Cyclometallated Compounds

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... 

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-... 

In cyclometallated compounds, the oxidation processes are displaced to less positive potentials because of the presence of the anionic ligands. Such processes... 

The hexanuclear cyclometallated compound 61 (Scheme 1 and Table 1) shows five reduction processes (Table 2). The first one, which occurs at a potential very close to the first reduction of the model compound [(bpyljRufdppnH)], is revcrs-... 

The organolithium compounds described here have proved to be valuable in the synthesis of a wide range of cyclometallated compounds, many of which are of current interest.7 The precise site of lithiation in these reagents has been determined from investigations of the air-stable products that they form with Me3SiCl and Me3SnCl.8... 

It is well known that nitrogen-containing ligands are readily metallated by paHadium(II). The first reaction of this type was described in 1965 by Cope et al.1 for azobenzene. Since then numerous examples of similar reactions have been described in the literature.2 These cyclometallated compounds have been shown to be reactive intermediates in organic synthesis, especially in the... 

Under the conditions leading to RuH(Cl)(L)(arene) complexes, the derivative 105 gives the cyclometallated compound 106 (55) [Eq (10)]. The bidentate complex 108 is formed from the evolution of the methyl derivative 49 in solution (Scheme 6). The reaction is consistent with ethylene... 

According to molecular orbital calculations, the main difference between bis(terpyridyl) complexes and cyclometallated ones lies in the tail of the wave functions with mainly metal character. In the case of cyclometallated compounds. 

Treatment of a benzene solution of the sterically crowded bis(aryloxide)zirconium dibenzyl complex 5 with BIC F5)3 leads to the facile formation of the corresponding zwitterionic species 6 (Scheme 3).44 The molecular structure of 6 features a three-legged piano-stool geometry about Zr with the 7/ -bound benzyl group and the tightly associated benzyl borate anion through r/ -arcne coordination this complex is stable in solution in the presence of propylene and phenylpropyne. In contrast, treatment of the 2,6-di-Buc-substituted aryloxide dibenzyl derivative 7 with B(C6Fs)3 forms the cyclometallated compound 8 after elimination of toluene (Scheme 3) 45 compound 8 was structurally characterized. 

A rather different chiral box is the trimeric cyclometallated compound reported by Ruttiman et al. [84], This triangular box (Figure 28) has a hydrophobic cavity in which a guest molecule such as acetonitrile fits perfectly. The chirality of this box... 

The reactions were rapid requiring s.f. techniques. Large negative values of the entropy and volume of activation were obtained for the reaction of TMTU with the cyclometallated compound, and were interpreted to indicate an associative substitution mechanism. The same conclusion had been reached in an earlier study of the substitution of the water molecule from the zwitterionic-Pt compound as shown in Scheme 5. 220... 

The field has been reviewed by several authors, from various points of view [23-31], and one monograph has been written [32], We give here, therefore, only an overview of those aspects which are important for photochemical and photophysical properties of cyclometallated compounds. 

From the photochemical and photophysical points of view, only those complexes are generally important, where the coordinating carbon atom is part of an aromatic system. Therefore, the numerous complexes where the coordinating carbon is sp3-hybridized will not be considered in detail. Many transition metals are capable of forming cyclometallated compounds with aromatic ligands. So far, however, only the groups 8, 9 and 10 of the second and the third row have been explored from a photochemical point of view. The metals considered here are therefore Ru/Os, Rh/Ir, and Pd/Pt. 

Pd(II), Pt(II), and Pt(IV) Complexes. A large number of cyclometallated compounds of these two elements have been described and also considered in several review articles. A recent one [23], just about Pd compounds, comprises 232 references. We will concentrate on bis-cyclometallated compounds with aromatic nitrogen and with aromatic C-donor atoms. 

In addition to the (CANAC)-type ligands, several examples of (NACAN) pincer ligands have been used in cyclometallated compounds [53,54]. None of these complexes seem to have interesting photochemical or photophysical properties due to the fact that all of them contain either sp3 hybridized N-ligands, six-membered metallacycles, or other ligating atoms, like halides, phosphines, and so on. 

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 ... 

A strong trans effect of the coordinated sp2 C-ligand is observed through elongation of the trans M-L bond. All multiple cyclometallated compounds have a strong tendency to avoid trans configuration of two M-C bonds. 

Detailed investigations of highly resolved spectra in single crystals at low temperature were carried out for bis-cyclometallated Rh(III) complexes [75]. They show that also in this species the lowest energy transitions are mainly MLCT in character, but they also indicate that the classification scheme for transitions in coordination compounds is sometimes no longer really adequate in the cyclometallated compounds. 

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