Cyclometallation comparison

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. 

As observed earlier, no information is available on cyclometallated complexes of Os(II). There is no reason to assume that this metal does not form such complexes and it would be worthwhile to synthesize a suitable selection for the sake of comparison with the corresponding species of the other metals in this region of the periodic table. 

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. 

Table 3 Comparison of structural and photophysical data for cyclometalated diimine Pt(II)/Pd(II) complexes... 

It is also noteworthy to mention that the A-adamantyl group forms a particularly stable chelate with these cyclometalated complexes. Catalysts bearing smaller substituents were found to be much more sensitive to air, moisture, and even temperature in comparison with 6. For example, catalyst 12 could not be isolated under the standard purification conditions, and catalyst 11 required storage under inert atmosphere below room temperature (—30°C) [55]. 

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