Cyclometalation, review

In connection with the activation of C—H bonds the cyclometalation has become a very general reaction, intensively investigated by Kaesz and coworkers4 and reviewed by Bruce.5 In transition metal complexes an organic ligand reacts with the transition metal resulting in the formation of a metal-... 

The Specialists Periodical Reports Organometallic Chemistry, Vol. 2 (1972)-Vol. 7 (1977) review the literature on the subject of cyclometalation and contain over 350 references. Other references include A. J. Carty, Organomet. Chem. Rev. Sect. A, 7, 191, (1972) M. I. Bruce and B. L. Goodall, Chemistry of Azo, Hydrazo, and Azoxy Compounds, S. Patai (ed.) Wiley, New York, 1974, Chapter 9. 

Discovered over a century ago, electrophilic mercuration is probably the oldest known C-H bond-activation reaction with a metal compound. The earliest examples of aromatic mercuration were reported by Volhard (mercuration of thiophene) [1], Pesci (mercuration of aromatic amines) [2], and Dimroth [3], who was the first to mercurate benzene and toluene, generalize the reaction, and assign the correct structures to the products originally observed by Pesci. Since the work of Dimroth electrophilic aromatic metalation reactions with compounds of other metals, for example Tl(III), Pb(IV), Sn(IV), Pt(IV), Au(III), Rh(III), and Pd(II), have been discovered [4], In this chapter, we will focus on intermolecular SEAr reactions involving main-group metal electrophiles and resulting in the formation of isolable metal aryls which find numerous important applications in synthesis [5], Well-known electrophilic cyclometalation reactions, for example cyclopalla-dation can be found in other chapters of this book and will not be reviewed here. 

Cyclometallation is a reaction in which a chelate metal alkyl or aryl is generated by cleavage of a C—H bond in a coordinated ligand, a new metal-carbon a bond being formed at the point of cleavage (e.g. compound (182) in Section 45.5.4.5.iii). Since most of the work on these types of complex is contained in another review (reference 1, p.734), only a brief survey is included here. 

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." ... 

In this section, we will highlight the development in the use of metal alkox-ides for the synthesis of new and interesting organometallic compounds, many of these are either inaccessible or difficult to synthesize by common synthetic procedures. We will not discuss (a) the chemistry of organometallic compounds containing alkoxides as supporting ligands, for which excellent reviews by Chisholm and co-workers (154, 513, 514) are available and (b) intramolecular cyclometalation (i.e., C—H bond activation) reactions of metal aryloxides due to the availability of an excellent account of this topic in a review article by Rothwell (515). Furthermore, a brief mention of the use of a related metal derivative (i.e., metal aryloxide) will be made merely for comparison. 

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. 

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. 

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. 

Numerous electrochemical measurements have been carried out with the ruthenium diimin complexes [15], mainly with the aim of comparing electron-transfer processes in the ground and in the excited state, and for the determination of the character of the frontier orbitals. Much less data are known for the cyclometallated analogs. By far the most popular method for the electrochemical measurements is cyclic voltammetry (CV). The measurements are mostly done in nonaqueous solutions (acetonitrile, dimethylfor-mamide, etc.). A general difficulty in such measurements is the reference potential, and the use of an internal standard like, for example, Ru(bpy)2 + is therefore highly recommended. Table 1 contains a compilation of electrode potentials of cyclometallated complexes of the type considered in this review. For comparison, the values of Ru(bpy) + are included in the table. 

Within this topic may be included agostic interactions and cyclometalation. A general review concerning the latter topic has appeared,as has a more mechanistically oriented review on the cyclometalation chemistry of aryl oxide ligands. ... 

Cyclometallation is an effective method for forming metallocarbon bonds, especially for d , square-planar complexes. This topic has recently been reviewed with the focus upon 8-methylquinoline and similar substrates. Further study of the reversible Pd—N based systems has revealed that a combination of factors determines the preferred attack site of bifunctional substrates. " Five-... 

The activation of intramolecular carbon-hydrogen bonds by metal ions in the gas phase first involves tt-coordination, followed by remote functionalization of C—H or C—C several carbons distant from the initial coordination site. Schwarz has reviewed his prolithic work in this area. High-resolution translational energy loss measurements on the remote functionalization of internal alkynes by Cr" have been made. These indicate that the ions involved are in the ground state and that no added collisional energy transfer is needed for cyclometallation to proceed. The reactions of Fe", Co", and/or Ni" have been studied with alkyl-substituted amines, " alcohols, ketones, nitriles, " ... 

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