Metal-ligand cooperation , pyridines

Deprotonation of a pyridinylmethylenic proton of pyridine- and bipyridine-based pincer complexes can lead to dearomatization. The dearomatized complexes can then activate a chemical bond (H-Y, Y = H, OH, OR, NH2, NR2, C) by cooperation between the metal and the ligand, thereby regaining aromatization (Figure 1.1). The overall process does not involve a change in the metal s oxidation state [6-8]. In this chapter, we describe the novel, environmentally benign catalytic synthesis of esters, amides, and peptides that operate via this new metal-ligand cooperation based on aromatization-dearomatization processes. 

A partioilar case of this type of metal-ligand cooperation (MLC), studied by Milstein et al, is based on dearomatization/aromatization processes on the ligand. These studies have been comprehensively covered in several recent reviews, but due to their relevance and fundamental importance they have been, although briefly, also included herein. These authors found that pyridine-based PNP and PNN (structures 33 and 34, Fig. 20), and bipyridine-based (structure 35, Fig. 20) pincer ligands can stabilize coordinatively unsaturated complexes. 

Milstein D (2010) Discovery of environmentally benign catalytic reactions of alcohols catalyzed by pyridine-based pincer Ru complexes, based on metal-ligand cooperation. Top Catal 53 915... 

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