Metal-Ligand Cooperativity

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. 

Figure 1.1 Metal-ligand cooperation based on aromatization-dearomatization.

In 2011, Gelman and coworkers [43] reported a PCjp P iridium pincer complex (17), which exhibited efficient catalytic activity for the conversion of benzyl alcohol to benzyl benzoate (Scheme 1.9) and was suggested to operate via a novel metal-ligand cooperation mode. Both electron-withdrawing and electron-donating substituents... 

The aminolysis of esters catalyzed by complex 8 is possibly initiated by N-H activation of the amine by metal-ligand cooperation involving 8. Ester coordination followed by intramolecular nucleophilic attack by the amido ligand at the acyl functionality is thought to be a key step [15]. Overall, in one catalytic cycle, two... 

Keywords Acceptorless alcohol dehydrogenatiOTi AUylic C-H activation Axial reactivity Carbenoid C-H insertion C-H activation Cycloprpanation Diruthenium compounds Metal nitride Metal-ligand cooperation Metal-metal bond... 

Scheme 25 Catalysts showing metal-ligand cooperativity...

In a similar fashion, so-called "solvent assisted H-H bond cleavage via metal-ligand cooperation" (TS, in Figure 1.21) that regenerates the catalyst represents a two-step process in solution first N-protonation of... 

This additional interaction increases the strength of the primary DHB-bonding interaction between an alcoholic substrate and a catalyst nonadditively via the so-called cooperative effect in hydrogen bonding, 2i and could explain the well-known catalytic efficiency of bifunctional catalysts in the dehydrogenation of alcoholic substrates as an alternative path to H-H bond formation via metal-ligand cooperation as shown... 

Metal-Ligand Cooperation Based on Aromatization/Dearomatization Processes 101... 

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. 

Zell T, Milstein D. Hydrogenation and dehydrogenation iron pincer catalysts capable of metal—ligand cooperation by aromatization/dearomatization. Acc Chem Res. 

Askevold B, Nieto JT, Tussupbayev S, et al. Ammonia formation by metal—ligand cooperative hydrogenolysis of a nitrido ligand. Nat Chem. 2011 3 532-537. 

Synthesis of H-Ru(II)pincer Complexes and Activation of Dihydrogen via Metal-Ligand Cooperation Approach... 

Our approach towards activation of molecular hydrogen and hydrogenation reactions is based on the metal-ligand cooperation concept (MLC). In this section we describe the synthesis of Ru(ll)-H pincer complexes prepared by our group and their activity towards activation of dihydrogen. 

Like in the case of 2, the bipyridine-based Ru(II)-PNN pincer complex 6 plausibly displays a novel type of metal-ligand cooperative activity through aromatization-dearomatization processes. Recently, a DFT calculation on classical C-O cleavage vs our newly reported C-N bond breaking in the amide hydrogenation reaction was reported by Cantillo [69]. 

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