Ru-pincer complex

Many interesting reactions and catalytic applications are known especially for Pd and Ru pincer complexes [7]. Ir and Rh pincer complexes are less common, but for some catalytic applications that require high temperatures, such as alkane dehydrogenation, pincer complexes seem to be irreplaceable [8]. 

The cationic Ru(ii) phosphine pincer complex 76 [95], which contains an agos-tic Ru-H-C bond, possesses four different P-atoms and thus demonstrates the geometric dependence of J( P, P). The fairly small value, 112 Hz, helps in... 

Though in moderate yield, the reported conversion of ethanol to ethyl acetate by complex 14 and a related osmium dimer complex [37] by Gusev generated interest in the catalytic synthesis of ethyl acetate from ethanol because it is a widely used fine chemical. Very recently, Beller et al. [38] screened the catalytic activity of various known pincer complexes for this transformation and found thatTakasago s complex 15, known as Ru-MACHO catalyst, is very efficient and the reaction in the presence of a base resulted in very good TON (Table 1.4). 

In addition to these transformations by well-defined pincer complexes, the complexes assembled in situ from the metal precursor [Ru(GOD)(methylallyl)2] (COD, cyclooctadiene) and PNP or PNN pincer ligands (used for complexes... 

Complex 26 reacted with LiEtj BH to form a mixture of three Ru hydride species (27a-c) (Scheme 6.7) [38]. The ratio of 27a b c observed in situ was approximately 2 1 1 and heating of the mixture did not change the observed ratio of these three complexes. Complex 27a was isolated from this mixture and assigned as a dinitrogen adduct of the type (PSiP)RuH(N2)(PPh3). Facile formation of dinitrogen adducts has been reported in related PCP- and PNP-Ru pincer chemistry [49-51]. 

This is currently a field of intensive investigation, particularly for asymmetric catalysts capable of promoting asymmetric syntheses. Metallophosphine complexes (favourite metals studied being Rh, Ru, Re, Ir, Pt, Pd, Co, Ni) are remarkable not only for the variety of reactions they may catalyse, but also in some cases for their high specificity of action. Transition metal pincer complexes are particularly fashionable since they are very stable and can impose unusual reaction pathways [9,31]. These properties have greatly stimulated molecular structure studies, but their reaction mechanisms often remain speculative. 

Pincer-ligated metal complexes have displayed extraordinarily rich chemistry and have found widespread use in catalysis. Pincer complexes of numerous transition metals have been synthesized, but the most well-studied probably involve Ru, Rh, Ir, and Pd [1-7], Our group has largely focused on pincer-iridium complexes, which have shown a strong tendency toward the activation of C-H bonds. These complexes have been found to effect the oxidative addition of a variety of C-H bonds including those with sp - and sp-hybridized carbon [8-10], Most notable, however, has been the activation of C(sp )-H bonds, leading to alkane dehydrogenation [6, 7],... 

Tridentate ligands of the pincer-type (PNP, PNN, or NNN) have also been successfully employed as ancilary groups for synthesizing the Ru-vinylidene complexes, e.g. 9-14. It should be noted that some of these complexes... 

Within the last years, a variety of Rh and Ir bis(NHC) pincer complexes could be synthesized. After pubUshing the first Rh bis(NHC) pincer complex 32, Peris et al. showed that this complex class is highly valuable for catalytic applications and worth investigating. Although the activity of their complex in transfer hydrogenation of ketones is far less than that of the most active Ru PCP complexes [44], the activity was nevertheless comparable to that of other Rh NHC complexes at that time. 

Several pincer complexes of this type, mainly based on Ru which are able to participate in MLC processes, have been described showing their abiHty as versatile catalysts for hydrogenation, dehydrogenation, and related reactions. It has to be highlighted that the first appHcation of dearomatization-reprotonation was the employment of complex 36 for the dehydrogenation of alcohols, wherein the combined metal and dearomatized ligand act in cooperative manner to convert, for example, primary alcohols to esters or secondary alcohols to ketones, with concomitant extrusion of H2 (Scheme 38). ... 

A number of examples of coupling reactions have been reported that involve either isolable or postulated vinylidene complexes. Addition of PhG=GH to the the pincer complex [Ru(PGP)(PPh3)Gl] generates the coupling product... 

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

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. 

The electron-rich bipyridine-based Ru(II)-PNN pincer complex 4 was synthesized by the reaction of the tridentate ligand, BPy- PNN (5) with [RuHCl (PPh3)3(CO)] in THF at 65°C (Scheme 2). The single-crystal X-ray structure of... 

Scheme 2 Synthesis of bipyridine-based H-Ru(II) Pincer complexes (4 and 6)...

Fig. 4 Hydrogenation of esters to alcohols catalysed by Ru(II)PNN pincer complex (2)...

Scheme 5 Ru(II) pincer complexes for homogeneous hydrogenation of esters to alcohols...

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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