CCC ligands

The monoanionic CCC ligand type was also used by Chianese and coworkers to prepare the Ir complexes 47a-f with benzimidazolin-2-ylidene moieties to prevent the formation of abnormal carbenes. They treated the imidazolium salt 8 2HBr with 30 equiv of triethylamine as base and 0.5 equiv of [(p-Cl)Ir(COD)]2 in acetonitrile at 80 °C to obtain the Ir(III) complexes 47 (Scheme 9.8). However, a clean reaction was obtained only in the case of 47 bearing Al-mesityl (a) or A/ -2,6-di-isopropylphenyl (b) substituents at the NHC moiety. With A/ -3,5-dimethylphenyl (c), A/ -3,5-di-tert-butylphenyl (d), N-tert-hutyl (e), and Al-l-adamantyl (f) substituents, they had to use a slight excess of CsF as base to obtain the respective Ir(III) complexes 47 [16]. For complex 47b, the group had to raise the temperature in an autoclave up to 150 °C [16b]. 

Some of the most stable NHC-Pd complexes reported to date feature a pincer type CNC or CCC ligands, where the two terminal ligands are NHCs (Figure 3.2). For example, complex 63 decomposed in refluxing A, A -dime-thylacetamide (DMA) (bp = 165 °C) depositing Pd black after 8 h, while 55 was unchanged after 24 h at this temperature. In Heck reactions catalyst 55 had no induction period and no loss of catalytic activity was found in the presence of metallic mercury. 

It is appropriate to identify our approach to developing the present review in the context of the Co chapter in CCC(1987). The first-edition chapter on Co featured a focused discussion and tabulation of synthetic methods, and many of these basic methods are still employed in synthesis today. Consequently, to avoid repetition, there will be diminished description here where prior appropriate methods have been provided, and only newer developments featured. The last two decades feature the development of many mixed-donor and sophisticated multidentate and macrocyclic ligands, which found limited coverage in the previous edition, and these will be discussed in more detail herein. Reaction kinetics and mechanism were also described thoroughly in the previous edition. We shall not reiterate this material, since the core mechanisms of many reactions involving Co compounds are now adequately defined. 

The dithioacid family of ligands includes dithiocarbamates (R2N CS2 ), xanthates (RO-CS2 ), thioxanthates (RS CS2 ) and dithiocarboxylates (R CS2 ), which have been described in CCC(1987, Section 47.8.10).1 The ligands are prepared by addition of a suitable nucleophile to the carbon center in CS2, usually in the presence of a base. It has been established for some decades that they bind to Co almost exclusively as bidentate chelates, including S-donors acting as bridging ligands in dinuclear systems. 

The material included in this chapter has been organized by oxidation state, with further subdivision into ligand donor type, and was obtained from reports published in primary research journals. The article will cover coordination complexes of copper in three oxidation states Cum, Cu11, and Cu1. The sections dealing with specific ligand donor types cut across several structural types. In view of the all-inclusive nature of the previous review of CCC(1987), no effort will be made here to present a comprehensive account. Instead, specific cases will be chosen for discussion because they exemplify important concepts concerning the relationship of ligand structure to metal complex properties. 

This volume presents a survey of significant developments in the chemistry of Groups 7 and 8 of the transition metals since the publication of Comprehensive Coordination Chemistry (CCC) in 1987. The material for each element is organized by oxidation state of the metal and also by the nature of the ligands involved, with additional sections covering special features of the coordination chemistry and applications of the complexes. 

In addition to water molecules the coordination chemistry of leaching generally involves simple inorganic anionic ligands, ammonia, or acetonitrile. Many of the well-established processes (see Table 2) were considered in CCC (1987),4 and are also described in a recent comprehensive text on hydrometallurgy.2... 

This section summarizes work carried out on polynuclear complexes containing M(bpy)2 units, an area in which there is much interest, in particular with respect to energy transfer. Dendritic systems are excluded from this review, but are covered elsewhere in CCC The complexes-as-ligands strategy is commonly exploited for the controlled construction of multinuclear complexes and examples are seen in this section. 

Ruthenium(IV) and osmium(IV) phosphoraniminato complexes are formed by nucleophilic attack of phosphines on the nitrido ligand of ruthenium(VI) or osmium(VI). The first examples of this type of complexes are [Os (NPR3)(PR3)2(Cl)3] and [Ru (NPEt2Ph)(Cl)3(PEt2Ph)2], which have been documented in CCC (1987). While there are quite a few osmium complexes of this class, there appears to be only one structurally characterized ruthenium complex. 

Some other intermolecular C-H activations involving the NHC ligand have been observed during the synthesis of particular NHC-containing pincer -type complexes also called CCC-NHC complexes. In addition to zirconium- and rhodium-based complexes (210) and (211)/ several examples involving palladium of general structure (271) have been synthesized. Whereas Faller... 

One of the consequences of the large increase in the number of structurally characterized compounds reported since the publication of Comprehensive Coordination Chemistry (CCC, 1987) is that some of the long-standing expectations for Group 1 and 2 chemistry need to be qualified. A conventional generalization holds that the coordination number (c.n.) of a complex should rise steadily with the size of the metal ion, and there is in fact abundant data to support this assumption for small monodentate ligands. For example, analysis of water-coordinated ions indicates that the most common c.n. for Mg +, and Ca + are four, six, and six to eight,... 

Fig. 1 Separation by affinity-ligand pH-zone-refining CCC in the ion-exchange mode of the main components from a sample of D C Yellow No. 10 (Quinoline Yellow, Cl 47005). (a) HPLC analysis of the original mixture (b) pH-zone-refining CCC elution profile and HPLC analyses of the combined fractions 81-103 and 114-138, respectively. For experimental conditions, see text and Ref. 9.

Fig. 4 Some applications of pH-zone-refining CCC. (a) Separation of eight CBZ dipeptides (see Table 1) [4,8] (b) separation of amaryllis alkaloids using both the lower phase (upper chromatogram) and upper phase (lower chromatogram) as the mobile phase (see Table 1) [4,9] (c) separation of catecholamines using a ligand (see Table 2) [4,12] (d) separation of two groups of dipeptide each using an affinity ligand [4,13] (see Table 2). Continued)...

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