Chelation robust types

The results of some initial experiments indicated that the catalytic activity of 1 is strongly temperature-dependent. For instance, at 100 °C, the rates are relatively low but there is an appreciable turnover [turnover frequency (TOE) = 20.5 h" j. Increasing the reaction temperature to 200 °C increases the TOE to 720 h hence, the thermal robustness of the iridium catalyst is pivotal for optimal catalyst performance. The tridentate pincer-type ligands provide a particularly stable platform for metal confinement through covalent Ir—C bonding combined with the terden-tate chelating coordination. 

However, the most employed half-sandwich complexes with chiral chelating ligands were reported by Noyori et who designed a new type of active and robust Ru(II) catalyst precursors based on complexes of monotosylated diamines. The structure of one of these catalyst precursors 3.21 was determined by X-ray analysis and showed that the metal has (R) configuration (see Figure 3.6). 

The rich coordination chemistry of palladium provides a plethora of more or less readily available complexes, almost all applicable in type 1 or type 2 Mizoroki-Heck reactions. Only a limited series of truly robust and almost indefinitely stable ( dead ) palladium complexes, such as palladium porphyrinate or phthalocyanine complexes, as well as salen-type chelates with Schiff bases, are known not to release palladium even under the harshest conditions used in Mizoroki-Heck reactions. Results from the investigation of reactivity of... 

Seeking robust NHC-incorporating pincer-type chelating ligands, we designed a bis-phenoxide-NHC ligand that combines two phenoxide substituents directly connected to the nitrogen atoms [49]. It was envisioned that such a structure would be suitable for coordination to various metals across the periodic table. 

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