Pincer ligand

Pyridine complexes of Pd- and Pt-pincer ligands are also suitable substrates for olefin metathesis [116]. The first-generation catalyst 9 efficiently mediates the RCM of diallylphosphines and diallyl sulfide when the heteroatom is com-plexed by a cationic [C5H5(NO)(PPh3)Re] moiety [117]. This principle has been exploited in the same study for tungsten, rhodium, and platinum complexes. 

Reinhoudt and coworkers studied the synthesis of hyper-branched polymers composed of organopalladium complexes with an SCS pincer ligand [11]. Removal of acetonitrile ligands on palladium led to the self-assembly of dinu-clear palladium complex (9) to give hyper-branched polymer (10), which was... 

Palladium complexes of inexpensive, easily synthesized bis(phosphinite) PCP -pincer ligands show good activity in the Suzuki coupling of deactivated and sterically hindered aryl bromides.256... 

Scheme 3 Syntheses of metal complexes of fluorous PCP pincer ligands 10-Rf ...

The success of derivatives of 1 and 2 as dehydrogenation catalysts has led to the investigation of numerous different pincer ligands for iridium-catalyzed alkane dehydrogenation. The Anthraphos pincer iridium complex (3-H2) was expected to afford even greater thermal stability (Eig. 1), and indeed, the catalyst can tolerate reaction temperatures up to 250°C [42]. The catalytic activity of 3-H2, however, is much less than that of I-H2 under comparable conditions. 

Fig. 7.21 [Gl] Carbosilan dendrimer with pincer ligands (Ni"-complex).

Pincer ligand Carbosilane Den- Ni Kharasch reaction 0.03 mol% 1-79 Ultrafiltration Negative dendritic 68, 70,... 

Pincer ligand Hyperbranched Pd Aldol 0.8 mol% >95 Membrane separa- No dendritic effect 69... 

Pincer ligand scaffolds have been shown to possess an extraordinarily useful potential for directing the properhes and reactivity patterns of coordinated metal (M) centers. This is illustrated not only by the sheer number of publications on this topic, but also by the various applications that have been disclosed by using pincer-type ligands, such as highly efficient sensors, switches and catalysts [1]. 

Figure 12.1 General representation of an ECE pincer ligand coordinated to a metal fragment (MX L ), including the different functions that can be regulated by appropriate ligand modifications.

Figure 12.2 Bonding of a neutral PNP pincer ligand to an lrX E fragment.

The de- and re-aromatization of the pyridine moiety of the pincer ligand appears to be crucial for this process. This is also the key underlying feature in the oxidative addition of H2 by complex 19 in an apparent iridium(111) oxidation state, which results in the formation of the dihydride complex 20 (Scheme 12.9). Similarly, the addition of CO to the iridium(I) complex 18 formally results in oxidation of the metal center and provides the iridium(III) complex 21 (Equation 12.9). 

In spite of the success that Ir—PCP pincer compounds have enjoyed in the development of more efficient and promising alkane dehydrogenation systems, one caveat of using these complexes has been the sometimes difficult or tedious synthesis of the pincer ligands and complexes. An answer to this problem was... 

By the same method, the authors were able to synthesize dendrimers of the generations zero and one, bearing the para lithiated pincer ligands in their periphery. Compounds containing four and twelve lithium centres could be obtained cleanly and in almost quantitative yield (as an example, dodecalithiated carbosilane dendrimer 36 is shown in Figure 12). 

Van Koten and coworkers also applied this strategy to obtain intramolecularly stabilized 1,4-dilithiated pincer ligands of the type 72. l,4-Dilithio-2,3,5,6-... 

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