Zero-valent transition metal complexes

In preparation for research involving the synthesis of zero-valent transition metal complexes, new apparatus and techniques have been developed which allow otherwise complex and tedious procedures to be performed entirely within a glovebox. The following discussion will focus on the assembly and application of those new systems which provide capabilities potentially useful in many areas of research. No attempt will be made to review standard glovebox... 

Use. The oligomerization of olefins has generally been carried out with zero-valent transition metal complexes (mononuclear catalysts) and usually leads to an array of dienes see 1, 259). Schrauzer et at.1 of the Shell Development Co. reasoned that a dinuclear catalyst such as ZnfCo(CO).,J2 in which the two cobalt centers are connected close to each other will lead to new transition state formation from which different products can form. As a model, they examined the dimerization of nor-hornadiene and with the new catalyst obtained in almost quantitative yield a single dimer, m.p. 65-65.6°, shown unequivocably by elemental analysis (C14H16), infrared, nuclear magnetic resonance, and mass spectrometry to have the structure (2). 

Oxidative addition of transition metal-hydride and transition metal-carhon bonds to zero-valent transition metal complexes provides convenient method for preparation of homo- and heterodinuclear organometallic complexes. Oxidative addition of iron-hydride to zero-valent platinum complex giving Fe-Pt heterodinuclear complexes was demonstrated hy the reaction of HFe[Si(OMe)3](CO)3(/c -dppe) with zero-valent platinum complex such as Pt(C2H4)3 or Pt( 1,5-cod)2 giving eventually heterodinuclear ethyl or cyclooctenyl complex (Scheme 3.86) [175]. The resulting heterodinuclear structure is stahihzed hy the bridging dppe ligand and the siloxo moiety. 

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

The metal vapor technique, in which a metal is vaporized from a resistively heated tungsten container under high vacuum and is cocondensed with a potential ligand at -125 to -196°C, had proven useful in the synthesis of a variety of unusual low-valent transition metal complexes (67-71). With lanthanide metals, this method not only has generated low oxidation state species, but it has also provided the opportunity to study zero-valent lanthanide chemistry on an atomic/molecular basis for the first time. These studies have been important in identifying new directions in organolanthanide chemistry. 

Basically, we have shown thus far that we can produce a particular type of zero-valent transition metal atom. It matters little whether we call this an active zero-valent metal atom, a nacent metal, or highly excited reactive metal. The important fact remains that we may use this metal atom for (a) the catalytic formation of biphenyl, and (b) for the formation of various arene 7r-complexes. The difference is the reaction conditions and general environment surrounding its creation. We can now proceed further into the development of this concept, ultimately involving polymerization. 

It was later reported that the yields and nature e.g., density, molecular weight and terminal vinyl content) could be altered by varying hydrogen pressure in the oxygenated dibenzenechromium catalyst. This type of catalysis is not considered to be catalysis at the atomic level although dibenzenechromium arene 7r-complex was employed. The seemingly essential role of freshly prepared zero-valent transition metal is not apparent. 

The use of zero-valent transition metal (mainly nickel and cobalt) complexes as promoters of the homo-Diels-Alder reaction has been an important development. These catalysts allow 1,4-dienes to react with nonactivated alkenes and alkynes, broadening the scope of the homo-... 

Oxidation of trifluorophosphine by halogens is well known (282) and although corresponding reactions with alkyl or aryl fluorophosphines have received only little attention, it has become apparent that oxidation to the pentavalent phosphorus fluorides can be brought about by a wide variety of reagents. In certain cases the reducing property of the fluoro-phosphine has been utilized in the synthesis of zero-valent transition metal fluorophosphine complexes (Section IX). 

Exchange reactions of aromatic compounds catalyzed by base are also well-documented. The generation of benzyne intermediates by loss of HX (or DX) from haloaromatics in the presence of a strong base such as NH2 (or NDi) is responsible for H—D exchange. Complexation of aromatics to zero-valent transition metals to give complexes such as 7i-C6H6Cr(CO)3 enhances the acidity of the protons of the complexed aromatic and permits base-catalyzed exchange. 

Such a bell-like dependence of Wsp on the surface density of transition metal ions has also been observed in other catalytic reactions (hydroformylation, oxidation, polymerization, etc.), and is probably one of the specific features of catalysis by immobilized metal complexes. While there is no well-founded explanation of the rising branch of the plot, the diminishing trend may be coimected with the formation and fiirther growth of low- and/or zero-valent transition metal ion associations, diminishing the catalytic efficiency. Active centers of immobilized catalysts are localized on the boimdaries of cluster-like substances with stabilization by their electron systems. 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

The diethyl ester of phenylphosphonous acid (diethoxyphenyl-phosphine) provides an easy pathway to relatively stable telrakis complexes of zero- and low-valent transition metals.1,2 Anhydrous metal halides serve as the metal source for the complexes, avoiding the necessity of inconvenient starting materials such as nickel carbonyl. The nickel(O) complex is formed by reaction with the phosphonite in ethanol with the addition of sodium tetrahydroborate, relatively stable dihydridoiron(l I) and hydridocobalt(I) complexes are obtained. 

