a two-step process in which the precursor metal reagent (I) forms an intermediate alkene complex (II) that subsequently rearranges to the insertion product (III),... 

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

Ylides other than acceptor-substituted diazomethanes have only occasionally been used as carbene-complex precursors. lodonium ylides (PhI=CZ Z ) [1017,1050-1056], sulfonium ylides [673], sulfoxonium ylides [1057] and thiophenium ylides [1058,1059] react with electrophilic transition metal complexes to yield intermediates capable of undergoing C-H or N-H insertions and olefin cyclopropanations. 

Many of these systems employ charged polymers or polyelectrolytes that confer on them particular properties due to the existence of electrical charges in the polymer structure. Oyama and Anson [14,15] introduced polyelectrolytes at electrode surfaces by using poly(vinylpiridine), PVP, and poly-(acrylonitrile) to coordinate metal complexes via the pyridines or nitrile groups pending from the polymer backbone. Thomas Meyer s group at North Carolina [16, 17[ also employed poly(vinylpyridine) to coordinate Ru, Os, Re and other transition-metal complexes by generating an open coordination site on the precursor-metal complex. 

In addition to the previously mentioned most common methods some less frequently used reactions have also led to NHC complexes. In all these cases certain requirements for the metal, the complex precursor or the NHC itself, have limited the approach and have prevented broader applications. 

Generally, the preparation of these species is achieved by (a) coordination of preformed diynes to transition metal centers, and (b) coupling reactions of the alkynyl ligand in mono-alkynyl metal complex precursor. 

B. Coupling Reactions of the Alkynyl Ligand in Mono-Alkynyl Metal Complex Precursors... 

Metal complexes with M-heterocyclic carbene ligands were known long before the first stable NHCs were isolated. Wanzlick [5] and Ofele [6] demonstrated as early as 1968 that NHC complexes can be obtained by in situ deprotonation of azolium salts in the presence of a suitable metal complex without prior isolation of the free NHC ligand (Fig. 1). In these cases a ligand of the metal complex precursor (acetate or hydride) acted as a base for the deprotonation of the imidazolium cation. This method has been successfully transferred to other metal precursors containing basic ligands like [Pd(OAc)2] [97] and [(cod)lr(p-OR)2lr(cod)] [98, 99]. Alternatively, an external base such as NaOAc, KOf-Bu or MHMDS (M = Li, Na, K) can be added for the deprotonation of the azolium salt [100]. In general, the in situ deprotonation of azolium salts appears as the most attractive method for the preparation of NHC complexes as it does not require the isolation of the reactive free carbene or its enetetramine dimer. 

The chalcogenide precursors possess many talents. Apart from forming the chalcogenide ions, they also form complexes with metal ions. As noted at the beginning of this section, and ignoring the distinction between ion-by-ion and hydroxide cluster mechanisms treated previonsly, CD processes can be divided according to two basic mechanisms participation of free snlphide ions (the... 

The most straightforward route to heteroaldehyde and heteroketone complexes is the substitution of a heterocarbonyl compound for another coordinated ligand. This method is naturally restricted to heteroaldehydes and heteroketones stable in the uncoordinated form, i.e., is usually restricted to thioketones and a few stable seleno- and tellurocarbonyl compounds.141718,27118 In most cases, metal carbonyls or solvent complexes of metal carbonyls were used as the complex precursors. The photochemically or thermally induced loss of the ligand to be replaced is followed by coordination of the heterocarbonyl compound [Eq. (3)]. 

Instead of metal carbonyls, a variety of other complexes can also be used as complex precursors. [MC14]2 anions (M = Pd, Pt) were found to react readily with p,p -disubstituted thiobenzophenones to form a mixture of the bis(thiobenzophenone) complexes [MCbJV-S= C(C6H4R-p)2 2] (R = H, Me, OMe, NMe2) and the orthometallated chloride-bridged dimers. Cleavage of the dimers by triphenylphosphine afforded the monomeric orthometallated complexes ll.130... 

The reductive decomposition of thiocyanato complexes should be applicable to the electrodeposition of other metal sulfides. We have tried this with Pd2, Co2+, Ni2+, Zn2+ and In3+.I8 While thin films of PdS, CoS and NiS could be successfully electrodeposited, other metal sulfides such as ZnS and In2S3 could not be obtained. This is an interesting series of results when we think of the softness (hardness) of these metals as acid. TC coordinates with its sof basic S atom to soft acidic Cd2+ and Pd2+, while hard acidic In3+ only permits coordination with hard basic N atom to form an isothiocyanato-complex. Other metals are at the borderline accepting coordination of both S and N. Because reduction of TC is catalyzed by a central metal,75,76) such ligand reduction may result in the formation of metal sulfides only for thiocyanato-complexes. The difference in bahavior among Co2+, Ni2+ and Zn2+ could be reasoned as the consequence of efficient catalysis of the electron transfer reaction by the transition metals. Such trends fit nicely with the previous findings by electrochemical analyses. 7) It is therefore understood that the chemical structure of the active species is decisive to the film formation. Thus, designing such molecular precursors which are chemically stable but can be electrochemically decomposed to metal sulfides should broaden the possibilities of electrochemical thin film synthesis. 

Formazans or their precursors, tetrazolium salts, have been used in dye-bleach systems. Certain formazans102 such as (37) can complex with metal ions to give dye images with improved light stability. 

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