Bonding cyclometallation

Oxidative addition of H—C bonds to transition metals, which also is referred to as an activation of the H—C bond, is of scientific and technological importance. " (See 1.10.2 for activation of H—C bonds by metal-atom vapors.) Intramolecular oxidative addition of H—C bonds (cyclometallation) occurs, but organic functional... 

As in the case of 2-substituted pyridine adducts, activation of an aromatic C—H bond is observed when the adducts of 6-thbipy and of 6-Bnbipy are heated under reflux in aqueous media. A cyclic dimer (1) with bridging N,C ligands is obtained in the first case [24], while cyclometallated derivatives [Au(N, N,C)C1] (2) are formed in... 

The reaction between [PdCl2(MeCN)2] and ethylene diamine-A, 3-ethylguanine yields the analogous C8-cyclometalated compound [PdCl(en-Et-A-C< )]+ 28 (Pd-C bond length 1.974 A) (56) (Fig. 29). Again, proton transfer to the adjacent N7 accompanies C8-metalation and this is observed downfield in the NMR spectrum... 

Treatment of [IrCl(CO)2(/ -toluidene)] with azine phosphines of type Z, -PPh2CH2C( Bu) =N-N=C(Q)R, Q = H, Me, R = an organic group, activates aryl, heterocyclic, alkenyl, or aliphatic C—H bonds to give cyclometalated Ir111 hydrides.339... 

The current mechanistic understanding of these reductive cyclization processes is largely conjecture. Stepwise oxidative addition, migratory insertion, and reductive elimination (see Scheme 26) is a widely proposed mechanism. However, other mechanisms - such as initial cyclometallation - are to afford a rhodacyclopentadiene followed by either oxidative addition to a rhodium(v) intermediate or (perhaps more likely) bond metathesis with an additional molecule of silane (Scheme 28). 

Studies of intramolecular C-H bond activation reactions at Pt(II) (and Pd(II)) leading to cyclometalated products have typically found that these reactions are facilitated by ancillary ligands that can easily dissociate (20,21). In the absence of any weakly bonded ligand, unusually... 

Reactions that involve significant bond formation in the ratedetermining step are in general expected to exhibit large and negative volumes of activation. This was for instance found for a series of cyclometallation reactions of benzylidenebenzylamines, -anilines, and -propylamine with palladium acetate in toluene and acetic acid solution (171, 172). The cyclometallated compounds are formed via C-H electrophilic bond activation to produce different types of metalla-... 

The proposed reaction mechanism is as follows (Scheme 16.83). Zinc metal reduces Ni(II) species to Ni(0). A nickelacyclopentadiene may be produced via coordination of two molecules of propiolates and regioselective head-to-head oxidative cyclometallation. Coordination and subsequent insertion of an allene into the Ni(II)-carbon bond give rise to a nickelacycloheptadiene intermediate. Finally, a benzene derivative is produced via reductive elimination followed by isomerization. 

These bulky terphenolate ligands have proved useful in the prevention of cyclometallation of the ort/zo-substituents146 in early-transition-metal phenolates. The bulky raeta-substituents prevent the rotation of the ortho-phenyl ring from becoming coplanar with the central phenoxide core, which is a requirement for C-H bond activation. 

Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

A model accounting for the observed sense of absolute stereoinduction is based upon the coordination mode revealed in the crystal structure of the cyclometallated C,0-benzoate complex [280]. It is postulated that aldehyde binding by the a-allyl haptomer occurs such that the allyl moiety is placed between the naphthyl and phenyl moieties of the hgand, allowing the aldehyde to reside in a more open enviromnent. In the favored mode of addition, the aldehyde C-H bond projects into... 

The preparation of carbonyl-lr—NHC complexes (Scheme 3.1) and the study of their average CO-stretching frequencies [7], have provided some of the earliest experimental information on the electron-donor power of NHCs, quantified in terms of Tolman s electronic parameter [8]. The same method was later used to assess the electronic effects in a family of sterically demanding and rigid N-heterocyclic carbenes derived from bis-oxazolines [9]. The high electron-donor power of NHCs should favor oxidative addition involving the C—H bonds of their N-substituents, particularly because these substituents project towards the metal rather than away, as in phosphines. Indeed, NHCs have produced a number of unusual cyclometallation processes, some of which have led to electron-deficient... 

Later in 1965, Chart and Davidson [2] reported the first example of cyclometal-lation of an sp C—H bond in [Ru(dmpe)2] (3) dmpe = dimethyl phosphinoethane. These authors found not only that this complex spontaneously cyclometaUates at the phosphorus methyl groups to produce complex [Ru(H)(CH2P(Me)CH2CH2PM 62)(dmpe)] (4 see Scheme 13.4) (a later examination by Cotton and coworkers [9] of this compound provided crystallographic evidence that the cyclometalated form of [Ru(dmpe)2] is in fact a dimer (5) of the type shown in Scheme 13.3), but also that the system reacts with free naphthalene via the oxidative addition of a C—H bond to the zero-valent rathenium center to produce complex [cis-Ru(H)(2-naphthyl)(dmpe)2] (6). This species was in equilibrium with the r-coordinated naphthalene ruthenium complex [Ru(naphthalene)(dmpe)2] (7) (Scheme 13.4). 

Mdssbauer spectra of bonding and structure in, 15 184-187 reactions with diborane, 16 213 stabilization of, 5 17, 18-19 cyanates, 17 297, 298 cyanide complexes of, 8 143-144 cyclometallated bipyridine complex, 30 76 diazene complexes, 27 231-232 dinitrogen complexes, 27 215, 217 diphosphine complexes of, 14 208-219 dithiocarbamates, 23 253-254 -1,2-dithiolene complexes, 22 323-327 hydrogen bonding, 22 327 halide complexes with phosphine, etc., 6 25 hexaflouride, structure, 27 104 hydride complexes, 20 235, 248-281, see also Transition metal-hydride complexes... 

Also cyclometallated complexes involving an additional C-M bond with a remote carbon atom are described (Equation 5) <2003JOM(688)112>. 

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