PCP pincer systems

Recently, Kaska and coworkers ]37] reported a rigid PCP pincer system based on an anthracene backbone (29) (Scheme 13.17). The iridium derivative of this... 

The anionic, tridentate piticer stracture suppresses the formation of an electrophilic it-aUylpalladium intermediate as established by Prof Szabo [42] in the PCP-pincer system. It is assumed that the nucleophilicity of the o-allylpalladium is further enhanced by strong trans influence and electron-donating nature of the Si atom, reahzing a facile nucleophilic addition to less reactive COj. 

Thus, most of the information collected for this system, including catalytic and electrochemical experiments, led the authors to conclude that the PCP pincer system (16) is just the second best of the series tested in this study and that the ratedetermining step for the electrochemical reduction of CO2 using this compound is probably the formation of an intermediate (19) such as that shown in Scheme 2.12. However, most of the efforts to prepare this species (19) independently were unsuccessful. 

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... 

Johansson et al. [48] were interested in improving this bimetallic system, and proposed the existence of PCP-pincer ligands (2,6-bis[(di-t-butylphosphino)methyl]... 

Pincer complexes with other chelating donors other than phosphorus are also useful as SRPCs. NCN- or SCS-structures are likely to be less stable than PCP-pincer complexes. Therefore, catalytic turnover nsing these precatalysts is possible at lower reaction temperatures. Generally, precatalysts 194-198 were tested in type 1 reactions with activated substrates (Table 2.10, Figure 2.25) [250-256]. Attachment of pincer complexes to various polymeric supports enabled the design of recyclable systems for type 1 reactions [250]. Moreover, lower stability of NCN- or SCS-pincer complexes ensures higher effective concentrations of catalytically active species precatalyst 195 was shown to serve well in reactions with activated aryl chlorides (Table 2.10, entry 3), typical substrates for type 2 processes [252],... 

Pincer complexes catalyze a variety of other organic reactions [49-51]. Hence, this work is currently being extended to other metals, and other more readily accessible PCP systems. For example, as shown in Scheme 3, lO-Rfs can be converted to the iridium hydride chloride complex 15-Rfs. Closely related dihydride complexes catalyze dehydrogenations of alkanes at high temperatures [52], However, no efforts to develop recoverable catalysts have been reported to date. 

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