Oxidative-Addition Reactions Cyclometallation

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. 

Most cyclometallated compounds of Pt and Pd contain the metals in the + 2 oxidation state (d8 configuration) with its strong tendency for planar coordination. Other oxidation states, notably +4, are also possible. A series of Pt(IV)-cyclometallated complexes have been obtained [55] from Pt(II) compounds through oxidative addition reactions. Details of the photochemical and photophysical properties of these systems are discussed later in this review. Here we restrict ourselves to the discussion of the structural aspects of the Pt(IV) and, as far as applicable, to Pd(IV) compounds. 

Cyclometallated Pt(IV) complexes have, so far always, been obtained through oxidative addition reactions to cyclometallated Pt(II) complexes. In most cases bis-cyclometallated compounds have been used. The general reaction scheme is ... 

Pt(II) and Pd(II) form planar, bis-cyclometallated species only through exchange with another metal, for example, Li. The former undergoes facile thermal, or photochemically induced, oxidative addition reactions, yielding bis-cyclometallated Pt(IV) complexes. 

Oxidative addition reactions offer an alternative pathway for the synthesis of nickel cyclometallated complexes. Many derivatives with NGN or PGP pincer ligands have been prepared from the corresponding halogenated precursors and Ni(cod)2. These intramolecular oxidative addition reactions take place under very mild conditions and are usually preferred over cyclometallation when the suitable precursors are available. The formation of small amounts of Ni(iii) byproducts is not rare in the case of the NGN ligands. The synthesis of cyclometallated complexes containing bidentate n c1 51,107,108 q pi3,230 polydentate ligands by oxidative... 

Intramolecular photoinduced oxidative addition reactions can also occur. For example, when the platinum(0) complex Pt(C2H4)(PPh3)2 is photolyzed at 280 nm, the product is a cyclometalated platinum(II) complex formed by intramolecular oxidative addition of the ortho carbon-hydrogen bond, followed by ethylene insertion into the intmnediate platinum(II) hydride ... 

The overall reaction is best viewed as intramolecular oxidative addition of the C(l)—H bond to the Rh(I) center, causing cyclometalation (25), followed by reductive elimination of an enamine from the Rh(III) intermediate accompanied by allylic transposition. Notably, the allylamine ligand in the initial Rh(I) complex as well as the Rh(III) intermediate has an s-trans conformation with respect to the N—C(l) and C(2)—C(3) bonds, allowing the overall suprafacial 1,3-hydrogen shift to produce the is-configured enamine product. 

Synthetic routes to compounds containing M-C o bonds are fairly obvious. Substitution of, e.g., Cl by CH, can be effected by treatment with LiCH3 or CH3MgBr. A number of reaction types mentioned in Chapter 9 - oxidative addition, reductive elimination, insertion and cyclometallation (Sections 9.6 and 9.7) - have their uses in preparative routes to M-C bonds. The formation of organo-compounds of the lanthanides and actinides is an area of growing interest. Preparative methods are similar to those for other ER species where E is of relatively low electronegativity, e.g. ... 

Cyclometallation (also called oxidative coupling) is a rather special case of oxidative addition. In this reaction, two unsaturated molecules, X=Y and X =Y, add to the same metal atom M. One of the X—Y bonds and one of the X —Y bonds are broken, and new M X and M -Y bonds form. However, a new Y—Y bond also forms, and the overall result is a cyclometaUated compound (Figure 3.7a). As in oxidative addition, the oxidation state of the metal center increases by 2. Cyclometallation is common with alkynes (Figure 3.7b), as well as with alkenes activated by electron-withdrawing groups [21]. 

Cyclometallation refers to a process in which unsaturated moieties form a metallacyclic compound. It is sometimes categorized under oxidative additions, but we prefer this separate listing. Examples of the process are presented in Fig. 4.31. Metal complexes which actually have displayed these reactions are M = L Ni for reaction a, M = Cp2Ti for reactions b and c, M = Ta for d, and M = (RO W for e. The latter examples involving metal-carbon multiple bonds, have only been observed for early transition metal complexes, the same ones mentioned under the a-elimination heading. 

The acidification of H2 may also be involved in hydrogenase action, where H2 is beheved to bind to an Fe(II) center. Isotope exchange between H2 and D2O is catalyzed by the enzyme see Nickel Enzymes Cofactors Nickel Models of Protein Active Sites Iron-Sulfur Proteins). Similar isotope exchange can also occur in H2 complexes. Oxidative addition to give a classical dihydride is also a common reaction. [W(H2)(CO)3(PCy3)2] is in equilibrium with about 20% of the dihydride in solution. This can lead to subsequent hydrogenolysis of M-C bonds as in the case of a cyclometallated phenylpyridine complex of Ir(III). ... 

Cyclometalation/ one of the earliest reactions involving CH oxidative addition, has been reviewed recently. Apart from the Murai reaction, cyclometalation has also been incorporated into a number of other catalytic reactions of interest in organic synthesis." ... 

The cyclometallation reactions are best rationalized by a mechanism involving as key steps the oxidative addition of a C—H bond to a metal followed by reductive elimination, e.g., in the reaction of Mn(Me)(CO)5 and PhCH2NMc2 (100°C)] shown in Scheme 1. ... 

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