Metal-ligand cooperation complex

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. 

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... 

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. 

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]. 

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... 

Zeng G, Li S (2011) Insights into dehydrogenative coupling of alcohols and amines catalyzed by a (PNN)-Ru(II) hydride complex unusual metal-ligand cooperation. Inorg Chem 50 10572... 

Bond Activation by Metal-Ligand Cooperation Design of Green Catalytic Reactions Based on Aromatization-Dearomatization of Pincer Complexes... 

Keywords Alcohols Amides Amines Carbon dioxide Catalysis Dehydrogenative coupling Esters Hydrogen Imines Iridium Metal-ligand cooperation O-H activation Pincer complexes PNN PNP Ruthenium... 

Fig. 3 Proposed mechanism for the direct conversion of alcohols to esters catalyzed by complex 5 via metal-ligand cooperation...

While we concentrated mainly on the catalytic applications of ruthenium PNP and PNN pincer complexes described above, we have also established the metal-ligand cooperation in C-H and H-H activation reactions by Ir-PNP pincer complexes. The Ir-PNP pincer complex 27, which was formed by C-H activation of benzene by a dearomatized Ir(I) complex, reacts with H2 to provide a trans-dihydride complex exclusively. Use of D2 revealed, surprisingly, formation of D-Ir-H in 28, while one D atom is attached to pyridinylmethylenic carbon (Scheme 9). Further studies... 

Nozaki et al. applied the metal-ligand cooperation of Ir(III) PNP pincer complexes for the highly efficient catalytic hydrogenaticm of carbon dioxide to formats [83]. The chloroiriditim(ni) dihydride PNP complex 31 was synthesized by the reaction of [Ir(coe)2Cl]2 (coe = cyclooctene) with PNP ligand under hydrogen pressure. Reaction of complex 31 with excess of NaH resulted in the formation of stable Ir(III) trihydride PNP pincer complex 32 (Scheme 10). 

Fig. 14 Proposed catalytic cycle for the hydrogenation of carbon dioxide, facilitated by metal-ligand cooperation of Ir(ni) PNP pincer complex 32...

The unprecedented high catalytic activity associated with complex 32 in comparison to the other reported catalysts [84-92] for the same transformation is attributed to the stability (as a result of strong coordination between the metal and the PNP pincer ligand) of complex 32 and to metal-ligand cooperation. 

While redox chemistry of metal complexes typically takes place at the metal center, the use of metal species bearing so-called redox non-innocent ligands may promote a metal/ligand cooperation in a synergistic manner. Therefore such complexes offer interesting prospects for uncovering unprecedented stoichiometric... 

The transformation of alcohols to carboxyhc acids with no oxidant or hydrogen acceptor uses water as the oxygen atom source with concomitant emission of dihydrogen gas. The reaction is catalyzed by a ruthenium complex at a low loading (0.2 mol%) in basic aqueous solution (Scheme 23). The same or related complexes are active in many other tandem reactions which involve dehydrogenative oxidation the proposed mechanism of the catalysis involves a metal—ligand cooperation and both... 

In the case of polynuclear metal cluster SCO complexes in the solid state, there will be intra-cluster, as well as inter-cluster cooperativity. To eliminate inter-cluster effects totally, studies must be made in dilute solutions. Williams et al. have done just this for a dinuclear [Fe(II)2L3] helicate complex which does not contain a good superexchange pathway between the Fe(II) centre but, rather, three flexible bis-bidentate ligands. A very broad, two step, SCO was observed (LS-LS<->LS-HS<->HS-HS) and fitted to a model for negative cooperativity in which subtle structural changes around each Fe oc-... 

Figure 1. Plot of v/V ax versus the millimolar concentration of total substrate for a model enzyme displaying Michaelis-Menten kinetics with respect to its substrate MA (i.e., metal ion M complexed to otherwise inactive ligand A). The concentrations of free A and MA were calculated assuming a stability constant of 10,000 M k The Michaelis constant for MA and the inhibition constant for free A acting as a competitive inhibitor were both assumed to be 0.5 mM. The ratio v/Vmax was calculated from the Michaelis-Menten equation, taking into account the action of a competitive inhibitor (when present). The upper curve represents the case where the substrate is both A and MA. The middle curve deals with the case where MA is the substrate and where A is not inhibitory. The bottom curve describes the case where MA is the substrate and where A is inhibitory. In this example, [Mfotai = [Afotai at each concentration of A plotted on the abscissa. Note that the bottom two curves are reminiscent of allosteric enzymes, but this false cooperativity arises from changes in the fraction of total "substrate A" that has metal ion bound. For a real example of how brain hexokinase cooperatively was debunked, consult D. L. Purich H. J. Fromm (1972) Biochem. J. 130, 63.

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