Such complexes have been referred to as nitrene complexes but, strictly, nitrenes are RN species and while zero valent NR compounds have been reported, none has been isolated. Imido compounds are commonly found in transition metal complexes with oxidation states 3 and above. The high capacity for electron donation by the imido group does of course, like that of O2-, act to stabilize high oxidation states prime examples of this are Os(NBu )4 vs. 0s04 and (Bu N)3MnCl vs. 03MnCl. 

Consideration of the chemical nature of zero-valent lanthanide metals raises some intriguing questions. The stability of zero oxidation state transition metal complexes depends in large part on the capacity of the metal to transfer its excess electron density back to the ligands via backbonding. Given the limited radial extension of the 4/orbitals (8, 9),... 

Intermolecular oxidative addition of H—C usually involves activated H—C bonds. The weak acid HCN reacts with transition-metal complexes e.g., HCN and NiL lead to the hydride complexes HNi(CN)Lj (L = various phosphorus ligands). The versatile complex IrCl(CO)(PPh3)j adds HCN cleanly in CH Clj at RT to form HIr(CN)(Cl(PPhj)2. The zero-valent complexes Pt(PPhj) or Pt(PPh3)3 also add HCN to yield HPt(CN)(PPh3)j. Reactions of HMNp(dmpe)j (M = Fe, Ru, Os Np = 2-naphthyl dmpe = Me PCH CH PMej) with HCN and terminal acetylenes give HMR(dmpe)2 that contain new M—C bonds (R = — CN, — CjR ) . 

However, there are many stable rare-earth compounds that are low-valent. The rare-earth elements are also early transition metals, in which d-electrons are in the valence shell. Since transition metal complexes in general exist in diverse oxidation states in which the electronic configuration is traditionally expressed as [NG]d s° with n>0 (NG being Ar, Kr or Xe), by analogy, there are a priori no reasons why R°, R and R complexes should not exist. Considering for instance the adjacent ions Hf " " and La which have the same electronic configuration ([Xe]5d 6s°), there are a few compounds of Hf (Fryzuk et al., 1996) and, as we shall see later, several perfectly characterised La complexes precisely in this electronic configuration. Until now, monovalent (R ) molecular compounds have only been found in the case of scandium, and there are several rare earths that can form zero-valent (R°) complexes. 

Butadiene dimerisation is very sensitive to catalysis by zero-valent nickel complexes, which can direct the reaction towards 1,2-cycloaddition or (4-1-4) cycloaddition, through a bis-7r-allyl intermediate, with small amounts of Diels-Alder product . Larger quantities of the latter were obtained in other cases . This and other examples of activation of olefins by transition metal complexes have been associated with excitation of the coordinated 7r-system . 

Nowadays, the preparation of well-defined materials from synthetic polypeptides is feasible due to the development of initiators that allow living NCA polymerization. One strategy involves the use of transition metal complexes as active species to control addition of NCA monomers to polymer chain-ends. This approach leads to substantial advances in controlled NCA polymerization. Highly effective zero-valent nickel and cobalt initiators e.g. Ni(bipy)(COD) or Co(PMc3)4] were developed by Deming. They allow the... 

Chaudret and coworkers have demonstrated the use of low-valent transition metal olefin complexes as a very clean source for the preparation of nanostruc-tured mono- and bimetallic colloids (Co, Ni, Ru, Pd, Pt, CoPt, CoRh, and RuPt). Syntheses were carried out in the presence of suitable stabilizers using CO or Hj as reducing agents at room or slightly elevated temperature. A number of nanoparticulate metal oxide systems have also been successfully developed by this method. " Olefin complexes are similar to metal carbonyl complex, except the metal is in either low or zero oxidation state. The most commonly used ligands are 1,5-cyclooctadiene (COD), 1,3,5-cyclooctatriene (COT), dibenzylidene acetone (DBA), and cyclooctenyl (CgHjj). 

The route to carbene initiation for systems catalyzed solely by transition metal salts (55, 54), or their combinations with Lewis acids such as A1C13 (55), is not well established. Nevertheless, some evidence suggests reduction of the metal by the olefinic substrate (55). Zero-valent (56) and hexavalent (57, 55) tungsten complexes that promote metathesis when activated by UV radiation are the least-understood metathesis systems. 

Hydrides of Pt(II) are the most numerous of any transition metal hydride group. In addition to the presence of the hydride ligand, the complexes invariably have a coordinated phosphine, and synthetic routes to these compounds using both hydridic and protonic reagents have been reported (I). The pure complexes are usually both air stable and kinetically inert. The purpose of this chapter is to show the diversity of hydrides that can be obtained from protonation reactions on zero-valent and di-valent triphenylphosphine platinum compounds, and to rationalize the type and nature of the product formed from the character of the acid HX. 

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